Assessment criteria for solar cooling systems S. Ayyash, R. K. Suri and G. P. Maheshwari K e y w o r d s : r e f r i g e r a t i o n , solar cooling, e n e r g y saving
CritCres pour 1'6valuation des systCmes frigorifiques solaires Afin d'uti/iser /'#nergie solaire pour/e rafraichissement des b#timents i/ est utile de consid#rer l'ensemble du syst#me comme une combinaison d'un groupe de transformation de I'#nergie et d'un groupe frigorifique. Le sous-syst~me de transformation de I'#nergie peut consister en un r~cha uffeur
Solar cooling systems should be able simultaneously to generate refrigeration and save conventional energy. An assessment of the potential of various cooling systems as refrigerators and energy savers is presented in the paper. For this purpose, a solar cooling
Nomenclature Cia, Cif Cw
system has been considered as a combination of an energy conversion and a refrigeration subsystem. Overall efficiencies of the various solar cooling systems are defined for direct quantitative comparison of different systems.
apparent and realistic cooling indices of refrigeration generation subsystem (dimensionless or kWhc/kWhe) parasitic constant for water consumption in cooling tower in terms of electrical power needed per unit heat rejection (kWe/kWt,) Cw=0.O00: for naturally available soft water Cw=Oo017: for soft water production from seawater by solar MSF at 95°C Cw=0.034: for soft water production by RO process from seawater and for cogeneration power-water production power plants daily electrical energy consumption by auxiliary equipment (kWh. m -2 d -~) total converted energy ((kWht, or kWhe) m -2 d-l) generator output (energy) (kWhe m -2 d -1 ) total electrical energy saving or
Solar cooling can be defined as a thermodynamic system powered by solar energy used for space cooling. Depending on the type of system employed, The authors are from the Department of Energy, Engineering Division, Kuwait Institutefor Scientific Research, PO Box 24885, Safat, Kuwait. Paperreceived26 July 1983. Volume 7 Num~ro 5 Septembre 1 984
utiliser avec un groupe frigorifique ~ absorption ou, soit un cycle Rankine, soit un g~n#rateur photovolta)'que ~ utiliser avec un r#frig#rateur ~ compression de vapeur. Les rendements des divers sous-syst~mes sont consid#r#s s#par#ment, puis en combinaison. Les d#finitions de tousles param#tres sont regroup#es clans le tableau 1, Les sources de consommation d#nergie parasite sont pr#sent#es aux figures 3 et 4. L "analyse des trois principaux syst#mes appliqu6s un jour de juin au Koweit pour un capteur de 1 m E de surface est pr#sent#e figures 5 et 6 en fonction des indices de refroidissement proposes.
Pc 'De Oc Qc Q.
~a, Vd (SCPI) ~, (SCPL),
generation (kWhe m -~ d -1) solar insolation on collector surface (kWhsola , m - 2 d-l) compressor input power (kWe) total electrical power consumption of standard system (kWe) rate of cooling production (kWc) daily cooling production (kWhc m -2 d -1) daily thermal energy collection (kWht, m -2 d-l) thermal loss efficient factor of collector (W m-2 oc -1 ) ~=5.3: for flat plate collectors using plate absorber ~= 2.0: for evacuated tube collectors standard system COP dry and wet sink (kWc/kW e or kWhc/kWhe) efficiency of energy conversion subsystem (kWhJkWh,o~ or kWhth/kWhsol) COP of absorption or desiccant chiller (dimensionless) Solar cooling performance indices, apparent and realistic (kWhc/kWhso~)
the solar cooling system may receive the whole or part of its energy needs from solar sources. The distinction stems from the fact that such cooling systems require two energy inputs which are not necessarily of identical forms. Energy needs for refrigeration can either be thermal or mechanical/electrical while the energy
0140-7007/84/050327-0653.00 © 1984 Butterworth Et Co (Publishers) Ltd and IIR
needed to circulate the thermal media (water and/or air) are normally of electrical/mechanical type. The latter, commonly termed the parasitic energy is of smaller magnitude but nevertheless critical in determining the overall usefulness of solar cooling systems 1-3. Solar cooling systems are categorized as thermal or electrical depending upon the type of energy input to the refrigeration cycle. Solar cooling systems in general can be viewed as combinations of energy conversion and refrigeration generation subsystems~. The energy conversion subsystem collects solar energy to power the refrigeration cycle. Th u s, it could be either thermal energy for vapou r absorption (or desiccant) applications or mechanical/ electrical energy produced by a Rankine cycle power generator (or photovoltaic) to drive a vapour compression refrigeration unit. Vapour compression cooling is the most widely used refrigeration method amongst miscellaneous known techniques for comfort cooling. Therefore, a cooling system comprising of conventional electricity-fed vapour compression
system has been chosen as standard 4. The electrical energy needs of such a standard system per unit of cooling effect, for a given set of operating parameters, forms the basis for comparing different solar cooling systems. Fig. 1 shows the energy conversion and the refrigeration generation subsystems in concept form, using both solar and the conventional sources of energy. The diagram indicates the electrical energy input for auxiliary equipment for the two solar based paths to produce cooling. The refrigeration subsystems are commercially proven and widely used and the solar energy conversion subsystems also enjoy well established operational credibility. ' One of the important criteria in assessing the potential usage of solar cooling systems is whether such systems can make a positive contribution in conserving fossil fuel. This can be done by first estimating the total electrical energy needed by the standard system to produce one unit of cooling output. Solar cooling systems could then be analysed and compared with the standard system to establish their electrical energy saving or generation capability. This paper outlines the basis for such an assessment.
Power plant ~'larfrlrnl
W EnergyconversiondlBlillb Refrigerotiongeneration /\/\
E, Fig. 1
Energy conversion and refrigeration generation subsystems
Fig. I Sous-syst~me de transformation de I'~nergie er de production de froid
. . . . [ ~ ~ ~ , ~ " F
1Coolingdelivered }'to.air_-hgndling jun'ts- 0¢
S t a n d a r d c o o l i n g s y s t e m and its e n e r g y needs The refrigeration generation subsystem of Fig. 2 shows the conventional vapour compression cooling system with two different energy rejection (sink) techniques. The electrical power needs for each component have been estimated by the authors in separate studies ~~. The energy expense of the airhandling unit which transports cooling to the conditioned space has not been indicated in the diagram since it does not form a part of refrigeration generation subsystem. It may be mentioned that for the wet sink case, equivalent electrical power (Pw) used for production of make-up water for the cooling tower has also to be considered as a part of the total power requirement. This is of relevance to Kuwait and other Gulf countries where fossil fuel based energy sources have to be used for producing soft water from seawater. Coefficient of performance (COP) of the system under steady state is determined as the ratio of cooling production rate (0o) to combined electrical power needs (P~). For the two sink cases of Fig. 2, the expressions for COP (s) are given below: For dry sink ~d :
= Oc/(Pc + Pchw+ PEP), kWc/kWe
For wet sink: I
/;w = (~c/(Pc + Pchw Jr- PEW+ P c w + Pw),
S o l a r c o o l i n g s y s t e m s - t y p e s and t h e i r energy analysis
Fig. 2 Standard cooling system;vapour compression Unitwith two alternate energy rejection choices Fig. 2 Syst#me frigorifique normal; groupe ~ compression de vapeur avec deux solutions possibles de rejet de I'#nergie
We have considered solar thermal systems, i.e. solar absorption cooling system and solar desiccant cooling system (see Fig. 3), and solar mechanical/electrical system, i.e. solar Rankine powered vapour compression cooling system and solar photovoltaic powered vapour compression cooling system (see Fig. 4). The diagrams also indicate the electrical power International Journal of Refrigeration
/P.. / ~r
P" 5, V -
~ _ .
_ ~ -
, ! !
11,12J,I Fig. 3 Solar Rankine and photovoltaic powered vapour compression cooling systems Fig. 3 Syst~mes frigorifiques ~ compression de vapeur solaires b cycle de Rankine ou ~ bnergie photovolta/que
needs of different components of the system. The performance assessment criteria are defined extending the earlier work 78, and the solar absorption cooling system of Fig. 3 is taken as an example to define different parameters. Definition of performance parameters (solar absorption cooling system): the energy conversion subsystem in this case consists of a collection of solar energy and its storage in thermal form at a constant temperature t h as shown in Fig. 3. If the cooling water input temperature to is constant, the chiller will produce chilled water at tch for supply to chilled water storage or directly to the air-handling units. In performing this task of converting solar thermal energy into cooling effect, parasitic energy is consumed by electric motors of pumps and fan besides equivalent energy needed for production of soft water from sea source as a cooling tower make-up. The capability of the solar cooling system to save electrical energy in comparison with an equivalent capacity vapour compression system can be determined by analysing the subsystems for a single day insolation and ambient data and basing the calculations for energy conversion and refrigeration on unit collector area. The unit collector area (1 m 2) has been taken as a standard for all systems. The cost factor will have to be separately considered if economics of energy conversion to cooling is of interest. The cost/economics does not form the subject matter for this paper. Various parameters which have to be evaluated and which define the subsystem and total system performance for such a generalized analysis are listed below: Converted energy (Ec): In the vapour absorption case, the converted energy is the total daily thermal energy collected (QH) at a fixed temperature th; given by: Volume 7 Number 5 September 1984
Eo=QH, kWh,h m -2 d -~
Efficiency of energy conversion subsystem (r/c): This parameter is the commonly used definition of daily average collector field efficiency, given by: (4)
where /d is the daily insolation in kWhso ~m -2 d -1. Cooling output (Qc): The daily cooling output Q c in kWhc m-2 d -~ is given by the absorption chiller design COP (va) at operating temperature of t h, t c and tch (see Fig. 3).
Electrical energy saving or generation (Es): The daily saving of electrical energy is estimated by evaluating the energy need of a standard system in producing the daily cooling output Qo. Using (1) or (2), the electrical energy saving can be evaluated in terms of ~a or sw depending upon the choice of heat rejection sink for the standard system. Since ~w>S~ for ambient conditions in Kuwait, the minimum saving in electrical energy will be given by: E,= (Oc/~w)-E,
where Ea is the daily electrical energy needs of the auxiliary equipment of the total system measured in kWh e m 2 d-~. Cooling indices of refrigeration generation subsystem (C,~ and C,r): These parameters define respectively the apparent and the realistic cooling output of the refrigeration subsystem for the energy input equal to QH. C,a, the apparent cooling index is defined as the ratio of cooling output to thermal energy input; given by C,==Oc/O.
Hot wafer tank
Energy conversion subsystem
Solar absorption and desiccant cooling systems
Cir, the realistic cooling index is defined as the ratio of the net equivalent cooling output to thermal energy input; given by: C,,= ( O c - ~ E ~ ) / O H
Solar cooling performance indices: Solar cooling performance index is the ratio of cooling output to solar energy input. Corresponding to the two cooling indices defined above, the two solar cooling performance indices are given by: (SCPI) a= apparent solar cooling performance index = Oc/Id = P/c"Cia ( S C P I ) , = realistic solar cooling performance index = (Oc-~wEa)/l~=~lcCi, (1 O)
Summary of results The definition of the parameters given in the previous section are summarized in Table 1. The table gives similar definitions for the solar desiccant cooling system and also for the other two electrical/mechanical systems (Rankine and photovoltaic). It should be noted that for the electrical/mechanical systems, the total daily converted energy is the difference between generator output Eg fed to the refrigeration generation subsystem and the total parasitic energy requirements 330
S ystbmes frigorifiques so/aires ~ absorption et b d#shydratan t
of the auxiliary equipment of energy conversion subsystem (Ea). The value of Eafor the photovoltaic system may only consist of losses in the power conditioning or the battery storage units; whereas its value for the Rankine case will include the power needs of pumps, motors and equivalent energy for water production for cooling tower (Pw)- Sources of parasitic energy consumption in the various systems are shown in Figs. 3 and 4, and defined in Table 1. Estimates of the energy needs of above sources have to be derived either from fundamentals or field experience.
Case study: comparative performance of absorption, Rankine and photovoltaic systems The analysis and definition of assessment parameters, defined earlier in the text, were applied for evaluating the performance of the three systems. The single day (in June) insolation and ambient temperature data in Kuwait 9 was used for evaluating the total energy collection and its conversion to cooling by the three systems. Since the efficiency of the energy conversion system keeps changing with the time of day, the analysis was done on an hourly basis using the thermodynamically derived analytical expressions. The Revue Internationale du Froid
T a b l e 1. D e f i n i t i o n
of solar cooling systems
Tableau 1. D ~ f i n i t i o n des paramdtres en vue de I'~valuation des syst#mes frigorifiques so/aires
Efficiency of energy conversion subsystem
(kWht~ or kWh~) m -2 d -~)
(Dimensionless or (kWh c ' kWhe/kWhso~) m -2 d -~)
(kWh~ m -2 d -~)
Absorption cooling system QH
Desiccant cooling system QH
Rankine powered vapour compression cooling system E g - E a
Ec(t,w or ~:d)
~ or 8d
Photovoltaic powered vapour compression cooling system E g - E a
Solar cooling systems
o r 8d)
Electrical energy saving/ generation
Overall system efficiency
Cooling index apparent
(Dimensionless or (kWhc/kWhe)
ew o r 8d
sw - Cooling output of standard system (wet sink) =Oc/(Pc+Pchw+PFw+Pcw+Pw); 6d-- cooling output of standard system (dry sink) = Qc/(Pc+Pchw+PFD); Ea - parasit!c power for Rankine (wet sink)=PR+PHc+PHM+PFw+Pcw+Pw; ~'a -- parasitic power for Rankine (dry sink)=PR+PHc+PHM+PFD; Ea - parasitic power for absorption system=PHc+PHM+Pchw+Pw+PFw+Pcw; va. Vd -- coefficients of performance of absorption chiller or desiccant cooler;/d -- solar insolation in kWhsoI m -2 d -1
SCPI)a SCPI)r for Cw=O.O00 SCPI)r for Cw =0.017 SCP/)rfor Cw = 0,034
OH in kWhth m-z d -I kWe/kW~
3olar energy = 7.64 kWh=o~. m-z d -I
c, = 0.ooo kW,/kW for Cw = 0Ol7 for C. : 0.034
Olor energy : 7.64 kWh=ol=r m-2 C
* Represents enerQy collection temperature in °C (thermal)
~Ooling production = Qc
= Zd[ (scm)= or (scm),]
1.0 I -
2.0~a---5.3 20---oP--5.3 Photovoltaic Vopourabsorption Ronkine (mechanical drive) Fig. 5 Cooling production capability of different solar cooling systems Fig. 5 Production de froid de diffdrents syst#mes frigorifiques solaires
Volume 7 Numero 5 Septembre 1984
2.0---a--5.3 Vopour absorption
?_0-- a --5.3 Ronkine
Fig. 6 Electrical energy saving/generation capability of different solar cooling systems
Fig. 6 Aptitude ~ dconomiser de I'dnergie dlectrique ou ~ en produire pour diffdrenrs syst#mes frigorifiques solaires
results, based on one square metre of collector area for daily cooling output and electricity saving are shown in Figs. 5 and 6.
Conclusions Solar cooling systems are used to generate cooling effect and simultaneously, save conventional energy. The systems should therefore be able to either save electricity w h e n compared with an equivalent cooling capacity standard system or else generate electrical/mechanical energy for operating a conventional cooling system. Criteria of cooling production and electrical energy saving proposed in the paper are a realistic and an engineering approach to evaluate solar cooling systems. The proposed realistic cooling index has a special significance for thermal systems (absorption and desiccant) as it defines the breakeven point; [ C , = ( S C P / ) r = O ] ; when the solar and the standard system consume the same amount of electrical energy in producing the cooling output. Such situations can easily be expected in a number of commercial or demonstration plants with degraded operation conditions. A similar situation can occur in electrical systems (Rankine) operating at low input thermal energy temperature conditions: [Ec = (SCPI)a = ( S C P I ) , = 0].
References 1 VanHaten, D., Dato, P. A. Description and performance of an active solarcooling systemusing a LiBr H20 absorption machine Energy and Buildings 3 (1981 ) 169-196 2 Bilgen, E. Solar power refrigeration in A. E. Dixon and T. D. Leslie (Eds), Solar energy conversion, Pergamon Press (1979) 1223-1243 3 Suri, R. K., Ayyash, S. Solar absorption cooling effect of operational parameterson power saving /nt J Refrig 5 (1982) 274 279 4 Ayyash, S., Suri, R. K., Maheshwari, (3. P. An approach to solar cooling assessmentstudy. KISR 1025, Kuwait Institute for Scientific Research, Kuwait (1983) 5 Suri, R. K., Ayyash, S., Maheshwari, G, P. Energyanalysisof solar cooling systems. KISR 1146, Kuwait Institute for Scientific Research, Kuwait (1983) 6 Suri, R. K., Ayyash, S., Maheshwari, G. P. Effect of parasitic power losseson the performance of organic Rankinecycle plants /nt J Ambient Energy (accepted for publication) 7 Curran, H. M. Coefficient of performance of solar powered space cooling systemsSolar Energy 19 (1977) 601-603 8 Barber, R. E. Solar Rankine air conditioning systemsAshrae Transactions 85 2 (1979) 9 Ayyash, S., Suri, R. K., Salman, M., Maheshwari, (3. P.
Performance deteriorating of solar absorption cooling systems due to conventional energy inputs Proc Solar World Congress Perth, Australia (1983)
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