cooling with solar heat - bine · 20 solar power instead of heat 24 outlook straight to the point...

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Themeninfo III/2016

Cooling with solar heat Concepts and technologies for air conditioning buildings

www.bine.info/en

2

Authors Dr Alexander Morgenstern Dr Mathias Safarik Edo Wiemken Peter Zachmeier

Editor Dr Franz Meyer

CopyrightText and illustrations from this publication can only be used if permission has been granted by the BINE editorial team. We would be delighted to hear from you.

Cover image: Kramer GmbH

All images are provided by the authors unless otherwise indicated.

Lead photos:P. 3 s-power GmbHP. 4 Klingenburg GmbHP. 8 Klingenburg GmbHP. 12 Festo AG Co. KGP. 14 TU-BerlinP. 20 Claus Ableitner (CC-BY-SA 3.0)

BINE-Themeninfo III/2016

Content 3 Solar heat replaces grid power

4 Closed and open methods

8 Applications and system selection

9 En passant: Initial attempts with solar cooling

11 Points of view: What are the chances for solar cooling?

12 Planning, cost and integration

14 The research starting points

18 Inpractice:Absorptioncoolingfordifferentregions

19 In practice: Success stories

20 Solar power instead of heat

24 Outlook

Straight to the pointThe cooling or air conditioning of buildings with solar heat has a particular appeal because the heat demand and supply are usually consistent with each another. Cold stores in southern climates and many process refrigeration systems also require considerable energy when the sun shines intensely. The use of solar cooling systems instead of electric chillers also eases the grid, particularly at peak load times. Solar thermal cooling could develop sales markets, for example, in the Mediterranean region but also play a small part in Germany‘s „Energiewende“ – itsenergytransition.Thisisreflectedintheresearchfundingprovidedby the German federal government.

In technological terms, German research institutions and SMEs have established a leading international position thanks to their intensive research. However, this young industry sector is facing double competition: on the one hand, well-known companies from the cooling sector, particularly from Asia, are surging onto the international markets; on the other hand, new competition is growing withregardtothetechnology,sincethesignificantdecreaseinthecostof photovoltaics has also made solar electric systems with compression chillers increasingly attractive.

Most scientists, however, see scope for both technologies. In Europe alone, the need for cooling will quadruple between 1990 and 2020 according to a report for the European Commission. An advantage of solar thermal systems is that they canbeflexiblycombinedwithotherheatsources.Forexample,industrialwasteheat or energy from cogeneration could also be used. Compared with conventional cooling technology, solar cooling requires high initial investment but has subsequently lower operating costs. It is therefore particularly suitable when the cooling load, solar radiation and price of electricity at the location are high. More research needs to be conducted on the components as well as on the system technology. The following sections provide an overview of the various open and closed methods and provide an insight into the research.

Your BINE editorial team wishes you an enjoyable read

„“

Kaiserstraße 185-197, 53113 Bonn, Germany Phone +49 228 92379-0 Fax +49 228 92379-29 [email protected] www.bine.info

3BINE-Themeninfo III/2016

Even in temperate climates, numerous buildings

have to be air conditioned. In conference centres,

theatres, department stores or high-rise buildings,

only indoor air handling units are usually capable

of ensuring a comfortable indoor climate. Solar-based

methods, however, can particularly lower the

electricity needs at peak load times.

Solar heat replaces grid power

Many countries in sunny regions suffer from high loads on the electricity grid for handling cooling and air conditio ning tasks. In some Mediterranean countries, more than half of the total electricity produced is used for air conditioning buildings in summer. Even a significant increase in building standards would not change any-thing in the short term. The use of solar energy for cool-ing and air conditioning would seem obvious here, as there is a high correlation between sunlight, ambient heat and the cooling requirement. Solar cooling can effec tively reduce the electrical energy consumption for cooling and air conditioning and thereby counter the growing burden on power grids in sunny countries.

Less clear, however, is the situation for central European climates. Less than 5 per cent of the total electricity produced is used in Germany for air conditioning build-ings. However, there is a significantly greater cooling requirement as part of food production and storage, as well as in industrial refrigeration. In these areas the cool-ing requirement is not temperature- and irradiation- dependent to the same extent. Nevertheless, a growing demand for comfort air conditioning can also be expect-ed in Germany, even if the number of full load hours per year will be low in many applications.

Solar cooling and air conditioning refers to a process in which solar energy directly supplies a cooling or air conditioning process with energy. There is therefore a direct correlation between solar energy and the cooling process.

There are three basic approaches for providing cooling with solar energy:

• The photovoltaic generation of electricity and its subsequent use in compression chillers

• Thermo-mechanical systems (Vuilleumier and Rankine processes)

• Solar thermal systems (desiccant and evaporative cooling (DEC)), ab- and adsorption cooling processes, steam jet cooling process).

This Themeninfo brochure focuses on solar thermal sys-tems. Here it is differentiated between closed and open methods. Closed methods use ab- or adsorption chillers to provide chilled water that is used, for example, in chilled ceilings. Open sorption methods, on the other hand, condition the supply air. Here they not only reduce the temperature but also ensure a pleasant indoor air humidity.

In both cases, the system technology and collector sys-tem must be matched in terms of the size, suitability and control of the components. Ideally, the solar heat is used for other tasks, such as for domestic hot water or auxiliary space heating. Solar thermally driven cooling and air conditioning technologies have the following advantages:

• They relieve the power grid, since they use little electricity.

• The refrigerant (e.g. water) does not have global warming potential.

• They are mostly operated at temperatures below 100 °C and are therefore suitable for stationary collector technology.

• They can be combined with waste heat recovery.• They do not produce noise and vibrations.

4 BINE-Themeninfo III/2016

Closed chillers provide chilled water. The chilled water temperature depends on whether devices are supplied that are also used for dehumidification (latent loads) or if the connected room-side components are only used for removing sensitive loads, i.e. to control the temperature. In central air handling units or decentralised circulating air units that are used for controlling both the tempera-ture and humidity of the indoor air, the air is cooled below the dew point. As a result, part of the water vapour from the air is condensed and the absolute humidity decreases. To achieve sufficient dehumidification requires chilled water temperatures in the 6–9 °C range. If, however, the chiller is only going to be used for removing sensitive loads, considerably higher chilled water temperatures in the 15–20 °C range are sufficient. Examples of room-side components include surface cooling systems, i.e. chilled ceilings, underfloor cooling, wall panels with integrated capillary tube mats as well as component cooling or con-crete core cooling. Also suitable are other systems for providing silent cooling such as circulating air coolers that work with natural air circulation.

Absorption chillers

Absorption chillers are the most widely used technology for thermally driven cooling. They can utilise heat sources at a low temperature level, for example solar heating, district heating, industrial waste heat or waste heat from cogeneration plants. Like compression chillers, they utilise the dependence of the refrigerant’s boiling point on the pressure. However, the refrigerant is compressed in a dissolved, liquid form in a sorbent. This uses less electricity for the cooling. The most commonly used refrigerant/sorbent pairs are H2O/LiBr and NH3/H2O. H2O/LiBr is usually used for applications above about 4 °C (air conditioning buildings), as this achieves greater efficiency. The advantage of NH3/H2O systems, on the other hand, is the lower freezing point of NH3, which means that useful temperatures considerably below 0 °C can be achieved.

The following section, by way of example, examines the H2O/LiBr working pair. The evaporator (E) is at a low pressure level of about 10 mbar. The water refrigerant therefore already evaporates between 4 and 7 °C, and generates the usable cooling capacity by absorbing the necessary evaporation energy. The refrigerant vapour is absorbed by the concentrated LiBr solution in the ab-sorber (A) and, because it is once again in the liquid state, it can be pumped with little energy to a higher pres-sure level with a solvent pump (SP). By supplying driving heat with a temperature of about 60–95 °C, the refriger-ant vapour in the generator (G) is expelled again from the H2O/LiBr solution and is liquefied in the condenser by added cooling water supplied at a temperature of about 30 °C. After being throttled down to the low pressure level, the refrigerant can now be evaporated again in the evap-orator. The concentrated solution created in the genera-tor is fed back via a solution heat exchanger (HX) into the absorber where it can once again absorb the refrigerant. Cooling the concentrated solution and preheating the di-luted solution in the solution heat exchanger significantly improve the efficiency of the system.

Heat assisted systems for air conditioning

canbedifferentiatedbythetypeofmethodused.

Closed chillers provide chilled water that is used,

for example, in chilled ceilings.

Open sorption methods are used for direct

air conditioning, i.e. temperature reduction

anddehumidification.

Closed and open methods

Fig. 1 Principle of an absorption chiller Source:FraunhoferISE

10 10 20 30 40 50 60 70 80 90 100

10

100

Pres

sure

[mba

r]

Temperature [°C]

E

C G

A

HX

SP

0 % 40 % 50 % 60 %


Adsorption chillers

In adsorption chillers, the refrigerant vapour generated in the evaporator is attached to adsorbents. The refrigerant vapour flow and thus the cooling is maintained until the adsorbent is saturated. This then requires regeneration of the adsorbent in order to start the cooling process again. Adsorption chillers therefore operate cyclically. During the regeneration phase, driving heat drives the refrigerant vapour from the adsorbent. The vapour condenses in the condenser. A refrigerant circuit is required to cool the condenser and dissipate the adsorption heat. Compared with absorption chillers, adsorption chillers have a slightly lower thermal efficiency. However, they have the advan-tage that no pumps are needed in the vacuum range. In addition, there is no danger of crystallisation as in ab-sorption chillers, so that there are fewer constraints on the cooling water temperature.

Steam jet cooling

Steam jet cooling technology uses water as the refriger-ant and propellant. Heat supplied at high pressure gener-ates motive steam. This vapour is fed through a nozzle and expanded. The accelerated steam generates a nega-tive pressure in the nozzle, whereby water vapour is drawn off by an evaporator. In the evaporator, water can therefore evaporate at low pressure and absorb heat. The depressurised motive steam, which is mixed with the re-frigerant vapour from the evaporator, condenses at a me-dium temperature level and rejection heat is dissipated. The efficiency is significantly influenced by the condensa-tion temperature. In normal operating conditions the thermal EER is less than 1. Steam jet cooling technology is currently only used for a few industrial applications in a very high output range. In research projects, however, at-tempts are being made to scale the technology down to a small output range and to couple it with solar thermal drive units. Refrigerants other than water are also being investigated.

Open, desiccant and evaporative methods

Open methods are based on a combination of sorptive air dehumidification with evaporative cooling. They make it possible to condition the supply air through an air handl-ing unit. Not only the air temperature but also the air humidity can be adjusted to a comfortable range. This method, which is known in German-speaking countries as “sorption-assisted conditioning” (sorptionsgestützte Klimatisierung (SGK)), is generally otherwise known as

Fig. 2 Schematic comparison between the absorption refrigeration principle (left) and the adsorption refrigeration principle (right). Whereas in the absorption process steady state conditions can be achieved in the hydraulic circuits by continuous circulation of the liquid solution, cyclic temperaturefluctuationsoccurintheadsorption process. The nominal values for theoutputsandtemperaturesaredefinedhere as mean values over several operating cycles.Source:FraunhoferISE

Evaporator

Generator

Condenser

Continuous transport of the solution

Heat rejection

Heat rejection

Driving heat

Heat extracted from'useful cooling'

Refr

iger

ant

Refr

iger

ant

Absorption

Evaporator

Absorber Adsorber

Generator

Condenser

Periodic functionreversal

Adsorption

Efficiency of the cooling output

Theefficiencyofthecoolingoutputismeasuredbythe EERvalue(EnergyEfficiencyRatio).Thisistheratioofthe cooling capacity to the power input. Air conditioning applications with compression chillers typically achieve EER values between 3 and 4.

In addition to thermal energy, thermal chillers also consume electricity, e.g. for control systems, solution circulators, etc. Therefore, two EER values are frequently used. Typical thermal EER values for adsorption chillers range between0.5and0.6;forsingle-effectabsorptionchillers theyrangebetweenabout0.6and0.8.Double-effectsystems even achieve EER values of up to 1.3. However, they need driving heat at a higher temperature level of about 140–160 °C. The electrical EER of thermal chillers can exceed 50, foradsorptionchillersitcanexceed100.However,thisfigure refers only to the chiller. The power required for the water circuitsandtheheatrejectionissignificantlylargerthanfor just the chiller itself, so that the entire system drops to a significantlylowerelectricalEERvalue.Itisverydifficultto provideaprecisefigurebecausetheyaresystem-specific. A value greater than 8 is also achievable and desirable for smallpowerratingstoensurethatsignificantelectrical energy savings can be achieved relative to compression refrigeration systems.


desiccant and evaporative cooling (DEC). The refrigerant is water and is in direct contact with the atmosphere, which is why it is described as an “open system”. In systems of this type, the supply air temperature and humidity are set according to the comfort requirements, and the nec-essary fresh air is also supplied simultaneously. Thermal driving energy is required to regenerate the sorbent (ex-pulsion of the water bonded in the sorbent), so that the dehumidification can be maintained. Desiccant evapora-tive cooling always uses the cooling potential of the exhaust air from the air conditioned rooms, which is relatively cool compared with the ambient air. It therefore requires a closed supply and return air system. The comprehen-sive air conditioning tasks that are handled using this technique make the direct comparison with water chillers difficult. Common to all technical implementations is a heat recovery unit, which is necessary for the efficient op-eration of the plant. The most common method is solid sorption using rotating sorption components (sorption wheels). The operating principle is shown in Fig. 4. The sorbent material, such as silica gel or lithium chloride, is embedded in the solid matrix of the sorption wheels. The systems consist of commercially available components; the technical challenge lies in selecting and sizing the components and establishing an appropriate control strategy for the entire system.

Analogous to closed refrigeration, in the DEC method the thermal coefficient of performance can be depicted as the thermal Energy Efficiency Ratio (cooling capacity/driving heat capacity), whereby this quotient is then defined only for the periods where the driving heat capacity > 0 (with active regeneration of the sorption components). The cool-ing capacity is calculated from the enthalpy difference between the ambient air and supply air.

An alternative to DEC systems with rotating sorption components is the application of liquid sorption. Such systems dehumidify the supply air using a liquid sorbent (e.g. lithium chloride), which is trickled into the absorber. The sorbent then absorbs water from the supply air. The sorbent is circulated in a cycle; through the introduction of heat, for example from a collector array, water vapour is

expelled and the sorbent is once again ready to dehumidify air. An advantage of this method is that the water- enriched sorbent and the desorbed sorbent can be temporarily stored separately. This enables the air condi-tioning to be operated outside the operating hours of the collector array. This was utilised, for example, in the construction of the Energy Efficiency Centre in Würzburg (project DEENIFDEENIF, FKZ 0327879A) or in Project Sara, FKZ 0329662D (Storage and conversion of industrial waste heat for air conditioning through open absorption). Until now there have only been a few suppliers of com-plete systems for DEC systems with liquid sorption.

Another, new method uses a supply air-side, sorptively coated crossflow air-to-air heat exchanger. Here the sup-ply air is dehumidified through contact with the sorbent during its passage through the heat exchanger. The sorp-tion heat thereby released is transferred to the exhaust air side of the heat exchanger, is absorbed by the air flow-ing through and is then delivered by the exhaust air to the environment. Through evaporative cooling on the exhaust air side, the effect is amplified so much that the supply air is cooled and its temperature is lower than the ambient air temperature. This method simultaneously cools the sorption process and increases the dehumidification capacity. The process is operated cyclically to periodically regenerate the sorbent. The method has been tested in pilot plants (example: ECOS, FKZ 0327406 A) and is aimed at air conditioning segments with smaller airflow rates < 1,000 m³/h.

In DEC systems, the additional components relative to conventional ventilation technology create a higher pres-sure loss in the air channel, which increases the electrical power requirements for the fans. For a realistic comparison with conventional technology, for example when assessing the primary energy savings, the entire auxiliary energy consumption therefore needs to be considered.

Fig. 3 Since 2001 in operation: Solar-powered, open, desiccant and evaporative cooling (DEC) of seminar rooms at the Southern Upper Rhine IHK, Freiburg.Thedehumidificationrotorisdirectlyregeneratedviaa100-m²aircollectorarray,whichwasinstalledonthebuildingroofatlowcost.Source:FraunhoferISE


Regenerationheat

Humidifier

Dehumidifier HR

Heating

Exhaust air

12 11

1 2 3 4 5

10 9 8

Ambient air Supply air

Return air

6

7

Building

10080

60

40

20

70

80

60

50

40

30

20

10

0

1

2

3

4 – 6

7

8

11, 129

10

-.

SA

RA

AA

EA

Tem

pera

ture

[°C]

0 % relative humidity

0

Absolute humidity [g/kg]

2 4 6 8 10 12 14 16 18 20

Fig. 4 Diagram of a desiccant and evaporative cooling (DEC) system with a sorption rotor and rotating heat recovery (HR) component. Theregenerationheat(drivingheat)canbeprovidedbyasolarcollector,wherebyatemperaturelevelbetween60and75°Cissufficient.Standardcyclewithevaporativecoolinginthesupplyairandindirectevaporativecoolinginthereturnairtract.Source:FraunhoferISE

1→2 Sorptivedehumidificationofthesupplyair;theprocessisexothermicandtheairisheated by the adsorption heat freed in the matrix and by the residual heat from the exhaust air tract.

2→3 Pre-coolingofthesupplyairinthecounterflowtothebuildingreturnairintheheatrecoveryrotor 3→4 Directevaporativecoolingofthesupplyairwithasimultaneousincreaseofthesupplyairhumidity 4→5 Heatingregisterforheatingthesupplyairinwinter 5→6 Lowtemperatureriseduetothefan 6→7 Increaseofthetemperatureandhumidityoftheairsupplythroughinternalloadsinthebuilding 7→8 Coolingofthebuilding‘sreturnairbydirectevaporativecooling,preferablyneartosaturation 8→9 Preheatingofthereturnairinthecounterflowtothesupplyairintheheatrecoveryrotor 9→10 Supplyofregenerationheattothereturnair,e.g.fromasolarthermalsystem10→11 Desorption(stripping)ofthewaterboundintheporesofthesorbentmaterialbythehotreturnair11→12 Withthefan,thereturnairisreleasedtotheenvironment(nowexhaustair)

TheprocessflowinaDECsystemisdepictedintheformofatemperature-humiditydiagraminFig.5.

Fig. 5 Temperature humidity diagram showing the process course in a DEC system for the following values: Ambient air (AA) 32 °C, 40%rel.humidity;Supplyair(SA)20°C,60%rel.humidity.Source:FraunhoferISE


Ab- and adsorption chillers can provide low chilled water temperatures for air dehumidification or even work at higher temperatures to provide sensitive cooling, e.g. via chilled ceilings. Absorption refrigeration technology can also be used for process cooling in the temperature range < 0 °C. Several examples demonstrate the high bandwidth of the applications:

Chilled water provision from 6 °C for operating circulating air, surface or fresh air cooling: • Only solar thermal cooling provision

(without cooling backup system) for increasing the indoor air comfort. Can be used for the residential sector and commercial applications in the low output range. Stationary collector technology is used. The solar thermal system also supports the domestic hot water supply and space heating if required.

• Fuel-saver mode, i.e. the conventional (electrically operated) part of the chilled water supply is partially or completely shut down when there is a sufficient solar heat supply. A good correlation between the cooling load and solar thermal output reduces not just the electric power demand but also the peak load. This mode of operation is interesting for both large-scale individual applications as well in cooling networks. In the large output range and in sunny locations, the use of tracking collector technology and multi-effect, thermally driven cooling technology is also possible.

• Combination with existing waste heat (from production or cogeneration), taking into account that the existing waste heat utilisation is not displaced.

Refrigerant < 0 °C:• Used in industrial process cooling, usually in

fuel-saver mode. Commercially available are absorption chillers with ammonia water as the working medium. Driving temperatures > 100 °C are generally required that need at the very least stationary evacuated tube collectors or even tracking, concentrating collector technology.

• This technology can also be used for cooling buildings, for example in combination with phase change storage systems (e.g. ice storage systems), whereby it may be possible to dispense with backup systems.

Suitable collectors can generally be differentiated between stationary collectors and tracking collectors with a high concentration ratio. The stationary collectors used include covered flat-plate collectors with selective coatings and various types of evacuated tube collectors.

Depending on the cooling and air

conditioningtask,differentsolarthermal

assisted systems can be used in the building

design.Asimplifieddecisiontreehelps

designers choose between using a chilled

water system, desiccant ventilation or a

combination of both systems.

Applications and system selection

Collector

Storage system

Supply air

Return air

Loads

Electriccompressionchiller

> 15 °C

Regenerationheat

Humidification

Dehumidification(summer)

Heatrecovery Heating

(winter)

Exhaust air

Ambient air

Fig. 6 Desiccant and evaporative cooling with solar thermal regenerationofthedehumidificationunit.Theexampleshows a standard rotor process for moderate climates. A conventional indoor refrigeration system handles the remaining sensible cooling loads; this can be operated with a high evaporatortemperatureandthuswithhighefficiency. Source:FraunhoferISE


In designing the collector array, it needs to be taken into account that there is a small temperature differential in the chiller’s drive circuit (usually about 10 K between the supply and return) and that there are high mass flows. This has implications for the collector connections and the collector control system. Stationary collector technol-ogy is used in single-effect, thermally driven refrigeration with driving temperatures < 100 °C. The method is there-fore also suitable for regions in central Europe. In most cases, conventional wet heat rejection is still deployed using open or closed cooling towers. However, there is an increasing focus on potential applications with dry heat rejection.

Regions with high solar irradiation open up the possibility for using multi-effect absorption refrigeration technology, which requires driving temperatures well in excess of 100 °C and is therefore dependent on tracking, concentrating collector technology. In several pilot and demonstration projects, linear, concentrating collectors have therefore been used for this purpose for the middle temperature range (up to 250 °C). These are parabolic trough or Fresnel collectors.

The temperature levels decide

When initially selecting the refrigeration and collector technology in any given application, important aspects include the relationship between the three respective temperature levels for the driving temperature (Thigh ) and the thermal efficiency of the refrigeration as well as the dependence on the temperature lift (Tmean – Tlow ), whereby the temperature lift corresponds to the temperature difference between the rejected heat and the chilled water. Fig. 9 depicts this relationship.

Their high thermal coefficient of performance makes double- effect absorption chillers particularly interesting because the required heat input for the drive and the re-quired thermal heat rejection capacity are reduced. This leads to smaller collector arrays and lower investment costs for the heat rejection. This contrasts with the in-creasing driving temperature, which requires the use of concentrating collector technology and thus limits applications to sunny locations. In addition, the use of dry heat rejection is hardly possible, as this leads to an increased temperature lift. However, with a high thermal coefficient of performance, a high temperature lift causes driving temperatures that lie outside the working range of medium temperature collectors and outside the specifi-cations of absorption chillers.

In some countries in the southern Mediterranean, the use of desalinated water for the heat rejection in refrigeration systems is problematic or even prohibited; dry heat rejec-tion is therefore usually used. Absorption refrigeration with the NH3/H2O working pair is therefore interesting for these applications with increased temperature lift. This technique offers the advantage that it can also be used in process cooling with temperatures < 0 °C.

Until recently, the market availability of thermally driven refrigeration technology was still limited to nominal cool-

En passant

Fig. 7 Solar furnace from Mouchot Source:London Permaculture CC BY-NC-SA 2.0 viaFlickr

Initial attempts with solar cooling

As a result of the shortfall in fuel supplies caused by energy-intensive industrialisation, considerable interest in using solar heatdevelopedinFranceduringthesecondhalfofthe19th century. Unfortunately, this interest quickly dissipated once new coal deposits were developed and transportation problems for procuring fuel were solved through new railway connections. Nevertheless,solartechnologyflourishedduringthistime. The beginning of solar cooling can perhaps be dated to 1878. This was the year in which Augustin Mouchot created an ice blockwithsolarheatforthefirsttime,whichhecarriedouton 29 September at the Universal Exhibition in Paris. He used what was then the largest solar mirror device of its time with a roughly 20m²apertureanda2-metre-longboilerfordirectlygeneratingsteam as an absorber. The cooling energy was produced using a periodically operating absorption refrigeration process in a Carré Apparatus. This was named after the brothers Edmond andFerdinandCarré.Theydevelopedandoperatedabsorptionprocesseswithdifferentworkingpairs(sulphuricacid/water,ammonia/water).MouchotreportedonhisexperimentattheWorldFair:„On29Septembertheskycleareduparound1 1:30 am, and by noon I had brought 75 litres of water to boiling point. Despite some temporary clouds, the steam pressure gradually rose within two hours to 7 atmospheres; this was the maximum shown by the pressure gauge. I managed to repeat the experiment from 22 September and direct the steam into a CarréApparatus,whichenabledmetoreceivethefirstblockoficeever to be produced by the sun.“

Fig. 8 Generation of an ice block on 29 September 1878 during the Universal Exhibition in Paris with a concentrating mirror system and the use of an absorption refrigeration process in a Carré Apparatus. Source: Public domain via Wikimedia

10

Fig. 9 The type of solar cooling application and the heat rejectiontechnologyusedhaveaneffectonboththerequireddriving temperature as well as the choice of collector and thermally driven refrigeration technology. By way of example, two curves are shown for the driving temperature for thermally drivenchillers(TDCs)withathermalcoefficientofperformance(=EER)of0.7(single-effecttechnology)and1.2(double-effectabsorption technology). The driving temperatures and working ranges of the collectors and refrigeration technology should onlybeviewedasindicativefiguresthatcanvarydependingonthespecificproductandlocation(duetothetemperatureleveloftheheatrejection).Source:FraunhoferISE FPC=flat-platecollectorETC=evacuatedtubecollector, LFC=linearfocussingcollector Ad = adsorption, Ab = absorption technology (H2O/LiBr),Ab.D-effect= double-effectabsorption,AbNH3 = NH3 /H2O absorption refrigeration Ad *, Ab * = ad- or absorption technology, which under certain conditions can be operated with dry heat rejection

Ad*, Ab*

FC

Ad, Ab Ad, Ab

AbNH3

Examples

Heat rejection

AbD-effect

Temperature lift Tmean – Tlow [° C]

Driv

ing

tem

pera

ture

Thi

gh [°

C]

FCETC

VRK

10 3020 40 50

200

100

LFC LFC

EERTDC= 1.2

Surface cooling Circulating air cooling Process cooling < 0 °C

wet/dry wet dry dry

EERTDC= 0.7

AbNH3


yes

no

no

All chilled water system

Distribution medium- TechnologyBuilding

no

Supply air system, TDC, chilled water network 6–9 °C

yes

temperate extreme

temperate and extreme temperate and extreme

DEC, chilled water network 12–15 °C

Conv. supply/exhaust air system, TDC, chilled water network 6–9 °C

DEC, special configuration for wet climates, chilled water 12–15 °C

temperate extreme

DEC Conv. supply/exhaust air system, TDC, chilled water network 6–9 °C

DEC, specific configuration for wet climates

yes

Climate

Start: Cooling load calculat-ion (building parameters:materials, wall structure,geometry, orientation,internal loads, meteoro-logical conditions) ▸ cooling/heating load, hygienic air change

Installation of centralised air handling unit feasible and desired?

Building construction appropriate for supply/exhaust air system (building tight enough)?

Hygienic air change able to cover cooling load?

Climate

ClimateClimate

TDC, chilled water network 6–9 °C

Supply air system + chilled water system

Supply/exhaustsystem + chilled water system

All air system(supply/exhaust air system)

Fig. 10 Decision tree for determining methods for providing solar thermal assisted air conditioning in buildings. Based on building services aspects, the distribution medium is established and then the basic technology is selected. The technology marked with the coloured dot could be implementedasinFig.8withawheeldehumidifier,butalsowithmethodsusingliquidsorption.Source:FraunhoferISE TDC = thermally driven chiller (chilled water); DEC = desiccant and evaporative cooling


Points of view

The practicality of solar cooling and air conditioning systems has now been successfully demonstrated in practical operation when there has been a high quality in terms of the design, construction and operational management. Solar cooling is still a complex technology that requires considerable communication between the designers and the installation companies on the heating and cooling side; an interaction that requires greater standardisation of systems on a broad level. With regard to the competing options using renewable energy by means of photovoltaic systems, advanced concepts are necessary in order to underline the advantages of solar thermal cooling in economic terms: such as by additionally using the collector heat for other process purposes and through theefficiencygainsintheheatrejection.Highlypromisingaredevelopments for thermally driven heat pumps in cooling and heating operation. A general advantage of the solar thermal method compared with the solution with PV is that there are no high loads on the power grid. In addition to the considerable potential for avoiding harmful emissions, this provides another argument for trying to increase the market penetration.

Prof. Dr Hans-Martin Henning Head of Thermal Systems and Buildings at FraunhoferISEandDeputyDirectoroftheinstitute. Professor for Technical Energy Systems attheFacultyofMechanicalEngineeringat the Karlsruhe Institute of Technology (KIT)

What are the chances for solar cooling?

ing capacities above about 8 kW. Meanwhile, providers with devices from about 2.5 kW cooling capacity have now entered the market, which means that decentralised solar cooling for the low output range is now within reach. There are few limits to the upward capacity; standard absorption refrigeration technology with H2O/LiBr as the working pair is also available in the MW cooling range. The currently largest solar cooling system is located on the campus of the United World College in Singapore. The system consists of an absorption chiller with a 1.5-MW nominal cooling capacity and a 3,870 m² flat-plate collec-tor array.

Mixed systems for warm locations

Open, desiccant evaporative cooling is mainly suitable for supply air dehumidification (treatment of latent cooling loads) and also supports cooling in buildings to a limited extent. However, especially at warm sites the sensible cooling loads are often so large that they cannot be handl ed by the solar thermally driven air conditioning. Here a separation of the air conditioning tasks makes sense: a solar thermally driven sorptive part ensures the dehumidification of the supply air, while conventional compression refrigeration cools the supply air. The advantage of this method is that the compression refrigerat ion equipment can be operated at a high evapo-rator temperature level, as it is no longer necessary to undercut the dew point. It therefore works more efficient-ly and the power can be reduced. In addition, the subse-quent re-heating of the supply air that is often required when undercutting the dew point is eliminated. Fig. 10 shows an example of a possible system configuration with this mode of operation.

Selection criteria

A simplified scheme for pre-selecting the basic technolo-gy for solar thermal assisted building air conditioning was created a few years ago in SHC Task 25, “Solar Assist-ed Air Conditioning of Buildings”, as part of the Interna-tional Energy Agency’s (IEA) Solar Heating and Cooling Programme. The scheme is shown in Fig. 10 and presup-poses that the required hygienic air change and cooling loads are already known. Not considered in this simpli-fied approach is the need for backup systems, economic aspects as well as the more detailed selection of tech-nology (absorption or adsorption, collector type, etc.). The scheme provides guidance in choosing between a purely chilled water system, a purely desiccant and evap-oration cooling (DEC) system or a mixed system.

During the last ten years, it has been largely research institutions and innovative companies in Germany which have developed and marketed solar cooling from initial prototypes to marketable products. This means that we now have a diverse range of absorption and adsorption chillers with small and medium cooling capacities available on the market. In Germany, solar cooling is promoted through various subsidy programmes for the private, commercial and trade sectors. Recent estimates indicate that there are more than 1,200 installed solar cooling systems worldwide (end of 2014). This technology is therefore still a niche product but with huge potential. In its Solar Heating and Cooling Technology Roadmap from 2012, the International Energy Agency describes the potential for solar cooling as follows: it is intended that approximately 17 % of the total global demand for cooling shall be covered by 2050 by solar cooling, which corresponds toaround417TWh/p.a.Themarketsforsolarcoolingareseenmainly in Asia and the Middle East. Solar cooling is thus an export product for German companies.

Dr Uli Jakob Managing Director of the Green Chiller Association for Sorption Cooling, Berlin Director of dr. jacob energy research (JER) and Managing DirectorofSolemConsultinginEurope.Lecturerin the KlimaEngineering degree programme at Stuttgart University of Applied Sciences.


The detailed design of the hydraulic components and the control system has a significant impact on the energy ef-ficiency. The separate solar thermal, heating and cooling trades need to be brought together with corresponding care, whereby still incomplete standardisation makes it difficult to compare the performance and efficiency of components (see Standardisation Infobox). Optimisations at several levels reduce the planning and operating re-quirements. This requires:

• Simple design tools (tables, convenient software)• Quality assurance in installation, commissioning

and maintenance (recommendations and guidelines)• Standardised assessment methodologies at the

component and system level (criteria, guidelines, table and simulation-based methods)

• Permanent operational monitoring (e.g. with fault diagnosis)

These issues were addressed by scientists and companies in Task 48, Quality Assurance & Support Measures for So-lar Cooling Systems, which forms part of the Solar Heating and Cooling Programme run by the International Energy Agency (IEA), who have derived recommendations and framework documents for quality assurance. The Task was completed in 2015 and the documents are now available from the website. Particularly interesting for end users are energy contracting models similar to those already used for solar thermal heating. For solar cooling, the eco-nomic hurdles are still high. Nevertheless, such a model has already been implemented by the Austrian solar com-pany S.O.L.I.D. for a large solar heating and cooling system at the United World College in Singapore.

The still very small market volume strongly impacts the component prices and the lack of standardisation in-creases the planning outlay. Both increase the investment costs. With absorption or adsorption chillers with small cooling capacities, large differences in the specific com-ponent costs are also noticeable between manufacturers and technologies. Fig. 13 shows an example.

The prices of the principal components show that there are considerable economies of scale: in 2011, solar cooling kits (collector, cooling system, peripherals, without instal-lation and cooling distribution) cost around 4,500 euros per kW for systems with a nominal cooling capacity between 8 and 15 kW. With 100-kW systems, the specific costs, however, were around 2,000–2,500 euros/kW. In any event, the investment costs exceed those for conven-tional cooling several times over. However, it needs to be considered that solar thermal systems can cover additional heating requirements. It is therefore more fruitful to make a comparison based on comparative calculations, as shown in Section 3.

There is also potential to significantly reduce the plant and installation expenditure in regards to not just the component costs but also the system development: in the SolCoolSys project, researchers optimised low-capacity systems and tested them in field trials. A total of six ad-sorption chillers were installed with a cooling capacity

Correctly designing a complete system for

solar cooling is a challenging planning task.

Until now there have only been a few experts

who have experience with this technology.

This and the currently still high cost of individual

components are still barriers for the further

dissemination.

Planning, costs and integration

Fig. 11 SimplifiedsystemschematicfortheSolCoolSysinstallations. Theheatpumpmodeisnotactivatedinallplantsinthefieldtrial. Source:FraunhoferISE

Flat-plate collectors

Gas boiler(space heating, DHW)

Heating/Cooling

Hydraulic switching

Adsorption chiller 8 kWHeat rejection unit

Heat sink/source for the heat rejection, free cooling, heat pump operation

Storage system

Originalgröße bitte bessere Vorlage


between 8 and 15 kW in combination with a flat-plate collector system. A specially designed, pre-fabricated switching group contains all the main hydraulic elements and in some systems also allows the adsorption chiller to be operated as a heat pump and to provide free cooling

Fig. 13 Pricesearchescarriedoutforthermallydrivenchillersbetween2010and2012,convertedtothespecific costs per kW nominal cooling capacity. Prices exclude VAT, without heat rejection units and other peripherals. ThesolidlineswereusedascostcurvesintheSolarCoolinginBuildingscomparativestudyaspartofthejointEVASOLKproject. Source:FraunhoferISE

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EVASOLK; S-effect, T > 0 °C

EVASOLK; D-effectSearch 3; AbTDC T > 0 °C

via the air cooler with high-efficiency fan technology (Fig. 11). The last system commenced operation in summer 2013 at a vocational school in Freiburg (Fig. 12). There existing borehole heat exchangers are also used as heat sinks for the heat rejection.

Fig. 12 SolarcoolingaspartoftheSolCoolSysfieldtestsattheRichard-FehrenbachvocationalschoolinFreiburg. Left:Adsorptionchillerandhydraulicgroup;right:Collectorarray.Source:FraunhoferISE


In Solarthermie 2000plus, the operation of five systems with very different operating concepts were analysed. As part of the accompanying research (FKZ 0329605A), the researchers demonstrated that additional collector systems can also be successfully integrated into large-scale existing plants with a thermally driven cooling sup-ply.

An example is provided by the solar cooling of operation-al buildings belonging to Deutsche Telekom at its site in Rottweil. In Rottweil a solar thermal system reduces the use of gas boilers. Most of the heat used to drive the absorption chiller is also provided here by waste heat from a CHP plant. In contrast to the system at FESTO (see “In practice: Success Stories”), the heat supply systems are not operated here simultaneously but sequentially. Both concepts make different demands on the collector size, storage size and operating strategy. The diagram in Fig. 14 shows the main components of the system in Rottweil.

Two other installations demonstrate autonomous solar cooling in the low output range (without cooling backup; gas boiler used only for space heating and/or domestic hot water supply purposes). In the low-energy building belonging to the Vocational and Technical School in Butz-bach, two absorption chillers with 10-kW nominal cooling capacities air condition the classrooms. They provide chilled water at two different temperature levels: one for supply air cooling and dehumidification, and the other for operating the surface cooling system. The system is also used for training purposes in the technical field. A schematic of the system is shown in Fig. 15.

In Fürth, an absorption chiller with a 30-kW nominal cool-ing capacity has cooled the office building of the IBA AG since 2007. The heat is provided by a flat-plate collector with an 88 m² aperture area. Both in Butzbach and Fürth it was shown that small plants can be used to provide solar cooling for several hours a day (up to 8 hours in Butzbach) and that the indoor climate is effectively en-hanced in the buildings without the need for additional backup for the cooling provision.

Solar cooling became an integral part of energy

research at the latest with the launch of the Solarthermie

2000plusfundingconcept.Subsequently,differentsolar

power systems were successfully further developed as an

integral component of the building services technology

within research and demonstration projects. A current

focus is on the development of components.

Research starting points

In terms of the technology, the plant operation for the solar cooling in Solarthermie 2000plus ran largely trouble- free. Whilst the operation of the collector arrays was pos-itively evaluated, the researchers were not always satis-fied with the electrical performance factor for the solar cooling. They see potential for optimisation mainly in the design and operation strategy for the heat rejection, but careful sizing of the circulation pumps in all hydraulic cir-cuits is also essential.

Heat rejection with phase change storage systems

The efficiency of thermal chillers is highly dependent on the heat rejection temperature provided. Dry cooling tow-ers are pushed to their limits on very hot days when there are extremely high cooling requirements. In the Solar-Cool+PCM project, researchers from ZAE Bayern therefore developed a concept in which a phase change material storage system (PCM storage) absorbs such load peaks. The latent heat storage unit receives a portion of the waste heat at a constant temperature of 29 °C through melting the salt hydrate, calcium chloride hexahydrate. During the following night, the storage system discharges again through the heat rejection unit.

A demonstration plant was developed, installed and con-tinuously optimised in 2007. The PCM storage system is integrated on the heat rejection side of a 10-kW absorp-tion chiller, which is used via chilled ceilings for cooling offices. This therefore enables a sufficiently low condens-ing temperature of 32 °C to be ensured even at high am-bient temperatures. After the optimisation, the electrical EER for this system achieved an average value of 11, which is considerably more than comparable compression refrigeration systems.

Heat rejection in existing refrigeration systems

The heat rejection in solar thermal refrigeration systems greatly affects the performance and efficiency of the sorp-tion chiller unit and also often requires significant power.

15

Fig. 14 SolarthermalassistedcoolingofoperatingfacilitiesandofficesatDeutscheTelekom‘soperationsbaseinRottweil. The collector system is installed on a neighbouring building and commenced operation in 2011. Source:Schematic:FraunhoferISE;Photoright:OffenburgSecondarySchool

Fig. 15 Solarthermalcoolinginthelow-energybuildingbelongingtoButzbachVocationalSchool.Totestdifferentoperatingstrategies, the drive circuits for both chillers can be connected in series or parallel. The plant commenced operation in 2009. Source:Schematic:FraunhoferISE;Photoright:ButzbachVocationalandTechnicalSchool

Space heating, DHW

Evacuated tube collector 315 m² aperture

Storage tank20 m³

Storage tank25 m³

Absorption chiller2 x 340 kW

Cooling

Heat rejection(wet, open)

Storage tank5 m³

CHP 315 kWth

Gas boiler

Emergencycooler

Condensing boiler28 kW

HeatingEvacuated tube collector 60 m² aperture

Storage tank2.9 m³

Absorption chiller2 x 10 kW

Supply air(2 x 1,250 m³/h)

Heat rejection(wet, open)

Storage tank1 m³

Chilled ceilings(110 m²) + cooling shaft


In the SolaRück project (www.solarueck.de) coordinated by Fraunhofer ISE, scientists have analysed the operation of heat rejection units in existing refrigeration systems. They investigated how the heat transfer can be improved and auxiliary energy simultaneously saved. In addition, they developed generic management strategies for heat rejection units and complete systems in order to increase the energy efficiency of the overall system. The industrial partners experimentally tested different concepts for in-novative heat rejection systems:

• Adiabatic pre-cooling of the air to lower the cooling water temperature at high ambient temperatures

• Small-capacity hybrid coolers with minimal maintenance requirements

• Dry coolers based on plastic materials for reducing weight and costs

Collector-integrated sorption components

In collaboration with Fraunhofer ISE, the Vaillant company has investigated innovative system solutions for integrat-ing solar-generated heating and cooling as part of the KollSorp research project.

One concept is based on the cyclic operation of collec-tor-integrated sorption modules. Hygroscopic salts pro-vide a suitable working pair here, which through water absorption enable a heat transformation process. A func-tional prototype was tested in Fraunhofer ISE’s solar sim-ulator.

The technical objectives were achieved. In particular, the system configuration and the LiCl-H2O working pair pro-vide the following advantages:

16

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Main cooler Auxiliary cooler

Load Backup

PCM

Hot side

Heat rejection

Cold side


• Low system complexity• Desorption temperatures > 100 °C possible

without problems• High thermal efficiency (working pair, low loss)• High electrical efficiency with electrical performance

factor > 12 (heat rejection is distributed during the day and night, high heat rejection temperatures possible, no pump for high temperature circuit)

Other investigated concepts include the coupling of solar thermal systems, backup (gas burner) and sorption mod-ules to efficiently generate heat, hot water and cooling energy.

Plate heat exchangers replace shell and tube heat exchangers

In the “Absorption chillers based on compact plate devices” research project, researchers from ZAE Bayern are inves-tigating the extent to which absorption chillers with low outputs (3-5 kW cooling capacity) can be built with plate heat exchangers instead of the usual tube heat exchang-ers. They hope that this will significantly reduce the vol-ume and costs. This will therefore also enable absorption chillers with small capacities to achieve a better foothold in the mass market. For this purpose, various plate geom-

etries are being tested and a pilot plant based on plate heat exchangers is also being constructed.

Development of a solar thermal, heat pump-based heating and cooling system

Since with solar air conditioning a backup system for gen-erating cooling energy must always be maintained to en-sure specific comfort or process requirements, this re-sults in an increase in space requirements and equipment costs. ZAE Bayern is therefore researching the integration of this backup system in the form of a direct-firing gas burner in a multi-effect absorption chiller. It is planned to develop a prototype as part of the research project. Since the thermal EER of a single-effect absorption chiller is very low (0.7), it is intended that the heat shall be cou-pled via the gas burner in a second stage of the absorp-tion chiller. This enables a thermal cooling efficiency of 1.2 to be achieved. Furthermore, the integrated gas firing also enables efficient heat pump operation in winter. The gas burner can assume conventional heating tasks if the heat pump cannot be operated due a lack of ambient heat. The development also aims to furnish the integrat-ed solution with a pre-assembled control and hydraulic unit so that the system always operates in its most effi-cient mode and the integration of the system in the plan-ning and operation is simplified for end users.

Fig. 16 Schematic diagram of the SolarCool+PCM cooling concept By integrating a phase change storage tank in the heat rejection system, a low return temperature for the cooling water can also be ensured even with high outside temperatures. This therefore combines the advantage of a wet cooling tower, which has a low heat rejection temperature, with the advantage of a dry cooling tower, which provides ease of maintenance. The storage system is regenerated by releasing heat to the night-time air. This enables the heat rejection to be shifted to the night-time hours, which enables the use of low outside temperatures. Source: ZAE Bayern


Fig. 17 Operating principle of a collector-integrated sorption module: Sunlight activates (dries) the working medium in the absorber; thecondenserhastobecooled.Coolingcanbegeneratedatnightbyevaporationofthewater;thereactor/absorberneedstobeactivelycooled. The right image shows the functional prototype on the solar simulator. Source:FraunhoferISE

Fig. 18 Across-sectionthroughamodifiedplateheatexchanger;outwardlytheplateheatexchangerlookssimilartoaconventionalmodel. Theinternalstructuresandfixturesenablenotonlythetransferofheatbutalsothetransportofsubstances.Thesamefunctionalityistherefore achieved as with a shell and tube heat exchanger but in a more compact design and with less weight. Source: ZAE Bayern

Fig. 19 Operating modes for the solar thermal, heat pump-based heating and cooling system. Source: ZAE Bayern

Day

Night

Condensation

Evaporation Absorption

Desorption

Evaporator/condenser Reactor/absorber

40 °C90 °C

15 °C

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SE

COPTDC = 0.7

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15 °C

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35 °C

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Solar LT- heating

Or: LT-geo-thermal energy 10 °C

Heat pump operation

35 °C

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Boiler operation (fossil)

DE

COPHP = 1

Gas burner


Aus der Praxis

Fig. 21 380m²ofevacuatedtubecollectors(left) onaroofbelongingtoFriedrichBoysenGmbH&Co.KGinAltensteig provideheattodrivea150-kWwater/lithiumbromideabsorption chiller (right). Source: EAW Energieanlagenbau Westenfeld GmbH

Fig. 20 Pilot plant for solar cooling storage. The cold storage in a low-rise wing of the building also includesanicestoragesystem.TheFresnelcollectorandabsorption chiller are installed on the roof. Source: Kramer GmbH

In many sunny regions, high ambient temperatures combined with a lack of fresh water present a problem when operating solar cooling with wet heat rejection. The use of dry heat rejection, on the other hand, limits the thermally driven technology that can be utilised. Together with commercialpartners,FraunhoferISEhasthereforetested a concept for solar thermal process cooling in the temperature range between 0 °C and –10 °C whichisspecificallyaimedatcoldstorageinwarm,sunny regions. Here an air-cooled absorption chiller with the NH3 /H2O working pair is powered using solarheatfromaFresnelcollectorarray.Thepilotstructure for the plant was built by project partner KramerKühlraumbauGmbHinUmkirch/Freiburg.

The target market for later applications is southern Europe and North Africa.

A typical example of a solar thermal cooling system in Germany is the plant constructed in Altensteig in 2012. The sloping roof of the technical centre for a production facility is used for generating solar heating, cooling and electricity. The solar heat drives a low-temperaturewater/lithiumbromideabsorptionchiller.Thesystems, which were initially designed for utilising waste heat from motorised CHP plants and district heating systems, can be operated with comparatively low driving temperatures (e.g. heating water at 86/71°Cwithchilledwaterat9/15°C),andarethereforealsoidealfor using solar thermal heat. The continuous mode of operation and theflexibleoperationoverawiderangeofexternaltemperatures are further advantages of the absorption technology.

Absorption cooling for different regions

In practice

Air-cooled sorption chillers

Indirectly heated sorption chillers are required for solar thermal cooling, i.e. systems that can be operated with heating water or heating steam. Until now, indirectly heated sorption chillers have only been available with water-cooled condensers and absorbers/adsorbers, and require an additional heat rejection unit. In comparison, air-cooled systems have the following advantages:

• Reduced system complexity• Reduced installation costs• Reduced auxiliary energy demand by eliminating the

cooling water circuit.

In the “Air-cooled Sorption Chiller” project, the Institute for Air Handling and Refrigeration (ILK Dresden) devel-oped and built a directly air-cooled absorption chiller with the water/LiBr working pair. After it was measured on a test rig, a field trial was carried out in summer 2015 which was able to verify its functionality and output capa-bility.

Solar heating and cooling in northern and central Europe (FKZ 03ET1231A)

Since heating is the dominant energy mode in northern and central Europe, ZAE Bayern is developing a system that – without additional auxiliary units – meets not just the cooling demand but also in particular the space heat-ing and domestic hot water requirements. Here an ab-sorption heat pump is combined with a solar thermal col-lector and a biomass-fired boiler plant. The solar energy is used both as driving energy for the sorption chiller as well as a low-temperature heat source for the absorption heat pump in heating mode. An integrated wood pel-let-fired boiler plant is used as backup for the driving en-ergy. This can provide the driving energy for both the heating and cooling modes if there is insufficient solar energy.


Success stories

In practice

One of the solar cooling success stories is a wine warehouse inBanyuls,southernFrance,whichiscooledpurelyusingsolarthermal cooling. The plant commenced operation in 1991 and has therefore been working for over 20 years. An absorption chillerwitha52-kWnominalcoolingcapacityanda215-m²evacuatedtubecollectorarraycoolthethreefloorsofthewinebottle warehouse via a ventilation system. About 3 million wine bottlesarestoredinthewarehouseacross3,500m²ofusablespace. A one-cubic-metre hot water storage tank is designed to provide only short-term storage. The wine bottles themselves act as the cooling storage system.

Another example is the solar assisted cooling of the technology centrebelongingtoFESTOAG&Co.KGinEsslingen-Berkheim.An adsorption chiller with a 1.05-MW nominal cooling capacity has air conditioned the building complex since 2001. Waste heat from the production facility and heat from gas boilers is used to drive the three adsorption chillers, which each have a 350-kW nominal cooling capacity. As part of the Solarthermie 2000plus funding programme, the plant has been expanded to include a large evacuated tube collector array that largely reduces the use of the gas boiler to drive the cooling supply. Thecollectorareais1,218m²insize(aperturearea).Withatotal of 17 m³, the hot water tank here is also designed to provide just temporary storage; the cooling technology‘s high number of operating hours means that solar heat is always absorbedimmediately.Onlywaterisusedasthecollectorfluid;a special frost protection switch in the collector control system prevents damage to the collectors during the winter.

Fig. 22 In operation since 1991: Solar cooling of a wine bottle warehouseinBanyulsinsouthernFrance. Two absorption chillers with a total nominal cooling capacity of 52 kW coolapproximately3,500m²of warehouse space. The thermal inertia of the bottle store means that a cold storage facility is not necessary. The plant is driven using an evacuated tube collector array; there is no backup system. Source:Tecsol,FR

Fig. 23 above: Inoperationsince2001/2007:

Solar assisted air conditioning of the technology centre atFestoAG&Co.KG.Oneofthethreeadsorptionchillers,whicheach

have 350-kW nominal cooling capacity, can be seen on the left. Source:FraunhoferISE

below: The collector array was added in 2007. Waste heat and

solar heating utilisation have priority; heat is additionally used from the gas boilers if there is an increased cooling demand.

Source:OffenburgSecondarySchool


In the EvaSolK project, which stands for “Evaluation of the opportunities and limitations of solar cooling in comparison to reference technologies”, the researchers demonstrated the potential for different applications. Criteria for the eval-uation were the primary energy demand, CO2 emissions and the economic efficiency. As reference, the researchers in-vestigated scenarios with conventional compression chillers.

In comparing the solar thermal and photovoltaic systems, the scientists examined the overall balance of the building supply at the annual level, i.e. for heating, cooling and do-mestic hot water heating. For their model calculations, they chose five sites representing the most common cli-mate types in central and southern Europe. Other calcula-tions cover very sunny and warm locations (Antalya, Turkey and Bechar, North Africa). Here concentrating collectors and double-effect absorption chillers can be used.

The researchers analysed three applications or uses:A Buildings whose usage structure roughly

corresponds to an apartment building with six residential units. In addition to the cooling and heating load, the hot water requirement is also taken into account.

B Office buildings where the use concentrates on the workspaces used during workdays. The hot water demand is low. It was differentiated between (B) small buildings with two floors and (B+) larger buildings with eight floors and a ventilation system.

C Buildings with an increased use in the evenings, including at weekends. A hotel is used as a model. Here there is also an increased domestic hot water requirement. The calculations differentiate between buildings with two floors (C) and buildings with eight floors and a ventilation system (C+).

Specifically, the researchers simulated the following configurations: Versorgung

ST – solar thermal assisted building supply• Single-effect ad/absorption

(double-effect at two locations)

• Flat-plate collector, evacuated tube collector (double-effect: concentrating collector)

• Backup cooling supply: Water chiller; in appropriate applications also solar thermal autonomous cooling

• Heating backup: Gas boiler (only space heating, DHW)

Reference• Cooling system: Electrically driven compression

refrigeration; in accordance with the building type and size: Multi-split units, water chiller,

• Heating supply: Gas boiler.

Reference + PV (grid-connected)• Building supply: As in reference,• Additionally: Grid-connected PV generator;

no additional components (storage system),• PV rated power: 50 % of the electrical rated

power consumption of the refrigeration.

In the simulations, the collector array size was optimised for minimal solar surpluses. The nominal capacities of the PV systems were limited to 50 % of the rated power con-sumption of the electrical compression refrigeration to achieve a high economic efficiency. More than 70 % of the PV electricity can therefore be absorbed directly by the supply technology and used by other loads. The surplus electricity fed into the grid was considered in primary energy and emission terms.

Economic assessment

The evaluation parameters, such as the relative primary energy savings ΔPErel and the costs of the primary energy savings ΔCPE, were calculated for an operating period of 20 years. The evaluation variables are related to the refer-ence. Example: Relative primary energy savings ΔPErel = 0.2 means 20 % less primary energy costs compared with the reference. ΔCPE indicates how the costs of the primary energy saved (kWh) in the solar variant compare with the costs of the reference (> 0: no amortisation within the operating period; < 0: the life cycle costs are lower than those of the reference).

Thanks to the falling cost of photovoltaic energy,

solar-electric cooling with compression chillers is

becoming increasingly attractive. In the joint EvaSolK

project, researchers analysed the competitive situation

ofsolarthermalsystemsfordifferentbuildings,

climatezonesandconfigurations,includingin

comparison to conventional cooling technologies.

Solar power instead of heat


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Fig. 25 System simulations for the application area C+. Thethermallydrivenchiller(TDC)intheSTconfigurationwasdesigned here not at full capacity but at 33 % of the maximal cooling load. The plant has a compression refrigeration backup. Source:FraunhoferISE

Fig. 24 System simulations for the application area A. The ST configurationisoperatedherewithoutabackupsystemforthecoolingsupply. The solar coverage of the cooling demand with solar thermally driven refrigeration is always > 70 %. Within the cooling season, this may mean that the target values for the indoor temperature are exceeded to a limited extent. This is tolerable for residential buildings. The Reference+PV configurationshavelowercosts,butsavelessprimaryenergy. Source:FraunhoferISE

For small residential buildings (A), solar thermal assisted cooling turns out to be uneconomical with current costs. There is also little potential for cost reductions. This ap-plies in particular to less radiation-rich locations with rela-tively few cooling operation hours. However, the CO2 sav-ings are high. Whenever possible, a backup system for the cold supply should be dispensed with.

With the office buildings (B), the solar thermal variants are also far from achieving cost neutrality. The potential for primary energy savings is considerably less than in appli-cation A. Noticeable here are the small domestic hot water requirement and the low load requirement at weekends. In terms of primary energy savings and the specific costs of the PE savings, the Ref+PV option is more advantageous here.

Much more favourable is the load profile of a hotel (C). Here the high domestic hot water requirement makes the solar thermal variants more advantageous. The cooling backup makes it possible to design the thermally driven cooling to meet about 1/3 of the cooling peak load (Fig. 25). This significantly improves the economic efficiency. Since the peak power is only rarely required, the primary energy savings reduce only moderately.

Here the solar-electric variant is also more economically attractive as a whole. However, the solar thermal systems come close to the reference in southern European loca-tions. In particular the CO2 savings are advantageous.

Assuming cost reductions realisable in the medium term of –25 % for the solar collector and –33 % for the cooling system, the efficiency at southern locations is comparable to conventional and photovoltaic systems. Solar thermal systems, however, possess the greater potential for primary energy savings.

In summary, the researchers conclude that, owing to the still high costs, solar thermal cooling is the most promising when an additional heat demand leads to a uniformly high utilisation of the collector. Examples include hotels, hospital departments, etc. Further calculations show that


in regions with very high solar radiation, highly efficient double-effect absorption refrigeration achieves costs comparable to the reference. The solar electric variant is, however, slightly more economical. In addition to carefully dimensioning the thermally driven cooling, it is therefore also essential that the costs for the main components are reduced.

Better evaluation of environmental effects

The evaluation parameter ΔCPE solely measures the costs of the primary energy savings. However, the actual primary energy saved remains unconsidered. As a compromise be-tween purely economic and purely environmental consid-erations, the researchers therefore defined a dimension-less parameter Opt+. They used this to total the cost and primary energy savings with equal weighting (each stand-ardised to the costs in the reference). The evaluation pa-rameter therefore compensates for the cost disadvantages caused by high primary energy savings. Advantageous compared with the reference are values > 0 for Opt+.

Grid relief

Especially in southern climates, the impacts on the often weak electricity grid play an important role. The investiga-tions show that solar thermal methods reduce “grid stress”, whereas PV systems tend to increase it – despite the high proportion of self-consumption. In the solar thermal vari-ant, in particular the configuration without cooling backup (solar thermal autonomous cooling) has a lower maximum. In Ref+PV, no additional storage systems (thermal or elec-tric) were considered. These can reduce the grid fluctua-tions with appropriate regulation, but require noticeably higher investment costs.

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rel. PE savings


Reference * PV



Fig. 27 ApplicationasshowninFig.25,butloweredinvestment costsintheSTconfiguration:Collectorsystem–25%; thermally driven cooling system (incl. heat rejection) –33 %. Source:FraunhoferISE

Fig. 26 Solar thermal cooling competes with systems with conventional or solar electric driven compression chillers. Left:AbsorptionchillerSource:KramerGmbH;Right:CompressionchillerSource:GerdHirn,BINEInformationService


Standardisation

Extensive standards and test specificationsexistforconventional components for heating and cooling systems thatenabledifferentmanufacturerstobecompared with one another. In the (solar) thermally driven cooling sector, however, the regulations are incomplete. Even just at the component level there are still no standards for hot water-driven cooling devices that are applicable within Europe.

Forthisreason,auniformevaluationmethod is currently being developed as part of EA-SHC Task 48 (Quality Assurance and Support Measures for Solar Cooling Systems), wherebytwodifferentapproachesarebeingcompared.

• The BIN method, which links part load values of the components with operating frequencies as part of a tabular procedure. This method is currently used, for example, in the standard for assessing the seasonal efficiencyofelectricheatpumps (DIN EN 14825: 2013).

• The CTSS method (Component Testing System Simulation): Here the key system components are measured individually and parameters set that are then used in simulations to determine the system performance under reference conditions. This is used, for example, to calculate the yield from solar thermal hot water systems (DIN EN 12977: 2012).

The results can be drawn upon, for example, in subsequent DIN activities. Australia was the firstcountrytopublisha„SolarCoolingStandard“ in September 2013. This works according to the CTSS method and so far only includes open methods.

– 0.2

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Fig. 28 ApplicationC+:ComparisonofΔCPE and Opt+. WiththeoptimisationparameterOpt+,notjusttheconfigurations fromRef+PVbutalsotheSTconfigurationslieinthepositiverange at several locations. The investment costs were not lowered here. Source:FraunhoferISE

Fig. 29 Grid interaction index fGrid, shown relative to the maximum valueoftheelectricalpowerin/fromthegrid.Thevaluesarestandardisedto the reference values. fGridrepresentsameasureofthefluctuationsintheelectricity exchanged to and from the grid (grid input and output): an increasing value corresponds to greater „grid stress“. Two data points areincludedfortheRef+PVvariant:forthestandardconfigurationusedinthe comparative study for the PV generator, covering 50 % of the electrical power consumed by the compression chiller (n = 0.5), and for covering 100%oftheelectricalpowerconsumption(n=1).Source:FraunhoferISE

Fig. 30 Solar cooling of the wholesale market building in Oberkirch, Germany Source: p-power GmbH

ImpressumProjektorganisation Bundesministerium für Wirtschaft und Energie (BMWi)11019 Berlin

Projektträger Jülich Forschungszentrum Jülich GmbH 52425 Jülich

Förderkennzeichen 00327430M0327430H0327387A-D0335007P

ISSN 1610-8302

Herausgeber FIZ Karlsruhe · Leibniz-Institut für Informationsinfrastruktur GmbH Hermann-von-Helmholtz-Platz 1 76344 Eggenstein-Leopoldshafen

24 BINE-Themeninfo I/2015

Links und Literatur>> www.XXX.de >> www.XXX.de >> www.XXX.de >> www.XXX.de >> www.XXX.de >> Literaturhinweis>> Literaturhinweis >> Literaturhinweis>> Literaturhinweis >> Literaturhinweis>> Literaturhinweis >> Literaturhinweis>> Literaturhinweis

Mehr vom BINE Informationsdienst>> XXX. BINE-Projektinfo XX/20XX >> XXX. BINE-Projektinfo XX/20XX>> Dieses Themeninfo gibt es auch online und in englischer Sprache unter

www.bine.info/Themeninfo_X_20XX

BINE Informationsdienst berichtet aus Projekten der Energieforschung in seinen Broschürenreihen und dem Newsletter. Diese erhalten Sie im kostenlosen Abonnement unter www.bine.info/abo

ÜberschriftWeit hinten, hinter den Wortbergen, fern der Länder Vokalien und Konsonantien leben die Blindtexte. Abgeschieden wohnen sie in Buchstabhausen an der Küste des Seman-tik, eines großen Sprachozeans. Ein kleines Bächlein namens Duden fließt durch ihren Ort und versorgt sie mit den nötigen Regelialien. Es ist ein paradiesmatisches Land, in dem einem gebratene Satzteile in den Mund fliegen. Nicht einmal von der allmächtigen Interpunktion werden die Blindtexte beherrscht – ein geradezu unorthographisches Leben. Eines Tages aber beschloß eine kleine Zeile Blindtext, ihr Name war Lorem Ip-sum, hinaus zu gehen in die weite Grammatik.

Der große Oxmox riet ihr davon ab, da es dort wimmele von bösen Kommata, wilden Fragezeichen und hinterhältigen Semikoli, doch das Blindtextchen ließ sich nicht beir-ren. Es packte seine sieben Versalien, schob sich sein Initial in den Gürtel und machte sich auf den Weg. Als es die ersten Hügel des Kursivgebirges erklommen hatte, warf es einen letzten Blick zurück auf die Skyline seiner Heimatstadt Buchstabhausen, die Headline von Alphabetdorf und die Subline seiner eigenen Straße, der Zeilengasse. We-hmütig lief ihm eine rhetorische Frage über die Wange, dann setzte es seinen Weg fort.

Die Copy warnte das Blindtextchen, da, wo sie herkäme wäre sie zigmal umgeschrieben worden und alles, was von ihrem Ursprung noch übrig wäre, sei das Wort „und“ und das Blindtextchen solle umkehren und wieder in sein eigenes, sicheres Land zurückkehren. Doch alles Gutzureden konnte es nicht überzeugen und so dauerte es nicht lange, bis ihm ein paar heimtückische Werbetexter auflauerten, es mit Longe und Parole betrunk-en machten und es dann in ihre Agentur schleppten, wo sie es für ihre Projekte wieder und wieder mißbrauchten.

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Kontakt · InfoFragen zu diesem Themeninfo? Wir helfen Ihnen weiter:

0228 92379-44 [email protected] Informationsdienst Energieforschung für die PraxisEin Service von FIZ Karlsruhe

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ImprintProject organisation Federal Ministry for Economic Affairs andEnergy (BMWi)11019 BerlinGermany

Project Management JülichForschungszentrum Jülich GmbH52425 JülichGermany

Project number 0325966A,B,C0325979A032599403259970327406A0327875A0327879A0329662D03ET1107A03ET1213A

ISSN 1610-8302

Publisher FIZ Karlsruhe · Leibniz Institute for Information Infrastructure GmbH Hermann-von-Helmholtz-Platz 1 76344 Eggenstein-Leopoldshafen Germany


Links and literature>> Project reports on EVASOLK, Solarthermie 2000plus, ECOS, AgroKühl

www.solare-kuehlung.info >> Quality Assurance and Support Measures for Solar Cooling Systems

http://task48.iea-shc.org/>> Green Chiller – Association for Sorption Cooling, Berlin

www.greenchiller.de/indexeng.php >> Henning, H.-M. (Ed.); Motta, M. (Ed.); Mugnier, D. (Ed.):

Solar Cooling Handbook. A Guide to Solar Assisted Cooling and Dehumidification Processes. Vienna (Austria): AMBRA/V, 2013. 3rd issue, ISBN 978-3-99043-438-3 (print issue)

More from BINE Information Service>> Kühlen und Klimatisieren mit Wärme. Hrsg.: FIZ Karlsruhe GmbH,

BINE Informationsdienst, Bonn. Stuttgart: Fraunhofer IRB Verlag, 2015. 2., vollständig überarb. Aufl., ISBN 978-3-8167-8324-4

>> This Themeninfo brochure is also available as an online document at www.bine.info

BINE Information Service reports on energy research projects in its brochure series and the newsletter. Youcansubscribetothesefreeofchargeatwww.bine.info/abo.

Outlook Solar cooling and air conditioning has been shown to be particularly advantageous in sunny locations with high operating hours. For German companies, the best opportunities are therefore provided by exports to tropical countries. Researchers and companies expect that the economic viability of the various technologies will improve. They see particular development potential, for example, in increasing the thermal and electrical performance factors of sorption chillers. Decisive, however, will be further cost reductions in the components and system technology. Researchers and providers are working on simplifying the system and thus reducing costs, especially with devices in the low power range. Starting points include, for example, the integration of circulation pumps (driving heat, heat rejection, chilled water) as well as integrated dry heat rejection.

In the project planning, the economic and ecological benefits can only be achieved if optimum use is made of the collector array. Comparative calculations show that:

• The environmental impacts are generally high: depending on the type of application, up to 80 % primary energy savings can be achieved in sunny locations. If, however, only electric energy is saved by the renewable supply, it is currently difficult to achieve economically beneficial operation.

• The economic efficiency improves if the solar thermal system is additionally used for supporting space heating and, in particular, domestic hot water heating. The costs become closer to conventional supply technology, whereby the life cycle costs of solar thermal variants are still generally higher. In particular, applications with very high additional domestic hot water demands (e.g. hotels, hospitals) are favourable in this respect.

• The correct configuration of large systems plays an important role: if cooling backup is available (conventional refrigeration unit), it makes sense not to configure the absorption or adsorption chiller to cover peak loads. Optimised configurations to meet cooling capacities significantly below the peak load make effective cost savings without significantly losing primary energy savings.

• Solar thermal configurations generally help to reduce fluctuations in the electricity drawn from the grid connection. This should be especially taken into account in regions where there is a high utilisation of the electricity grids through air conditioning.

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Contact · InfoQuestions regarding this Themeninfo brochure? We will be pleased to help you:

+49 228 92379-44 [email protected] Information Service Energy research for application A service from FIZ Karlsruhe

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