tin

orno

y ansedl Enrograinseaned an o

ing boiler and an external airair heat pump. With this study, energy and exergy ows are investigated.

can beding (rtries, tighestin thgs is q

and Herkel [4] evaluated statistical simulation of user behaviourin low-energy ofce buildings. Yang et al. [5] investigated energyperformance of building envelopes in different climate zones inChina. Sjgren et al. [6] studied on monthly consumption data reg-istered by the property holders for over 100 multifamily buildingsin Sweden. They used an approach, based on the energy signaturemethod, which was developed for evaluating the energy perfor-

[11] studied analysis of the design and technology of residentialand non-residential energy-efcient solar buildings, their thermaland energy behaviour and their comparison with the thermaland energy behaviour of conventional, non-solar buildings. Perssonet al. [12] investigated inuence of window size on the energy bal-ance of low-energy houses. Eskin and Turkmen [13] presented howthe energy demands in an ofce building vary with changing con-ditions and control strategies by energy simulation. They used En-ergy Plus, an energy simulation program. They examined effects ofparameters like climatic conditions (location), insulation and ther-mal mass, aspect ratio, color of external surfaces, shading, window

* Corresponding author. Tel./fax: +90 232 3888562.

Applied Energy 86 (2009) 19391948

Contents lists availab

lseE-mail address: abdullah.yildiz@ege.edu.tr (A. Yildiz).carefully considered.Calculations for all kinds of energy utilization, including heat-

ing/cooling loads of rooms in buildings and temperature distribu-tions, are based on energy balances. This is done in reference tothe rst law of thermodynamics, which states that energy is con-served in every device and process and energy can neither be de-stroyed nor consumed [2]. There are many studies in theliterature dealing with energy analysis in buildings. Gratia andHerde [3] studied design of low-energy ofce buildings. Pfafferott

cations for building energy efciency. Durmayaz and Kadoglu [9]presented calculations of the heating energy requirements for achosen apartment building and fuel consumptions in some of thebiggest city centers of Turkey in terms of degreehours dependingon the outdoor weather conditions in the heating season and a pre-determined indoor design temperature. Chandel and Aggarwal [10]studied performance evaluation of a passive solar building in Wes-tern Himalayas based on energy analysis. Filippin and Beascochea1. Introduction

In general, energy consumptionmain sectors such as industrial, builtion and agriculture. In many counspace heating in buildings has the habout 40% of total energy consumedThus, energy consumption in buildin0306-2619/$ - see front matter 2008 Elsevier Ltd. Adoi:10.1016/j.apenergy.2008.12.010Energy and exergy losses in the whole system are quantied and illustrated. The highest efciency valuesin terms of energy and exergy were found to be 80.9% for external airair heat pump and 8.69% for LNGcondensing boiler, respectively.

2008 Elsevier Ltd. All rights reserved.

examined under fouresidential), transporta-he energy required forshare of all which his

e residential sector [1].uite high and must be

mance of multifamily buildings in terms of the overall heat losscoefcient. Chowdhury et al. [7] studied thermal-comfort analysisand simulation for various low-energy cooling-technologies ap-plied to an ofce building in a subtropical climate. Lam et al. [8]investigated the thermal and energy performance of ofce build-ings and identied major energy-efcient strategies in the differ-ent climatic zones in China using energy simulation techniques.They presented the work and its ndings and discussed the impli-Space heatingEfciency

example of application. Indoor and exterior air temperatures are 20 C and 0 C, respectively. It isassumed that the ofce is heated by a liquid natural gas (LNG) red conventional boiler, an LNG condens-Energy and exergy analyses of space hea

Abdullah Yildiz *, Ali GngrDepartment of Mechanical Engineering, Faculty of Engineering, Ege University, 35100 B

a r t i c l e i n f o

Article history:Received 14 August 2008Received in revised form 3 December 2008Accepted 9 December 2008Available online 14 January 2009

Keywords:Energy analysisExergy analysis

a b s t r a c t

In the present study, energbuildings. This study is bawork for the Internationaand Community Systems Psuch as heat losses and gaccordance with the Europbut cooling load is neglectThe analysis is applied to a

Applied

journal homepage: www.ell rights reserved.g in buildings

va, Izmir, Turkey

d exergy analyses are presented for the whole process of space heating inon a pre-design analysis tool, which has been produced during ongoingergy Agency (IEA) formed within the Energy Conservation in Buildingsamme (ECBCSP) Annex 37. Throughout this paper, in all of the calculationswere taken according to Turkish Standards Institution TSE, which is inStandard TS EN ISO 13789. In the analysis, heating load is taken accountnd the calculations presented here are done using steady state conditions.fce in Izmir with a volume of 720 m3 and a net oor area of 240 m2 as an

le at ScienceDirect

Energy

vier .com/locate /apenergy

EnNomenclature

A area (m2)b temperature factor ()Cp specic heat (kJ/kg K)COP coefcient of performance ()_E energy rate (W)_Ex exergy rate (W)f approximation factor ()F factor ()g total transmittance ()I radiation intensity (W/m2)l length (m)N percentage of equipment resistancend air exchange rate (ach/h)

Subscriptsaux auxiliary energy requirementcirc circulation

1940 A. Yildiz, A. Gngr / Appliedsystems including window area and glazing system, ventilationrates and different outdoor air control strategies on annual build-ing energy requirement. Rey et al. [14] analyzed the different stepsof BEA methodology (heat and cooling load, energy demand, en-ergy consumption and CO2 emission). They presented a practicalstudy of a small health centre that is analyzed with BEA methodol-ogy and they compared it with other energy simulation programslike Hourly Analysis Program (HAP) and PowerDOE. They foundthat the results of energy labeling are very similar for both simula-tion programs. Wittchen and Aggerholm [15] studied calculation ofbuilding heating demand in EPIQR based on energy analysis. Prageret al. [16] investigated the effect of painted facades with spectrallyselective properties on the energy balance of a building and com-pared real measured data from an outdoor test of facade sampleswith data calculated using the ESP-r simulation program. Feng[17] studied the rationale for dening thermal design of energy-efcient buildings and made discussions based on results of eldtests on pilot buildings and calculations for typical buildings.

To obtain a better understanding of the energy ow processes,in addition to the rst law of thermodynamics, the second law ofthermodynamics, in which the entropy concept plays the key role,can be applied. It is stated that in every process where energy or

const constantdis,D distribution systemdt design temperaturee external surrounding, equipmentE,emis emission systemenv environmentew external wallex extraf window frame, parameterG,Ge generationgp generator positionh heatheat heateri inside surrounding, surface, counting variablein input, inletins insulationj counting variablel lightingno number ()P power (W)p specic power, pressure (W/m2, N/m2)_m mass ow (kg/s)R pressure drop of the pipe (Pa/m)T temperature (K)U thermal transmittance (W/m2 K)V volume (m3)

Greek symbols_Q heat transfer rate (kW)g energy or rst law efciency ()q density (kg/m3)w exergy or second law efciency ()D differenceloss thermal lossesmax maximumN nettono effect of non-orthogonal radiationo occupantsp primary energy

ergy 86 (2009) 19391948matter is dispersed, entropy is inevitably generated. All real andnon-ideal processes are irreversible and there is an increase inthe irreversibility of a closed system [2]. Several studies have beenconducted on exergy analyses of buildings. Meester et al. [18] stud-ied exergetic life-cycle assessment (ELCA) for resource consump-tion evaluation in the built environment. Gong et al. [19]conducted schemeselection optimization of cooling and heatingsource systems of air-conditioning systems for buildings basedon exergy analysis. In the analysis, the product exergy cost is con-sidered as the objective function by which to evaluate the air-con-ditioning systems cooling and heating sources. Shukuya andKomuro [20] applied exergy analysis to thermal storage in build-ings. They used concepts of entropy and exergy to investigate therelationship between the building, the passive solar heating sys-tem and the environment. Saito and Shukuya [21] reported the re-sults of a pattern of human body exergy consumption related tothe exergy balance of various heating and cooling systems andfound out which were the low exergy systems. Franconi andBrandemuchl [22] evaluated two HVAC systems using both therst and second laws of thermodynamics. They determined theuseful work produced by these systems using exergy analysismethod. Zmeureanu and Wu [23] investigated energy and exergy

prim primary energyq qualityref referencerenew from renewable sourceret returnS solar, supply and storages source, surfacesh shading effectsT transmissiontd temperature droptot totalV ventilationw windowo reference

AbbreviationsECBCS Energy Conservation in Buildings and Community Sys-

tems ProgrammeIEA International Energy AgencyLNG liquid natural gasTSE Turkish Standard Institution

performance of residential heating systems with separate mechan-ical ventilation. Chengqin et al. [24] studied principles of exergyanalysis in HVAC and evaluation of evaporative cooling schemes.

In these studies, external wall of buildings were taken asboundary conditions and energy and exergy analyses were calcu-lated for buildings. But, in the actual analysis, energy and exergyanalyses must be calculated in all of the systems from the primaryenergy transformation till the building envelope including theenvelope. Firstly, Schmidt [25] presented a detailed report for thedesign of low exergy buildings and gave an example of energyand exergy analyses of an ofce heated by liqueed natural gasred high temperature boiler from power plant till building enve-lope. Balta et al. [26] studied a heating system from the powerplant through to the building envelope and used a ground sourceheat pump with a COP of 2.32 as the heating system.

In the literature, only one system such as LNG red high tem-perature boiler or ground source is analyzed. In this study, exergyand energy analyses for three different heating systems that are inwide use in _Izmir namely (i) liquid natural gas (LNG) red conven-tional boiler, (ii) LNG condensing boiler and (iii) an external airairheat pump are conducted from the power plant through to thebuilding envelope. The methodology which used in this studybased on a pre-design analysis tool, which has been produced dur-ing ongoing work for the International Energy Agency (IEA) formedwithin the Energy Conservation in Buildings and Community Sys-tems Programme (ECBCSP) Annex 37. Throughout this paper, de-sign temperatures such as indoor and outdoor temperatureswhich are used in calculation of heat losses and gains were taken

A. Yildiz, A. Gngr / Applied Eneaccording to Turkish Standards Institution TSE, which is in accor-dance with the European Standard TS EN ISO 13789. Analysis is ap-plied to an ofce which has a dimension of 15 m 16 m 3 m.Heating load is taken into account but cooling load is neglected.In these analyses, the systems energy and exergy losses and ef-ciencies were calculated and compared. Further, the best systemis proposed.Fig. 1. Plan of the heated ofce.2. Denition of ofce structure and system

A plan of the ofce exampled in this study is shown in Fig. 1.The ofce is heated by an LNG red conventional boiler, an LNGred condensing boiler and an external airair heat pump. The of-ce has one window with double glazing facing the south and itstotal conductance Uwindow is 2.2 W/m2 K. The ofce has a one doormade fromwood and its total conductance Udoor is 3.5 W/m2 K. Theofce has a total oor area of 240 m2 and a total volume of 720 m3.External wall comprises two layers of horizontal bricks, in betweenwhich the insulation material (glass wool) is placed. Both the in-door and outdoor faces of this wall are covered with a layer of plas-ter and the walls total conductance Uew is 0.816 W/m2 K. In thisstudy, in all of the calculations such as heat losses from walls, ceil-ings and gains Turkish building standard was taken as reference[27,28].

3. Determination of energy demand in the buildings

An important step in the entire analysis is the estimation of theenergy demand of the actual building. The energy demand is a keygure in the analysis, as it corresponds to the buildings exergyload. The energy requirement for the service equipment is thenestimated. The calculations presented here are done using steadystate conditions. They provide an instantaneous view of the pro-cesses and are not meant for estimations of annual energy demand.

For the balance of energy ows through the building, all possi-ble effects must be taken into account, even the extraction and pro-duction of the energy carrier (Fig. 2). The calculation of energyows caused by a building starts much earlier during the produc-tion of energy by means of primary energy sources.

For a deeper analysis of the energy ows in a building, a closerfocus on the buildings services systems is needed. The entire owfrom the source to the sink, as indicated in Fig. 3, must be takeninto consideration. All energy ows from the left-hand side, i.e.from the source, via a number of HVAC-components and the build-ing structure itself, to the ultimate sink, the outdoor environment.Imperfections and losses in the different steps throughout thebuilding are regarded, as well as the need for auxiliary energy. En-ergy, mainly in the form of electricity, is needed to drive additionalpumps and fans, for the operation of the system [25].

In the analysis, general project data and boundary conditionssuch as internal volume of the ofce (V), the net oor area (AN)were taken 720 m3 and 240 m2, respectively. The indoor tempera-ture (Ti) and outdoor temperatures (Te) are design temperaturesand were taken as 20 C and 0 C, respectively for Izmir accordingto Turkish Standard Institution [27,28].

For the estimation of the design heat demand of a building,rstly heat losses caused by transmission and ventilation mustbe calculated.

Transmission heat losses from buildings occur from externalwalls, ceilings, windows, and basements and by inltration. Ther-mal bridges are neglected in this study. The total transmission heatloss is the sum of the losses from all surfaces i and may be calcu-lated from Eq. (1)

_QT X

Ui Ai 1 b Ti Te 1

where _QT (W) is the total transmission heat loss, Ui (W/m2 K) is thetransmission coefcient of the surface i and Ai (m2) is the transmis-sion area of the surface i. b is temperature factor of the ofce con-struction elements and it takes into consideration that thetemperature of the external surface of a building construction ele-

rgy 86 (2009) 19391948 1941ment can be different from the external temperature. The tempera-ture factor is consequently 0 for construction elements facing theoutdoor air [15].

En1942 A. Yildiz, A. Gngr / AppliedThe ventilation heat loss _QV (W) can be calculated by using thefollowing equation:

_QV CP q V nd 1 gV Ti Te 2where nd is the air exchange rate (ach/h), gV is the efciency of heatexchanger if a mechanical balanced ventilation system with heatrecovery has been installed. In this study, since no heat exchangeris used, efciency of the heat exchanger is taken as zero and sincethere is natural ventilation in the ofce, air exchange rate is takenas 1.5 [25].

After heat loss is calculated, now heat gain must be calculated.Similar to the heat loss, surpluses or gains of heat have to be takeninto account for the heat balance. They are divided into two majorclasses namely; solar gain and internal gain.

The solar gain is

_QS X

Is;j 1 Ff Aw;j gj Fsh Fno 3where _Qs (W) is the solar gain, Is,j (W/m2) is the solar radiation andis given for different orientations. As it can be seen from Fig. 1, the

Fig. 2. Energy demand in buildings for differe

Fig. 3. Schematic view of energy utilizatioergy 86 (2009) 19391948ofce has a single window facing the south. For Izmir, the averagevalue of Is,j in the south orientation can be taken as 44W/m2 [29].Ff is the window frame fraction, Aw,j (m2) is the total window area.gj is the total energy transmittance of the glazing and can be esti-mated to be 0.75 for double glazing. Fsh is the possible shading ef-fects of other surrounding buildings and the Fno correction fornon-orthogonal radiation on the windowpanes and both of themcan be estimated to be 0.9 for most cases [15,27].

The internal gain is specied by two groups. One group is theheat gain of occupants, i.e. people staying inside the room, andthe other is the heat gain from equipment, like computers, printers,or other appliances, like television sets. In this study, it is assumedthat there are 15 people, 15 computers, one refrigerator, one pho-tocopy machine, one television and bulbs with a total power of200 W in the ofce. _Q 00i;o is set to 80W per occupant and with thenumber of present occupants, noo; the gain from occupants canbe estimated from Eq. (4). _Q 00i;e is the gain caused by each equip-ment. Then, the internal gain caused by equipments can be esti-mated by Eq. (5). Heat gains per equipment are given Table 1

nt applications from source to sink [25].

n in building services equipment [25].

Theat Tin Tretln TinTiTretTi

12

24

35 Ti

0@

1A 9

where Tin and Tret are inlet and return temperatures of the emissionsystems and their values are given in Table 2.

Table 1Heat gains per equipment [30].

Equipment Computer Refrigerator Photocopy machine Television

Heat gain (W) 75 150 200 75

A. Yildiz, A. Gngr / Applied Energy 86 (2009) 19391948 19434.2. Room air system

The room is assumed to be either heated by a warm surface(radiator) or a warm air (emission for external airair heat pump).The temperatures of the warm surface or warm air and of the roomalso give the exergy content at the heater surface and of the room.The effects of different surface and air temperatures, and of radia-tive and convective heat transfer processes between them, are ne-glected. The surface temperature of the heater is estimated by Eq.(9) utilizing the logarithmic mean temperature of the carrier med-ium with the inlet- and return temperature of the emission system[2].4. Energy and exergy analyses of systems

The calculation must be performed in the direction of the devel-opment of demand, in other words from building envelope to pri-mary energy transformation as indicated in Fig. 3. First, thedemand of the last subsystem (building envelope system) mustbe satised by the one before (room air system). In this subsystem,losses may occur and the demand increases, again it must be sat-ised by the next one [25].

4.1. Building envelope system

Firstly, heating load of buildings must be determined from Eq.(6) which is called energy balance. The quality factor of the roomair Fq,room is estimated by means of the Carnot efciency

Fq;room 1 T0Ti 7

where T0 and Ti are the reference temperature and room tempera-ture and they are taken as 0 C and 20 C, respectively.

Then, the exergy load, i.e. the exergy demand of the room to besatised by the following system:

_Exroom Fq;room _Qh 8_Qi;o _Q 00i;o noo 4

_Qi;e Xmi1

_Q 00i;e nieo 5

All heat ows, heat losses via the envelope, and internal gains,occurring inside the ofce have to be summed up to create the fol-lowing energy balance which refers to the rst law ofthermodynamics:

(Heat demand) = (sum of heat losses) (sum of heat gains)_Qh _QT _Qv _QS _Qi;o _Qi;e 6Table 2Characteristic data of heat emission systems [31].

Heat emission system Design supply temperature, Tin (C) Return temp

Radiator DIN 255 90/70 90 70Radiator DIN 255 70/55 70 55External airair heat pump 35 254.4. Distribution system

The distribution is characterized by a specic heat loss or ther-mal efciency gD and by the possible use of auxiliary energy, re-garded as a fraction of the distributed heat and given by anauxiliary energy factor Paux,D. The thermal efciency of the distribu-tion system is calculated by

erature, Tret (C) Thermal efcient, gE (%) Auxiliary power, paux,E (W/kW)4.3. Heat energy emission system

Heat energy emission system is a space heater from which heatis emitted to the space to be heated. Typical emission systems areradiators, oor heating systems and fan coil units. Heat energyemission system is a subsystem of the distribution system. For thisreason it is named as the emission subsystem. It has to be designedso as to satisfy the room air systems energy and exergy demands.

Inlet temperature Tin and return temperature Tret is very impor-tant for exergy analysis. There is a thermal efciency value gE andthere can also be auxiliary energy demand Paux,E for each heat en-ergy emission system (for example; radiator or convector) andcharacteristic data of heat energy emission systems are given inTable 2.

The heat losses are calculated for heat energy emission systemsfrom Eq. (12)

_Qloss;E _Qh 1gE 1

12

The demand on auxiliary energy or electricity of the heat energyemission system is

Paux;E paux;E _Qh 13where paux,E is the specic power for heat energy emission systemsand is given in Table 2.

The exergy demand of the heat energy emission system is de-rived as given in Eq. (14)

D _Exemis _Qh _Qloss;ETin Tret Tin Tret To ln

TinTret

14

The exergy load of the heat energy emission system is

_Exemis _Exheat D _Exemis 15Using this temperature, a new quality factor at the heater sur-face is dened

Fq;heat 1 T0Theat 10

The exergy load at the heater is calculated by

_Exheat Fq;heat _Qh 110.95 00.95 00.95 0.81

energy to be covered by the generator is

Table 3Values of parameters of fi [31].

Criteria Possible choices Parameter fi Comments

Position of the generator (fgp) Inside the heated space 1.00Outside the heated space 0.90

Insulation (fins) No insulation 0.70Bad insulation 0.90 Losses > 32.9 d + 0.22Good insulation 1.00 Losses > 2.6 d + 0.20

1944 A. Yildiz, A. Gngr / Applied Energy 86 (2009) 19391948gD 0:98 fgp fins fdt ftd 16values of the parameters fi used in Eq. (16) are given in Table 3.

For the electrical auxiliary power demand of the pump in thedistribution system, an approach, which takes the inuence ofthe temperature drop and the temperature level of the system intoaccount, has been chosen.

paux;D Dp _m=gcirc 17where gcirc can be taken as 0.27 [25]. Pressure drop in the distribu-tion system is calculated from

Dp 1 N R lmax AN pex 18where N is the percentage of equipment resistances and R is thepressure drop of pipe with typical values of 0.3 and 100 Pa/m,respectively. The maximal pipe length of the distribution is givenas specic value lmax per net oor area with a typical value of0.25 m/m2. pex is the extra pressure losses occurring in the system[25].

The average mass ow under design conditions _m can be calcu-lated from

_m 11:163 DTdis 0:0036

m3

s

19

where temperature difference in the distribution system DTdis is ta-ken as 5 K [25].

The heat loss of the distribution system is calculated from

_Qloss;D _Qh _Qloss;E 1gD 1

20

where gD is the energy efciency of the distribution system and wascalculated to be 0.96 from Eq. (16) .

Mean design temperature (fdt) LowMidHigh

Design temperature drop (ftd) LowMidHighThe demand on auxiliary energy or electricity of the distributionsystem is

Paux;D paux;D _Qh _Qloss;E 21where paux,D was calculated as 9.31 W/kWheat from Eq. (17).

Exergy demand of the distribution system is similar to emissionsystem. But the inlet temperature of the distribution system is the

Table 4Values of characteristics for heating systems [31].

Heating system Thermalefciency/COP

Max. supply temperature,Ts,max (C)

Auxiliary energy, P(W/kWheat)

LNG condensingboiler

0.95 90 1.8

LNG conventionalboiler

0.80 70 1.8

External airairheat pump

3.20 80 10_QGe _Qh _Qloss;E _Qloss;D _Qloss;S 1 FS 1gG24

Since, there was no contribution from solar energy FS was takenas zero in this study and efciencies of generating systems were gi-ven in Table 4.

The demand on auxiliary energy of the generation system todrive pumps and fans is

P p _Q _Q _Q _Q p 25mean design temperature Tdis and the return temperature is the de-sign temperature minus the temperature drop DTdis

D _Exdis _Qloss;DDTdis

Tdis To ln TdisTdis DTdis

22

where DTdis and Tdis are taken as 5 K and 308 K, respectively.The exergy load of the distribution system is

_Exdis _Exemis D _Exdis 23

4.5. Storage system

In this study, it was assumed that no energy storage was used.

4.6. Generation system

The generation system has to satisfy the demands of all previ-ous systems such as building envelope, room air, emission, distri-bution and storage systems. If a seasonal storage is integratedinto the system design, some of the required heat is covered bythermal solar power with a certain solar fraction FS. The required

1.00

ing is calculated fromergy

/exergyo

wan

den

ergy

/exergylosses

rate

ofthesystem

compo

nents.

ent

LNGconde

nsingbo

iler

LNGconvention

albo

iler

External

airairheatpu

mp

Totalen

ergy

rate

(kW)

Totalexergy

rate

(kW)

Relativeen

ergy

loss

(kW)

Relativeexergy

loss

(kW)

Totalen

ergy

rate

(kW)

Totalexergy

rate

(kW)

Relativeen

ergy

loss

(kW)

Relativeexergy

loss

(kW)

Totalen

ergy

rate

(kW)

Totalexergy

rate

(kW)

Relativeen

ergy

loss

(kW)

Relativeexergy

loss

(kW)

9.54

9.09

0.00

0.00

11.25

10.71

0.00

0.00

7.5

7.5

0.00

0.00

wer

7.15

6.65

2.39

2.44

8.46

7.90

2.79

2.81

2.5

2.31

5.00

5.19

ating

m6.76

2.12

0.39

4.53

6.77

2.52

1.69

5.38

7.51

1.35

5.01

0.96

orage

6.76

2.12

0.00

0.00

6.77

2.52

0.00

0.00

7.51

1.35

0.00

0.00

ibution

6.39

1.98

0.37

0.14

6.39

2.38

0.38

0.14

6.39

1.13

1.12

0.22

ission

6.07

0.79

0.32

1.19

6.07

0.93

0.32

1.45

6.07

0.5

0.32

0.63

om6.07

0.41

0.00

0.38

6.07

0.41

0.00

0.52

6.07

0.41

0.00

0.09

velope

6.07

0.00

0.00

0.41

6.07

0.00

0.00

0.41

6.07

0.00

0.00

0.41

Energy 86 (2009) 19391948 1945_Extot _Eprim;tot Fq;S _Erenew Fq;renew 29

4.8. Denitions and efciencies of the energy and exergy

Energy analysis is the traditional method of assessing the wayenergy is used in an operation involving the physical or chemicalprocessing of materials and the transfer and/or conversion of en-ergy. This usually entails performing energy balances, which arebased on the rst law of the thermodynamics, and evaluating en-ergy efciencies. This balance is employed to determine and re-duce waste exergy emissions like heat losses and sometimes toenhance waste and heat recovery. However, an energy balanceprovides no information on the degradation of energy or resourcesduring a process and does not quantify the usefulness or quality ofthe various energy and material streams owing through a systemand exiting as products and wastes [32].

The exergy method of analysis overcomes the limitations of therst law of the thermodynamics. The concept of exergy is based onboth the rst law of the thermodynamics and the second law of thethermodynamics. Exergy analysis clearly indicates the locations ofenergy degradation in a process and can therefore lead to im-proved operation or technology. Exergy analysis can also quantifythe quality of heat in a waste stream. A main aim of exergy analysisis to identify meaningful (exergy) efciencies and the causes andtrue magnitudes of exergy losses [32].

Energy and exergy balances can be written for steady state(Energy in) = (Energy output in product) + (Energy emitted with

waste)(Exergy in) = (Exergy output in product) + (Exergy emitted with

waste) + (Exergy loss)The energy efciency is calculated from4.7. Primary energy transformation system

The overall energy and exergy loads of the building are ex-pressed in the required primary energy and exergy inputs. Forthe fossil or non-renewable part of the primary energy, the result is

_Eprim;tot _QGe FP Pl PV Paux;G Paux;S Paux;D Paux;E Fp;electricity 27

where Fp is the primary energy factor and is given in Table 4. Aux-iliary energy used in the generation as electricity needed for drivingpump is effective in the part of heat generation. This effect is givenby auxiliary energy factor Paux,G. The typical maximum supply tem-perature of the boiler Ts,max is needed to check the consistency of theoverall system design. Pl and PV are specic power of specic light-ing and specic ventilation, respectively, in Eq. (27) and they are ta-ken as zero in this study. Values of characteristics for systems ofheat production are given in Table 4. Fp,electricity is primary energyfactor for electricity and it can taken as three [25].

If the generation utilizes a renewable energy source or extractsheat from the environment, as heat pumps do, the additionalrenewable energy load is

_Erenew _QGe Frenew _Eenv 28In this study, when external airair heat pump is used for heat

generation Frenew is taken as 2.20. If a boiler is used as heating sys-tem, Frenew is equal to zero.

Quality factor of energy source Fq,s needed for exergy analysisare specied for energy source in the primary energy conversionand its value is given in Table 4. Finally, total exergy load for build-

A. Yildiz, A. Gngr / Appliedg Energy output in product=Energy input 1 Energy loss=Energy input 30 Table

5To

talen

Com

pon

Inpu

tAfter

poplan

tAfter

he

syste

After

stAfter distr

After

emAfter

roAfter

en

The exergy efciency w can be written as

w Exergy output in product=Exergy input 1 Exergy loss=Exergy input 31

5. Results and discussion

The results for the numerical examples of energy utilizationsand exergy consumptions for the whole process of space heatingare given in Table 5. The process begins with the power plant, thencontinues sequentially with the following steps; generation of heat(the boiler and external airair heat pump), storage and distribu-tion, heat emission and transmission of heat via the room air andacross the building envelope to the outside environment. For thisofce, project data and boundary conditions are as follows: the

volume and the net oor are of the ofce are 720 m3 and 240 m2,respectively. Indoor and exterior air temperatures are 20 C and0 C, respectively, which are reference temperatures of TSE forIzmir.

The transmission and ventilation are calculated to be 3.33 kWand 7.23 kW, respectively, using Eqs. (1) and (2). Solar and internalheat gains are found to be 0.275 kW and 4.22 kW, respectively,using Eqs. (35). According to these data, heat demand of the ofceis calculated to be 6.07 kW from Eq. (6).

The graphical presentations of the calculated results are shownin Figs. 46, which indicate where losses occur. In Fig. 4, the use-able ow of energy and exergy through the space heating processfrom source to sink is presented. The largest input energy of sys-tem occurs when the LNG conventional boiler is used and its valueis 11.25 kW. The lowest input energy of system occurs when theexternal airair heat pump is used and its value is 7.5 kW. As a

Fig. 4. Total energy and exergy rates of systems.

1946 A. Yildiz, A. Gngr / Applied Energy 86 (2009) 19391948Fig. 5. Relative energy losses.

ve e

Eneresult of the energy analysis, energy production of the externalairair heat pump is remarkable. According to the rst law of ther-modynamics, energy production is impossible but explanationhere is that renewable environmental heat is included in the pro-cess. In the generation section, an increase in the energy ow isdue to the external airair heat pump, which produces 5.01 kW.Exergy is consumed in each component till it reaches dead state.While the ow of energy leaves the building envelope, there is stilla remarkable amount of energy left, but this is not the same forexergy. At the ambient environment, energy has no potential ofdoing work, so all exergy has been consumed. The exergy owon the right side of the diagram has to be zero.

Fig. 6. Relati

A. Yildiz, A. Gngr / AppliedRelative energy and exergy losses occurring in each componentare presented in Figs. 5 and 6, respectively. The largest energylosses occur in the primary energy transformation. The lowest en-ergy loss occurs the in the primary energy transformation whenLNG condensing boiler is used and its value is 2.39 kW. The largestenergy loss occurs in the primary energy transformation whenexternal airair heat pump is used and its value is 5.00 kW. Asmentioned above, an energy production of 5.01 kW is subject inthe external airair heat pump heating system due to addedrenewable environmental heat and this shown below the x-coordi-nate in the diagram. Unlike the energy losses, the largest exergylosses do not occur in the primary energy transformation, butoccur in the heating system due to the combustion process whichconsumes a lot of exergy that is indispensable when extractingthermal exergy from the chemical exergy contained in LNG.The largest exergy loss occurs when using LNG conventionalboiler and its value is 5.38 kW. The lowest exergy loss in the heat-ing system occurs when using the heat pump and its value is0.96 kW.

6. Conclusions

As shown in this paper, through analyses and examples, the en-ergy conservation concept alone is not adequate in gaining a fullunderstanding of all the important aspects of energy utilizationprocesses. Thus, a method for exergy analyses based on a combina-tion of the rst and second laws of thermodynamics is presentedhere for a better understanding and design of energy ows inbuildings. The advantages of the analyses and the difference be-tween energy and exergy analyses are demonstrated.

We can extract some concluding remarks from this study asfollows:

(a) While a part of energy leaves from building envelope, someof it still remains. However, all of the exergy is consumed inthe building envelope.

(b) When the external airair heat pump is used as the heatingsystem, input energy to the generation system is 2.50 kWbut output energy is 7.51 kW. Therefore, the added renew-able environmental heat is 5.01 kW.

xergy losses.

rgy 86 (2009) 19391948 1947(c) The largest exergy loss occurred during the combustion pro-cess when the boilers are used as the heating systems, butwhen the external airair heat pump is used as the heatingsystem, the largest exergy loss occurred in the primaryenergy transformation.

(d) Total energy efciencies of systems using LNG condensingboiler, LNG conventional boiler and external airair heatpump (energy demand room/total energy input) are calcu-lated to be 63.6%, 53.9% and 80.9%, respectively.

(e) Total exergy efciencies of systems using LNG condensingboiler, LNG conventional boiler and external airair heatpump (exergy demand room/total exergy input) are calcu-lated to be 8.69%, 8.68% and 6.66%, respectively.

(f) The best system in respect of environmental aspects is thesystem with the heat pump. There is no combustion in thissystem and the lowest input energy to power plant is in thissystem.

(g) For the optimum system choice not only the environmentalaspects but also economical aspects must be taken intoaccount.

(h) The exergy load and auxiliary energy demand of each systemis affected by its subsystem characteristics such as heatemission, distribution and heat generation subsystem char-acteristics. These values are given in Tables 24.

(i) For the future studies, an ergonomic analysis, which is acombination of energy and economic analyses, and exergo-nomic analysis, which is a combination of exergy and eco-nomic analyses are recommended.

(j) In this period where global warming is a hot issue, it is veryimportant to take not only the building envelope but thewhole system into account during the energy consumptionanalyses of buildings which has a big share in the globalenergy use.

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Energy and exergy analyses of space heating in buildingsIntroductionDefinition of office structure and systemDetermination of energy demand in the buildingsEnergy and exergy analyses of systemsBuilding envelope systemRoom air systemHeat energy emission systemDistribution systemStorage systemGeneration systemPrimary energy transformation systemDefinitions and efficiencies of the energy and exergy

Results and discussionConclusionsReferences