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Exergy Analysis of Solar Air-ConditioningSystems and their Applicability

Presentation at the C23 Conference, WP1, September 24 2008, Munich

Christopher J. KoroneosEvanthia A. Nanaki

George A. Xydis

Strategies for a Low Carbon Built Environment

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The exergy concept

Exergy is defined as the property of the system, whichgives the maximum power when it is brought to athermodynamic equilibrium state. It is the maximumavailable energy.

Ex = ?H - T ?SExergy Analysis & Solar Cooling Systems: Analysis andDesign of Innovative Systems in the Built Environment. A Alowlow -- exergy heating or cooling system was defined by IEAexergy heating or cooling system was defined by IEA

Annex 37 as a system that allows the use of low valued Annex 37 as a system that allows the use of low valuedenergy as a source. Theenergy as a source. The greatest exploitation of solarcooling potential is achieved in buildings with high thermalgains during the day and consequently high cooling load .

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Cooling energy consumptionCooling energy consumption

During the last few decades energy consumption for cooling hasDuring the last few decades energy consumption for cooling hasincreased dramatically in most European countriesincreased dramatically in most European countries

During the summer months the demand for electricity in GreeceDuring the summer months the demand for electricity in Greeceincreases (extensive use of heating ventilation and air conditioincreases (extensive use of heating ventilation and air conditio ningning

systemssystems increasesincreases peak electric loadpeak electric load causing major problems incausing major problems inthe electric supply)the electric supply)

In the current practice, air conditioning is exclusively based iIn the current practice, air conditioning is exclusively based i n the usen the useof electric energy, while the use of solar energy is limited toof electric energy, while the use of solar energy is limited to heating ofheating ofdomestic hot water, and in limited applications for space heatindomestic hot water, and in limited applications for space heatin g andg andfewer for cooling.fewer for cooling.

It has been estimated that the total electric energy consumptionIt has been estimated that the total electric energy consumption ininGreece, in 2003, for central air conditioning systems was 2909Greece, in 2003, for central air conditioning systems was 2909 GWh/yGWh/y ,,whereas the per capita consumption of electricity was estimatedwhereas the per capita consumption of electricity was estimated at 371at 371KWh/ yKWh/ y

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Solar Collectors in Greece

Sales & Energy production of solar collectors in Greece

0

50000

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1 9 7 8

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m2

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GWh

Sales Energy Production

95% of the collector area installed up today : households264 m 2 / 1,000 inhabitants

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Solar Cooling System

Cooling tower

Generator Condenser

Expansion valve

Evaporator

Solution heat exchanger

Absorption

L i B r

LiBr/H 2O

Refrigerantliquid

Refrigerantliquid

Refrigerantsteams

Refrigerantsteams

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Exergy AnalysisExergy balance of the systemExergy balance of the system ?? XinXin == ?? XlostXlost ++?? XoutXout

(kW)(kW)Exergy efficiency of the system

n = ? Xout / ? Xin = 1 ? Xlost / ? XinTotal exergy input into the absorption refrigeration cycle, in each component:

Xin,total = Xin,g + Xin,c + Xin,exv + Xin,e + Xin,a+ Xin,she

Total exergy output of the absorption refrigeration cycle :

Xlost,total = Xlost,g + Xlost,c + Xlost,exv + Xlost,e + Xlost,a +Xlost,she

g=Generator,c=Condenser, exv=Expansion Valve, e=Evaporator, a=Absorber,she=Solution Heat Exchanger

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Exergy Analysis

The conditions used are the following: Ambient temperature of ?0 = 25 C,Heat exchanger efficiency e= 0.5,Temperature of cold water ?17 = 16 ?C,Temperature of cold water ?18 = 10 ?C,Temperature of hot water ?11 = 100 ?C,Mass rate of flow m7 = 0,005 kg/s

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SHEGenerator

Condenser Expansion

Valve Evaporator Absorber

-5

0

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Ein (kW) Elost (kW)

Exergy efficiency of thdifferent solar thermalsystems components

xergy inputs andergy losses at

ifferent solar thermalstems components

0.000.100.200.300.40

0.50

0.600.700.800.901.00n

SHE Generator Condenser ExpansionValve

Evaporator Absorber

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Application of solar cooling system inGreece: the case study of Igoumenitsa

A medical center located near A medical center located near IgoumenitsaIgoumenitsa -- a city in northa city in north --westernwesternGreece with average rainfall of 1,100mm/ year Greece with average rainfall of 1,100mm/ year -- having units for thehaving units for thetreatment of patients, offices, labs and auxiliary spaces.treatment of patients, offices, labs and auxiliary spaces.The building was designed in 1984 and consists of a ground floor The building was designed in 1984 and consists of a ground floor area of 1,240marea of 1,240m 2

and height of 3.60 m and a basement area of 600mand height of 3.60 m and a basement area of 600m 2 and height of 2.8 m.and height of 2.8 m.

The structural components of the building consist of:The structural components of the building consist of: double casing external walls with insulation 4cm, K= 0.55 kcal/double casing external walls with insulation 4cm, K= 0.55 kcal/ mm 22 00 CC internal walls : 10cm, K= 1.5 kcal/ minternal walls : 10cm, K= 1.5 kcal/ m 22 00 CC insulated roof, K= 0,47 kcal/ minsulated roof, K= 0,47 kcal/ m 22 00CC marble insulated floor 5cm, K= 0.52kcal/ mmarble insulated floor 5cm, K= 0.52kcal/ m 22 00 CC aluminum window frames, K= 0.52kcal/ maluminum window frames, K= 0.52kcal/ m 22 00 CC sunlight protection of the building consisting of opensunlight protection of the building consisting of open --colored walls and internalcolored walls and internal

curtaincurtain

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Cooling LoadsThe calculation of the buildingThe calculation of the building s cooling loads took into consideration the following:s cooling loads took into consideration the following:

internal temperature 26internal temperature 26 ooCC internal humid 50%internal humid 50% climatic data for the city ofclimatic data for the city of IgoumenitsaIgoumenitsa calculation method : Carriercalculation method : Carrier checking times: 08:00checking times: 08:00 18:0018:00

reporting months: 5 (Mayreporting months: 5 (May -- September)September)

KWatt

0

20

40

60

34

8p

38

9p

42

10p

46

11p

50

12p

56

1

63

2

68

3

69

4

68

5

63

68am 9am 10am 11am 12 pm 1pm 2pm 3pm 4pm 5pm 6p

Cooling loads of July 2006

Di i i h

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Dimensioning o the ystemChillerChiller ((YazakiYazaki type WFCtype WFC --20)20) Cooling capacity: 20RT (70kW, 240,000Btu/h)Cooling capacity: 20RT (70kW, 240,000Btu/h) COP: 0.7COP: 0.7

Temperature of water: 9Temperature of water: 900

CC Temperature of water at the generator: 75Temperature of water at the generator: 75 -- 100100 00CC Nominal operating temperature :88Nominal operating temperature :88 00CC Temperature of water at the condenser: 29.5Temperature of water at the condenser: 29.5 00CCCooling tower (wet type) Cool ing tower (wet type) Capacity : 70kWCapacity : 70kW Rate of air flow: 130Rate of air flow: 130 --170 m3/h per kW of cooling capacity170 m3/h per kW of cooling capacity Electrical consumption: 6Electrical consumption: 6 --10 W per kW of cooling capacity for axial fans and 1010 W per kW of cooling capacity for axial fans and 10 --20 W per kW of20 W per kW of

cooling load for radial fans.cooling load for radial fans.Solar thermal collector field (flat plate co llector)Solar thermal collector field (flat plate co llector) Collector Collector s azimuth: 0s azimuth: 0 ?? Collector Collector s slope :30s slope :30 ?? (Latitude: 39,53(Latitude: 39,53 ?? )) Average daily radiation for the month of July 2006: H=7 kWh/m2 / Average daily radiation for the month of July 2006: H=7 kWh/m2 / d for a slope of 30d for a slope of 30 00 Average daily collector Average daily collector s performance : n=0.5s performance : n=0.5 Ac Ac == QUQU // nn**?? Ac Ac = 800 / 0,5*7= 800 / 0,5*7 Ac Ac = 230= 230 m2m2Hot water storage tank Hot water storage tank 50lt/ m2 of collective area.50lt/ m2 of collective area. V=50 * 230=11,500lt.V=50 * 230=11,500lt. H=2.0m , W=2.5m , L=2.3mH=2.0m , W=2.5m , L=2.3m

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Energy Benefits

5,401 Price of oil

9,002 (7,472)lt (kg)Oil used for heating

16.85 Price

177.00kWhElectrical energy for cooling*

Solar System

8,310 Price of oil13,850 (11,495)lt (kg)Oil used for heating

1,500 Price

15,560kWhElectrical energy for cooling

SystemUnitsType of EnergyConventional System

E i l B fiE i l B fi

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Environmental BenefitsEnvironmental Benefits

kg/year 0.140.0080.032.741500.177lectriccooling)

kg/year 2.1361.424.275.2323,4777,472il

? . Overall emissions of solar systemolar System

kg/year 12.450.772.80241.1813,22615.56lectric(cooling)

kg/year 3.282.196.578.0436,11711,495il

?. Overall emissions of conventional system

onventionalSystem

kg/MWh0.80.050.1815.58501 MWhlectric

g/kg0.2860.1910.5720.73,1421 Kgil

UnitsParticlesNO xCOSO 2CO 2Quantitykg, MWh?

ype ofenergy

Gas Emissions

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Solar Cooling Systems CostEvaluation

12 yearsPayback time having a 50% subsidy

24 yearsPayback time

107,640 Difference between cost of solarsystem conventional system

138,000 Cost of solar thermal system

4,409 Price of energy saving per year

C l i

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ConclusionsThe use of conventional cooling equipment has introduced

several drawbacks such as high electric loads, increase ofelectrical energy consumption and environmental problemsresulting from the use of refrigerants and electricity production.

As the rate of air-conditioned demand is expected to grow, the

exploitation of solar energy, especially in South Europeancountries, like Greece, seems to be a valuable option to mitigatethe consumption of conventional fuels.

Solar air conditioning systems can be a reasonable alternative to

conventional air-conditioning systems. No long- term intermediatestorage is necessary. The sun can provide a substantial part ofthe energy needed for air-conditioning. This can help to reduceprimary energy consumption

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Thank you!Thank you!koroneos@aix.meng.auth.gr koroneos@aix.meng.auth.gr

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I invite you to the conference:I invite you to the conference:International ExergyInternational Exergy --LifeCycleLifeCycle

Assessment andSustainabilityAssessment andSustainabilitySymposium(ELCAS)Symposium(ELCAS)

June 4June 4 --6 20096 2009NisyrosNisyros -- GreeceGreece