report · 2017-11-08 · “exergy assessment guidebook for the built environment“ edited by...

“Exergy Assessment Guidebook for the Built Environment“Edited by Herena Torio and Dietrich Schmidt

reportECBCS Annex 49

Low Exergy Systems for High-PerformanceBuildings and Communities

A n n e x 4 9 S u m m a r y r e p o r t

FRAUNHOFER VER LAG

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© Copyright Fraunhofer IBP 2011

All property rights, including copyright, are vested inFraunhofer IBP (Germany), Operating Agent for theECBCS Executive Committee Support Services Unit,on behalf of the Contracting Parties of the Internatio-nal Energy Agency Implementing Agreement for aProgramme of Research and Development on EnergyConservation in Buildings and Community Systems.

In particular, no part of this publication may bereproduced, stored in a retrieval system or transmit-ted in any form or by any means, electronic, mecha-nical, photocopying, recording or otherwise, withoutthe prior written permission of Fraunhofer IBP.

Published by Fraunhofer Verlag Nobelstraße 12 D-70569 Stuttgart Germany

Disclaimer Notice:This publication has been compiled with reasonableskill and care. However, neither Fraunhofer IBP northe ECBCS Contracting Parties (of the InternationalEnergy Agency Implementing Agreement for a Pro-gramme of Research and Development on EnergyConservation in Buildings and Community Systems)make any representation as to the adequacy oraccuracy of the information contained herein, or asto its suitability for any particular application, andaccept no responsibility or liability arising out of theuse of this publication. The information containedherein does not supersede the requirements given inany national codes, regulations or standards, andshould not be regarded as a substitute for the needto obtain specific professional advice for any parti-cular application.

ISBN: 978-3-8396-0239-3

Participating countries in ECBCS:Australia, Austria, Belgium, Canada, P.R. China,Czech Republic, Denmark, Finland, France, Germa-ny, Greece, Italy, Japan, Republic of Korea, theNetherlands, New Zealand, Norway, Poland, Portu-gal, Spain, Sweden, Switzerland, Turkey, UnitedKingdom and the United States of America.

Further information about the ECBCS programmemay be obtained from www.ecbcs.org

reportECBCS Annex 49CONTENT

TABLE OF CONTENTS

Preface 4

1 Introduction 7Background and motivationThe exergy approachBenefits and outcome: why exergy?Target groupMain objectives and layout of this report

2 Method and models for exergy analysis 11Applied fundamentalsThe reference environmentDescription of the method for exergy analysisExergy and thermal comfortExergy in building systems

3 Tools for exergy analysis 23Annex 49 pre-design toolCascadia toolSEPE: an Excel calculation tool for exergy-based optimisationsDesign Performance Viewer (DPV)Tool for calculating exergy in thermal comfortDecision tool for energy efficient cooling for building retrofit

4 Low exergy design strategies 33General design strategies for building systemsGeneral design strategies for community systems

5 Exergy benchmarking parameters 39Parameters for exergy performanceBenchmarking for components of building systemsGraphical representations for characterising the exergy performance of community supply systemsPre-normative proposals

6 Application of the exergy approach to building systems 49IntroductionInnovative building case studies

7 Application of the exergy approach to community case studies 55IntroductionInnovative community case studies

8 Conclusions 69

9 References 71Appendix A: ECBCS Annex 49 participating countriesAppendix B: Additional information from ECBCS Annex 49

summaryECBCS Annex 49 PAGE 4

PREFACE

International Energy Agency (IEA)The International Energy Agency was established in1974 within the framework of the Organisation forEconomic Co-operation and Development (OECD)to implement an international energy programme. Abasic aim of the IEA is to foster co-operation amongthe twenty-four IEA participating countries and toincrease energy security through energy conserva-tion, development of alternative energy sources andenergy research, development and demonstration(RD&D).

Energy Conservation in Buildings and CommunitySystems (ECBCS)The IEA co-ordinates research and development in anumber of areas related to energy. The mission ofone of those areas, the ECBCS - Energy Conserva-tion for Building and Community Systems Program-me, is to develop and facilitate the integration oftechnologies and processes for energy efficiencyand conservation into healthy, low emission, andsustainable buildings and communities, throughinnovation and research.

The research and development strategies of theECBCS Programme are derived from research dri-vers, national programmes within IEA countries, andthe IEA Future Building Forum Think Tank Workshop,held in March 2007. The R&D strategies represent acollective input of the Executive Committee membersto exploit technological opportunities to save energyin the buildings sector, and to remove technicalobstacles to market penetration of new energy con-servation technologies. The R&D strategies apply toresidential, commercial, office buildings and com-munity systems, and will impact the building industryin three focus areas of R&D activities: • Dissemination • Decision-making• Building products and systems

The Executive Committee (ExCo)Overall control of the program is maintained by anExecutive Committee, which not only monitors exi-sting projects but also identifies new areas wherecollaborative effort may be beneficial. To date, thefollowing projects have been initiated by the execu-tive committee on Energy Conservation in Buildingsand Community Systems (completed projects areidentified by (*) ):

Annex 1: Load Energy Determination of Buildings (*)

Annex 2: Ekistics and Advanced Community Energy Systems (*)

Annex 3: Energy Conservation in Residential Buildings (*)

Annex 4: Glasgow Commercial Building Monitoring (*)

Annex 5: Air Infiltration and Ventilation Centre

Annex 6: Energy Systems and Design of Communities (*)

Annex 7: Local Government Energy Planning (*)

Annex 8: Inhabitants Behaviour with Regard to Ventilation (*)

Annex 9: Minimum Ventilation Rates (*)

Annex 10: Building HVAC System Simulation (*)

Annex 11: Energy Auditing (*)

Annex 12: Windows and Fenestration (*)

Annex 13: Energy Management in Hospitals (*)

Annex 14: Condensation and Energy (*)

Annex 15: Energy Efficiency in Schools (*)

Annex 16: BEMS 1- User Interfaces and SystemIntegration (*)

Annex 17: BEMS 2- Evaluation and Emulation Techniques (*)

Annex 18: Demand Controlled Ventilation Systems (*)

Annex 19: Low Slope Roof Systems (*)

Annex 20: Air Flow Patterns within Buildings (*)

Annex 21: Thermal Modelling (*)

Annex 22: Energy Efficient Communities (*)

Annex 23: Multi Zone Air Flow Modelling (COMIS) (*)

Annex 24: Heat, Air and Moisture Transfer in Envelopes (*)

Annex 25: Real time HVAC Simulation (*)

Annex 26: Energy Efficient Ventilation of Large Enclosures (*)

Annex 27: Evaluation and Demonstration of Domestic Ventilation Systems (*)

Annex 28: Low Energy Cooling Systems (*)

Annex 29: Daylight in Buildings (*)Annex 30: Bringing Simulation to Application (*)

Annex 31: Energy-Related Environmental Impact ofBuildings (*)

Annex 32: Integral Building Envelope Performance Assessment (*)

Annex 33: Advanced Local Energy Planning (*)

Annex 34: Computer-Aided Evaluation of HVAC System Performance (*)

Annex 35: Design of Energy Efficient Hybrid Ventilation (HYBVENT) (*)

reportECBCS Annex 49PAGE 5

IEA ECBCS ANNEX 49

ECBCS Annex 49 was a three year internationalresearch project which arose from the discussionsheld in a Future Building Forum in Padova in April2005. The project began on November 2006 andran until November 2009. It involved 22 researchinstitutions, companies and universities from 12countries, many of which are also members of theInternational Society of Low Exergy Systems in Buil-dings (LowExNet). The main objective of this projectwas to develop concepts for reducing exergydemand in the built environment, thus reducing theCO2-emissions of the building stock and supportingstructures for setting up sustainable and secure ener-gy structures for this sector.

Specific objectives are to:• to use exergy analysis to develop tools, guideli-

nes, recommendations, best-practice examplesand background material for designers and deci-sion makers in the fields of building, energy pro-duction and politics

• to promote possible energy/exergy cost-efficientmeasures for retrofit and new buildings, such asdwellings and commercial/public buildings

• to promote exergy-related performance analysisof buildings, viewed from a community level.

Countries which participated in the IEA ECBCSAnnex 49: Austria, Canada, Denmark, Finland, Ger-many, Italy, Japan, the Netherlands, Poland, Swe-den, Switzerland, and the United States of America.

The work within Annex 49 is based on an integralapproach which includes not only the analysis andoptimisation of the exergy demand in heating andcooling systems, but also all other processes whereenergy/exergy is used within the building stock. Inorder to achieve this, the project worked with theunderlying basics, i.e. exergy analysis methodolo-gies. The work items were aimed at development,assessment and analysis methodologies, and thedevelopment of a tool for the design and performan-ce analysis of the regarded systems.

Annex 36: Retrofitting of Educational Buildings (*)

Annex 37: Low Exergy Systems for Heating and Cooling of Buildings (LowEx) (*)

Annex 38: Solar Sustainable Housing (*)

Annex 39: High Performance Insulation Systems (*)

Annex 40: Building Commissioning to Improve Energy Performance (*)

Annex 41: Whole Building Heat, Air and Moisture Response (MOIST-ENG) (*)

Annex 42: The Simulation of Building-Integrated Fuel Cell and Other Cogeneration Systems (FC+COGEN-SIM) (*)

Annex 43: Testing and Validation of Building EnergySimulation Tools (*)

Annex 44: Integrating Environmentally Responsive Elements in Buildings

Annex 45: Energy Efficient Electric Lighting for Buildings (*)

Annex 46: Holistic Assessment Tool-kit on Energy Efficient Retrofit Measures for Government Buildings (EnERGo)

Annex 47: Cost-Effective Commissioning for Existingand Low Energy Buildings

Annex 48: Heat Pumping and Reversible Air Conditioning

Annex 49: Low Exergy Systems for High Performance Buildings and Communities

Annex 50: Prefabricated Systems for Low Energy Renovation of Residential Buildings

Annex 51: Energy Efficient Communities

Annex 52: Towards Net Zero Energy Solar Buildings

Annex 53: Total Energy Use in Buildings: Analysis &Evaluation Methods

Annex 54: Analysis of Micro-Generation & RelatedEnergy Technologies in Buildings

Annex 55: Reliability of Energy Efficient Building Retrofitting (RAP-RETRO)

Annex 56: Energy and Greenhouse Gas OptimisedBuilding Renovation

Working Group - Energy Efficiency in Educational Buildings (*)

Working Group - Indicators of Energy Efficiency in Cold Climate Buildings (*)

Working Group -Annex 36 Extension: The Energy Concept Adviser (*)

Working Group - Energy Efficient Communities

(*) – Completed

Structure of the ECBCS Annex 49

Exergy analysis methodologies

Knowledge transfer and dissemination

Exergy supply and renewable resources

Community Level

Low exergysystems

Building Level


With this basis, the work on exergy efficient commu-nity supply systems was focused on the developmentof exergy distribution, generation and storage systemconcepts, as well as a collection of case studies. Forthe course of the project, both the generation andsupply of and the use of energy/exergy were impor-tant issues. Resulting from this, the development ofexergy efficient building technology depends on thereduction of exergy demand for the heating, coolingand ventilation of buildings. The knowledge transferand dissemination activities of the project are focu-sed on the collection and spreading of informationon ongoing and finished work.

OPERATING AGENT:Dietrich SchmidtFraunhofer Institute for Building PhysicsKassel (Germany)[email protected]

Further information can be found in internet under:www.annex49.com or www.ecbcs.org/annexes/annex49.htm

The Guidebook of ECBCS Annex 49 is the result ofa joint effort of many countries. We would like togratefully acknowledge all those who have contribu-ted to the project by taking part in the writing pro-cess and the numerous discussions. A list of the par-ticipants within Annex 49 and their correspondingcountries can be found in the Appendix. All partici-pants from all countries involved have contributed tothe guidebook. However, the following annex parti-cipants have taken over the responsibility of writingthe chapters:

This report is the summary version of the final reportof ECBCS Annex 49. The full and extended version,the ECBCS Annex 49 Guidebook, is available as aCD-ROM and also freely available on the internet(www.annex49.com).

Dietrich Schmidt editor, operating agent and Subtask D coordinator, specially chapters 1,4 and 8

Herena Torío editor, specially chapters 1, 2, 4 5 and 8

Sabine Jansen specially chapters 2 and 5

Masanori Shukuya specially chapter 2

Adriana Angelotti specially chapter 2

Petra Benz-Karlström specially chapter 3

Toshiya Iwamatsu specially chapter 3

Gudni Jóhannesson specially chapters 4, 6 and Subtask C coordinator

Marco Molinari specially chapters 3, 4 and 6

Forrest Meggers specially chapters 3, 6 and 7

Michele de Carli specially chapter 4

Pier Giorgio Cesaratto specially chapter 4

Lukas Kranzl specially chapter 4

Paola Caputo specially chapters 4 and 7

Ken Church specially chapters 3, 7 and Subtask B Coordinator (2006-2008)

Peter Op’t Veld specially chapter 7 and Subtask B Coordinator (2009)

Mia Ala-Juusela Subtask A coordinator

David Solberg specially chapters 6 and 7


1. INTRODUCTION

Background and MotivationEnvironmental problems that have been linked toextended energy use, such as global warming, haveraised a growing concern which has emphasisedboth the importance of all kinds of so-called “ener-gy saving measures”, and the necessity for an incre-ased efficiency in all forms of energy utilisation.

The consumption of primary energy in residentialand commercial buildings accounts for more thanone third of the total world energy demand. Thismeans that buildings are collectively a major contri-butor to energy related problems on a global scale.Despite the efforts made to improve energy efficien-cy in buildings, the issue of gaining an overallassessment, and comparing different energy sour-ces still exists (Schmidt and Shukuya 2003). Currentanalysis and optimisation methods do not distin-guish between different qualities of energy flowsduring the analysis. In the building codes of severalcountries, this problem has been solved by thetransformation of all energy flows to the primaryenergy demand. An assessment of energy flowsfrom different sources is first carried out at the endof the analysis by multiplying the energy flows bythe so-called primary energy factors. The primaryenergy factors necessary for the calculation havebeen derived from statistical material and politicaldiscussion and are not based on thermodynamicprocess analyses. All energy conversion steps fromthe extraction of energy sources (e.g. fuels) to thefinal demands are assessed in the primary energymethod; however, no information on the quality ofthe supplied energy and its relation to the requiredenergy demands can be obtained through thisassessment.

The quantity of energy is given by the first law ofthermodynamics, and is calculated from energybalances for a system. Current energy systems inbuildings are designed and improved based on thislaw. This means that of course the quantity of ener-gy supplied is matched with the quantity of energyrequired. Highly efficient condensing boilers, withefficiencies of up to 98% are a straightforwardresult of such an analysis framework.

The quality of energy, is given, in turn, by a combi-ned analysis of the first and second laws of thermo-dynamics. From these combined analyses, the ther-modynamic concept of exergy is derived. Exergyrepresents the part of an energy flow which can becompletely transformed into any other form of ener-gy, thereby depicting the potential of a given ener-gy quantity to perform work or, in other words, itsquality.

In every energy system, some part of the exergysupplied to the system in question has to be “cons-umed” or destroyed to make the system work. In thecase of the highly efficient boilers mentioned abovewhen used to supply low temperature heat, thepotential to produce work (exergy) of the fuels fedinto the boiler is almost completely lost in the com-bustion process. Due to this loss of energy potential,a large consumption of exergy occurs. Exergy effi-ciencies for such building systems are lower than10%.

A combined energy and exergy assessment permitsa understanding of the importance of moving awayfrom burning processes for supplying the energydemands in buildings, and paves the way for a newtechnique of designing energy supply systems inbuildings based on the use of renewable and lowtemperature heat sources.

The exergy approachMost of the energy used in the building sector isrequired to maintain constant room temperatures ofaround 20°C. Since the required temperature levelsfor the heating and cooling of indoor spaces arelow, the quality of the energy demanded for appli-cations in room conditioning are naturally low(q ≈ 7%)1.

Different levels of energy quality are needed for dif-ferent appliances within a building. If the productionof domestic hot water is considered as heating waterup to temperatures of about 55°C, the energy qua-lity needed is slightly higher than that of heating aroom to 20°C (q ≈ 15%). For energy applicationssuch as cooking or heating a sauna, an even higherquality level is needed (q ≈ 28%). For the operationof different household electrical appliances and ligh-ting the highest possible quality of energy is needed(q ≈ 100%).

Today’s energy supply structure is not as sophistica-ted as today’s energy demands. Energy is common-ly supplied as electricity or as a fossil energy carrier.Thereby, the energy quality of the supply for all dif-ferent uses is at a constant value of (q ≈ 100%), avalue that is unnecessarily high (see Figure 1.1, left).Similarly as in the case of a boiler mentioned above,the typical primary energy efficiency of the heatingprocess in Germany is approximately 70% for hea-ting newly erected dwellings that are equipped withgood building service systems. This level of efficien-cy decreases to approximately 10% when conside-ring exergy.

In turn, an adaptation of the quality levels of supplyand demand could be managed by covering, for

Figure 1.1: Left: Energy supply by means of high quality energy sources for a typical building with severaluses at different quality levels. Right: Energy supply with sources at different quality levels for the samebuilding with uses at different quality levels.


example, the heating demand with suitable energysources, as available waste district heating with aquality level of about 30%. Other appropriate lowexergy sources (i.e. low temperature sources) aresolar or ground source heat. By using these sources,quality levels of the energy demanded and suppliedare adapted to each other as shown in the right dia-gram of Figure 1.1.

New low temperature heating and high temperaturecooling systems are required to make energy use inbuildings even more efficient by supplying energywith low quality and creating the possibility of usingrenewable energy sources. There is a large varietyof such emission systems solutions on the market,e.g. water borne floor heating systems, that can beused to supply buildings with the lowest possiblesupply temperatures (q ≈ 13%). Furthermore, it hasbeen found that when low temperature systems areapplied to buildings, the thermal indoor comfort isimproved at the same cost level as by using conven-tional, less comfortable building service systems (IEAAnnex 37, 2003).

In this report a summary of the methodology andmodels behind complex dynamic exergy simulationswhich have been developed within the Annex 49project is presented. A detailed version of thesemodels and method, with detailed equations anddiscussions, are presented in the full version of theECBCS Annex 49 final report.

It should be noted that a simplified steady-state ana-lysis has proven to be adequate for the first estima-tions on the performance of different buildingsystems. Additionally, several simplified and user-friendly tools that grant building planners, archi-tects, and other decision makers of the built environ-ment access to an exergy-based building approachhave been developed within Annex 49 and are alsointroduced in this report.

Benefits and outcomes: Why exergy?As previously stated, common assessments of ener-gy utilisation in buildings are based solely on quan-titative considerations. The exergy analysis appro-ach will take the methods of building energy assess-ment a step further by considering not only thequantitative aspects of demand and supply, but thequalitative aspects as well.

The following simple example shows how an exergyanalysis can help building designers choose moreefficient energy supply systems.

Figure 1.2 shows the results from a simplified exer-gy analysis on a building case study. Different ener-gy supply systems have been considered to supplythe same low-exergy demands for space heatingand domestic hot water (DHW) production. Primaryenergy analyses focus on the maximum possible useof renewable energy sources. Based on the criteriafrom primary energy analysis the input of fossilenergy sources needs to be minimized. Following,the wood pellet boiler would be the best performingsolution, allowing minimum use of primary fossilenergy.

Exergy analyses aim additionally at minimizing boththe fossil and renewable exergy input for a givensystem. An exergy analysis promotes an efficientenergy supply, while highlighting that even renewa-ble energy sources need to be used efficiently. Figu-re 1.2 depicts the exergy input of a wood-basedboiler as being the largest of the four options. Thisis because wood is a high-quality energy sourceeven though it is renewable and the efficiency ofwood boilers is not yet as high as that of conventio-nal liquefied gas condensing boilers. The fact thatthe exergy input is the largest indicates that such anenergy supply does not promote an efficient use ofthe potential of the energy sources used. As a high-quality energy source, wood could be used insteadfor supplying high exergy demands such as electri-city generation. In this way, wood as a fuel would beused to its fullest potential.


The use of wood in a combined heat and power(CHP) unit, using the waste heat from the powergeneration process for low-exergy applications,allows minimum exergy input and minimum fossilenergy input.

A deeper understanding of the nature of energyflows and/or conversion processes in buildingswould enable building designers and architects toachieve an improved overall design.

Based on the example and considerations above,the main benefits of joining considerations on thequantity and quality of energy supply, i.e. of exergyanalysis, can be summarized as:• Exergy analysis clearly shows the importance of

promoting a more efficient use of fossil fuels.Systems such as highly energy efficient boilerswould always be avoided following this appro-ach. CHP systems, using better the high potential(i.e. quality) of fossil fuels would be promotedinstead.

• The above conclusion is also valid for renewableenergy sources with high thermodynamic potenti-al (i.e. high exergy sources), as shown in theexample presented (Figure 1.2). Thereby, exergyanalysis also promotes an efficient use of limitedavailable renewable sources such as biomass.

• Exergy analysis highlights the importance of usinglow temperature renewable energy sources avai-lable to supply heat demands in buildings.

Low temperature heating and high temperature coo-ling systems in buildings are exergy efficient emis-sion systems, that allow the integration of renewablesources. The exergy performance of these systems issignificantly higher than for conventional air basedor high temperature heating and low temperaturecooling systems. Thereby, exergy analysis is anappropriate tool for integral building design, allo-wing to see the benefits of efficient energy use inevery conversion step of the supply chain.

Target groupThis report is a summary version of the full Annex49 report. This short version is oriented to buildingplanners, architects and decision makers, and triesto bring them closer to the exergy concept by givingan overview on the main features and benefits fromthis approach. Therefore, the technical detailsbehind the exergy concept are explained in a sim-plified and applied manner, focusing more on theoutcomes of exergy analysis and its importance forbuilding systems design. In addition, the main featu-res of several building and community case studieshighlight the importance and main benefits of thisanalysis approach.

The full version of the ECBCS Annex 49 report,which is freely available under www.annex49.com,is mainly oriented, in turn, to scientists and resear-chers working in the field of energy efficient building

Figure 1.2: Calculated primary energy demand(fossil and renewable) and the related exergy fordifferent supply options of a building case study.Results correspond to steady state calculations per-formed with the Annex 49 pre-design tool (seechapter 3).

Where:

Cond. Natural gas fired condensing boiler

Wood. Wood pellet boiler

GSHP Ground source heat pump

DH (Waste) District heat

Floorh. Floor heating system

systems. The technical background and thermody-namic concepts related to the exergy analysis inbuilding systems are explained thoroughly in a clearand detailed way in the full length report which isintended to present state-of-the-art exergy analysesin buildings. Over the past years, exergy analyses ofbuilding systems have become more prevalent inscientific literature; however, exergy analysis in buil-dings (particularly dynamic exergy analysis) is acontroversial issue and is very sensitive to theassumptions made by the user. The full version of theECBCS Annex 49 report is intended to be a referen-ce for further analyses so that comparability can beguaranteed between the results of exergy analysesof different building and community case studies.

Main objectives and layout of this reportIn this context, the main objectives of the ECBCSAnnex 49 are to: • Develop design guidelines regarding exergy

metrics for performance and sustainability• Create open-platform exergy software for buil-

ding design and performance assessment • Show best practice examples for new and retrofit

buildings and communities• Document benefits of existing and developed

demonstration projects • Set up a framework for future development of

policy measures and pre-normative work inclu-ding the exergy concept

The topics mentioned above are treated in detail inthe following chapters. Chapter 2 gives a brief des-cription on the first unitary methodology for perfor-ming dynamic exergy analysis on building systems.Some fundamental concepts and the thermodynamicbackground of the exergy approach are highlighted.Detailed equations for analysis of several buildingsystems as well as an extended version of the thermo-dynamic background of exergy analysis can befound in the full version of the Annex 49 report. Inchapter 3 the tools developed within the work ofAnnex 49 are presented. A brief description of themain features, calculation approach and usability ofeach tool is also given. Chapter 4 highlights andsummarizes the main strategies for an exergy orien-ted design of buildings and community systems.Chapter 5 presents the main parameters developedor used here for characterising exergy performanceof any building or community. Based on these para-meters, the first discussions and bases for setting pre-normative proposals that include the exergy conceptare also included. Chapters 6 and 7 show the mainbuilding and community case studies analyzed wit-hin the research activities of the ECBCS Annex 49.


1Quality factors q are explained in chapter 5

2. METHOD AND MODELS FOR EXERGYANALYSIS

In this chapter a brief overview of the method andmathematical models developed for exergy analysisof building systems within the ECBCS Annex 49group is introduced. A detailed description of themathematical models and fundamentals of the exer-gy analysis method can be found in the full versionof this report.

Exergy analysis is a thermodynamic method whichhas commonly been applied since the 1950s to, forexample, complex power generation systems. Yet,energy processes in power plants and buildings aresignificantly different. In power stations energy pro-cesses are operated often far away from environ-mental conditions. Whereas, thermodynamic varia-bles in the energy processes in buildings are veryclose to (outdoor) ambient conditions. Thus, forapplying the exergy method to the built environment,several adjustments and adaptations are required.

This chapter begins with a brief introduction to theexergy concept and a review of the main thermody-namic fundamentals behind exergy analysis, e.g.the reference environment or sign convention app-lied are explained in direct relation to energy pro-cesses in buildings.

The input-output approach followed for developingthe mathematical models for exergy analysis is alsobriefly introduced. Additionally, main differencesbetween steady-state, dynamic and quasi-steadystate assessment methods are presented.

Buildings are erected to be comfortable living spa-ces and provide adequate shelter to their occupants.Thus, models related to the exergy of human thermalcomfort have also been developed and are brieflyintroduced here.

Like the energy demand of buildings, the exergydemand of buildings is one of the most importantvariables of exergy analysis in buildings. The exer-gy demand represents the minimum amount of workthat would need to be provided to the building inorder to maintain acceptable conditions in theindoor environment. Within ECBCS Annex 49, twodifferent approaches have been developed fordetermining the exergy demand of buildings: a sim-plified approach, suitable for analysing the efficien-cy and performance of building systems, and adetailed approach suitable for analysing the perfor-mance of the building design and envelope.

A brief summary of the models developed for dyna-mic exergy analysis of building systems is to follow.


Finally, a simplified method developed for exergyanalysis of community supply systems is introduced.Main results of such method, applied to a case studyin Germany, are also introduced.

Applied fundamentals The aim of this chapter is to provide an overview ofthe main fundamental concepts for exergy analysis.For a more general introduction to the exergy con-cept, we refer to the final report from the previousannex on the topic, the Annex 37 guidebook (Ala-Juusela, 2003).

Brief introduction to the exergy conceptExergy is a measure of the quality of energy. Workis energy with the highest quality, which can betotally converted to any other type of energy. In ther-modynamics, exergy can be defined as the maxi-mum theoretical work that can be obtained from aquantity of energy or matter by bringing this energyor matter into equilibrium with a reference environ-ment. The maximum theoretical work will be obtai-ned if the considered energy or matter is convertedin a system in which only reversible processes takeplace, in such a way that equilibrium with the envi-ronment is finally achieved.

This definition shows that all systems in a state diffe-rent from the environmental state contain exergy, or,in other words, have the ability to produce work.The exergy of a system can consist of the followingcomponents:• chemical exergy (due to a difference in chemical

composition)• thermal exergy (due to a difference in temperature)• mechanical exergy (due to a difference in pressure)

For heating and cooling purposes, thermal exergy isof most importance. Therefore, in this chapter thefocus is on thermal exergy. However, as chemicaland mechanical exergy can play a role in certainsituations such as human thermal comfort, they arealso mentioned in some cases.

Heat is the transfer of energy between two systems,resulting from a difference in temperature. This ener-gy is not related to matter. When analysing onesystem, heat is the transfer of energy across thesystem boundary, taking place at the temperature ofthe system boundary. The word “cold” is used torefer to heat at temperatures below the environmen-tal temperature T0.

The ratio between the exergy (Ex) and energy of theheat transferred (Q), is defined as the quality factor.In scientific literature, the quality factor is also called

“exergy factor” or “exergetic quality factor” (Dincerand Rosen 2007). The quality factor illustrates thework potential per unit heat and thereby indicates itsquality.

Thermal energy is contained by matter. It is the partof its internal energy associated with temperature,including both sensible and latent heat. As descri-bed by Shukuya (Shukuya 2009), there can be botha surplus of thermal energy relative to the environ-ment (the system is warmer than the environment) ora lack of thermal energy relative to the environment(the system is colder than the environment). A ther-mal energy surplus relative to the environment canbe called “hot thermal energy” and a thermal ener-gy deficit relative to the environment can be called“cold thermal energy”.

The sign convention used in this report accords withthat used in most textbooks on thermodynamics(Bejan, et al. 1996; Moran and Shapiro 2004; Din-cer and Rosen 2007):• Q > 0 = Heat transfer to a system;

Q < 0 = heat transfer from a system. • W > 0 = Work done by a system;

W < 0 = work done on a system.

The sign of the heat or work is thus dependent on thedefined system, thus showing the importance of defi-ning the system under consideration and its bounda-ries. The sign convention for exergy accompanyingheat is: • ExQ > 0 = Exergy transferred to the system; • ExQ < 0 = exergy transferred from the system

The reference environmentThe thermodynamic reference environment for exer-gy analysis is considered as the ultimate sink of allenergy interactions within the analysed system, andabsorbs all generated entropy within the course ofthe energy conversion processes regarded (Baehr,2005). The environment needs to be in thermodyna-mic equilibrium, i.e. no temperature or pressure dif-ferences are to exist within different parts of it (ther-mo-mechanical equilibrium). Chemical equilibriummust also be fulfilled. Furthermore, intensive proper-ties of the environment must not change as a resultof energy and mass transfer with the regarded ener-gy system (Baehr, 2005). In addition, the referenceenvironment is regarded as a source for heat andmaterials to be exchanged with the analyzed system(Dincer, Rosen, 2007), i.e. it must be available andready to be used by the system under analysis.

The reference environment can also be described asthat portion of the surroundings of a system, ofwhich the intensive properties of each phase are

uniform and do not change significantly as a resultof the process under consideration (Bejan, et al.1996). It can thus act as either an unlimited sink orunlimited source.

Discussion: Possible choicesSeveral choices for the reference environment canbe found. However, Wepfer and Gaggioli (1980)clearly state that the reference environment for exer-gy analysis, unlike reference variables for thermody-namic or thermo-chemical tables, cannot be chosenarbitrarily. The reason is that energy analysis isbased on a difference between two states and, thus,the chosen reference levels out in the balance. Inturn, in exergy analysis the chosen reference doesnot level out in the balance and values of the abso-lute temperature chosen as reference, for example,strongly influence results from exergy analysis.

In this section, a discussion on the physical and ther-modynamic correctness of different reference envi-ronments for exergy analysis is presented. In orderto show the influence of choosing one referenceenvironment or another for exergy analysis, steady-state exergy analyses have been carried out on abuilding case study. Analyses with the four differentoptions for the reference environment introducedbelow have been performed with the pre-designAnnex 49 tool (see chapter 3). Exergy and energyflows obtained with the different reference-environ-ments are shown graphically in Figure 2.1.

a) The universe (nearly zero Kelvin) as referenceenvironmentThe temperature of the universe is very low, around3 degrees Kelvin. This allows radiative energy trans-fer from the earth and, thus, the discarding of entro-py produced as a result of energy processes onearth (Shukuya and Komuro, 1996). From a firstlaw of thermodynamics (or energy conservation)perspective, the earth is an open system receiving anet energy flux from the sun in the form of high qua-lity solar radiation, tidal energy from celestialbodies, and geothermal energy from nuclear pro-cesses within the crust of the Earth (Sørensen, 2004),with the energy from solar radiation being the gre-atest input. All incoming energy from the sun is ulti-mately radiated (or reflected) back into the universe(Rosen, 2002): about one quarter is reflected in theform of light (high quality, short wave radiation) andthree quarters are emitted in the form of low-tempe-rature heat (low quality, long wave radiation) (Szar-gut, 2005; Shukuya and Komuro, 1996). Exergybalances for these processes can be found in Szar-gut (Szargut, 2003 and 2005). The emission of lowtemperature heat, occurring since the sky tempera-ture is lower than the mean temperature of the Earth,


allows for the discarding of the entropy producedthrough the degradation of the incident solar radia-tion and maintains the so-called “exergy-entropy”process on the global environment (Shukuya andKomuro,1996). It could be regarded, therefore, asthe ultimate sink of energy processes within a buil-ding. It is infinite and undergoes no variation in itsintensive properties as a result of heat and masstransfer processes within the building.

However, cool radiation from the universe is notalways directly available and ready to be used bythe built environment (otherwise no cooling energywould be required). This is shown in Figure 2.1 (a).Thermal energy and exergy flows from storagesystem to the building envelope are equal. Differen-ces in the “generation” and “primary energy trans-formation”2 subsystems occur due to quality factorsfor liquefied natural gas (LNG), regarded as 0.94(Szargut and Styrylska, 1964)3.

b) Indoor air inside the building Indoor air within the building has also been propo-sed as a reference environment for exergy analysis.However, indoor air is neither an infinite sink nor inthermodynamic equilibrium. In addition, its tempe-rature varies as a result of energy processes withinthe building. Therefore, it does not fulfil the necessa-

ry requirements for being regarded as a thermody-namically correct reference environment.

Results using indoor air as a reference environmentare shown in Figure 2.1 (b). As with this approachthe exergy demand of the building (input in the“envelope” subsystem) is zero, for it is regarded asthe reference environment. In consequence, thisapproach does not allow for the derivation of effi-ciencies for the overall energy supply in buildingssince the desired output would always be zero.

c) Undisturbed ground The undisturbed ground can also be proposed as areference environment for building exergy analysis.It can be regarded as an infinite sink, whose proper-ties remain uninfluenced by interactions with thebuilding. Yet, the main objection for regarding it asa reference environment, similarly as absolute zero,is that it is not always directly available and readyto be used within the built environment4.

d) Ambient air surrounding the building Most energy processes in the building sector occurdue to temperature or pressure differences to thesurrounding air. Thus, the air surrounding the buil-ding can be regarded as the ultimate sink (or sour-ce) for the energy processes occurring in the buil-


Figure 2.1: Energy andexergy flows for a buil-ding case study with thefour introduced referen-ce environment options.

ding. On the other hand, the air volume around thebuilding can be assumed as being large enough(infinite sink) so that no changes in its temperature,pressure or chemical composition occur as a resultof the interactions with the building. In addition, out-door air surrounding the building is naturally avai-lable and ready to be used5.

Conclusion & recommended reference environmentIt is recommended to use the (current) surroundingoutdoor air as the reference environment for theexergy analysis of buildings and their energy sup-ply systems since this is the only system that is unli-mited (either acting as a sink or a source), unchan-ged by the processes that are regarded, andalways available.

However, outdoor air temperature and pressure dovary with time and space, i.e. external air is not ahomogeneous system in thermo-mechanical or che-mical equilibrium. As stated in Dincer and Rosen(Dincer and Rosen, 2007), “the natural environmentis not in equilibrium and its intensive propertiesexhibit spatial and temporal variations. Consequent-ly, models for the reference environment are usedwhich try to achieve a compromise between theore-tical requirements and the actual behaviour of thereference environment”. In order to model the out-door air surrounding a building as a thermodyna-mic reference environment, temperature and pressu-re are assumed to be uniform for the air surroundingthe building (thermal and mechanical equilibrium).Concentration of different chemical species in theatmospheric air is also regarded as homogeneous.

Exergy of different thermal heat transfers Exergy can be calculated for different thermal heattransfer processes, e.g. conductive, convective andradiative. The commonly known expression for cal-culating the exergy of heat (shown in equation 2.3),related to the so-called Carnot cycle, is suitable forcalculating the exergy of conductive and convectiveheat transfer processes.

The exergy of heat is based on a reversible thermalpower cycle (e.g. Carnot cycle) operating between ahot and a cold reservoir, see Figure 2.2. Heat (QH)is transferred to the system from the hot reservoir.From the second law of thermodynamics, it is knownthat not all heat to the system can be converted intowork, but a certain amount (-QC) must be rejected tothe cold reservoir.

The work obtained from this cycle can be calculatedusing equations 2.1 and 2.2, based on the first andsecond law of thermodynamics respectively. For

these equations QC is regarded as negative, accor-ding to the sign conventions.

The maximum amount of work for a given QH andgiven temperatures can be calculated with (2.3). Thefactor (1-TC/TH) is called the Carnot efficiency.


W Ex Q QH C= = + (2.1)

Q

T

Q

TH

H

C

C

= − (2.2)

Ex Q Q Q QT

T

QT

T

H C H HC

H

HC

H

= + = − ⋅

= ⋅ −⎛

⎝⎜⎞

⎠⎟1

(2.3)

Exergy of radiative heat transfer processesAs stated above, equation 2.3 is only valid for con-ductive or convective heat transfer processes. In turn,the exergy of radiative heat transfer process needsto be assessed by a different expression, being thequality (i.e. exergy level) of a radiative heat transferlower than that of a conductive or convective pro-cess. In the full and extended version of this reportthe mathematical expression used for exergy ofradiative heat transfer is presented. Besides, diffe-rences between both assessments and implicationsfor exergy analysis of building systems are also ana-lysed and explained in detail. It can be generallysaid that if the goal of exergy analysis is to assessthe performance of building systems as a whole (notat a component level) the exergy of radiative heattransfer can be assessed with the Carnot factor.Otherwise, if the main focus is to optimise one sin-gle component (e.g. floor heating system); assessingthe exergy of radiative heat transfer as such mightbe relevant.

Exergy of matterAn amount of matter which is not in equilibrium withthe environment contains a certain amount of exer-gy. The exergy of matter consists of a thermal,mechanical, and chemical component, due to a dif-ference in temperature, pressure and chemical com-position respectively. Unlike the transfer of energyby heat, the thermal energy of matter can be regar-ded as a state of this matter. This state can bebrought to equilibrium with the environment by heattransfer with the environment.

For latent heat, the heat transfer takes place at con-stant temperature and therefore equation 2.3 can be

Figure 2.2: Scheme of areversible thermal powercycle (e.g. Carnot cycle).

used. For sensible energy however, the temperatureof the heat transfer changes as the system (or mat-ter) comes closer to equilibrium. Equation 2.4 mustbe used in this case.

Heating and cooling processes: Exergy input oroutput?In the above statements and paragraphs, the focusis on the exergy ’available’ from heat. However, itdepends on the direction of the heat flow and on thetemperatures of the system and of the referenceenvironment T0, whether heat has the ability to pro-duce work or requires the input of work. In generalthe following rules are valid:• Heat transfer which brings a system into equili-

brium with the environment (and thus closer to T0)can theoretically produce work. This means heattransfer that takes place spontaneously could pro-duce work.

• Heat transfer bringing a system further from T0

requires work. (All non-spontaneous heat transferrequires work).

It can be helpful to picture an imaginary Carnotcycle between the environment and the heat transferthat is considered, in order to visualise if work hasto be supplied or work can be obtained.

In the image below, the different options are shown:Heating a system (A) of which T>T0

→energy input →exergy input / required

Cooling a system (A) of which T>T0

→energy output →exergy output / available

Heating a system (B) of which T<T0

→energy input →exergy output / available

Cooling a system (B) of which T<T0

→ energy output →exergy input / required

The negative value for the exergy should not beunderstood as “negative work”, but as an indicationof the direction of the exergy into the system (= exer-gy supply).

As it can be seen from Figure 2.3, the exergyaccompanying heat transfer is in the same directionof the heat transfer in the case of T>T0, and in theopposite direction of the heat transfer in the case ofT<T0. By using equation 2.3 (with the natural Carnotfactor), the signs of the heat and exergy values willdemonstrate whether they are inputs or outputs tothe system.

Description of the method for exergy analysis

Input-output approachIn order to improve the energy and exergy perfor-mance of energy supply in buildings, the wholeenergy supply chain needs to be assessed. Thisapproach can also be found in many energy regu-lations and standards (DIN 4701-10, 2003; EnEV,2007; DIN 18599, 2007, CEN EN 13790:2004).For this, the energy supply chain in buildings is divi-ded into several subsystems. Figure 2.4 shows thesubsystems of such an energy supply chain, fromprimary energy conversion to the building envelope,for the particular case of space heating applications. In order to assess the energy performance of thecomplete energy chain, a simplified input/outputapproach is usually followed. A similar approachcan be used for exergy analysis. This whole exergychain analysis is implemented in an Excel basedpre-design tool developed by Schmidt (2004) withinthe framework of the IEA ECBCS Annex 37 project.The tool has been improved and enhanced withinthe framework of Annex 49.

The input-output approach followed with Annex 49pre-design tool for a steady-state assessment canalso be applied for dynamic analysis. Equations fora dynamic analysis based on this input-output sche-ma are shown in detail for each of the subsystems inFigure 2.4 in the full version, the guidebook, thisreport.

All the conversion steps in the energy supply chainare directly related to each other and their perfor-mance often depends on one another. While analy-sis of single components happens as part of theenergy supply chain, an overall optimisation of com-plete building energy supply systems, as a whole,can also be accomplished. Optimisation of singlecomponents is desirable and required, but theinfluence of optimising one component based on theperformance of the following and previous onesshould always be taken into consideration (Torío et


Figure 2.3: Direction of the exergy transfer relatedto energy transfer and temperatures T and T0.

Ex c m T T TT

T= ⋅ ⋅ − − ⋅

⎛⎝⎜

⎞⎠⎟0 0

0ln (2.4)

al., 2009). With the holistic energy and exergy ana-lysis of the whole supply chain optimisation of singlecomponents which might have a negative influenceon other steps of the energy supply process is avoi-ded.

The models for exergy assessment presented herefollow the sign convention mentioned above, i.e.exergy inputs are regarded as positive and exergyoutputs as negative.

Steady-state, quasi-steady state and dynamicapproaches Steady-state and quasi-steady state estimations ofthe energy demands and flows in buildings are pro-posed and used by building regulations in severalEuropean countries (EnEV, 2007; EN 13790,2008). However, exergy is a parameter that refers toboth the state of the reference environment and thatof the system under analysis. Exergy flows haveshown to be very sensitive to variations of the cho-sen reference environment6 when the variables of thesystem and those of the environment do not differvery much from each other, as in the case of spaceheating and cooling in the built environment. Thus,an estimation of the error of steady-state exergyassessment as compared to dynamic approaches ismandatory. Results from investigations comparingboth methods (Angelotti and Caputo, 2007; Ange-lotti and Caputo, 2009; Sakulpipatsin, 2008) showthat steady-state exergy analysis might be reasona-ble for an initial estimation of the exergy flows inspace heating applications, particularly in colder cli-mates. The error is expected to be higher, the milderthe climatic conditions are. Yet, exergy flows in coo-ling applications can only be assessed by means ofdynamic analysis, where variations in outdoor refe-rence conditions are taken into account (Torío et al.,2009).

The impact of variable climatic conditions is alsoexpected to be different in different energy systems.


Figure 2.4: Energy sup-ply chain for space hea-ting in buildings, fromprimary energy transfor-mation to final energy,including all intermediatesteps up to the supply ofthe building demand(Schmidt, 2004).

For example, the exergy input and exergy losses ofa condensing boiler are expected to be rather con-stant even under varying outdoor reference condi-tions, since high quality fossil fuels with a constantquality factor of 0.94 is used. In turn, the tempera-

ture of the heat output from a solar thermal systemor a ground source heat pump (GSHP) varies signi-ficantly depending on outdoor conditions. Thus,strong variations in the quality factor associated tothe exergy flow from the solar thermal and GSHPsystems are expected and greater variations bet-ween stationary and dynamic exergy analysis canbe presumed. Therefore, if the goal of exergy ana-lysis is to compare different energy systems, dyna-mic exergy analyses are preferable, so that errorsarising from the steady state assessment can beexcluded and the differences between energysystems can be solely attributed to improved or opti-mised performance.

Alternatively, a quasi-steady state assessment canbe performed. Quasi-steady state represents ahybrid approach between fully dynamic and fullysteady-state calculation methods. Quasi-steady stateexergy assessment is performed by using steady-state equations for exergy assessment combinedwith dynamic energy simulations. The exergy flowsare evaluated following a steady-state approach,i.e. storage phenomena are not regarded separate-ly in the exergy balance, over discrete time-steps.This simplified quasi-steady state evaluation methodhas been compared to a full dynamic approach bymeans of two building case studies.

Results from analysis comparing both assessmentmethods show that if the main aim of exergy ana-lysis is to improve, study or optimise a storagesystem, the dynamic behaviour of the exergy storedand consumed needs to be analysed dynamically. Aquasi-steady state approach is not accurate enoughto depict the dynamic behaviour of the exergy flowsaccurately. However, if the aim is to perform exergy

analysis on a system level, the dynamic behaviour isnot that relevant, but total required input and outputover a certain period of time might be enough. Aquasi-steady state exergy assessment method com-bined with dynamic energy analysis (including sto-rage phenomena) is suitable in this case.

Exergy and thermal comfort Low exergy systems for heating and cooling of buil-dings, similarly as buildings themselves, should notsolely be designed to be energy or exergy efficient.Above all, they need to provide adequate comfortconditions in the built environment. Physics withrespect to the built environment and its technologymust be in harmony with human physiology andpsychology. Thus, it is vitally important to have aclear understanding of the exergy balance of thehuman body in order to understand in which waythermal energy demands in buildings could be pro-vided with minimum losses while guaranteeing com-fort conditions.

This section gives an introduction on the exergy pro-cesses in the human body. Based on these mathema-tical models for human body exergy balance, it hasbeen determined that minimum exergy consumptionwithin the human body occurs at thermally neutralconditions.

Figure 2.5 shows the human-body exergy consump-tion rates for winter and summer conditions: the for-mer is shown as a function of mean radiant tempe-rature and air temperature and the latter as a func-tion of mean radiant temperature and air move-ment. During the heating period minimum exergyconsumption can be achieved at higher mean radi-ant temperatures and lower air temperatures. Theexperience of many building engineers and scien-tists indicates that these conditions of minimum exer-gy consumption in the human body are coherentwith maximum level of thermal comfort. During sum-mer conditions, minimum exergy consumption hap-pens at higher mean radiant temperatures and airvelocities. Natural ventilation based concepts forcooling allow to achieve these indoor conditions.

The conditions for minimum exergy consumptionmight be achieved with low-temperature heatingand high temperature cooling systems (i.e. radiantsystems) which supply the required energy demandsat a temperature very close to the indoor temperatu-re, thus being low-exergy heating and coolingsystems. These findings suggest that the developmentof so-called low-exergy systems for heating andcooling are on the right track also from the perspec-tive of providing good thermal comfort.


Figure 2.5: (left): Relationships between human body exergy consumption rate, represented by the unitW/m2 (body surface), and the human body’s environmental temperature under a winter condition (0°C;40% relative humidity). There is a set of room air temperature (18 to 20°C) and mean radiant temperature(23 to 25°C) which provides the body with the lowest exergy consumption rate; (right): Relationships between human body exergy consumption rate, of which the unit is W/m2 (body sur-face), and the combination of mean radiant temperature and air movement under a summer condition(33°C; 60% relative humidity).

Human body exergy consumptionAnimals and human beings live by feeding on orga-nic matters containing a lot of exergy in chemicalforms. They move muscles by consuming exergy, notonly to obtain their food but also not to be caught asfood by other animals. All such activity realised bytheir body structure and function is made possibleby chemical-exergy consumption.

The chemical-exergy consumption brings aboutquite a large amount of “warm” exergy. In fact, thisis the exergy consumed effectively by animalsknown as homeotherms, including human beings, tokeep their body-core temperature almost constant.At this temperature, various bio-chemical reactionsare necessary for life to proceed smoothly. This tem-perature level, as we know by our own (usuallyunconscious) experience, is generally higher thanthe environmental temperature.

There are two kinds of animals, from the viewpointof thermoregulation of their body temperature:homeotherms (endotherms), as described above,and poikilotherms (ectotherms). Homeotherms inclu-de animals which maintain their body temperatureat an approximately constant level regardless oftemperature variations in their environment. Poikilo-therms include those animals whose body tempera-ture fluctuates in accordance with temperature vari-ations in their environment.

Both homeotherms and poikilotherms generate acertain amount of entropy in proportion to the exer-gy consumption inside their bodies in the course oftheir life and they must excrete the generated entro-py into their environmental space by long-wave-length (LW) radiation, convection, conduction, andevaporation.

To stay alive, it is vitally important for the homeo-therms to be able to get rid of the generated entro-

py immediately and smoothly due to their relativelylarge rate of exergy consumption. We humans areno exception. Energy processes in the human bodyare complex and involve a great number and varie-ty of heat transfer processes, e.g. through blood cir-culation, and moisture transfer through sweatingand breathing or radiative heat exchange. Exergyanalysis of human thermal comfort is in consequen-ce also complex. In the full version of this report andin the Annex 49 report on the exergy of the humanbody, the equations and their derivation for the pro-cesses involved can be found (see appendix B).

Exergy demand of a buildingThe energy demand of a building can be defined asthe amount of energy required to keep the indoorenvironment within the comfort ranges required byits users. Similarly, the exergy demand is the amountof exergy required to keep the indoor environmentwithin the comfort ranges required by its users. Thisis equal to the exergy content of the required ener-gy. In order to achieve a more clarifying description,exergy demand is defined as the minimum amountof work needed to provide the required energy.

The minimum amount of work depends on the qua-lity of the energy that is required. This means for theminimum amount of work, the energy should be pro-vided at the lowest quality possible. In practice,however, it happens very often that energy is supp-lied at a higher quality (= more exergy) than neces-sary, as is the case when heating is supplied at 90°Cto obtain a room temperature of 20°C. While provi-ding more energy than required leads to overhea-ting or under-cooling, providing more exergy thanrequired does not lead to overheating or under-cooling; it only leads to the destruction of exergy,since mixing (in this case of higher and lower tem-perature energy) involves exergy destruction.

The exergy demand of a building can be calculatedaccording to two different approaches: • Simplified approach: the exergy demand is calcu-

lated as if all energy would be provided at theminimum possible temperature level (i.e. room airtemperature). The exergy demand is, thus, obtai-ned by multiplying the energy demand of the buil-ding with the Carnot factor (i.e. quality factor,equation 2.3) for indoor air temperature.

• Detailed approach: in this calculation method theexergy demand is divided into exergy whichcould be provided by ventilation and exergy pro-vided directly by heat. The exergy of ventilation isassessed as exergy related to matter (equation2.4), i.e. air, and that of heat is calculated with theCarnot factor (equation 2.3).

summary

Figure 2.6: Simplified representation of the exergyand entropy processes in the human body

ECBCS Annex 49 PAGE 18

Exergy

Entropy

Neither of the methods includes humidification. The(small) difference between the exergy of convective andradiative heat exchange between surfaces with smalltemperature differences is also ignored in both cases. Adetailed description of both calculation approaches canbe found in the full version of this report.

Figure 2.7 shows the differences in both assessmentmethods for a building case study. Different thermalinsulation level and ventilation losses have been con-sidered in order to check the influence of these para-meters on the detailed assessment approach.

The detailed exergy demand (which is the sum of thedemand related to ventilation air and the demandrelated to heat) is always smaller than the simplifieddemand calculation, as a result of the lower qualityfactors for the exergy of matter as compared to thatof heat.

Since the energy demand and the temperatures areconstant, the simplified exergy demand is also con-stant. However, due to the different contribution ofventilation and transmission losses, the detaileddemand is different in each case.

The simplified calculation method is easy to use,whereas the detailed method requires much moreequations. Therefore it is recommended to use thesimplified method when looking at a complete ener-gy supply system of a building (from building to pri-mary energy source) in a preliminary design phase.When zooming in to the building in a detailed way,it is recommended to use the detailed calculationmethod. The detailed approach should be usedespecially when trying to optimise heating and ven-tilation systems at the building level.

Exergy in building systemsExpressions for evaluating the dynamic exergy flowsin different building energy systems have been deve-loped. In the full version of this report the equationsfor several energy systems can be found: conden-sing boiler, ventilation unit, storage tank, solar col-lectors, emission systems, distribution pipes, etc.These equations have also been implemented in anadd-on calculator allowing quasi-steady and dyna-mic exergy assessment on TRNSYS. A screenshot ofthis calculator included in a TRNSYS model is shownin Figure 2.8. The energy flows and temperaturescalculated by TRNSYS at every time step are givenas inputs to the calculator in order to evaluate thecorresponding thermal exergy flows.

Exergy in community systemsCommunities are complex energy systems where awide diversity of energy supply chains are ofteninterconnected. A great number of data with hightime resolution would need to be obtained and eva-luated for depicting the dynamic behaviour of acommunity system. A detailed and dynamic assess-ment of the energy and exergy flows in them wouldthus be very accurate, but also very time consuming.In this section, some simplifications are introducedwhich could be used for the exergy analysis methodof energy supply systems for community structures.


Figure 2.7: Results fromthe simplified and detai-led exergy demand cal-culation methods for foursituations with equalenergy demand but dif-ferent characteristics(insulation value and AirChange Rate)

Figure 2.8: Screenshotshowing an example ofadd-on TRNSYS equa-tions for calculating exer-gy flows.

Dynamic or steady-state assessmentDynamic exergy analysis of community systemsmight be required if the purpose is, for example, tooptimise the performance of a system or to size theenergy systems involved in an appropriate manner.For this aim, the detailed equations presented in thefull version of Annex 49 report for the analysis ofbuilding systems can be used. A great number ofinput data with good time resolution are required forthis purpose, e.g. results from dynamic energy simu-lations.

However, if the main aim of the analysis is to obtainan initial idea of the general performance of anenergy supply concept for a community, preliminarysteady-state assessment might be used.

To assess the accuracy of simplified steady-stateanalysis, results with such an approach are compa-red to results from dynamic analysis for the casestudy of Oberzwehren (Germany), which is alsoincluded as a case study in chapter 7.

The case study corresponds to a small neighbour-hood whose space heating (SH) and domestic hotwater (DHW) demands are supplied with a low-tem-perature district heating system. District heat corre-sponds to waste heat and is supplied to the hydrau-lic distribution network inside the neighbourhood bymeans of a centralised heat exchanger. The buil-dings and energy supply systems, i.e. heat exchan-gers, pumps and thermal losses in the hydraulic net-work, have been dynamically simulated withTRNSYS using a time step of 3 minutes. Following

the simplified input-output approach mentioned atthe beginning of this chapter, the exergy associatedto main energy inputs into the system are analysed.These energy inputs are the heat input from the pri-mary side of the district heating heat exchanger,pumping energy in the secondary sides and auxilia-ry energy to power the back-up electric heater forDHW supply.

Since it is a waste heat district heating system, theexergy associated to the primary side heat transferfrom the heat exchanger can be evaluated as a func-tion of its inlet and return temperatures (i.e. as exer-gy of matter as expressed in equation 2.4). In turn,if it were heat from a heat plant, the quality factor ofthe fuel used to supply the heat would need to beassessed.

For steady-state exergy analysis, average outdoorair temperature during the heating period is usedhere as reference temperature. Estimated annualenergy demands for SH and DHW are used in com-bination with design conditions for the supplysystems chosen, i.e. design inlet and return tempera-tures and mass flows for the district heating heatexchanger.

Figure 2.9 (a) shows results for the exergy supplyand demand following a dynamic and steady-stateassessment. Figure 2.9 (b) shows the exergy efficien-cies for both analysis methods.

With the conditions considered here for stationaryanalysis, steady-state evaluation gives a reasonably

summary

Figure 2.9: (a): Exergy demand and supply following dynamic and steady-state assessment methods fortwo different hydraulic configurations (system x and system y) as well as only for SH and for combined SHand DHW supply; (b): Seasonal dynamic exergy efficiency and steady-state exergy efficiency calculatedassuming design operating conditions and yearly energy demands..


accurate first estimation of the trend of the exergyefficiency of the system. Mismatching between theexergy efficiency for different configurations of thesupply systems is lower than 10%. In turn, differen-ces for the exergy demanded and supplied with bothassessment approaches (dynamic and steady-state)are higher, amounting to as much as 22%. Thetrend, however, is similar for both assessmentmethods.

It can be concluded that steady-state exergy analy-sis as performed here gives correct insight into thetrend of the exergy performance for differentsystems and can therefore be used for comparingthem. Absolute values of the exergy performanceobtained with this simplified evaluation method are,however, not accurate.

Simplified input-output approach for communitiesAs shown above, simplified steady-state analysis issuitable for giving an idea of the exergy behaviourof community supply systems. Energy systems incommunities often enclose a great diversity of ener-gy supply systems. A detailed assessment of everyenergy conversion step in the different supplysystems involved would require a great number ofdata and time consuming analysis. To avoid this, asa first step, communities can be depicted as a set ofdemands to be provided, with their correspondingquality level (i.e. exergy content associated to them)and an available set of possible energy supply sour-ces. With this simplified approach, the suitability ofdifferent supply options can be depicted in a relati-vely simple manner by assessing the matching levelbetween the demands and the sources used.

In Figure 2.10, some possible energy sources andenergy uses present in a community system are clas-sified according to their quality level (i.e. exergycontent). Ideally, high quality sources are used onlyto provide high quality applications, whereas lowquality sources should be used for low quality appli-cations.

Sources Quality Uses

OilCoalUranium

(fossil fuels)

Wind energy

High tempwaste heat, e.g.from industrialprocesses(200°C)

Low temp.waste heat, e.g.from CHP(50-100°C)

Ground heat

Lighting

Electricalappliances

Cooking

Washing machine

DHW

Space heating

High

Medium

Low


Figure 2.10: Simple classification of energy sourcesand uses (i.e. demands) in a community accordingto their quality level.


2Primary energy transformation” and “Generation” subsy-stems referred to here correspond to the modular method forexergy analysis developed by Schmidt (2004)

3The quality factor of 0.94 is referred to a reference tempe-rature of 25°C. However, the variation of this value due todeviations in the reference environment chosen are expec-ted to be lower than the uncertainty in the calculated value.Consequently, the same value is assumed here for all refe-rence environments chosen.

4Directly available in the sense that if a heating load is pre-sent, i.e. Tindoors < 20°C, having an undisturbed groundtemperature greater than 20°C would not directly reducethe heating load unless a suitable energy system (e.g.ground heat exchanger) was installed.

5On the contrary as the undisturbed ground temperature, ifa heating load is present, i.e. Tindoors <20°C, having anoutdoor air temperature greater than 20°C would directlyreduce the heating load (by e.g. opening the window).

6The reference environment here is assumed to be outdoorair, as concluded in the previous sections.

7Quality factors for the solar thermal system are referred tothe heat output from the solar collectors. The conversion ofsolar radiation in low temperature heat is not taken intoaccount.

8See the full version of the final report of ECBCS Annex 49for detailed insight into the results.

Annex 49 pre-design toolThe Annex 49 pre-design tool is a MS Excel-basedcalculation tool intended to analyze energy supplysystems in buildings. It is based on a simplified ste-ady-state approach for energy and exergy analysis.The tool allows to depict the overall performance ofenergy supply systems in buildings, as well as theexergy performance of single components of suchsupply systems (i.e. boiler, solar collectors, floor hea-ting systems…).

This tool is based on the German energy savingStandard (EnEV, 2007), which targets the limitationof the energy consumption of buildings. Thereby, thefield of application is focusing mainly on buildingswith normal and low internal temperatures respecti-vely, as e.g. residential buildings, day-care facilitiesfor children and office buildings.

The objective was to develop a simple and transpa-rent tool which is easy to understand and compre-hensible for its users, such as architects and con-struction engineers. Further assumptions have beenmade to compose exergy analysis as clearly as pos-sible, and to limit the required input data.

This tool is based on the MS Excel tool developedwithin IEA ECBCS Annex 37. Main changes introdu-ced in the ECBCS Annex 49 pre-design tool are:• Two different energy sources, or energy supply

systems for DHW and space heating demandscan be combined, e.g. solar thermal collectorsand heat pumps, boilers, etc.

• Renewable energy flows are accounted for, bothin energy and exergy terms, in the generation andprimary energy transformation subsystems.

• Renewable and fossil energy and exergy flowsare regarded separately, allowing good traceabi-lity of different energy sources on the energy sup-ply chain.


3. TOOLS FOR EXERGY ANALYSIS

Tools to facilitate exergy analysis of buildingsIn order to promote the use of the exergy conceptamong building planners and decision makers avariety of software tools have been developed withinECBCS Annex 49. These have different levels ofcomplexity and can be used in various applications.These tools are at the forefront of the use of exergyin the building sector. They provide a unique view-point that simple analysis based on energy balancesalone might overlook. Many designers may be una-ware or incapable of performing an analysis thatconsiders exergy flows through buildings. Thesetools provide designers with a range of options toproduce results pertaining to the exergetic perfor-mance of a particular design. These can lead thedesigner to subsequent optimizations that wouldotherwise not be applied.

Application of toolsThe tools developed have a wide range of applica-tions and are focused on the analysis of differentparts of energy supply systems in buildings. ThreeMS Excel based tools are available: Annex 49 pre-desing tool for exergy analysis of building systems,SEPE performing exergy analysis of system compo-nents and Cascadia which can be used for exergyanalysis of community systems. The exergy calcula-tions have also been implanted into a Building Infor-mation Modeling (BIM) tool, allowing energy andexergy calculations for the three-dimensional com-puter designs of architects. Beyond system analysis,another MS Excel software tool has been developedto improve comfort analysis, which models the exer-gy of the human body. Finally a graphical tool hasbeen created, which acts as a decision tree to provi-de a very simple guide for owners and designers inthe selection and integration of low exergy buildingcooling systems.

Tool Ideal User Calc Level Interface Programming Availability Manual Repository

Annex 49 pre-design tool

Engineer/Architect

System/Building Excel BASIC Public Yes Annex 49

Cascadia Eng./Planer Community Excel BASIC Public No Annex 49

SEPE EngineerSystem/Component

Excel BASIC Public Yes Annex 49

DPV Arch./Eng. Building GUI C Private Yes (DE) Keoto

Human Body Engineer Occupant Excel (GUI) BASIC (FORTRAN) Public Yes Annex 49

Decision Tree Owner/Planer System/Building Graphical -- Public Yes Swiss BfE

Table 3.1: Summary of tools for exergy analysis in the built environment developed during the Annex 49 project.

All relevant building data as well as heating ventila-tion and air conditioning (HVAC) systems can beselected directly by the user on the first page. Resultsare presented graphically and analytically at theend of the first page. Main assumptions are summa-rised in tables on further pages. The analysis consi-ders all steps of the energy chain – from the prima-ry energy source to the building and the environ-ment (i.e. the ambient climate).

Potential uses of this tool are for studies of, e.g.,effects of improving the building envelope compa-red to improving the building equipment, or systemflexibility and the possible integration of renewableenergy sources within the building system.

A definition of the building details (e.g. buildingenvelope, air tightness,…) is required from the user.By means of several drop-down menus, differentbuilding systems can be chosen to supply the requi-red building demands. The amount of input datarequired can be limited in this way. In Figure 3.1, ascreenshot of the menu for required input data todefine the building and the drop-down menus forselecting building services are shown.

Energy and exergy assessments follow a steady-state approach. Based on the energy flows obtai-ned, and depending on the temperature levels cho-sen for the building systems, an estimation of theexergy flows is carried out on a steady-state basis.The equations for each of the performed exergy cal-culations are directly shown in the calculation sheet.Furthermore, all required assumptions, such as ener-gy efficiencies and temperature levels regarded forthe operation of the building systems, are introducedin tables displayed in different worksheets and refer-red to in the calculations. The user can modify thedefault values for these parameters, allowing him toadjust the parameters to his particular system. Inaddition, this allows transparency and intends toenhance understanding of the thermodynamic back -ground and calculations within the tool.

summary

Figure 3.1: Fields forinput data to define thebuilding envelope in theAnnex 49 pre-designtool (a) and drop-downmenus for selecting buil-ding services (b).


(a) (b)

In the energy and exergy analyses, all steps of theenergy chain – from the primary energy source tothe building and the environment (i.e. the ambientclimate) are considered and displayed, following theenergy chain as shown in Figure 2.4.

The calculated energy and exergy flows are illustra-ted in two diagrams. Here, a separation occurs forthe heating system and the DHW production. There-by, an energy and exergy analysis for each specificenergy demand is possible (see Figure 3.2).

Additionally, several parameters allow a direct andquantitative comparison between the performancesof different building systems. The main parametersincluded for this use are:• overall energy and exergy efficiencies for the

complete energy supply chain• exergy expenditure figures for the generation

and emission systems and exergy expenditurefigure for the energy demand (i.e. its quality fac-tor). This parameters provide an idea of the “mat-ching” between the quality levels of the energydemanded and supplied.

In Figure 3.3, all energy and exergy losses areshown separately for each subsystem. Negativevalues of the energy and exergy losses in a compo-nent indicate gains in this component, e.g. solargains. Since all energy flows are regarded in thebalance (i.e. fossil and renewable), the only systemwhere energy and exergy gains are possible is thebuilding envelope. Here, energy gains through thebuilding envelope are taken into account and contri-bute in compensating for the total transmission andventilation losses.


Figure 3.2: Exergy and energy flows in each sub-system of the energy supply chain.

Figure 3.3: Exergy and energy losses in each sub-system of the energy supply chain.

Cascadia toolThe MS Excel-based tool “Cascadia” intends to pro-vide insight about the exergy performance of diffe-rent energy supply systems for communities. The toolaims thereby at introducing the exergy concept tomunicipal planners and decision makers, so thatmain conclusions from exergy analysis on a commu-nity level can be integrated on the design process.Cascadia is based on the calculation method imple-mented in the spreadsheet Annex 49 pre-designtool. While the model in the pre-design tool is focu-sed upon individual building components, radiators,heat transfer equipment, etc, the model used inCascadia represents the building as a simple ther-mal load and emphasises more in the form of theenergy supply and its distribution network.

The model of the neighbourhood, shown in Figure3.4, consists of a centralised energy plant supplyinga district heating pipe network. Heating loads arefrom a typical neighbourhood, including high riseapartment buildings, low-rise or detached residenti-al homes and a retail sector comprised of strip mallsor single storey retail buildings. Individual buildingsare connected to the district energy system in aparallel configuration with the supply and returnlines, whereby the three categories of buildings –high rise, residential and retail, are connectedsequentially.

The model includes an allowance for both spaceheating and internal electrical loads. Building detailsprovided to the model relate to the heat loss andventilation requirements of the building and the elec-trical loads (pumps, fans, plug loads, etc.) associa-ted with the building and distribution system.

Different building designs (high rise, residential, andcommercial) can be chosen. They are consideredonly representative for the purposes of this analysisand serve only to provide nominal thermal and elec-trical loads. Since the quality of the energy demandsis independent of the absolute quantity of the loads(i.e. a quality can be assigned to each load inde-pendently of their energy value), the load itselfaffects only the demand for primary energy and notthe energy or exergy efficiency. The number of buil-dings in each category enables the temperaturedrop in the district energy system to be determinedby balancing the water flow rate required.

For the evaluation process the district energy supplytemperature has been selected, based upon thecapabilities of the supply technology. Five technolo-gies were included within the model:1. a medium efficiency gas fired boiler2. a high efficiency, condensing gas fired boiler3. a reciprocating gas fired engine based co-gene-

ration system4. an electrically driven ground source heat pump5. flat plate solar thermal collectors

For options 1 to 3, the initial exergy level is relatedto the combustion temperature of the fuel. Electricalpower, where not provided by the neighbourhoodsystem (i.e. in options 1, 2, 4 & 5), is assumed to ori-ginate from a utility owned gas fired simple cycle co-generation system.

In the district energy loop, the supply temperature isconsidered to be 90°C for the first three options andreduced to 54°C for the heat pump and solar paneloptions. Heat distribution within the buildings can beeither forced air or waterborne radiators.

Keeping in mind the concerns relating to urbandesign and energy use, and, in particular, the desi-re to reduce the need for fossil fuels as outlined ear-lier, the model can be used to examine the implica-tions of different energy supply technologies, urbanformats and heating techniques in terms of their ove-rall energy and exergy usage.

The results of the analysis are presented in terms ofthe primary energy requirements, i.e. the fossilbased energy required for the creation of all thermaland electrical needs of the system. Since the intent ofthe tool is to demonstrate the impact of both the tech-nology and the reduced demand for fossil fuel,information is provided on:• the energy efficiency of the system – heating and

electrical generation as a percentage of input pri-mary energy. This illustrates the amount of energyusefully deployed as space heating or as availa-ble electricity.

• the exergy efficiency of the heating system – hea-ting and electrical generation exergy as a percen-tage of available exergy. This illustrates the exer-gy consumed in space heating and in the genera-tion of available electricity.

• the exergy efficiency of the overall system – thetotal exergy consumed in the process of spaceheating and power generation as a percentage ofthe overall exergy available. This illustrates theexergy lost in the delivery system.

• the fossil fuel efficiency – heating and electricalgeneration energy as a percentage of fossil fuelenergy input. This illustrates the potential for areduction in fossil fuel.

summary

Figure 3.5: Graphs displaying the results of investigated options. Left: district heating network is suppliedby means of a condensing boiler. Right: network is supplied with flat plate solar collectors.


Results are also displayed graphically in terms of theexergy flow through the district heating network andthe temperature level for each use within the supplystructure. In Figure 3.5, two examples of suchgraphs are shown for a supply with a condensingboiler (above) and solar flat plate collectors (below).

Figure 3.4: Scheme for neighbourhood design imple-mented in Cascadia.


Table 3.2: Available models in SEPE

SEPE an Excel calculation tool for exergy-basedoptimisationsSEPE (Software Exergy Performance Assessment) isa MS Excel-based tool, with which it is possible tomodel and analyze the most common heating andcooling system components. It uses the iterativepotential of MS Excel to perform steady state energyand exergy analyses. The fact that the software per-forms iterative loops and calculates the outputs on aphysical basis increases the model reliability. Thevarious components of the heat production chainare modelled here as black boxes, each one withindependent internal equations. By copying andpasting these equations and connecting the inputand output variables for each component, it is pos-sible to create a whole space heating and coolingsystem. Since the dependent variables are calcula-ted by the single absolute temperatures, this ensuresa quick connecting process and control over theoperation itself. Figure 3.6 shows as an example theconnection of two system components.

To perform loops, once the required componentshave been placed and connected in the MS Excelsheet, the input variables (absolute temperature andpressure) need to be connected to the output ones ofthe loop. Once the iteration options have been ena-bled, the program automatically updates the valuesuntil convergence.

So far, the following models have been developedand included in the software:

The ability to perform thorough and meaningfulsimulations depends on the ability of the user tomodel the system and adapt the existing componentsto his own needs. A heat exchanger, for instance, isused to allow heat exchange from the primary to thesecondary loop but it is equally effective as a heatrecovery system in the air handling unit. In addition,a saturator can be used both for evaporative coolingand as a cooling tower.

Basically, all the systems share the same structure.Each model is divided into three areas: an inputarea on the right, an output area on the left and thecentral area. All the defining equations, which defi-ne the transfer function (how the values are proces-sed from the input to the output value) of the model,are included in the central area. The user is reque-sted to insert sizing and characteristic parameters todefine the model: for example, a heat exchanger ismodelled by the type and the mass flow of the ener-gy carriers in the first and the second loop (air orwater), and by the exchange surface and the type ofheat exchanger itself (i.e. parallel or counter flow).

Figure 3.6: Example of atwo-systems layout andconnections

Figure 3.7: Exergy flows calculation steps.

Generation Emission Distribution Other Models

Boiler Air handling unit Air ducts Room model

Heat pump Floor cooling/heating Water pipes Heat exchanger

Chiller Radiator Fans

Adiabatic saturator (forevaporative cooling)

Pumps

The calculation of the exergy flows is performed byevaluating inlet and outlet pressures and temperatu-res in the nodes, given the reference temperature. Inthis way, specific thermal and pressure exergy is cal-culated in two different ways according to whetherthe medium is water or air. The calculation of theexergy flows and exergy losses is then made possi-ble by multiplying them for the mass flow passingthrough the system. This calculation process isshown schematically in Figure 3.7.

The program opens up vast possibilities of analysisand optimisation within the whole chain – exergylosses can be detected from generation to roomsystem, through primary and secondary loops heatexchange, and within distribution and emissionsystems. This allows for a better understanding of theweaknesses of the different systems.

Design performance viewer (DPV)The Design Performance Viewer takes a step towarda more simplified and accessible tool for the analy-sis of building performance. Not only that, but thetool takes the procedures developed by the ECBCSAnnex 49 group to include an exergetic performan-ce aspect that has never before been integrated intoa Building Information Model (BIM) model. The tool,which is integrated with Autodesk Revit software,allows planners, designers, and architects to obtainan easy-to-understand graphic display of the ener-getic and exergetic performance of their buildings.

The energy and exergy performance of a buildingdesign is shown using the BIM system interface (seeFigure 3.8). This allows the various parts of the buil-ding design to be compartmentalised within themodel itself. Thus, the designer sees the buildingform along with its function in one model. Still, formost BIM systems the input is not automated andretrieving significant information from the model canbe difficult.

The tool can be implemented in all phases of designand, most importantly, allows the user to observe thepotential impacts of changes during the earliest andmost influential phases of the design process. Thisfacilitates an awareness of energy and exergy per-formance throughout a project, instead of energyanalysis just being an afterthought. It is not easy todesign a modern building, and as buildings havebecome more complex so have the tools used todesign them. Nearly all buildings today rely onsome form of Computer Aided Design (CAD) tool intheir creation.

The need for higher performance buildings has ledto the development of energy simulation tools thatnot only show the construction of a building, but alsoits operation. Yet, these simulation systems oftenrequire complicated inputs, making analysis ofvarious constructions or multiple design possibilitiesvery difficult. The development of object orientatedCAD models has facilitated the development of moreaccessible energy analysis systems. These BIM’sinclude both geometric data as well as other infor-mation about various components of the building,such as wall thermal resistance and room orienta-tions. The information stored in the BIM can be useddirectly to perform calculations for the design, forexample shading and lighting, as well as energycalculations. The DPV tool uses an API from a Revitbuilding model to take information and use it todetermine the performance factors for a buildingand display them in a simple graphical interface.A Sankey diagram of the energy flows is also auto-matically generated (Figure 3.11). There is also aflow chart of the relative exergy destruction as cal-

culated with Annex 49 pre-design tool (also shownin Figure 3.11). The inputs are also defined bymeans of drop down menus (see Figures 3.9 and3.10).

The exergy aspect of the tool illuminates the impor-tance of the type of system chosen by the designer,especially with respect to the temperature of opera-tion. The value of low temperature heating and hightemperature cooling is demonstrated through the useof the DPV tool. The interest in the DPV tool is gro-wing rapidly and the start-up keoto AG(www.keoto.net) will soon begin consulting in theuse of the tool.

At present, the third version of the DPV is under finaldevelopment. The interface has been completelyrevamped and dynamic calculations including weat-her data are now being implemented. Also, exergycalculations are able to be re-evaluated more effi-ciently to show the direct impact of certain designdecisions, not just in the system drop down boxes,but within the parametric model itself. This is wherethe real preliminary design decisions are made, andif exergy can play a role here it would become eveneasier to reduce the primary energy demand of buil-dings.

summary

Figure 3.8: Screenshot from DPV tool with spidergraph for comparing the performance of differentparts of a building design.



Figure 3.9: This tab allows theuser to input various systemparameters.

Figure 3.10: This tab shows thebuilding data acquired from theRevit Model.

Figure 3.11: This panel containsthe energy balance showing theSankey diagram. Below: theexergy flow through the buildingsystems.

Tool for calculating exergy in thermal comfortIn this section a MS-Excel based calculation tool forestimating the exergy associated with thermal com-fort and energy processes within the human body isintroduced.

People spend a lot of time in the built environment.As thermal environment has a great influence onquality of life, it is important to investigate the builtenvironment from the viewpoint of human-bodyexergy balance.

The development of the theory of human-body exer-gy balance began in the middle of the 1990’s bySaito and Shukuya. About 15 years before theystarted to develop the theory, Prof. Dr. I. Oshida, aJapanese scientist, who was one of the pioneeringresearchers in the field of solar exergy utilisation,mentioned in his essay (Oshida, 1981) that a rela-tionship between the input exergy and output exer-gy of human body and thermal sensation must existbut he himself did not create the theory of human-body exergy balance.

The first version of the human-body exergy balancemodel, initiated by Saito and Shukuya, combines theenergy balance and entropy balance equations forthe human body, using the, at that time, state-of-the-art knowledge. They calculated human-body exergybalance under a thermally steady-state environmen-tal condition assuming that the environmental tem-perature equals the ambient air temperature andmean radiant temperature. They found that the exer-gy consumption rate within the human body is at itsleast under the conditions when the metabolic heatgeneration is equal to the outgoing heat. This sug-gests that the thermally neutral condition is provided

with the lowest exergy consumption rate within thehuman body (Saito and Shukuya, 2001). The second version of human-body exergy calcula-tion model was developed by Isawa, Komizo andShukuya in the early 2000’s. They made a fewmodifications to the human-body exergy balanceequation. First of all, the overall sensible thermalexergy transfer was split into radiant exergy andconvective exergy. This enables us to calculate radi-ant exergy flux and convective exergy flux separate-ly. The other modification was to improve the mathe-matical expression for sweat secretion and its eva-poration, thus making the calculation for cases whenindoor relative humidity varies from outdoor relativehumidity possible. Following these modifications,some theoretical re-examination on the derivation ofliquid-water exergy and moist-air exergy has takenplace more recently, since 2006, and the model hasreached its present version.

The original version of this calculation tool wasdeveloped as a FORTRAN code by Saito and Isawa.We converted this FORTRAN code to a Visual-basicversion and added a graphic user interface for thespreadsheet application.

Figure 3.12 shows the appearance of the calculationtool when opened with MS Excel. It consists of twoparts: the upper is to fill in the input values and thelower to display the results of the calculation. A greybutton in between is to execute the calculation. The eight input values for calculating the human-body exergy are: balance, metabolic energy gene-ration rate, clothing insulation level, mean radianttemperature, surrounding air temperature, relativehumidity, air velocity, and outdoor air temperatureand relative humidity.

The twin-bar graph shown at the bottom of Figure3.12 indicates the whole exergy balance of thehuman body. The upper bar shows all exergy inputrates and the lower bar shows the sum of the ratesof exergy consumption, exergy stored and outgoingexergy. That is, the upper bar indicates the first termof the left-hand side of equation (3.1) and the lowerbar, the second term of the left-hand side, exergy-consumption rate, and two terms of the right-handside of equation (3.1). The height of the bars indica-tes the exergy rate, while the width of the bars indi-cates the percentage of the exergy rates of eachcomponent. The quantities on the bar graph are thepercentages of each component.

This tool enables us to find out the thermal exergeticaspect of human body in relation to given indoorand outdoor environmental conditions. This calcula-tion tool can be used by those interested in low-exer-gy system solutions for concentrating on achieving avariety of thermal environments for human body inlow-exergy, high performance buildings.

The human-body exergy balance equation is deri-ved by combining the energy and entropy balanceequations with the environmental temperature forthe exergy calculation, which is outdoor air tempe-rature. In this calculation model, the human body isassumed to consist of two subsystems, the core andthe shell, because the temperature of the core is sta-ble due to its homoeothermic mechanism, while thetemperature of the shell, the peripheral part, variesdue to its surrounding thermal conditions.

The tool consists of two sub-programs: one to calcu-late the temperature of the core, the skin and clo-thing surface based on a two-node energy balancemodel (Gagge et al., 1972); the other to calculateincoming and outgoing exergy fluxes together withthe exergy consumption rate within the human body.The calculation proceeds as follows:

1) Calculate the core temperature, skin temperature,clothing-surface temperature and sweat secretionrate using the first six input values: metabolicenergy generation rate, clothing insulation level,mean radiant temperature, surrounding air tem-perature, relative humidity, and air velocity.

2) Calculate incoming and outgoing exergy usingthe three calculated temperature and sweat-secretion rates given by 1), together with outdoorair temperature and humidity.

3) Calculate the exergy-consumption rate, the lastunknown variable, substituting all of the incomingand the outgoing exergy fluxes obtained from 2)into the exergy-balance equation.

As you fill in the eight input values and push the but-ton, “Execute Calculation”, then you will immediate-ly find the results of the calculation at the lower partof the calculation tool. The quantities displayed onthe drawing of the human body in Figure 3.12 areincoming and outgoing exergy fluxes and the exer-gy consumption rate. The unit of these quantities isWatt per one square meter of human-body surfaceW/m2. The three values appearing on the upperright side of the human-body picture are the calcu-lated corresponding values of PMV*, skin-surfacetemperature and clothing-surface temperature.PMV* is a thermal comfort index which so far seemsto take into rational consideration the effect of sweatevaporation, in particular in hot and humid condi-tions (Gagge et al., 1986).

In general, the exergy balance equation for a systemis expressed as follows:

summary

Figure 3.12: Appearance of a spreadsheet tool forthe human-body exergy balance calculation.


[Exergy input]-[Exergy consumed]= [Exergy stored]+ [Exergy output].

(3.1)

Decision tool for energy efficient cooling for buil-ding retrofitThe decision tree tool is intended to provide an over-view on the different possibilities for energy efficientcooling when retrofitting a building. It shall serve asan instrument in the early phase of design in thediscussion with HVAC designers.

Only systems available on the market in Switzerlandat the time of the study are discussed. An importantprecondition is that the supply cooling water tempe-rature to the rooms shall not be lower than 18°C, i.e.low exergy cooling emission systems are a prerequi-site. The presented emission systems are all able tosecure the required indoor climate with the highcooling temperature. Consequently, only generationsystems which can produce a supply temperature of18°C are considered. Due to this precondition main-ly low exergy systems, such as natural heat sinksand chiller units with high COP, are considered.

The boundary conditions for the tool are set by theSwiss energy code SIA 382/1 (2007) which isbased on EN 13779 (2007). Here the minimumrequirements for glazing, shading, available buil-ding mass etc. which have to be fulfilled to be allo-wed to cool a building are defined. The design roomtemperature is not allowed to be lower than 26.5°C.


A standard office environment with a cooling loadof 30 W/m2 was chosen as reference. The datagathered derives from system producers as well asfrom empirical values from Basler & Hofmann Con-sulting Engineers, Zürich.

The decision tool is comprised of two main parts:The choice of emission system and the choice ofgeneration system. To find the appropriate emissionsystem suitable for the existing building, the first stepis to analyse the building design post retrofit and thedesired range of indoor climate variation. This isdone by means of a rose (Figure 3.13, to be readfrom inside and out): • Is regulated indoor air humidity desired? • What is the basic structure of the building?• Which type of ceiling is intended?• Are the ceilings constructed flat or with ribs?• Does the building have window parapets?

By answering these questions, the number of poten-tial systems is reduced. A table (Figure 3.14) showswhich systems are available for the respective result-category A1-A7 and B1-B7. A comparison of thedifferent systems is possible by comparing thesystem descriptions and the provided characteristics:efficiency, investment costs, annual energy costs andrequired surface area. The efficiency is defined asthe emitted cooling energy relative to the electricalenergy needed for the water circulation and, ifapplicable, unit fans.

The choice of generation system depends on theavailable heat sinks and whether waste heat of hightemperature is available. Here a description of thedifferent systems is given as well as characteristics.Of high importance is the efficiency, defined as thecooling energy produced divided by the electricalenergy required. It defines a reference point as tohow high efficiency should be expected.

The characteristics investment costs, annual energycosts and required surface area are given related tom2 cooled floor area, resulting in a number which iseasy for buildings owners and architects to recalcu-late to their specific case.

As every building is different, the calculated charac-teristics can not be generally applied. Especially thecosts vary according to the specific situation, andthe overall efficiency also depends among otherthings on the energy needed for distribution. Thetool however shows the relation between the systemsat the given boundary conditions.

Figure 3.13: Rose for finding appropriate emissionsystems in the Decision Tree tool.


Figure 3.14: Table showing available systems in the Decision Tree tool.

4. LOW EXERGY DESIGN STRATEGIES

To make exergy analysis reach a wider public ofbuilding planners and decision makers it is impor-tant to clearly state and summarize the central stra-tegies that can be derived from this new approach.A wider use of the method will contribute to a signi-ficantly more rational and efficient use of fossil fuels,while promoting the integration of renewable ener-gy sources in the built environment.

As stated in chapter 2, exergy analysis has alreadyproved to be successful in optimising power plantsand is making its way into building analysis (see thetools presented in chapter 3). The targets of exergyanalysis applied to power plants and buildings areof course different in scope and aims. The optimisa-tion of a power plant aims at increasing the output,i.e. the electricity produced. The reduction of theexergy losses in buildings aims, instead, at decrea-sing the exergy input to maintain the required out-puts, i.e. the comfort conditions.

The core and first principle of the exergy methodapplied to the design of energy systems is to matchthe quality levels of the energy supplied and theenergy demanded. In this sense, exergy can be

understood as optimisation tool for the use of ener-gy sources.

Applying the exergy method to energy systems inbuildings contributes to increase their efficiencyusing both fossil and renewable energy sources, asit is shown for several building and community casestudies in chapters 6 and 7.

An example showing the additional informationoffered by exergy analysis is the use of biomass orphotovoltaic (PV) panels to provide space heating inbuildings, as it is shown in the graphs in chapter 1.Although both are renewable energy systems andthus have a low environmental impact and CO2

emissions allocated, the exergy quality of biomassand that of the electricity output from the PV panelsis very high. Exergy analysis helps showing thatthese renewable energy sources should rather beused for equally high quality applications (e.g. ligh-ting, mobility, etc.) instead of using them for lowexergy demand heating purposes.

In this chapter strategies for a general design ofenergy supply systems in buildings and communitiesare introduced. Based on these strategies, imple-mentation technologies presented both on a buildingand community level. Aspects related to control stra-tegies and costs of the systems are also brieflydiscussed.

General strategies for building systems Buildings are major energy consumers (energy usefor space and domestic hot water heating). Due tothe low temperature level of most of these demands,their quality is very low (approximately a qualityfactor of 7%). The energy approach, in this context,intends to reduce energy demands in buildings byincreasing insulation levels or increasing the airtightness of the building envelope, i.e. optimizingthe building shell and later also an implementationof renewable energy sources. The exergy approachadditionally requires the use of low quality sourcesfor these equally low quality demands, i.e. by mat-ching the quality levels of energy demand and sup-ply (as shown in Figure 1.1).

Condensing boilers are considered highly efficientenergy supply systems. Their energy efficiency isclose to 100%. However, their exergy efficiency canbe as low as 5-10% because they degrade highexergy natural gas to rather low temperature heat.The core conclusion from exergy analysis on thisbasic level is that, in an exergy efficient energysystem combustion processes should not be used forthe production of low temperature heat.


Figure 4.1: Power plant optimization aims at incre-asing the exergy output.

Figure 4.2: Building exergy optimization aims atdecreasing the exergy input.

Instead, low quality sources should be used forspace heating and cooling applications in buildings.Examples of available low exergy sources are solarthermal heat, geothermal heat or process wasteheat.

Additionally, low temperature heat flows existingwithin the built environment, such as heat in wastewater or exhaust ventilation air, could also be usedto supply a share of the energy demands via heatrecovery systems. The use of these waste heat flowsrequires the use of innovative heat recovery con-cepts. Some examples of such systems are shown inchapter 6. These concepts already play a significantrole in low energy building concepts. Taking intoaccount exergy balancing, the mostly electrical auxi-liary systems become more important. In order tominimize the high-exergy input in terms of electrici-ty required for pumps and fans in these concepts,heat recovery connected to highly efficient energysystems, such as heat pumps is beneficial10.

However most of the low exergy sources mentionedare not constantly available and almost important,are available in very limited power. Reducing ener-gy demands in buildings consequently reduces therequired peak power for space heating and coolingapplications, making the use of low exergy sourcesmore favorable.

In addition, lower specific power demands for spaceheating and cooling also allow the use of surfaceheating and cooling systems such as floor heating,chilled ceilings or thermally activated building com-ponents. Surface heating and cooling systems ope-rate at lower temperature levels than conventionalunits (radiators or fan coils), thereby making also theuse of low exergy sources more effective. Since theselow temperature heating and high temperature coo-ling systems deliver the required heating or coolingenergy at temperature levels closer to that of theenergy demand in the building, they can be calledlow exergy emission systems. The use of these lowexergy emission systems is a necessary step for awider and more efficient integration of low exergysources in building supply systems. In consequence,low exergy emission systems are “more flexible”since they allow the efficient integration of low exer-gy sources, but could also be supplied with highexergy sources. In turn, systems requiring highersupply and return temperatures such as old radia-tors with temperature levels of 90/70°C cannot beefficiently coupled with low exergy systems such asground source heat pumps (GSHP) or solar thermalsystems.

Yet, it is important to stress that the use of low-exer-gy emission systems is only a prerequisite for a low-

exergy building, since low exergy needs with a highexergy supply would not improve the outcome froma standard building solution significantly.

For instance, a low-temperature floor heating systemwith a gas boiler would not perform much betterthan a high temperature radiator supplied by thesame boiler (Figure 4.3).

The main focus to achieve an exergy efficient buil-ding supply is to decrease the quality of the sourceused and to find low exergy sources to be exploitedfor buildings.

As energy demands for space heating and coolingare reduced, the share of other uses within buildingssuch as domestic hot water (DHW) demands increa-ses. The exergy quality factor of DHW energydemand is about 13%, being almost twice as high asfor space heating applications. Energy systemsusing low energy sources show lower efficiencies forthese demands on higher temperature levels. Strate-gies aiming at improving the performance of lowexergy systems for DHW supply are desired and apromising future research topic.

In addition, higher and lower exergy demands wit-hin a building can be supplied one after another, fol-lowing a cascading principle. This means that appli-ances needing higher exergy levels are served priorto appliances with lower exergy demand, makinguse of the same energy flow several times. Casca-ding of thermal energy flows in buildings is also apromising field which can be directly derived from

summary

Figure 4.3: Comparison of exergy flow. The lowerexergy need in the floor heating system gives noadvantage since the boiler requires high exergyinput regardless of the emission system.


the exergy approach and where future research isalso required.

In addition to the concept and design of systemsmaking use of appropriate energy sources for lowexergy demands, control strategies of buildingsystems to minimize exergy losses in the supply pro-cess are necessary. The first step is the applicationof good building physics to reduce the energydemand of the building. Here a good insulationlevel, air tightness of the building envelope, the useof daylight and the passive use of solar energy areimportant factors.

Economic aspects in Low Exergy building designCost efficiency is a key issue in all building projectsand in turn an important part of the development oflow exergy solutions for buildings. Prototype solu-tions will always be more expensive compared tocommon market technologies. The potential of anew technology to perform better than a standardsystem the development of energy prices is a keyfactor. The cost efficiency of solutions is, in the longrun, determined by the quality of the system design.High costs can therefore indicate that maybe betterand more economic alternatives to reach the sameresult have been overlooked.

The development of components for low energy andlow exergy buildings has been rather slow in the past.Solutions such as concrete core heating and cooling,waterborne solar collector systems and various heatpump solutions have been commercially successful ontheir own merits. There are solutions that have beencommercially successful due to dual functionality suchas floor heating where customers have probably beenmore often interested in the increased comfort ratherthan the exergy aspects. Waterborne radiative panelsor chilled beams have often been chosen instead ofair heating and cooling because of the comfortaspects, the reduction of fan electricity and operatio-nal costs being a positive side-effect. Offering a morecomfortable system, integrating additional functionsand advantages or positive side-effects (e.g. savedconstruction costs by reduced floor heights because ofthe integration of thermally activated floor slabs) is akey issue for the success of new systems. In recentyears, efforts were made to integrate collector panelsinto façade or roof structures where the collector ele-ments replace the normal cladding and, thereby,some costs can be saved. These technologies are stillmostly in a prototype stage. There is also a knownpotential of saving energy by better control and avariable operation of the energy system in buildings,especially in commercial buildings. But, the costs ofsensors and actuators and the wiring in residentialbuildings were still far too high to motivate private

investment. With further development of the compo-nents and using wireless technology this could chan-ge if larger market potentials are identified.

General optimization strategies for communitiesAt the community level, generally speaking, twodirections can be taken to address building relatedenergy issues: • the first focuses on the single building and aims at

energy self-sufficiency (e.g. by designing zeroenergy buildings ZEBs)

• the other direction, characterized by higher com-plexity, aims at taking advantage of the variety ofdemand structures and available energy sourcesof a whole city by an integral energy supply andadjusted use profile

Very often, main efforts are directed to technologi-cal improvements for low-energy, self-sufficient andlow-exergy buildings (e.g. by the development of so-called zero energy buildings (ZEBs), but this strategycan not have the same potentials as using synergiesin communities instead of individual buildings. Com-munities are intrinsically characterized by a level ofcomplexity and by a efficiency potential respective-ly higher than single buildings. At the communityscale, however, it is possible to adopt deep-reachingchanges in the supply structures, enabling the use oftechnologies that make a more rational and efficientenergy use possible on a wider scale.

The core of the exergy approach for communities issimilar to the building approach: the quality levels ofthe energy demanded and supplied shall matcheach other (see Figure 1.1). To accomplish this, theuse of low exergy sources for supplying low exergydemands in buildings has to be promoted.

However, additionally to the similarities with thebuilding level, communities supply strategies canoffer synergies for an exergy optimized supplysystem design which can not be found in buildings,e.g. several demands with different quality levels arepresent, several low exergy sources can be linked toeach other more efficiently and economically than ina decentralized supply, or a more efficient use offossil fuels can be promoted more cost effectivelyand efficiently on the community scale.

The first step for a more exergy optimized communi-ty supply is, similar as for buildings, to promote awider integration of low-temperature renewableenergy sources, such as solar thermal or groundsource heat. Higher solar fractions are generallyachieved if solar collector fields are used in combi-nation with heat networks, connecting several supplysystems (e.g. collector or borehole fields) with diffe-


rent users. As the solar fraction increases, i.e. theshare of low exergy supply increases, the exergyefficiency of the energy supply also rises. Similarly,the use of ground source based systems in combina-tion with heat networks will increase the energy effi-ciency (i.e. COP) of heat pump units, if demands ofhigher temperatures, such as DHW supply, can besupplied by solar thermal heat. Solar thermal heatcan be used in winter to reduce the required tempe-rature lift from the heat pump units, allowing signifi-cant increase of the COP. This way high exergy inputin terms of electricity required for operating the heatpumps can be reduced.

On the other hand, a more exergy efficient use of fos-sil fuels needs to be promoted. Decentralized supplywith individual boilers should be substituted by elec-tricity driven CHP units, maximizing the exergy outputobtained from the high-quality fuels used. Distributedor centralised generation with CHP units can reducethe demand of fossil fuels and thus reduce the use ofcombustion processes for heat production in total,characterized by a high level of exergy losses.

As stated above, heat networks can play a signifi-cant role in a more exergy efficient energy supply oncommunity level. They allow combining severalrenewable energy sources with waste heat from anexergy efficient use of fossil fuels. Heat networksalso allow cascading energy flows according to theirtemperature, to supply high temperature applica-tions, such as process heat, first followed by mediumtemperature demands such as DHW and finally lowtemperature heat can be directly used for spaceheating. In this way, pumping energy, i.e. high exer-gy input, into the network can be minimized and theexergy efficiency of the energy supply increases.

Exergy analysis can be a useful tool for improvingthe design of heat networks. Coming back to the twomain strategies mentioned at the beginning of thissection, and bearing in mind the main directions forpromoting a more exergy efficient supply at thecommunity scale, it can be concluded that designingmore “sustainable” buildings could be regarded anecessary but not sufficient condition for reachingenergy and exergy efficient communities. Innovativesupply structures allowing the application of the stra-tegies mentioned above are required.

From an exergy perspective pumping energy inpipes and ducts shall be minimized. This is alsovalid for the design of heat networks. For this pur-pose, the diameter of the pipes in the networks canbe increased. Thereby, lower heat losses can befound in the network and lower maximum fluidspeeds occur. In turn, as a result of the greater pipediameter, thermal losses in the network increase.First results on the sizing criteria of small scaledistrict heating systems show that actually a tradeoff between the increase of (low exergy) thermallosses in the network and the decrease in pumpingenergy for its operation can be found. While fromthe perspective of energy analysis lower target fluidvelocities for sizing the network, i.e. smaller pipes,seem always advantageous, exergy analysis showsthat an optimum between both criteria can be found(see Figure 4.4).

Storage units are also a key component in low exer-gy supply systems, particularly if based on heat net-works to integrate a higher share of fluctuating rene-wable energy supplies (e.g. thermal solar power)into the system.

summary

Figure 4.4: Pumping energy required for the operation of the heat network, thermal losses in the pipes andresulting required net energy input to supply both demands in energy (a) and exergy (b) terms (Torío, 2010).


Economic issues in LowEx community designDue to the different scale and the large number ofdecision makers involved, different technologiesmight be cost- efficient at the building and commu-nity level.

Considering technical-economic feasibility, solarphotovoltaic and solar thermal systems, if properlyintegrated, can be well applied in the urban context.However, implemented community case studiesshow that the cost of these technologies as compa-red to their energy yield is still relatively high ascompared to other systems such as district heatingsystems or heat pumps (Jank, 2009).

For this report the integration of biomass plants hasbeen investigated in terms of economics and is out-lined here as an example. The urban scale applica-


bility of biomass is challenging because of plant fea-ture problems, their location in the cities andmanagement difficulties as well as supply and stora-ge issues. On the basis of the actual conditions, bio-mass plants are more suitable in low-density urbanenvironments and as close to the source as possible.Of course, in all cases with combustion (or pyrolysisor gasification) processes, CHP are recommendedfor improving both the energy and exergy perfor-mance. This is shown in Figures 4.5 and 4.6.

Figures 4.5 and 4.6 combine the exergy efficiencyand the capital costs of several investigated systemsfor the biomass energy use chains. If we are sepa-rating the areas (1) thermal plants and CHP and (2)mobility (because the latter shows clearly additionalcosts for different reasons) we can observe that thereis a clear trade-off between exergy output (efficien-cy) and capital costs (for the selected woody bio-mass chains this is an almost linear relation, for theselected biogas chains the situation is not that clear).This shows that there are higher investments neces-sary for a CHP compared to a thermal plant in orderto make use of the full exergetic potential of biomassresources.

If we would follow the objective to gain a highestpossible exergetic use of biomass resources with aminimum of capital cost, we would have to draw anenvelope line in these figures connecting thosepoints situated on the left hand and top side of Figu-res 4.5 and 4.6. This would lead to the conclusion,that using biomass for transport purposes in anycase is not efficient, both from an exergetic and froman investment costs point of view. But, biogas plantsfeeding biogas into the gas grid and for combinedheat and power production are an efficient option.

However, if we are considering that currently there isa high demand for individual transport systems, thelowest exergy losses result from bio-based e-mobili-ty models compared to combustion engines. Thiswould require clearly higher investment costs (whichare partly offset, at least for the case of 2nd genera-tion liquid biofuels by lower running costs).

Figure 4.5. Exergy efficiency and capital costs (woody biomass)

Figure 4.6: Exergy efficiency and capital costs (biogas)


10In chapter 6 an example of such an innovative system is shown.

5. EXERGY BENCHMARKING PARAMETERS

To bring the method for exergy analysis of buildingsystems to the wider public parameters able to cha-racterize the exergy performance of differentsystems are required.

A great number of parameters can be found in theliterature (Cornelissen, 1997; Dincer and Rosen,2007; Tsatsaronis, 1993). In this chapter the set ofparameters considered as relevant by the ECBCSAnnex 49 group are presented. These parametersare used to characterise the performance of the buil-ding and community case studies found in chapters6 and 7. The diagrams used for graphically showingthe exergy performance of energy systems in buil-dings and communities based on the parametersintroduced, are also presented. Additionally, abenchmarking proposal for characterising the per-formance of building systems is introduced.

The main added value of the exergy approach isshown through the parameters and diagrams pres-ented. Including exergy assessment in building ener-gy codes would be an important step towards amore energy efficient built environment and wouldhelp bringing the exergy approach to the public anddecision makers. Therefore, at the end of this chap-ter, a proposal on how to include exergy in energycodes is suggested.

Parameters for exergy performance

Quality factorsQuality factors allow depicting the exergy associa-ted to a given energy flow (i.e. energy transfer) orthe energy content of a given system. They representthe convertibility of an energy flow into mechanicalwork, i.e. high valued energy with high exergy con-tent. Thereby, they allow high exergy sources anddemands to be characterised and distinguished fromlow exergy sources and demands. They allow a sim-ple yet thermodynamically correct representation ofthe matching in the quality levels between energysupplied and demanded, and are used for this pur-pose in the “arrow diagrams” presented below inthis chapter.

Quality factors are defined as the ratio between theexergy and energy of a given energy system. Froma thermodynamic point of view, they represent theproportion of work that can be obtained from anenergy conversion process which brings an energysystem into equilibrium with its environment11 asrelated to the energy input in the process (i.e. theenergy present in the system before the conversionprocess takes place).

Thermal, chemical, mechanical, potential and kine-tic exergy derived from different temperature, com-position, pressure, height and velocity between asystem and its reference environment might be pre-sent. Exergy flows can be derived from quality fac-tors related to all of the above items.

Equation (5.1) shows the general expression of qua-lity factors which can be applied to any energy flowor source.

However, this report, as well as the method introdu-ced in chapter 2, focuses on thermal exergy. Themost popular expression of quality factors for ther-mal energy transfers are Carnot factors (or Carnotefficiencies). Carnot factors can be applied if an iso-thermal heat flow happens via a heat engine bet-ween two temperature levels. Equation 5.2 showsthe expression of Carnot factors for a temperature Tof the system and a reference temperature T0. Car-not factors are used to calculate the so-called “exer-gy of heat” (see chapter 2).

If the heat transfer is not isothermal, as it is in thecase of, for example, storage processes, Carnot fac-tors cannot be applied. Instead, the quality factorshown in equation 5.3 needs to be used12. The so-called exergy of matter (see chapter 2) can be obtai-ned using the quality factors defined in equation 5.3.

Exergy efficiencyExergy efficiencies are a suitable and appropriatebase for comparing the performance and optimisa-tion of different heating and cooling systems. As anyother efficiency, exergy efficiencies are defined asthe ratio between the obtained output and the inputrequired to produce it. Exergy efficiencies help inidentifying the magnitude and point of exergydestruction within an energy system (Cornelissen,1997). Therefore they allow to quantify how close asystem is to ideal performance or where the energyand exergy inputs to the system are better used(Torío et al., 2009).


FEx

EnQ ii

i, = (5.1)

FT

TQ Carnot, = 0 (5.2)

FT

T T

T

TQ matter, ( )ln= −

−

⎛

⎝⎜⎞

⎠⎟1 0

0 0

(5.3)

Different definitions of exergy efficiency parameterscan be found in the literature. At least two types ofexergy efficiencies can be identified and differentia-ted: “simple” or “universal” and “rational” or “func-tional” (Cornelissen, 1997; Tsatsaronis, 1993). In(Schmidt and Torío, 2009; Torío et al., 2009) adiscussion on the differences and suitability of thesetwo efficiencies can be found.

The mathematical expressions of the simple andrational exergy efficiencies are shown in equations5.4 and 5.5.

The main difference between both exergy efficien-cies is the way the exergy output is considered. Therational efficiency considers the difference between“desired output” and any other kind of outflow fromthe system. In turn, the simple exergy efficiency con-siders any kind of output as such, be it desirable ornot for the investigated use. In most building systemsundesirable outputs are present, e.g. in a waterbor-ne heat or cold emission system in a building, outletwater flows back via return pipes into the heat/coldgeneration system. In consequence, the simple exer-gy efficiency works better when all the componentsof the incoming exergy flow are transformed intosome kind of useful output. In turn, the rational effi-ciency shows how much exergy is getting lost whileproviding a specific output. Exergy losses regardedin the rational efficiency are due to both irreversible(not ideal) processes present and to unused outputexergy flows. Therefore, the rational exergy efficien-cy is a more accurate definition of the performanceof a system and can be better used without leadingto false conclusions.

The rational exergy efficiency is the parameter usedin the “PER-Exergy efficiency” diagram presentedbelow to characterise the exergy performance ofcommunity supply systems.

Depending on whether the exergy efficiency refersto a single component or process of an energysystem, or whether it refers to all processes and com-ponents constituting the system, so-called “single”and “overall” exergy efficiencies can be defined.

An example of single and overall efficiencies for theroom air subsystem and complete energy chain inFigure 2.4 (in chapter 2) is shown in Equations 5.6and 5.7. Overall efficiencies are derived from an

input/output approach13 for the analysis of a givenenergy system and can be calculated as the productof the single efficiencies of the single processes orcomponents comprising the system (Torío et al.,2009).

Exergy expenditure figureTo clearly show the relation between the exergyrequired for supplying a given energy demand, andthe energy demand itself, Schmidt, et al. (2007)defined the “exergy expenditure figure”. Exergyexpenditure figures can be used to characterise theperformance of components in energy supplysystems. This figure can be seen as an enhancedversion of the quality factors (exergy to energyratio), where both the energy and exergy losses in acertain energy conversion unit are depicted.

In Equation 5.8, the exergy expenditure figure isdefined for a component i of an energy system. It iscalculated as the ratio between the exergy input (eff-ort) required to supply a given energy demand andthe energy demand itself (use). Auxiliary energy foroperating the component is also included as input(i.e. effort) in the parameter.

For supplying a given energy demand due to ineffi-ciencies in the supply systems a greater amount ofenergy needs to be supplied. Ideally, however, theenergy supplied should have a similar quality as thedemand. Providing smaller amounts of energy withhigher quality would not be sufficient. Therefore, inthe exergy expenditure figure the “use” of a givencomponent (e.g. heat loads to be supplied by radi-ators in buildings) are regarded in terms of energyand not in exergy terms.

Comparing the exergy expenditure figure to thequality factor of the demand provided (use) the levelof matching between the quality levels of energysupplied and demanded can be obtained.

Figure 5.1 shows the energy and exergy flows usedfor the general definition of the exergy expenditurefigure for a component i.


Ψ simpleout

in

Ex

Ex= (5.4)

Ψratdes out

in

Ex

Ex= , (5.5)

Ψ single,rin env

in ra

Ex

Ex= ,

,

(5.6)

Ψ single,rout env

in prim

Ex

Ex= ,

,

(5.7)

εηi

in i

out i

q in i

i

Effort

Use

Ex

En

F= = =,

,

, , (5.8)

Energy and exergy losses happening in the compo-nent are implicitly taken into account by the ratio ofprovided output to required input. Energy losses aretaken explicitly into account by means of the energyefficiency in equation 5.8. Exergy losses are takeninto account by comparing the exergy expenditurefigure of the component with the quality factor of thefinal demand to be provided. In consequence, if theenergy losses in the component are high, i.e. lowenergy efficiency, the exergy expenditure figuremight reach values higher than 1 (see equation 5.8).

For the particular application of space heating andcooling of buildings, the quality factors of the ener-gy demanded are very low. Figure 5.2 shows that forspace heating applications assuming an ambienttemperature (i.e. reference temperature) of 0°C andan indoor air temperature 21°C the quality factor ofenergy demand is 7%. Therefore, for space heatingof buildings, the closer the exergy expenditure figurefor a given system to 7% , the better the system exer-gy performance is. Consequently, in space heatingand cooling applications, energy supply systems withlow exergy expenditure figures shall be used.

The definition of the exergy expenditure figure usedhere is not equivalent to the expenditure figure in theGerman Standard (DIN 4701-10, 2001), despitesimilar nomenclature. The main difference is that theexergy expenditure figure as proposed here repre-sents a ratio between an energy output and an exer-

gy input. In the German Standard the expenditurefigure is the inverse of the energy efficiency of agiven component, i.e. a ratio between the requiredinput and the provided output.

Again, the exergy expenditure figure regards thequality level of the energy supplied (effort), whereasthe output (use) is regarded in energy terms (i.e.quantity). Therefore, as long as a certain energysource with its corresponding quality level is usedwith the same energy efficiency, the exergy expen-diture figure would be the same and the parameterwill not vary. By comparing the exergy expenditurefigures for different steps or subsystems of the ener-gy supply to the exergy level of the energy demand(e.g. 7%) the suitability of each component for thatparticular use can be checked. Therefore, it is a bet-ter indicator of the good matching between the qua-lity level of the energy used by a given componentand the final energy demand, i.e. of the suitability orappropriateness of the energy system for providinga given use. In (Schmidt and Torío, 2009) a casestudy comparing the exergy expenditure figure andthe single exergy efficiency for different componentsof the energy supply chain is presented. Results sho-wed the suitability of the exergy expenditure figurefor providing insight on the appropriateness of usinga given component for a certain energy use.

Primary energy ratio, PERExergy assessment provides information on the mat-ching between the quality levels of the energy dem-anded and supplied. It allows a common and scien-tifically grounded approach for analysing differentenergy sources, be they renewable or fossil. In turn,exergy does not provide any information on therenewability of a certain energy source. To connectthese considerations with exergy analysis, a furtherparameter is required. For this purpose, the prima-ry energy ratio (PER) is introduced.

PER is calculated as the ratio between the usefulenergy output, i.e. the energy demand to be supp-lied, and the fossil energy input required for its sup-ply. The analytical expression of PER is shown inEquation (5.9). High PER values indicate that theproportion of fossil energy in the supply is low, and,thus greater share of renewable energy sources ispresent in the supply.

PER’s are used in the PER-Exergy efficiency diagramintroduced below in this chapter for characterisingthe performance of community systems.


Figure 5.1:Graphicalrepresentation of theexergy flows included inthe exergy expenditurefigure for a general com-ponent of an energysystem (Schmidt et al.,2007).

Figure 5.2: For a refe-rence temperature of0°C, the exergy contentof the energy in theroom air, assuming anindoor air temperature of21°C, is 7%.

PEREn

Eniout i

in fossil i

= ,

, ,

(5.9)

Benchmarking for components of building systemsThe exergy benchmarking proposed here for compo-nents of building supply systems is based on the exer-gy expenditure figure. Here the benchmarking methodand its applicability are presented by means of anexample where several building systems are analyzed.

The case study consists on a building heating case.Several building systems are considered for supply-ing the same space heating demand. In particular,different heat generation and emission systems areregarded: condensing boiler (Cond. in Figure 5.3)without and with solar thermal systems, wood pelletboiler (Wood. in Figure 5.3), ground source heatpump (GSHP in Figure 5.3), district heating (DH inFigure 5.3), radiators (radiator in Figure 5.3) withsupply and return temperatures of 55/45°C andfloor heating systems (floorh in Figure 5.3) with sup-ply and return temperatures of 28/22°C, respective-ly (see also Figure 1.2).

A component, e.g. a radiator, is designed to supplya specified heating power. An appropriate buildingsystem should perform this task with the smallestpossible amount of exergy input. Furthermore, theuse of high quality (auxiliary) energy, e.g. electricalpower, and losses to the environment, should be low.

As described above (see Figure 5.2), the exergyfraction of the energy needed to heat a room is onlyaround 7%. This value can be directly compared tothe exergy expenditure figures of the building servi-ce systems discussed above (Figure 5.3). They satis-fy the heat demand with a more or less well-adap-ted heat supply. Heat generators that utilise a com-bustion process use much more exergy than requi-red, and are thus less efficient from an exergy per-spective. As for emission systems, for heating thesame room, the radiator system uses more exergythan the floor heating system, which is closer tobeing ideal in terms of exergy use.

Benchmarking for buildingsAll parameters presented until now in this chapterrepresent different ratio between effort invested anduse obtained. In consequence, they state the mat-ching level between energy supplied and demanded,but do not give any information on the total energyor exergy demand of a building. For benchmarkingthe performance of buildings, similarly as it is donecurrently in terms of energy, a limitation of the exer-gy of the primary energy demand is suggested.

An ideal line can be drawn based on the real exe-getic demand of the regarded zone. The exergysupplied by different building systems should becompared with the exergy of the demand. Ideally

they should be as similar as possible, i.e. for lowexergy demands such as space heating or domestichot water production (DHW) low exergy should besupplied. To promote the use of building systemsmaking use of low quality energy sources, i.e. whichrequire low exergy inputs, the upper limit of theexergy of primary energy input should be limitedaccording to the demand of a good building serviceequipment solution, similarly as it is done for thelimitation of fossil primary energy demands. Thelimit is set here close to the exergy demand of a con-densing boiler, regarded as an available and ener-gy efficient state-of the art technology.

summary

Figure 5.3: Assessment of the components “heatgeneration” and “emission system” with the exergyexpenditure figure for the chosen variants of thebuilding service system.


Figure 5.4: Calculated exergy of total primarydemand (fossil and renewable) for the chosen vari-ants of the building service equipment (steadystate) and a suggested benchmarking classification.

However, it can be clearly seen in Figure 5.4 that acondensing boiler demands higher exergy inputs asneeded for satisfying the demand. The exergy inputcan only be reduced if building systems which donot use combustion process are used to provide lowtemperature heat.

As the supply matches the needed demand and theexergy destruction in the regarded building is keptto a limit, the building can be regarded as a“LowEx”-building.

Four main design principles can be extracted fromthe examples shown in Figure 5.4:1. The limitation of the (fossil) primary energy

demand is a useful tool to reduce energy con-sumption and the related CO2-emissions frombuildings. The exergy approach needs to be com-bined with the primary energy approach in orderto include insight on the renewability of energysources used. This is already mandatory in anumber of European countries (e.g. Germany).

2. Maximal heat transmission losses through thebuilding envelope should also be limited (e.g. asit is done in German regulations by limiting themean transmission heat loss coefficient) in orderto ensure a good building envelope construction.The energy demand should be reduced. Therebyexergy demands would automatically be reduced.

3. To assess and use properly the thermodynamicpotential of the utilised energy, the exergydemand of fossil and renewable sources shouldbe limited. This limitation could be done in a simi-lar manner as already known from the procedu-re of limiting the primary energy demands.

4. The exergetic demand of a zone should be satis-fied with a suitable supply system, e.g. the exer-gy expenditure figure should be oriented to theactual exergetic demand of the zone.

Exergy fingerprint diagramThe “Exergy fingerprint” diagram depicts the energydemanded and supplied against the quality of eachenergy demand (Jentsch et al., 2009)15. It allows aquick graphical overview on the matching betweenthe quantity and quality levels of the energy suppliedand demanded. The calculation algorithm corre-sponds to a steady-state approach similar to thatimplemented on the Annex 49 pre-desing tool (seechapter 3). The diagram is shown here for comple-teness but has not been used to characterise the casestudies from ECBCS Annex 49 work.

Figure 5.5 shows an example of two exergy finger-print diagrams, for two different energy supply sce-narios. The grey areas represent exergy losses in theenergy supply. The rest of colours represent the dif-ferent energy demands considered: electricity, ligh-ting, process heat, DHW and space heating, respec-tively. The length of the coloured areas (its value onthe X-axis) represents the share of the respectiveenergy use on the entire demand. Its height (i.e. itsvalue on the Y-axis) represents the quality of thegiven demand, i.e. its quality factor. By the meredefinition of quality factors (see equation (5.1) withthe product of the quantity of the energy demand(i.e. value on the X-axis) and its quality (i.e. value onthe Y-axis) the exergy associated to the energydemand can be obtained.


Figure 5.5: (a): Exergy fingerprint diagram for a reference scenario consisting of an average residentialbuilding with an energy supply via a gas fired condensing boiler; (b): Exergy fingerprint diagram for animproved scenario consisting of a well insulated building supplied with CHP units and district heating.

Graphical representations for characterising theexergy performance of community supply systemsGraphical representations for characterising theexergy performance of community supply systemsenable to visualize the performance of a given casestudy and make different community energy supplyconcepts comparable. The characterisation of theexergy performance of different case studies andcommunity concepts is presented here by means ofdifferent diagrams included under the section“LowEx Diagrams” in the respective case study (seechapter 7).

Arrow diagramsThe arrow diagram shows the matching between thequality levels of the energy supplied and demanded.The diagram is a qualitative representation of thequality and quantity of energy demands and supplyin buildings. Figure 5.6 shows an arrow diagram asan example.

The position of the arrows on the Y-axis (i.e. “Ener-gy quality, q”, with a scale from 0 to 1) representsthe quality factor of the energy supplied and deman-ded and thereby depicts the exergy content of theenergy flow. The thickness of the arrows representsthe amount of energy demanded or supplied. Bythese means both the quality and quantity of the dif-ferent regarded energy flows is shown. Thus, simi-larly as the exergy fingerprint diagram introduced inthe previous section, the matching between thequantity and quality levels of the energy suppliedand demanded can be seen.

summary

Figure 5.6: Example of an arrow diagram.


Exergy losses, associated to the energy losses pre-sent in the energy supply systems used, are shown atthe right of the diagram for the different energydemands analyzed.

Figure 5.5(a) shows the diagram for a referencescenario consisting of an average residential buil-ding in Germany whose demands are supplied withelectricity from the German grid and a gas firedcondensing boiler.

By comparing the diagram of different supplyoptions with this reference scenario, improvementscan be recognized. An ideal supply system wouldimply firstly a reduction of the demands, i.e. of thelength of the coloured areas (on the X-axis). Further-more, exergy losses, i.e. grey areas, also need to bereduced. An improved insulation level for the buil-ding shell and the use of suitable energy supplysystems, such as CHP units and waste district heat,allow achieving these aims as shown in Figure 5.5(b). The better performance is also shown at aglimpse through the a traffic light complementing thediagram, where the exergy savings in percentage ascompared to the reference scenario can be read.

The diagram gives similar information as that deli-vered by the Annex 49 pre-design tool. However,the performance of the different components in buil-ding supply systems cannot be assessed individuallywith this diagram. The Annex 49 pre-design toolallows to obtain such information on a quick andeasy way. Different building energy demands (e.g.DHW, space heating or lighting) are also includedin the tool.

PER – Exergy efficiency diagramThe Primary Energy Ratio (PER)-Exergy efficiencydiagram characterises the exergy performance anduse of renewable energy in the supply of a commu-nity project. Exergy efficiency is represented in theY-axis. PER ratio is represented in the X-axis. Eachcase study is represented by both factors (white dotsin the diagram). Ideally, high values for the exergyefficiency and PER ratio should be obtained. Whitedots show both parameters for different supply con-cepts, characterising the performance of the casestudy. Dots in the upper right corner indicate goodexergy performance and high use of renewableenergy sources. Supply concepts on the area closeto the upper right corner would correspond to“LowEx” community concepts. In turn, dots close tothe lower left corner depict case studies with lowexergy efficiency and high fossil fuel share on theenergy supply.

Pre-normative proposalsBuildings are major contributors to the final energydemands in many industrialized countries (Eurostat,2007). Therefore, to reduce CO2 emissions from thebuilt environment and thereby contribute to a moresustainable development and to international targetstrying to avoid or limit climate change, energy direc-tives for buildings have been developed.

Generally all current energy laws are based onenergy (i.e. the first law of thermodynamics), not onexergy. In this sections some thoughts and sugges-tions on including the exergy concept in energylegislation are presented.

There are five important questions to be asked whendesigning energy legislation:Which objectives are to be obtained by this legisla-tion?

Which parameters are the right indicators of the(energy) performance the building in relation to theforeseen objectives?

Which analysis method should be used?

Which ‘requirements’ should be set to the building?(e.g. benchmarking against comparable buildingsor set a maximum value to the chosen parameters)

Which administrative instrument could best be utili-zed? (i.e. energy tax, ‘force’, subsidies)

The following section gives a brief introduction tocurrent European energy legislation and tries to givesome thought and suggestions on including the exer-gy concept, hereby also addressing the five ques-tions mentioned above.

Current status of energy laws and exergy in ener-gy lawsReducing energy consumption and eliminating wasteare among the main goals of the European Union(EU). There is significant potential for reducing ener-gy demands, thereby limiting consumption of energysources. The EU has introduced legislation to imple-ment energy efficiency measures in the built environ-ment. According to the Energy Performance BuildingDirective (EPBD, 2002) the Member States mustapply minimum requirements as regards the energyperformance of new and existing buildings, ensurethe certification of their energy performance andrequire the regular inspection of boilers and air con-ditioning systems in buildings (EU, 2010). Regardingthe five main questions raised in the previous section,the following answers can be given related to currentenergy legislation on a European level:Objectives: the final aim is to promote secure andsustainable energy supply systems for the built envi-ronment. Related to sustainability this objective istranslated to the aim to reduce primary energy useand CO2 emissions. This objective should be obtai-ned by reducing energy demands in buildings andenhancing the use of renewable energy sources wit-hin the sector.

Parameters: the secondary objectives formulatedabove already determine primary energy use andCO2 emission as indicators for the performance of asystem.

Method: The analysis method must be determinedby the member countries. However, it should beaccording to some defined standards to allow cer-tain comparability and based on common frame-work methodology for all member states. It is alsomentioned that “the calculation shall also include anumeric indicator of carbon dioxide emissions andprimary energy use” (van Dijk, 2008).


Figure 5.7: Example ofan “PER – exergy effi-ciency“ diagram.

Requirements: the Member States are responsiblefor setting their own minimum thresholds and bench-marks.

Administrative instrument: compulsory energy per-formance certificates should be made availablewhen buildings are constructed, sold or rented.

Including exergy in energy legislationAs it can be concluded from the previous paragraphthere are no exergy requirements (using exergy as amethodology or as an indicator) in current energylegislation.

Some literature about including exergy in energylegislation (Van Gool 1997; Dincer 2002; Favrat etal. 2008) can be found. These works mainly focuson the importance of including exergy in energylegislation. Favrat, et al. (2008) describe a practicalapproach determining fixed exergy efficiencies forvarious energy conversion processes. In this work itis also mentioned that the Canton of Geneva requi-res that documents from city developers include anexergy performance evaluation of their project.

In this paragraph the main motivation and contribu-tion gained by including exergy in energy legisla-tion is presented. The same structure used in the pre-vious sections is followed: Objectives: the main motivation for including exergyin building energy regulations is that this conceptcan contribute to design and operate more efficientenergy supply systems in the sector since exergydepicts the real thermodynamic efficiency (and the-reby the improving potential) of energy systems. Thiscould contribute to the main objective of the energylegislations available of achieving more sustainableenergy supply systems. In addition, exergy allowsanalyzing all kinds of energy sources on a commonand scientifically grounded basis (be they renewableor fossil). It can be argued that in the future, whenall energy supply is based on renewable energy, itwill also be important to use renewable energies inan efficient way, since they are limited in time orspace, and the conversion of energies will never befree of materials. Therefore it can be argued that asecondary objective can be to reduce exergydestruction by designing more intelligent systems,even if these are based on renewable energies.

Parameters: exergy is in the first place related to thethermodynamic performance of a system . An exer-gy analysis can, therefore, determine how muchpotential has been lost and thus how well the perfor-mance of an energy supply system really is, compa-red to the ideal performance. In this way exergy hasan added value to energy:• Exergy efficiency is always <100% (different from

COP), thus a real improvement potential can bedetermined, while energy analysis is only able tocompare systems;

• Exergy analysis shows quality losses that are notshown with energy analysis, being thereby a truemeasure for the thermodynamic efficiency andperformance of a given system or process;

• Exergy assessment is not limited to the consumptionof “primary energy” as in energy from fossil fuels(as the primary energy approach is), but it alsoincludes the analysis of the potential of renewableenergy sources used. Therefore it is also a tool todesign intelligent systems using renewable energies.

Method: using exergy analysis as a method to deter-mine the exergy losses and the improvement poten-tial is the most obvious application of the exergyconcept, which is already used by many designersof energy systems. An exergy analysis can supportmeeting the objective to reduce the consumption ofprimary energy sources by making available moreefficient building systems. Furthermore, by matchingthe quality, i.e. exergy, level of the energy suppliedand demanded suitable energy sources and energysystems can be identified for providing different useswith different quality levels within the built environ-ment (e.g. space heating and lighting). At thismoment no standard tool at the level of most natio-nal energy analysis tools is available. A first idea ofthe exergy performance can be obtained with theAnnex 49 pre-design tool (see chapter 3). Alternati-vely the calculation methods as explained in chapter2 can be applied. The development of a generallyapplicable tool as are the current energy tools willrequire additional work.

Requirements: since exergy analysis is relatively newin the built environment it is difficult to set minimumstandards at this moment. By now, a common scien-tifically grounded methodology has been developedand agreed upon by the ECBCS Annex 49 group(see chapter 2). The next step would be to apply themethodology to several case studies and definebenchmarks based on the results. For a first idea theparagraph on benchmarking (chapter 5) can beconsidered. In addition, requiring an exergy analy-sis as is done by the Canton of Geneva (Favrat et al.2008) can be a good first step, since this will giveinsight in the losses and motivate designers to comeup with more intelligent systems. It is also a way ofintroducing the concept to many professionals wor-king in the built environment.

Administrative instruments: once the benchmarksare set, administrative instruments to make sure thatthey are met can be defined. However, as statedabove, further research needs to be conducted toreach this stage. Thus, the discussion of possibleadministrative instruments is not further treated inthis work


Exergy as sustainability indicatorSustainable development can be defined as a deve-lopment that “meets the needs of the present withoutcompromising the ability of future generations tomeet their own needs” (Bruntland, 1987). Severalauthors have linked the exergy concept with insightson sustainable energy supply and sustainable deve-lopment (Cornelissen, 1997; Rosen and Dincer,2007; Wall, G. and M. Gong, 2001). This link isbased on the fact that exergy is a thermodynamicconcept that clearly identifies the improvementpotential of an energy system, thus opening up thepossibility of increasing its efficiency (Rosen et al.,2008). For this aim, all energy flows involved, fossiland renewable, must be analyzed. This allows sho-wing the thermodynamic efficiency of using differentenergy sources, independently of their renewable orfossil character, and allows a common basis for thecomparison of different energy sources and uses(Schmidt et al., 2007). Since energy sources, andparticularly fossil fuels, are limitedly available,increasing the efficiency of their utilization leads toincrease the time span in which they can be utilizedand reduce negative environmental impacts derivedfrom its use, thus increasing sustainability of energysystems.

However, it must be clearly stated that systems basedon renewable energy sources are more “sustaina-ble” than fossil fuel based ones, even if the exergyefficiency of the first might be lower than that of anequivalent fossil-based alternative. The exergy con-cept does not distinguish between renewable andnot renewable energy sources. This distinction, cru-cial for finding options towards a more “sustaina-ble” energy supply, must always be regarded addi-tionally to the exergy analysis.

Therefore, within research group of ECBCS Annex49 consensus was agreed upon that exergy canNOT be understood as an indicator able to depictsustainability on its own. Exergy performance andsustainability are not equivalent concepts, and exer-gy analysis can only be seen as a further indicatorto complement existing analysis methods in order todevelop more “sustainable” energy systems.

Main conclusionsSeveral parameters usable to depict the matching ofthe quality levels between the energy supplied anddemanded have been introduced. Through theirapplication to case studies it has been shown thatexergy analysis adds information to conventionalenergy analysis: the supply of high quality exergy inbuildings for space heating and DHW supply purpo-ses needs to be minimized. This implies avoidingcombustion processes in building supply systems

and substituting them by low temperature systemsand sources. In consequence, the core of the bench-marking proposal is to minimize the exergy of pri-mary energy supply. With the parameters andbenchmarking proposal presented in this chapter,this information is made directly and clearly availa-ble to building planners on a scientifically groundedbasis.

Additionally, exergy analysis provides a commonlyand scientifically grounded base for comparingenergy systems using different energy sources, rene-wable and fossil, giving a true measure of the effi-ciency of their performance. In this sense, exergyanalysis contributes to promote the efficient use ofrenewable energies. Since they are limited in time orspace, and the conversion of energies implies alsomaterial use and process, reductions in the exergydestruction contributes to design more intelligentand efficient energy systems, even if these are basedon renewable energies. However, it is important toremark that, despite the great added value of exer-gy, it cannot depict the sustainability of an energysystem.

Including exergy analysis in energy legislation mightbe useful for two reasons: it supports meeting theobjective to reduce primary energy consumption,and it supports the design of intelligent energy sup-ply systems based on renewable energy, which willalso become important in the future. The practicalimplication as to standardised methods and mini-mum requirements must be further developed.However, the methodology presented in chapter 2 ofthis work and results from case studies presented inchapters 6 and 7 can be a very valuable contribu-tion for this purpose.



11So-called “reference environment”, see chapter 2.

12The quality factor in Equation 5.3 corresponds to a ther-mal heat transfer, where the temperature of the system chan-ges from T to T0 reversibly, i.e. via several heat enginesworking respectively at temperatures infinitesimally smallerthan T (i.e. T-dT; T-2dT; etc. ) up until T0..

13A description of this input/output approach can be foundin chapter 2.

14For cooling systems the quality factor of the demand to beprovided is even lower due to the closer temperature levelto outdoor air.

15This diagram has been kindly supplied by the researchgroup from the Fraunhofer UMSICHT Institute, Germany,Jentsch, A. et al. (2009).

16By some authors (Wall and Gong, 2001) exergy is alsoused as an environmental indicator, meaning it is also ameasure for sustainability in a broader sense than just ‘redu-cing energy consumption’’. However, this vision is not sup-ported by the ECBCS Annex 49 group, as it is clearly sta-ted further below in this chapter.

6. APPLICATION OF THE EXERGY APPRO-ACH TO BUILDING SYSTEMS

IntroductionIn the present chapter, some building case studiesare shown where the general exergy-based designstrategies for buildings presented in chapter 4 areapplied. Emphasis is put on the reduction of energyuse by means of innovative approaches for cold andheat storage as well as energy recovery. Six casestudies of innovative concepts or technologies arepresented here. Three of them are related to air-con-ditioning systems, in order to reduce both the ener-gy required for cooling and for the air circulation toensure proper indoor air quality. The need for coo-ling, in fact, is becoming increasingly high in buil-dings: since there are less alternatives for producingcold than for heat generation – cold is commonlyproduced with air heat pumps or compression chil-lers with relatively poor efficiency – the possibility ofusing natural ventilation or evaporative coolingwould be beneficial, where ever possible and withsuitable ambient conditions.

Similarly to the seasonal storage systems, groundheat helps improving the performance of the buil-ding system by using a renewable and freely avai-lable source: its exploitation is particularly intere-sting with heat pumps, raising their COP to a valuethat makes the use of a high exergy source like elec-tricity convenient. The use of hybrid technologies,coupling the use of renewable and non-renewableenergy is in fact one of the most promising trade-offbetween availability and exergy efficiencies.

Waste heat utilisation can be considered anothertype technology particularly efficient form an exergypoint of view: its use in the cogeneration approachis now widespread but it has to cope with problemslike the matching of heat and electricity demand inthe power plant, the need of an extensive planningand energy loss due to the heat distribution. Aninnovative approach that would partially solve theseissues is the local heat recovery in the building, as itwill be shown in the before last case study.

Two cases are about seasonal storage systems bothfor cooling and for heating (see the fourth and fifthbuilding case studies). They are mandatory for aneffective exploitation of renewable sources but theyare also useful to lower the peaks in the supplysystem and to make it work preferentially in the bestpossible conditions. By letting the demand and thesupply not being directly matched, they pave theway for a flexible energy use management.

A further case is about a waste water system to reco-ver heat from buildings waste waters similarly to the

heat recovery systems in the Air Handling Units(AHU): a rational energy use, in fact, would compri-se the recovery of all valuable types of energy.

The following is the list of the cases presented in thisreport:1. Innovative Concepts for Exergy Efficient Air-con-

ditioning Systems and Appliances in Buildings2. Temperature and Humidity Independent Control

(THIC) air-conditioning system3. Adjustment of the ventilation rates based on the

variation in time of the actual needs4. Seasonal heat storage by Ground Source Heat

Pumps (GSHP) system5. Shallow ground heat storage with surface insula-

tion6. Exergy recovery from waste water in small scale

integrated systems7. Innovative configuration for cooling purposes:

series design for chillers

Innovative building case studies

Innovative concepts for exergy efficient air-condi-tioning systems and appliances in buildings17

By using outdoor dry air as the driving force, theindirect evaporative chiller is aimed at providing anovel air-conditioning concept for public buildings indry regions. In this manner, it takes advantage of theuse of “wet” exergy contained in liquid water (whichis very large) in order to produce cool exergy andsubsequently cool the air or water as a cool carrier.

It produces cold water with a temperature between~ 15 and 18°C, lower than outdoor wet bulb andinfinitely close to the dew-point temperature of theinlet air. As the heat carrier of the chiller is water rat-


Figure 6.1: Principle of the chiller

her than air, the energy consumption for transmis-sion is greatly reduced. An air conditioning systemis also designed using the indirect evaporative chil-ler, as Figure 6.2 shows, which can use outdoor dryair sufficiently by matching the temperature level ofthe cold water and the heat sources.

Relevance as low-exergy technologyExergy use in cooling has two big benefits: the exer-gy needed for heat conversion and the exergy forheat distribution and emission. In regards to heatconversion, cold water is produced at 16-18 °C, orhigh temperature and low-exergy cooling. In addi-tion, it is produced using dry air as the driving forceinstead of electricity, as used in common chillers. Theuse of water strongly contributes to lowering exergylosses, with respect to airborne systems, due to bet-ter heat vector behaviour.

Temperature and humidity independent control(THIC) air-conditioning system18

Temperature and humidity control are the two maintasks of air-conditioning systems. In most centralisedair-conditioning systems in China, the air is cooledat the temperature below the indoor dew point tem-perature, dehumidified by condensation, and thensupplied to the occupied spaces to remove both thesensible and latent load. The required chilled watertemperature should be lower than the air dry bulbtemperature or air dew point in order to remove thesensible load (control temperature, covers 50%-70%) or the latent load (control humidity, covers30%-50%), respectively. However, the same 7°Cwater is used to remove both sensible and latentload and, as a result, available energy is wasted.

summary

Figure 6.2: Structure of the air-conditioning system using the indirect evaporative chiller


The proposed THIC (Temperature and HumidityIndependent Control) system is composed of twoseparated systems, a temperature control systemand a humidity control system, as shown in Figure6.3. The temperature of chilled water in the tempe-rature control system is raised from 7°C in the con-ventional system to about 18°C, which also allowsfor the utilisation of some natural cooling sources.Even if the chilled water is still produced by amechanical chiller, the COP (Coefficient of Perfor-mance) increases greatly.

In the southeast of China, where many large buil-dings are located, the outdoor air is humid: the maintask of air-conditioning systems is to dehumidify theair. In this case, the liquid desiccant dehumidifica-tion method is recommended. In the northwest ofChina, the outdoor air is dry and the main task ofair-conditioning systems is to decrease its tempera-ture. Direct or indirect evaporative cooling is recom-mended.

Figure 6.3: Device scheme

Relevance as low-exergy technologyThis system allows the control of both humidity andtemperature by splitting the management of theminto two independent systems. Due to the increasedtemperature for cooling from 7° to 18 °C, much bet-ter performances in terms of exergy can be obtained.Referred to an outside reference environment at25°C, the exergy content is respectively 6.4% and2.4% of the produced and delivered heat. Similarly,a chiller ideally working in the same environmentwould perform almost three times more effectively.Consequently, relevant amounts of exergy can besaved, while still assuring good comfort conditions inthe cooled areas.

Adjustment of the ventilation rates based on thevariation in time of the actual needsVentilation plays a role of key significance in the ove-rall building performance in terms of energy con-sumption, indoor air quality and thermal comfort.Ventilation can be ensured by natural means or by amechanical system. The major challenges for thedevelopment of purely natural ventilation techniquesare the uncertainty about practicing real control onairflows and the unreliability related to the stochasticnature of its driving forces - wind and temperaturegradients. Mechanical ventilation may result in an

unnecessary use of energy. Hybrid technology repre-sents the attempt of combining the benefits of bothventilation strategies in a unique system by promo-ting interactions between occupants, indoor climateand outdoor conditions. Hybrid technology urges atechnological development of system components(supply inlets, exhaust grilles and control algorithmsin order to always make airflow rates consistent withactual ventilation needs (e.g. amount of fresh air).The energy required by air conditioning and distribu-tion also has, of course, to be minimised.

Relevance as low-exergy technologyEnergy use for air circulation in air unit systems is arelevant part of the overall energy balance. To over-come the pressure drops in air ducts, which impliesslight exergy destruction, electricity-driven fans areneeded as their exergetic efficiency is very low. Thisapproach limits the electricity consumption for air cir-culation by making use of the natural pressure diffe-rences in the environment that would be otherwisesupplied. Furthermore, active systems, such as chil-lers, can be switched off to maintain IAQ comfortrequirements. As a result, in intermediate seasons, itis possible to cut off the electricity consumption, thatis exergy, and make use of available environmentalsources.


Figure 6.4: Averagehumidity ratio of the mosthumid month in China.Southeast of the line: out-door air is humid. North-west of the line: outdoorair is dry enough.

China

Seasonal heat storage with ground source heatpump system19

Ground source heat pump (GSHP) systems with ver-tical ground source heat exchangers can be aneffective solution to heat and cool buildings withlow-exergy consumption. In the case of small buil-dings the tubes are normally installed satisfactorilyfar from each other and utilise the geothermal ener-gy of the constant temperature of far-away soil volu-mes. In this way the seasonal energy storage is notavailable. However, in the case of larger buildingswhere several boreholes have to be installed, a moreeffective conception can be used. In this case thetubes can be installed in a cylindrical branch. If thenumber of boreholes increases, the proportionamong the cylindrical boundary surface and theheat storage soil volume becomes smaller. As aresult, the heat storage soil volumes are in contactwith a relatively smaller surface with the far-awaysoil volumes. Consequently the effectiveness of theseasonal heat storage becomes higher (see Figure6.5). In order to decrease the exergy loss of the sto-red energy (the temperature drop can be decrea-sed), the heat exchangers are distributed into moregroups and used in a suitable sequence during theheating and cooling periods (Simón, 2008).

Relevance as low-exergy technologyThe main precondition to the exploitation of manyrenewable sources is the possibility to store energy,due to their inconsistent availability. The exploitationof renewable sources is considered as a low exergyapproach. Even though solar radiation has a theo-retically great exergy potential, the exergy destruc-tion of the solar radiation would take place anyway,regardless of human exploitation, and its use repla-ces high-exergy fossil fuels.

Seasonal heat storage has a two-fold positive effecton exergy consumption in buildings: it allows themassive exploitation of solar energy in an efficientway – thus collecting freely available exergy - and itimproves the performance of active, electricity-dri-ven systems, such as heat pumps.

Shallow ground heat storage with surface insulationCoupling solar panels and a heat pump with a pipesystem merged into the ground under the building,either warm or cool exergy can be stored and thenreleased to the building itself (see Figure 6.6). Bycovering large ground surface areas with insulationof sufficient thermal resistance the heat loss from thestorage will be closer to the solution for a semi-infi-nite solid heated on the surface. Such storage will befavourable compared to a single borehole, especial-ly when heat is supplied and extracted by the heat

summary

Figure 6.5: Estimated heat storage efficiency.

Figure 6.6: General view of the system.


carrier in an annual cycle. The aim is to combinesuch an annual storage with solar collector and alow-exergy heating system in order to minimize theuse of high quality energy for heating and/or coo-ling. The energy carrier can be as an example air inducts or in a gravel bed or a fluid in pipes with highconductivity flanges.

Relevance as low-exergy technologyThis technology opens up the possibility of providingheating and cooling with low exergy supply. Thereduced heat loss to the ground is also a way tominimize exergy losses in the system. However, spe-cial care will probably be needed to control the moi-sture from the ground.

Exergy recovery from wastewater in small scaleintegrated systemsIn order to create a truly low exergy building, thesources of unnecessary exergy consumption must beeliminated. These include the exergy consumed bywarm air being released to the external environ-

ment, as well as the warm water. Recovery systemsfor exhaust air are already common, but wastewa-ter has been overlooked. Most well insulated highperformance buildings now have nearly half of theirheat demand coming from hot water production. Inthis system, a recovery system is being analysed tomaximise the potential of warm wastewater to aug-ment the performance of a heat pump. The heatfrom showers and other hot water demands is cap-tured at the highest possible temperature and usedto reduce the temperature lift needed for the heatpump to produce hot water. Thereby, a low lift com-pressor can be used in the production of both lowtemperature (LowEx) space heating as well as hotwater, which requires a higher production tempera-ture, but now receives a higher source temperature.This concept is depicted below (see Figure 6.7) andthe potential change in COP is demonstrated in theT-S diagram (Figure 6.8).

Innovative configuration for cooling purposes:series design for chillersAlthough a few companies supply chilled water attwo temperatures, the industry standard design is toprovide a single temperature chilled water supply.Water cooled chillers are normally configured withevaporators in parallel and condensers in parallel.The supply to return temperature differential for bothevaporator and condenser water chiller flows is typi-cally between 5.6°C and 6.7°C. The industry largescale chiller plants average approximately 0.267system kWelectric/kWcooling at 24.2°C ambient tempe-rature.

The improvement potential achievable with an inno-vative chiller design consisting on a series connec-tion of several chillers is investigated here.

Figure 6.9 shows schematically the conventionaldesign (left) and the innovative configuration propo-sed here (right). Temperature levels assumed for theperformance of both designs are also shown in thediagram. Ideal exergy efficiencies for both configu-rations amount 8.33 and 12.14 respectively. Thisrepresents an improvement of 47%.

Such an innovative configuration has been checkedfor the cooling supply of a production plant inMalaysia (Solberg, 2010).

In the innovative configuration chilled water is supp-lied at 7.2°C and 13.3°C with a common return with11.1°C temperature differential. The design consistson eight centrifugal refrigeration compressors inseries and has four condensers in series for a tem-perature differential of 8.3 °C. The forecasted elec-trical energy demand for the chillers is then reducedfrom the conventional value of 0.267 system kWe-lectric/kWcooling to 0.135 kWelectric/kWcooling at24.2°C ambient air temperature.

Increasing the chilled water temperature differentialreduced the total flow of chilled water by 50%, mea-ning pipes, pumps, and valves were much smaller.A conventional design for the particular case of theinvestigated production plant required 27 pumps;the innovative design, in turn, required 9 pumps.

In the production facility studied the innovative con-figuration represents an improvement on the overallexergy efficiency of the production plant from 0.249to 0.293.

Relevance as low-exergy technologyIn this study case, the exergy improvement potentialfrom a cascaded use of thermal energy flows forcooling applications is shown.


Figure 6.7: View of the system

Figure 6.8: T-S diagram of the heat recovery process

Relevance as low-exergy technologyIn this study case, the recovery of waste energy has astrong influence on the performance of the heat pump,By increasing the source temperature, and conse-quently the COP, the demand of electricity decreases.


17This case study was kindly submitted to the ECBCS Annex49 working group by Xiaoyun Xie and Yi Jiang from Tsing-hua University (China) as guest participants.

18This case study was kindly submitted to the ECBCS Annex49 working group by Xiaoyun Xie, from Tsinghua Universi-ty (China) as guest participant.

19This case study was kindly submitted to the ECBCS Annex49 working group by Tamás Simon, a guest participant,from the Budapest University of Technology and Economics,Budapest (Hungary).

Figure 6.9: Conventional parallel configuration of chillers for cooling energy supply (left) and innovativeseries configuration for high efficiency cooling supply (right).

7. APPLICATION OF THE EXERGY APPRO-ACH TO COMMUNITY CASE STUDIES

Managing energy supply and costs within a commu-nity requires that community to have a vision for itsfuture development. Plans and strategies for develo-ping energy supply structures for communities wouldincorporate the development of programs and pro-jects that create resilience within the community andthereby a resistance to the impact of energy marketfluctuations.

In this chapter several community case studies whichhave decided to go through such a planning processand implemented development projects to modifytheir energy supply structures are presented. Table7.1 shows an overview of the community case stu-dies included in this chapter. Besides a general des-cription of the community and the innovative supplysystems used, the relevance of the deployed techno-logies as “LowEx” systems is explicitly stated foreach case.

Prior to the case studies a general introduction onthe community scale is given. Here, the concept ofcommunity as used in this report is introduced, follo-wed by some words on the operation and develop-ment of community supply structures.

The communityInterestingly the term “community” is commonly usedwith apparent disregard for a consensus on its mea-ning. Here, the term community refers to a predeter-mined study area over which the decision-makershave authority or influence. For a City Hall this may

be an entire municipality, although the evaluation ofan entire city might be complex or unwieldy: it couldalso be a more modest development such as adowntown rejuvenation project. To enable categori-sation of demands the study area should be hetero-geneous in its design and contain a mixture of buil-ding types with a variety of energy uses anddemand profiles. Such mixtures could include suchproperties as residential, commercial, retail, institu-tional, and even industrial uses.

The planning and decision making processFigure 7.1 suggests that changes in energy use pat-terns within a community may be initiated at a varie-ty of levels. At each level the decision-makers aredifferent. The simplest change is often at the level ofthe end-user. For example a manufacturer mightimprove the efficiency their refrigerators, his cars orlight bulbs. Each end-user would purchase this newproduct based upon anticipated cost savings, but forsignificant savings to be made, the number of end-users purchasing this new product must be large. On the other hand, a change in energy type at thesystem level would involve fewer stakeholders andtheoretically should be easier to initiate, but it wouldrequire increased investment. For example, a simplecycle plant might decide to recover its waste heatand employ this within a district energy system,displacing oil heating in community buildings. At thecommunity level, this change would likely be theexpensive but also environmentally the most far rea-ching of the alternatives. It is at this level of changetowards which the community case studies present-ed in this chapter are oriented.

As already stated in this report, exergy is a compre-hensive measure of the potential of an energy sup-ply to do work (Shukuya and Hammache, 2002),therefore offering users the ability to manage theavailability of energy. By knowing the characteristicsof the task to be undertaken (demand), one canselect the most appropriate energy stream for it (sup-ply). Energy sources within the community must beseparated and categorised according to their quali-ty (i.e. exergy content) before being aggregated toform specific energy supply groups. Similarly, cate-gories for energy demand types can be defined.

With an understanding of the capacity and capabili-ty of each category, supply and demand integrationcan follow, linking energy supplies and demands inthe most effective manner and where possible, usinglocal resources to generate that energy.

Often, it is also possible to align tasks in such amanner that the output energy stream from one taskbecomes the input energy stream for another, there-


Table 7.1: Summary ofcommunity case studies.

community country LowEx highlights

Alderney Gate Canada Sea water cooling coupled with borehole thermal energy storage

Andermatt Switzerland geothermal energy systems

Heerlen Netherlands low temperature emission systems, low temperaturedistrict heat from old coal mines

Letten Switzerland geothermal energy systems

Minnesota USA co-generation and district heating

Oberzwehren Germany utilisation of waste heat from CHP as low exergy supply source

Okotoks Canada solar thermal heating systems coupled with seasonalground thermal energy storage

Parma Italy Low temperature heating systems coupled with efficientventilation systems

Ullerød Denmark low energy district heating, ground source heat pump(GSHP) and air-to-water heat pump (AWHP).

by cascading through the activities and maximisingthe effectiveness of the supply. This line of thinking issimilar in some respects to Pinch Technology (Walland Gong, 1996), as used within an industrial pro-cess where the cooling and heating requirementsare coordinated to minimise the need for externalenergy. However, the fundamental difference bet-ween the use of exergy and energy in Pinch Techno-logy is that, for energy, a satisfactory solution isobtained when supply and demand are balanced ortheir difference is minimised. For a satisfactory exer-gy solution, supply and demand not only have to bebalanced, but the exergy level at the final step hasto be close to that of the ambient temperature – amuch more demanding requirement.

Diagrams for characterising community exergyperformanceThe characterisation of the exergy performance ofdifferent case studies and community concepts ispresented here by means of diagrams that enablevisualization of the performance of a given casestudy and make different community energy supplyconcepts comparable. They are included under thesection “LowEx Diagrams” in the respective casestudy. Arrow diagrams and PER-Exergy efficiencydiagrams introduced in chapter 5 are used here tocharacterise graphically the exergy and energy per-formance of community supply systems.

There are some projects which have already beenimplemented. Therefore monitoring results are avai-lable and the contribution of different energy sour-ces and technologies used to supply them is known.In this cases the PER and exergy efficiency figuresare shown for the mix of the different energy sour-ces used in the supply. Examples of this situation arethe Okotoks Drake Landing Solar Community andAlderney Gate projects. Some other projects arestill in planning or under development. Here, diffe-rent options regarded for energy supply are cha-racterised separately. An example of this situation isthe City of Parma.

Innovative community case studies

Alderney Gate (CA)This low-exergy project integrates demand sidemanagement within the Alderney Gate Complex inDartmouth, Nova Scotia, with a renewable energycooling supply (seawater) and in-ground seasonalthermal storage to eliminate the use of electricallydriven chilling equipment.

The overall objective of the project is to develop acooling system for a municipal building complexthat employs the cooling effect of sea-water, either

directly to the building’s cooling system or indirectlythrough a Borehole Thermal Energy Storage (BTES)system.

The project demonstrates a systems approach tobuilding energy management. It is the first of its kindof project in Canada, successfully representing theuse of borehole thermal energy storage for coolingpurposes.

Water is drawn from the harbour adjacent to theproject site and passed through a heat exchangerbefore being returned to the harbour. The extractedcold energy is then passed directly to the building’sown cooling distribution system and, during periodsof low cooling loads, passed through a series of ver-tical borehole heat exchangers and stored in theground.

summary

Figure 7.1: Hierarchy of energy-related decisions


Figure 7.2: Advanced coaxial energy storage: heatexchanger design..

systems, thereby minimizing exergy destruction. Byincorporating new heat pump technologies, muchhigher COP’s can also be achieved. The viability ofthe projects depends on the evaluation of the valuegained versus the extra infrastructure or transportrequired for implementation. These aspects are stillunder evaluation in ongoing research.

This case study demonstrates the transport and utili-zation of heat at what would be absolutely low tem-peratures (i.e. being low exergy sources availablelocally). However, the sources used in this projecthave still relatively high exergetic potential andminimal environmental impact compared to otherambient sources.

The energy masterplan includes a low temperatureloop around the resort with decentralized heatpumps. The loop is fed by a seasonal geothermalstorage field of borehole ground heat exchangers of300m length, with a temperature of 0-5°C. Theother source is the Furka tunnel, which has an ent-rance located 6 km away (see Figure 7.4). It sup-plies a constant flow of drainage water at 13°C,which can be piped to the resort.

The tunnel water is of special interest in the moun-tains. Because of the low ambient temperature, theexergetic value of this relatively low temperaturesource is actually quite high.

This project is ongoing and research includes thefeasibility of the low temperature hydronic networksupplying the heat pumps. Also of interest is theinterplay between the two reservoirs and the rela-tionship between the exergetic value of reservoirsversus the transport cost from the tunnel.

The coaxial heat exchanger improves thermal andexergetic efficiency by cutting the temperature diffe-rence between the fluid and the ground to 1-2°C,giving the fluid direct access to the borehole walland providing very low pumping resistance. Figure7.2 shows schematically the coaxial heat exchangerused. The design results in a smaller storage volumefor the same cooling load and eliminates the use ofmechanical chillers.

A custom designed control system optimises thesystem components, the storage temperature distribu-tion, and the activities within the Alderney 5 complex.

LowEx highlightsSea water cooling coupled with borehole thermalenergy storage is planned to be used for coolingpurposes in the project. Both thermal energy groundstorage as well as the cooling potential from the seawater have low temperature levels and are therefo-re suitable LowEx sources for supplying coolingdemands.

LowEx diagramsFigure 7.3 shows the Primary Energy Ratio andexergy efficiency for the energy mix used in theAlderney Gate complex.

Andermatt (CH)Andermatt is an alpine region of Switzerland wherean entirely new tourism resort is being built. The coldclimate implies a high heating demand with a lowcooling demand. One goal of the project is for theenergy use at the resort to be CO2-free.

The energy concept for this resort in the Swiss Alpsconsiders the high potential of deep geothermalenergy from mountain tunnels. The temperature levelof the heat reservoir is not high enough to supplybuilding energy demands directly. Instead, the con-cept is to use this low temperature reservoir to mini-mize temperature gradients in energy supply


Figure 7.3: PER ratio vsexergy efficiency dia-gram for the energy sup-ply mix in the AlderneyGate complex.

Figure 7.4: Energy plan for the new AndermattAlpine Resort, at 1447 m altitude.

Heerlen (Netherlands)This low-exergy project uses warm and cold watervolumes from abandoned mines. In the Mine WaterProject in Heerlen water from abandoned and floo-ded mines is used as a new sustainable energy sup-ply for heating and cooling of buildings. The tempe-ratures that have been found (16..30°C) are used invery well insulated buildings, with energy efficientventilation systems and low temperature emissionsystems, the thermal comfort is excellent during 365days/year. At the same time there will be a CO2

reduction of 50% in comparison with a traditionalsolution.

The project started in February 2006 in Heerlerhei-de by drilling the warm wells. In October 2007 thelast well was drilled, the cold well.

This project is situated on the concession of the ONIII pit in a relatively deep mined area with warmwater wells (30..35°C). The area of buildings inclu-ded in the project are:33,000 m² dwellings (single family dwellings and

residential buildings)3,800 m² commercial building2,500 m² public and cultural buildings

11,500 m² health care buildings2,200 m² educational buildings

The first new building and construction activities inHeerlerheide Centre have started in 2006. the totalplan will be realised between 2006 and 2011. Allplanned buildings will be connected to the energy

supply (heating and cooling) from minewater. Allthese buildings are planned in a very compact area,which is very favourable for energy distribution. Thebuilding location is situated between two warmwells. Next to it, the planned building functionsrequire heating as well as cooling. The energy sup-ply includes the building of an energy station and asmall scale distribution grid from this to the buil-

summary

Figure 7.5: Minewater energy concept: depth andtemperature level of the wells in the project.


Figure 7.6: Energy management system: temperature levels and lifts in the different parts of the energysupply concept planned in Heerlen.

dings. In the energy station the minewater is broughtto the necessary heating and cooling levels by heatpumps. In order to facilitate the process and to gua-rantee all real estate developers, involved in thisbuilding plan, the delivery of energy to the buildingthe main investor, is realising the exploitation of theenergy supply, including the building and construc-tion of the energy station and distribution grid. It isimportant to realise, that with minor modificationsthis energy supply can also be functional and ope-rational without the application of minewater.

Minewater is extracted in this project from four dif-ferent wells with different temperature levels. Theprimary energy grid transports the extracted mine-water from the warm wells (~30°C) to local energystations. In these energy stations heat exchangetakes place to the secondary energy grid (from theenergy station to the buildings). This secondaryenergy grid provides low temperature heating(35..45°C) and high temperature cooling(16..18°C) supply and one combined return(20..25°C) to an intermediate well. The differenttemperature levels of the wells considered can beseen schematically in Figure 7.5

The temperature levels of the heating and coolingsupply are guaranteed in the local energy stationsby a polygeneration concept existing of electric heatpumps in combination with gas fired high-efficiencyboilers (see Figure 7.6). The surplus of heat in buil-dings which cannot used directly in the local energystation can be lead back to the minewater volumesof storage. DHW is prepared in local sub-energystations in the buildings by heat pumps, small scaleCHP or gas fired condensing boiler, depending ontype of building and specific energy profile. The

total system will be controlled by an intelligent ener-gy management system including telemetering of theenergy uses/flows at the end-users.

LowEx highlightsIn this project, low temperature heating and hightemperature cooling systems are being used in com-bination with highly insulated buildings. As stated inthe design strategies mentioned in chapter 4 forbuildings and communities, this makes possible theuse of a low exergy source such as minewater at lowtemperature level for space heating and cooling. Furthermore, as back-up system to guarantee thesupply whenever the minewater temperature is notenough to provide direct heating or cooling, heatpumps operating at high COPs are used. Theseenergy systems allow, as stated also in chapter 4,minimizing the high exergy input required to supplythe demands.

LowEx diagramsFigure 7.7 shows the Primary Energy Ratio andExergy efficiency for the minewater-based supplytechnologies considered in the community of Heer-len (NL).

Letten (CH)This case study deals with one energy concept beingstudied for the supply of the ETH Zurich central cam-pus. One goal of the Energy Strategy to be imple-mented is to halve the CO2-emissions of its structu-res and buildings by 2020. The energy supply con-cept focuses on the potential exploitation of the tem-perature differences between a stratified lake andthe mixed river at the lake output. Similarly as casestudy “Andermatt”, the temperatures and tempera-ture differences are not enough for a direct supply ofthe required energy demands. Again, the idea is topromote and use this low temperature (low exergy)sources available to reduce the temperature diffe-rences in common building supply systems, therebyreducing exergy consumption in such systems.

The concept considers reopening an old train tunnelthat has been filled in using microtunneling. This tun-nel passes underneath the campus. This “Thermotun-nel” (see Figure 7.8) would then connect a down-stream part of the Limmat river directly with its tribu-tary, the Lake of Zurich. The temperature differencebetween the two would create an exergy potential atlow temperature. Compared to thermal networkswith only one reservoir, this system makes use of thetemperature differences between the two reservoirs.Due to potential environmental disturbance thesesources are not allowed to have their temperaturesdisturbed. Campus heating and cooling systemsfrom one source would cause a considerable distur-


Figure 7.7: PER ratio vs Exergy efficiency diagramfor the energy supply options chosen in Heerlen(represented by the white dots). For comparison,grey dots represent the performance of conventio-nal technologies.

bance. Instead of disturbing one source, the tunnelcan be used to extract or deposit heat between thetwo thanks to the temperature gradient between thefully mixed river current and the stratified layers inthe lake. The energy from the water can be used fordirect cooling and for heating with a central heatpump. With a typical extraction temperature ofaround 6°C combined with low temperature emis-sion systems on the buildings of the campus, highCOP systems are expected to minimize the renewa-ble electricity that must be supplied.

LowEx highlightsThis project also makes use of a strategy mentionedin chapter 4, namely the reduction of the high exer-gy input in highly efficient energy systems such asheat pumps. This is achieved by minimizing the tem-perature difference in the thermodynamic cycle ofthe heat pump by exploiting the potential of water ina lake as low exergy source.

LowEx diagramsA graphical representation of the quality levels ofthe energy supply and end-use categories conside-red in this case study is shown in the arrow diagramin Figure 7.9.

Oberzwehren (GER)The city of Kassel, situated in the centre of Germany,is aiming at carrying out an environmentally ambi-tious housing project within the coming years. Thebuilding site is situated on the property of the formerSchool for Horticulture of the University of Kassel inthe city district of Oberzwehren. It is bordered byaccess roads and private estates. To the north, amixed-use area borders the site. To the northwest,there is a university campus, to the west, multi-fami-ly buildings, and to the southwest and east, single-family houses can be found. Floodplains from asmall river can be found to the south. Bus and tramconnections to the city centre exist.

A district heating pipe from the local utility compa-ny circulates close to the residential area. The planis to use the return line of this district heating con-nection to supply domestic hot water and space hea-ting demands. District heating in Kassel is mainlywaste heat from co-generation power plants. Waste

summary

Figure 7.8: Thermotunnel Letten for the potentialenergy plan of the ETH Zurich.


Figure 7.9: Matching of the quality levels of energy demand and supply for the Thermotunnel Letten casestudy. The different energy supply options regarded as possible supplies are characterised separately.

heat available, e.g. from combined heat and powerproduction (CHP) plants, is a low quality energy flowsuitable for supplying the requested energydemands. The use of waste heat with low exergycontent allows suitable matching between the exer-gy level of the demand and supply sides and thusrepresents a very efficient manner of supplying ther-mal energy demands in buildings.

For the analysis of this case study dynamic energyand quasi-steady state exergy analysis have beenperformed using the simulation software TRNSYS(TRNSYS, 2007) with a timestep of 3 minutes. Space heating (SH) is supplied by floor heatingsystems operated with supply and return temperatu-res of 32-27°C. Small DHW storage tanks of 200litres are considered in each house. This allows asignificant reduction in peak loads for DHW supply.For DHW supply in single family houses, a tempera-ture of 50°C at the outlet of the DHW supply elementmust be ensured at all times (AGFW, 2009). Anelectric heater located at the outlet of the tank is fore-seen for this purpose.

A centralised heat exchanger unit is being plannedfor the supply of heat to the small neighbourhood,shown in Figure 7.10. In this way, the district hea-ting network from the local utility company is decou-pled from the installed building appliances andsystems, i.e. mass flow and temperature drop in thedistrict heating network are not directly determinedby the mass flows and temperature drops in the buil-ding systems (e.g. floor heating systems). All housesare connected in parallel to the local distribution net-work (secondary side of the heat supply), as shownin Figure 7.10.

LowEx highlightsIn the project, the utilisation of a low exergy supplysource, i.e. waste heat from CHP units, is being inve-stigated. Best case scenarios and hydraulic configu-rations have been derived based on dynamic exer-gy assessment performed. This clearly shows theadded value of exergy analysis in comparison toconventional energy assessment. In order to ensurea minimum supply of high exergy sources for DHWsupply, hydraulic configurations which ensure maxi-mum supply from the district heating network forthese demands have been analysed. Furthermore,low temperature (floor) heating systems have beenimplemented in the buildings.

LowEx diagramsHere, all possible supply options considered at thebeginning of the project are analysed. Thereby, thesimple graphical representations show, at a glimpse,how well the final supply chosen (district heatingreturn pipe) performs, as compared to other supplyoptions. A graphical representation of the qualitylevels of the energy demanded (energy use) andsupplied is shown in Figure 7.12. The height of thearrows gives an idea of the degree of matching bet-ween the energy supplied and demanded. In anideal case, supply and demand arrows would beequally thick (no energy losses) and equally high (noexergy losses). In Figure 7.11, primary energy ratioand exergy efficiency for the different energy supplyoptions regarded for the community of Oberzweh-ren are shown.

Quality levels of the energy supplied and demandedare calculated by using simplified steady state equa-tions assuming a reference temperature of 0°C (typi-cal winter space heating conditions in Germany), aswell as typical supply temperatures for the technolo-gies, sources and demands under regard. Supplyand return temperatures for the solar thermal collec-tors and district heating return pipe are assumed tobe 70/50°C and 50/30°C, respectively. Approxi-mate quality levels under these assumptions aredisplayed close to the corresponding arrows in thediagram.


Figure 7.10: Simplified scheme of district heat sup-ply to the studied neighbourhood of Oberzwehren(Germany).

Figure 7.11: PER ratio vs exergy efficiency dia-gram for the different energy supply options underconsideration for the community of Oberzwehren

The solar water heating system uses flat plate solarcollectors and provides at least 90% of the annualspace heating and 60% of domestic hot water(DHW) for the 52 individual dwellings. This wasachieved, despite winter temperatures of -33°C. Inthe scheme (Figure 7.14) of the solar thermalsystem, the borehole seasonal thermal storage andthe district heating loop is shown.

Okotoks (CA)The community of Okotoks (Figure 7.13), Alberta, ismore than 1,000 m above sea level, but its averagesummertime temperature exceeds 20°C. This allowssolar thermal collectors, facing due South at anangle of 45°, to generate up to 1.5MW (thermal) toheat the buildings at 55°C.

The plant started operation in June 2007 and it isestimated that it will take three years to fully chargethe underground storage to 80°C. Construction ofthe 52 homes is complete and all homeowners havemoved in. Performance indications from May 2008suggest that the solar energy system is performingas designed and that the 90% solar fraction will beachieved by year 5.

summary

Figure 7.12. Matching of the quality levels of energy demand and supply for the community of Oberzweh-ren. The different energy supply options regarded as possible supplies are characterised separately.


Figure 7.14: Solar seasonal storage and districtheating loop used as energy supply system.

Figure 7.13: Okotoks complex (Canada)

The array is mounted on garages, at the rear of thehouses, and uses a propylene glycol/water solu-tion, pumped through an underground pipe net-work to a heat exchanger and a high temperature,short-term thermal store (STTS) located within the‘Energy Centre’.

Two unpressurised epoxy-lined cylindrical steelwater tanks form the SSTS and internal bafflesencourage thermal stratification. The Energy Centrealso houses most of the pumps and controls.

In addition, there is long-term Borehole ThermalEnergy Storage (BTES) which encompasses 144boreholes. Each contains a single U-tube grouted inplace. Above them, layers of sand and insulationand a waterproof membrane are topped by clayand landscaping. The BTES is connected as 24strings of six boreholes in series and divided into


Figure 7.16: PER ratio vs. exergy efficiency dia-gram for the energy supply mix in the OkotoksDrake Landing solar community.

Figure 7.15: Heat emission: low temperature coo-ling air fan coils

four circuits, preventing the loss of any string or cir-cuit from having an impact on storage capacity. Bythe end of a typical summer, temperature in theearth surrounding the boreholes is expected to top80°C. When the STTS temperature exceeds that inthe BTES, pumps circulate hot water from the STTSthrough the boreholes. Figure 7.14 shows an sche-me of the supply system.

Because a power cut may overheat the glycol loop,an additional photovoltaic (PV) array and batterybank is incorporated to power the pumps.

In winter, with no glycol circulation, parts of the loopcan cool down to below freezing. Therefore, onstart-up, the glycol solution is recirculated through abypass loop until its temperature exceeds the STTS.This protects the heat exchanger in the energy cen-tre from freezing.

In winter, whenever the temperature in the STTS islower than that of the BTES, the system reverses andheat is transferred from the BTES to the STTS, and toa heat exchanger and the district heating loop.

This supplies heated water to individual houses andthe specially designed low temperature air-handlerunits in the basements (Figure 7.15). Warmed air isdistributed through the house via internal ductwork.

LowEx highlightsIn the project, solar thermal heating systems, cou-pled with seasonal ground thermal energy storage,are planned to be used for heating purposes in aresidential area. Both thermal energy ground stora-ge as well as solar thermal heat have low tempera-ture levels and are therefore suitable LowEx sourcesfor supplying heating demands in buildings.

LowEx diagramsFigure 7.16 shows the Primary Energy Ratio andexergy efficiency for the energy mix used at theOkotoks Drake Landing solar community.

Parma (IT)Parma is located in Northern Italy’s Emilia-Romagnaregion and has a population of approximately178,000 people and a balanced presence of thetertiary, industrial and agricultural sectors, a mildclimate and a notable historical buildings stock andcultural heritage. With these features, Parma repre-sents a typical city of the Pianura Padana.

In recent years, Parma has undergone many initiati-ves related to energy efficiency, with two energyplans (the last dates back to 2006), local regulationsfor mobility, and a mandatory building energy regu-lation with advanced quality certification tools andincentives for low energy and the implementation ofrenewable energy technologies.

An important aim of the present study is to modifyenergy choices in order to optimize energy and exer-gy efficiency. Renewable energies, distributed gene-ration, micro-cogeneration and micro-trigenerationmay represent important measures to that end.

In order to evaluate the quality and quantity of ener-gy uses within the built environment, the performan-ce of the whole city, sector by sector, must be consi-

dered. This holistic approach implies that during thedesign process not only single buildings but thewhole community must be analyzed.

This approach emphasises the use of low energysystems and leads to better environmental and eco-nomic effectiveness, exploiting the potential of distri-buted local resources. This research project is lea-ding the way in adapting energy systems to thischanged paradigm.

New energy systems should address the followingissues:• the use of technologies to minimize primary ener-

gy consumption by reducing end-users demand• the analysis of the whole energy supply chain,

from generation through distribution and storageto end-users.

The aim of this study is to provide some represen-tative experience with these issues.

In the future research will address the city of Parmaas a whole. So far energy fluxes have been analy-zed in detail for three different districts of the town,characterized by different energy end-uses:• a part of the historical city centre• an urban neighbourhood• an industrial and agricultural area.

Exergy loss minimisation will be one of the most impor-tant objectives of this study. Here, exergy analysis isonly focused on the urban neighbourhood because ofits large potential for energy system optimisation.

In a distributed poly-generation system, electricity,high and low temperature heat and refrigeratedwater are produced locally. In order to efficientlysupport the transition towards such a system theinteraction among customers’ demands for energyservices, available generation technologies, availa-ble renewable energy sources and utility tariffs haveto be investigated. For this reason, natural gas andelectricity use data was mapped in a GIS to visuali-se energy use pattern and identify land-use cons-traints that can prevent the implementation of distri-buted generation. Based on this real data and cons-traints, an energy and exergy analysis has beenperformed in order to define a realistic scenario.

For this purpose, energy demands were split into sixmain categories based on statistical data: electricity,end-use only (appliances, lighting, etc.), electricityfor refrigeration and building cooling, natural gasfor water heating, building space heating, processheat (industrial sector), and natural gas only end-use (cooking, etc). Alternative strategies for supply-ing thermal, electrical and cooling energy demands,


in a poly-generation framework, were highlighted tosuggest system concepts that improve energy andexergy efficiency, and reduce emissions and costs.Starting from these initial evaluations, hourly loadprofiles for electricity (utility statistical data) andthermal energy (simulated heating and coolingdemand of buildings) were determined.

A multi criteria procedure, currently in development,will take into account economic, energy and exergygoals in the design and optimisation of energysystems.

In this work, three scenarios have been analysed forthe town:- Scenario 0: Parma 2007. State of the artThe scenario Parma 2007 is based mainly on fossilfuels used for electricity generation and heating. Infact, currently in the city of Parma, fossil fuels are theonly energy source. Renewable energies are notused. The average energy demand to be assumedfor further planning was based on assumptions oftotal heat demands and heat loads. With this pro-cessed data, we were able to evaluate measures toadopt in the planning scenarios.- Scenario 1: Parma 2020 Here, the objective is to find a realistic path to reachthe 2020 European goals20 by introducing mandato-ry regulation for local energy planning concerningurban planning and the refurbishment of buildings.- Scenario 2: Parma 2050The target is to transform Parma into a renewablecity21 by the year 2050, adopting today’s best avai-lable technologies and practices as a benchmark.Here, the optimisation of exergy fluxes is also takeninto account.

LowEx highlightsIn the building energy regulation developed here,the use of “LowEx” technologies is strongly encoura-ged. Low temperature heating systems close to roomtemperature will be used, meaning that the energyand exergy supply to the indoor building spaces willbe very efficient, with minimal losses. Low tempera-ture renewable energy sources like solar energy andthe heating and cooling potential of undergroundheat exchangers are also considered as supply sour-ces. As stated in chapter 4, to utilise these sources,the overall building system has to be adjusted to lowprocess temperatures. Radiant heating and coolingsystems, ground water heat exchange, solar ther-mal, as well as building envelope performanceimprovement (insulation, thermal capacity and natu-ral ventilation) are suggested and economicallysustained. The new building energy regulation is anexample of the promotion of “LowEx” design strate-gies mentioned in chapter 4 for both the buildingand community levels.

Quality levels of the demands and energy suppliesare calculated by using simplified steady state equa-tions assuming a room temperature of 20°C for hea-ting and 28°C for cooling as well as typical supplytemperatures for the technologies, sources anddemands evaluated.

All calculations are done assuming a reference tem-perature of 5°C for winter and 32°C for summer.

Supply and return temperatures considered for thesolar thermal collectors and district heating returnpipe are assumed to be 70/50°C and 50/30°C,respectively. Supply and return temperatures for thedistrict cooling return pipe are assumed to be18/25°C.

Twin cities Minnesotta (USA)The energy supply of the Twin Cities of St. Paul andMinneapolis, located in Minnesota (USA) has beenanalyzed on the light of exergy principle. The ener-gy demands regarded include electrical powergeneration, home heating and cooling, and auto-mobiles. Besides the analysis of energy flows, harm-ful emissions and ground water use were also con-sidered.

Minneapolis and St. Paul receive most of their elec-tric power from three Xcel Energy district electricpower plants Riverside, Highbridge, and Black Dog.Riverside and Highbridge are natural gas fired com-bined Brayton and Rankine cycle plants. Black Dog iscoal fired Rankine cycle. All condensing heat energyis rejected to the Minnesota and Mississippi Rivers.25 MW of additional electric power is also genera-ted by Evergreen Energy which supplies downtownSt. Paul with electric power, district steam heating,and chilled water cooling. Evergreen Energy current-ly heats 80% of the commercial, residential and indu-strial buildings in downtown St. Paul and providescooling for 60 percent of downtown Buildings.

Minneapolis has district heating and cooling provi-ded by NRG and Hennepin County, and the Univer-sity of Minnesota also has district heating and coo-ling for their buildings. Hennepin County burns1,000 t/d of municipal waste to produce 40 MW ofelectric power in downtown Minneapolis. No heat isbeing recovered at the Hennepin County garbageburning facility. Rock-Tenn, a paper recycling plantis in St. Paul, is generating 9 MW for cogeneration.A study (HVAC S.T., 2009) of Rock-Tenn facility indi-cates that 20.2 MW of heat energy could be reco-vered from the Rock-Tenn paper exhaust stack.

As a rough approximation, automobile transporta-tion is considered essentially 100% powered by


Figure 7.17: Diagram of exergy efficiency of thesystems vs. primary energy ratio for scenario 1 -Parma 2020 in winter conditions.

Figure 7.18: Diagram of exergy efficiency of thesystems vs. primary energy ratio for Scenario 2 -Parma 2050 in winter conditions.

The “LowEx” measures, in this case study, include:− Low energy demand for heating, good insulation

and air-tightness− Radiant heating systems like floor and wall hea-

ting, slab heating, capillary tube systems− Solar energy systems for DHW− Heat pumps

LowEx diagramsA graphical representation of the quality levels of theenergy supply and that will be considered in the opti-misation study is shown in Figures 7.17 and 7.18.

The scenarios Parma 2020 and Parma 2050, inwhich district heating and cooling have been plan-ned, refer to hypothetical energy plans that are cur-rently being defined.

summary

Figure 7.19: PER ratio vs Exergy efficiency dia-gram for the investigated supply of the Twin citiesof St. Paul and Minnesota, based on district heatsupply from the power plant.


gasoline, and home heating is considered to be100% from indirect fired natural gas furnaces.

Based on exergy analysis several recommendationsand modifications of the existing energy supplysystem as explained above have been derived. As aresult, a different supply scenario based on electriccars and the use of waste heat from the power plantsfor district heating purposes has been developed.The main technical characteristics of this supplyoption are stated below.

Approximately 341 m3/hour of water would bedistributed through new distribution piping to homesand buildings throughout the city. With an averageyearly temperature of 9.4°C, the Twin Cities requiresheating for most of the year. In the winter all the heatrejected to the Minnesota and Mississippi Rivers byXcel Energy power plants (1049 MW) would beused to heat 300,000 homes (with average loads of4.4 kW). The power plant steam turbines would pro-duce 41.7 MW less electric power due to increasingcondensing temperature from 21.1 °C to 71.1°C.Hot water would be distributed at 71.1 °C and retur-ned at 26.7 °C.

In the summer low exergy cooling technologies suchas adsorption chillers or liquid desiccant systemswould be used to produce 331,000 MW of cooling.The steam turbine condensing temperature would beincreased from 21.1°C to 54.4°C reducing electricalpower output by 26.1 MW. Chilled water would bedistributed at 7.2°C and returned at 21.1°C. Districtcooling would be significantly more efficient than aircooled home direct expansion condensers, creatingan increase in peak electric capacity of 91 MW,representing 6% of the current capacity.

District electric would charge 88,200 automobilesbatteries for 12 h/d at a rate of 1.04 kW/car. Thisis based on 9.7 km/litre gasoline and an efficiencyof 22% fuel/engine power.

Potential heat recovery from Evergreen Energy, Hen-nepin County, or Rock-Tenn energy plants is notregarded in this retrofit scenario of the energy sup-ply in the communities. Detailed data showing theloads, supply and performance of the Twin cities canbe found in (Solberg, 2010).

LowEx HighlightsThe performance assessment simulation of the TwinCities Community of Minneapolis and St. Pauldemonstrates that major reductions in energy inputand ground water and environmental harmful emis-sions could be achieved by using electric cars andmodifying local power plants to recover waste con-denser heat for a district heating and cooling system.

The Twin Cities community systems exergy perfor-mance can be increased by 64% from 0.465 to0.762. Annual carbon emissions can be reduced by39% or 1,676,000 t/a and ground water use redu-ced by 73% or 15,870,000 t/a. Reductions in sul-phur dioxide and nitrous oxides would be of similarmagnitude as carbon. A substantial amount of theemissions reduction is because power plants havesignificantly less emissions than do automobile engi-nes and home furnaces.

LowEx DiagramsIn Figure 7.19 Primary Energy Ratio and Exergyefficiency for the district heat and electric powersupply regarded for the Twin cities are shown. Theexergy efficiency figure shown corresponds to acombined analysis of heat and power generationtogether, with the corresponding heat and electricaldemands supplied. A graphical representation ofthe quality levels of the energy demanded (energyuse) and supplied is shown in Figure 7.20. Theheight of the arrows gives an idea on the degree ofmatching between the energy supplied and deman-ded. In an ideal case supply and demand arrowswould be equally thick (no energy losses) and atequal height (no exergy losses).

Quality levels of the energy supplied and demandedare calculated by using simplified steady state equa-tions assuming a reference temperature of 9.4°C(average annual outdoor air temperature at theinvestigated locations), as well as typical supply andreturn temperatures for the district heating systemplanned (71/21°C). Resulting quality levels underthese assumptions are displayed close to the corre-sponding arrows in the diagram.

Ullerød (Denmark)22

In Denmark, the government has decided that ener-gy use in new buildings must be reduced step bystep by 25% in 2010, 2015 and 2020. With theincreasing number of new low-energy houses, thequestion is: "What kind of heat supply is economi-cally and environmentally most attractive?" In urbanareas with DH, it might be reasonable to connectsome new low-energy houses. Yet, in new subdivi-ded areas with lots of or only low-energy houses, itis interesting to know if it is feasible to use DH. Todayin Denmark, low-energy houses located in DHdistricts can be exempted from connection obliga-tion to the DH network. Therefore, it is relevant toresearch if DH is a good alternative to other heatingtechnologies, e.g. heat pumps.

The low heat demand in low-energy houses meansthat, with a traditional network design, the networkheat loss may be a very significant part of the totalheat demand. To solve this problem, the networkheat loss and involved costs must be reduced. Thesolution seems to be a low-temperature DH networkwith high-class insulated twin pipes in small dimen-sions, reference (Svendsen, et al. 2005; Svendsen,et al. 2006).

The advantages of a low-energy DH system are:• DH is a flexible system suitable for all kinds of

energy sources• renewable Energy (RE) sources can be used

directly or in combination with large-scale heatstorages. This means that DH can be an importantpart of the future energy supply system fully basedon RE

• great potential for utilisation of waste heat fromCHP plants, refuse incineration and industrial pro-cesses

• DH covers a large part (60%) of Denmark's hea-ting supply and is a well-known technology

• DH is reliable and easy to operate for the consum-ers.

An urban area has been selected for reference. Thearea is located in a new district called Ullerød-byenin the municipality of Hillerød, Denmark. The areahas a great focus on energy efficiency regardingboth buildings and energy supply. This area consistsof 92 low-energy houses with an energy demand of42.6 kWh/m2a including space heating, domestichot water, cooling and electrical auxiliary energy.

LowEx highlights In the project, the following “LowEx” technologiesare compared: low energy district heating groundsource heat pump (GSHP) and air-to-water heatpump (AWHP). Furthermore, these technologies areapplied to buildings that fulfil the requirements oflow-energy buildings, being already suitable forusing low exergy sources in their energy supply.

LowEx diagramsA graphical representation of the quality levels ofthe energy demanded (energy use) and supplied isshown in Figure 7.21. The height of the arrows givesan idea of the degree of matching between the ener-gy supplied and demanded. In an ideal case, sup-ply and demand arrows would be equally thick (noenergy losses) and equally high (no exergy losses).

Quality levels of the energy supplied and demandedare calculated by using simplified steady state equa-tions assuming a reference temperature of 0°C (typi-cal winter space heating conditions), as well as typi-cal supply temperatures for the technologies, sour-ces and demands taken into consideration.


Figure 7.20: Matching of the quality levels of energy demand and supply for the Twin cities of St. Paul andMinneapolis for the described energy supply scenario.

The temperature of the ground for the GSHP systemis assumed to be 8°C, equal to the mean annual out-side temperature of the air in Denmark. The sametemperature of the source is considered for theAWHP system, since the heat pump is used both inwinter (for space heating and domestic hot water)and in summer (only for domestic hot water). Supplyand return temperatures for the district heating net-work are assumed to be 50°C and 22°C, respecti-vely. Approximate quality levels under theseassumptions are displayed close to the correspon-ding arrows in the diagram.

The different systems which are assumed to providethe given demand are represented in the exergy effi-ciency vs. PER below (Figure 7.22).


Figure 7.22: Diagram of exergy efficiency of thesystems vs. primary energy ratio.

Figure 7.21: Matching of the quality levels of energy demand and supply.

20To reach 2020 goals for EU countries means cuttinggreenhouse gas emissions by 20% from 1990 levels; a20% share of renewable energies in EU energy consump-tion (17% for Italy); cutting energy consumption by 20%through improved energy efficiency.

21Not totally but almost entirely fuelled by renewable energy.

22This case study has been kindly supplied to the ECBCSAnnex 49 working group by Sven Svendsen, a guest parti-cipant, from the Department of Civil Engineering of the Tech-nical University of Denmark.

8. CONCLUSIONS

The thermodynamic concept of exergy allows depic-ting how the potential of a given energy flow is used,or lost, respectively, in the course of an energy con-version. Thereby, inefficiencies within energy supplysystems can be pinpointed and quantified. Applyingthe exergy method to energy systems in buildings cancontribute to increasing their efficiency significantly.

Within the ECBCS Annex 37 low exergy systems weredefined as “heating or cooling systems that allow theuse of low valued energy as the energy source” witha focus on space heating applications. However, thescope of ECBCS Annex 49 includes the various ener-gy demands in buildings as well as the integration ofmultiple buildings in communities or neighbourhoods.Thus, within the course of research activities in ECBCSAnnex 49 the definition has been extended to applyto this broader context. In this sense, low exergysystems are defined as “systems that provide accepta-ble thermal comfort with minimum exergy destruc-tion”. This allows to find the optimal match betweenquality (i.e. exergy) levels of supply and demand forany use or appliance within buildings.

The basis for exergy analysis in buildings is a com-monly accepted and scientifically grounded methodo-logy. Developing such a methodology was one of themain working items within ECBCS Annex 49 activities.Results are presented in chapter 2 including a detaileddescription of the methodology that can be applied toboth, heating and cooling processes analysis.

To obtain coherent and meaningful results, the signconvention adopted for energy and exergy analysisis of great importance. We argue that the thermody-namic reference environment for exergy analysis inbuilding systems should be the ambient air surroun-ding the building. Climatic data on a time depen-dent basis are required for dynamic as well asquasi-steady state assessments. Average outdoor airtemperatures during the heating season can be usedfor first estimations on the thermal exergy perfor-mance of heating applications following a steady-state method. Simple Input-output approaches (interms of sources and demands) can also employedto perform exergy analyses at the community level.

Quasi-steady state approaches for exergy analysisperformed on the basis of results from dynamicenergy simulations (or measurements) have provento be reasonably accurate. They require less inputdata than a fully dynamic approach and, being sim-pler, are less time consuming. Thereby, quasi-steadystate exergy analysis represents a reasonable com-promise between accurateness and complexity. It canbe used in exergy calculations in buildings aiming at

analyzing the performance of whole buildingsystems. However, if the main goal of the analysis isto optimize or study the performance of storage com-ponents dynamic assessments are required.

In any application steady-state exergy assessmentcan only be used to show the the approximate per-formance of a given system or get first comparisonsbetween systems. Steady-state analysis has proven tobe inadequate to obtain the absolute value of theperformance of building systems, even for spaceheating applications. Therefore, quasi-steady state ordynamic exergy analyses are required for an accu-rate comparison of building energy supply systems.

Space heating and cooling systems in buildings aimto provide comfort for the occupants. Thus besides theenergy efficiency, thermal comfort within buildings isthe main requirement that they must meet. Due to theimportance of human thermal comfort in the builtenvironment, a whole section is devoted to the exergyassessment on thermal comfort in chapter 2.

To make the exergy approach and calculationmethodology available to the public, several toolshave been developed within the project. A furtherimportant step in this direction would be the deve-lopment of pre-normative proposals including exer-gy as a performance indicator for building systems.In such a standard, the total exergy input requiredby a building should be limited according to state-of-the-art technologies available. In chapter 5 seve-ral concrete proposals on strategies for characteri-sing the performance of buildings and buildingsystems are presented.

The energy approach, both on a building and com-munity level, intends to reduce energy demands inbuildings by increasing insulation levels or increa-sing the air tightness of the building envelope, i.e.optimizing the building shell. The exergy approachat both levels focuses on matching the quality levelsbetween the energy supply and demand. Therefore,it requires the use of low quality sources for low qua-lity demands like space heating. Demands requiringhigher quality levels, such as lighting, electricalappliances or mobility, would in turn need the use ofhigh quality sources.

Exergy analysis shows that combustion processesshould not be used for providing the low temperatureheat demands in buildings. Fossil fuels have a highenergy quality and in intelligent energy systemsshould be used more rationally and efficiently withrespect to exergy. CHP units, providing equally highexergy outputs such as electricity, are a great exam-ple of an appropriate use of these energy sources.Similar conclusions to for biomass-based fuels: alt-


hough being renewable, their exergy efficiency ifdirectly used for space heating is extremely low. Inste-ad, low exergy sources should be promoted for heatand cold demands in buildings. Examples of suchsources are solar thermal or ground source heat.

For the exploitation of low exergy sources often highquality energy is also required, e.g. pumping or fanpower, electricity for powering heat pumps, etc.These high exergy inputs also need to be minimized.

Several case studies in this report highlight the diffe-rences between energy and exergy performance ofbuilding systems such as boilers or heat pumps. Theydemonstrate the necessity of designing new systemconcepts based on the use of low temperature heatsources for low temperature applications such asspace heating or cooling. Wastewater heat recovery,waste heat in district heating networks or solar ther-mal heat are some of the sources that should be usedfor meeting these demands. However, the availabili-ty of these sources varies strongly with time and oftenis not coupled with demand. Intelligent storage con-cepts, with maximum stratification and minimummixing are therefore a key component of low exergysupply systems in buildings.

On the other hand, as energy demands for spaceheating and cooling are reduced, the share of otheruses within buildings such as domestic hot water(DHW) demands increases. The exergy quality factorof DHW energy demand is about 13%, almost twiceas high as for space heating applications. Energysystems using low exergy sources show lower effi-ciencies for these demands at higher temperaturelevels. Further research is required to design systemconcepts for an exergy efficient supply of DHW.

In addition, higher and lower exergy demands wit-hin a building might be supplied in sequence, follo-wing cascading principles. Cascading of thermalenergy flows in buildings is a promising approachthat can be directly derived from the exergy analy-sis. Here future research is required.

District heating grids are a promising solution forcascading available heat flows to supply differentenergy demands in an intelligent way. The coordina-ted management and control of district heating andelectricity networks together with state-of-the-art sto-rage systems can be used to maximize the exergyefficiency of the supply. How to design and managesuch systems will require further research.

CHP units and heat pumps are very efficient energysystems which allow bridging heat and electricityproduction, making them promising technologies forfuture energy systems. Further research is required

to develop suitable storage concepts in combinationwith local heat and electricity networks on a commu-nity scale in order to reduce CO2 emissions and pri-mary energy use within the built environment usingthese technologies. The integration of solar thermalsystems in local district networks is also a very pro-mising low exergy technology, as shown in theCanadian case study for Okotoks (chapter 7).

Low temperature heating and high temperature coo-ling systems increase the efficiency of low-exergysources. Thus, improving building envelopes allowsusing surface heating and cooling systems and there-fore enables the efficient and cost-effective use of lowexergy sources available. Therefore the choice ofemission system restricts the options for low exergysources of energy in buildings. For example, the exer-gy approach shows that water-based systems areable to provide the same thermal comfort as airbornesystems. However, they require much lower exergyinput for pumps and fans and exergy losses in theemission process are also lower since the emissionsystem and the desired room temperature are veryclose for water-based system. An exergy efficientdesign in such cases would necessarily begin with achange of the emission systems – an important insightespecially in countries with a strong tradition of air-borne systems like the USA or Canada. In turn, incountries using mainly waterborne systems, e.g. mostof European, the important choices for exergy effi-cient building design in the choice of energy sources.

In either case, it has been shown that the exergy per-formance of a building does not increase significant-ly if the energy demand is reduced and surface hea-ting systems are used without changing the supplystructures and sources used (see chapter 4).

Within this report the methodology for exergyassessment of building systems, which was one ofthe main items within ECBCS Annex 49 activities, isdescribed and applied to several building and com-munity case studies. This represents a significant steptowards a wider application of this method for buil-ding related energy uses. The application of thisassessment method to further technology concepts(e.g. storage, control, cascading concepts) will helpidentifying optimum and suitable uses of the analy-zed technologies and represents a promising fieldwhere further research needs to be conducted.

Designing energy systems with the exergy approachwould increase the use of environmental heat andrenewable energy sources, leading to lower prima-ry energy consumption and CO2 emissions. To pro-mote all benefits shown in this report and summari-zed above, exergy should be included as a furtherindicator in building and energy regulations.


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APPENDIX A: ECBCS ANNEX 49 PARTICIPANTS

Austria: Lukas KranzlVienna University of Technology, Institute ofPower Systems and EnergyPhone: +43 1 58801 37351e-mail: [email protected]

Canada: Ken ChurchSustainable Buildings & Communities &Natural Resources CanadaPhone: +1 613 947 8952e-mail: [email protected]

Chris SnoekSustainable Buildings & Communities &Natural Resources CanadaPhone: +1 613 947 8952e-mail: [email protected]

Denmark: Bjarne W. OlesenICIEE – Department of Civil EngineeringTechnical University of DenmarkPhone: +45 45 25 41 17e-mail: [email protected]

Finland: Mia Ala-JuuselaVTT Technical Research Centre of FinlandPhone: +358 2 072 26947e-mail: [email protected]

Markku VirtanenVTT Technical Research Centre of FinlandPhone: +358 20 722 4064e-mail: [email protected]

Germany: Herena TorioFraunhofer-Institute for Building PhysicsPhone: +49 561 804 1834e-mail: [email protected]

Dietrich SchmidtFraunhofer-Institute for Building PhysicsPhone: +49 561 804 1871e-mail: [email protected]

Dirk MüllerRWTH Aachen UniversityE.ON Energy Research CenterPhone: +49 241 80 99566e-mail: [email protected]

Italy: Adriana AngelottiPolitecnico di Milano, BESTPhone: +39 02 2399 5183e-mail: [email protected]

Paola CaputoPolitecnico di Milano, BESTPhone: +39 022399 9488e-mail: [email protected]

Michele De CarliDipartimento di Fisica Tecnica University of PadovaPhone: +39 049827 6870e-mail: [email protected]

Piercarlo RomagnoniDepartment of Construction of ArchitectureUniversity IUAV of VeneziaPhone: +39 041 257 12 93e-mail: [email protected]

Japan: Masanori ShukuyaTokyo City UniversityPhone: +81 45 910 2552e-mail:[email protected]

Poland: Zygmunt WiercinskiUniversity of Warmia and Mazury,Chair of Environmental EngineeringPhone: +48 89 523 4576e-mail: [email protected]

Sweden: Gudni JóhannessonIcelandic National Energy AuthorityPhone: +354 569 6001 / +354 8930390e-mail: [email protected]

Marco MolinariKTH-The Royal Institute of Technology Building TechnologyPhone: +46 8 790 9026e-mail: [email protected]

Switzerland: Forrest MeggersETH Swiss Federal Institute of Technology ZurichPhone: +41 44 633 28 60e-mail: [email protected]

Luca BaldiniETH Swiss Federal Institute of Technology ZurichPhone: +41 44 633 28 12e-mail: [email protected]

Petra Benz-KarlströmBasler & Hofmann AGPhone: +41 44 387 13 38e-mail: [email protected]

The Netherlands: Peter Op´t VeldCauberg-Huygen R.I. B.V.Phone: +31 43 346 7842e-mail: [email protected]

Sabine JansenTU DelftPhone: +31 15 278 4096e-mail: [email protected]

Elisa BoelmannTU DelftPhone: +31 15 278 3386e-mail: [email protected]

USA: Dave SolbergThermo-Environmental Systems, L.L.C.Phone: +1 612-861-0468e-mail: [email protected]


you can find the

following additional

information on the

enclosed CD-ROM

and at the

homepage

www.annex49.com


APPENDIX B: ADDITIONAL INFORMATION FROMECBCS ANNEX 49

BrochureThe brochure gives an overview of the activities ofthe Annex 49 working group and a short introduc-tion of the exergy concept and its utilization withinthe built environment. The brochure is available inEnglish and German.

Annex 49 guidebook: full versionA printable .pdf version of the full and extendedAnnex 49 guidebook, the final report of this project,is available for those who prefer to get more detai-led information.

Annex 49 guidebook: summary reportThe summary version of the Annex 49 guidebook,the full and extended version of the final report isavailable on the CD-ROM

A framework for exergy analysis at the buildingand community levelThe Annex 49 midterm report entitled: “A frame-work for exergy analysis at the building and com-munity level” gives an overview of the basic princi-ples of exergy analysis within the built environmentand about the used models for the tool development.Furthermore, some case study examples are given.

Human-Body Exergy Balance and Thermal Com-fortThe Annex 49 working report on Human-Body Exer-gy Balance and Thermal Comfort outlines the rese-arch work with in the field of exergy and comfort.This report gives detailed information about thebasics and about the modelling and the developedcalculation tool.

Annex 49 newslettersAll seven issues of the biannual Annex 49 newslet-ter can be found on the CD-ROM. Starting from thefirst description of the work in the newsletter no. 1 inMarch 2007 to the summary of the results of theAnnex 49 work in newsletter no.7 in March 2010.

Conference proceedings: The Future for Sustaina-ble Built Environments- Integrating the Low Exergy ApproachThis conference about the future of sustainable builtenvironments was focusing on providing front-edgeresults on the field of exergy analysis of buildingsand communities. It was held in Heerlen/TheNetherlands on April, 21st, 2009.

Conference proceedings: The Future for Sustaina-ble Built Environments with High PerformanceEnergy SystemsThis conference about the future for sustainable builtenvironments and energy systems integrating amaximum amount of renewable energies providedfront-edge technologies and solutions for buildings,communities and energy supply. It was the finalAnnex 49 conference and took place inMunich/Germany on October 19th-21st, 2010.

ToolsIn total, six different tools have been developedduring the Annex 49 project. Ranging from a deci-sion support tool, via a tool for a pre-design of abuildings or the assessment of a community districtheating structure to a detailed building informationmodel (BIM) based platform. Five of them are enclo-sed in the CD-ROM, the DPV tool has been develo-ped to a commercial available tool, and you canfind an animation about this tool on the CD-ROM.

Tool manualsUser-Guides for the enclosed five tools are availableon the CD-ROM.

Technical presentationsA series of technical presentations were preparedfor the biannual ECBCS Executive Committee (ExCo)meetings during the working time of Annex 49. Therelated presentations can be found on the CD-ROM.

Published articlesA list of the exergy related articles published bymembers of the Annex 49 working group during thecourse of the project is given on the CD-ROM.

The Network of the International Society for LowExergy Systems in Buildings (LowExNet)The International Society for Low Exergy Systems inBuildings (short LowExNet) has been founded on the13th September 2003 to keep the members of the atthat time ending ECBCS Annex 37 together. Themain objective of this network is to formulate ourinterest in the regarded topics beyond the workingtime of the ECBCS Annex 37 and ECBCS Annex 49itself. A large number of workshops in connectionwith other international events have been organised.During this often industry related workshops techno-logies and applications of LowEx systems on a buil-ding and community level have been presented anddiscussed in detail. LowExNet is intended to coveralso applications in countries outside the IEA. Allinformation about this network is available on awebsite (http://www.lowex.net/).

ECBCS ANNEX 49

Annex 49 is a task-shared international research project initiated withinthe framework of the International Energy Agency (IEA) programme onEnergy Conservation in Buildings and Community Systems (ECBCS).

ECBCS Annex 49 is a three year project. 22 research institutes, universi-ties and private companies from 12 countries are involved.

The main objective of this project is to develop concepts for reducing theexergy demand in the built environment, thus reducing the CO2-emissionsof the building stock and supporting structures for setting up sustainableand secure energy systems for this sector.

Annex 49 is based on an integral approach which includes not only theanalysis and optimisation of the exergy demand in the heating and coo-ling systems but also all other processes where energy/exergy is used wit-hin the building stock. In order to reach this aim, the project works with theunderlying basics, i.e. the exergy analysis methodologies.

These work items are aimed at development, assessment and analysismethodologies, including a tool development for the design and perfor-mance analysis of the regarded systems. With this basis, the work on exer-gy efficient community supply systems focuses on the development of exer-gy distribution, generation and storage system concepts.

www.annex49.com

ISBN: 978-3-8396-0239-3

ISBN 978-3-8396-0239-3

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