Assessment criteria for solar cooling systems

S. Ayyash, R. K. Suri and G. P. Maheshwari

Keywords: re f r igerat ion , solar cool ing, energy 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

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 con- sid#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.

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

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.

N o m e n c l a t u r e

Cia, Cif

Cw

Eo

Ec

.E0

E~

/d apparent and realistic cooling indices of refrigeration generation subsystem Pc (dimensionless or kWhc/kWhe) 'De parasitic constant for water consumption in cooling tower in terms Oc of electrical power needed per unit Qc heat rejection (kWe/kWt,) Q. 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 co- generation power-water production power Sd, plants daily electrical energy consumption by r/c 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

~a, Vd

(SCPI) ~, (SCPL),

generation (kWhe m -~ d -1) solar insolation on collector surface (kWhso la , 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~)

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 Institute for Scientific Research, PO Box 24885, Safat, Kuwait. Paper received 26 July 1983.

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 Volume 7 Num~ro 5 Septembre 1 984 327

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 deter- mining 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 sub- system 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

Power plant ~'larfrlrnl F'~mr~r~c,~r (~.nnlinn

W Energy conversion dlBlil lb Refrigerotion generation / \ / \ subsystem ~'-!'-~ subsystem

E, E~

Fig. 1 Energy conversion and refrigeration generation subsystems

Fig. I Sous-syst~me de transformation de I'~nergie er de pro- duction de froid

. . . . ~ 1Cooling delivered [ }'to. air_-hgndling

~ ~ ~ , ~ " Qw jun'ts - 0¢

F Evopomtor I

I Dry sink Wet sink J

Fig. 2 Standard cooling system;vapour compression Unitwith two alternate energy rejection choices

Fig. 2 Syst#me frigorif ique normal; groupe ~ compression de vapeur avec deux solutions possibles de rejet de I'#nergie

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 estab- lished 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.

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 air- handling unit which transports cooling to the con- ditioned 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/Pe = Oc/(Pc + Pchw + PEP), kWc/kWe (1)

For wet sink:

/;w = (~c/(Pc + Pchw Jr- PEW + Pcw+ Pw), kWc/kW e (2)

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 e n e r g y a n a l y s i s

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

328 International Journal of Refrigeration

/ P . . P" 5, I \ ~ - _ ~ I / ~r V - O,ysi.kl \ _ . -

'P Wet slnk A / P~L, I

.ow ' _ ^ ' - -

conditioner ] ~ . , A

! ! pcw

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:

Eo=QH, kWh,h m -2 d -~ (3)

Efficiency of energy conversion subsystem (r/c): This parameter is the commonly used definition of daily average collector field efficiency, given by:

r/c = OH/Id (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).

Oo=Ec va~- O.v, (5) 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, (6)

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 sub- system 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. = V a (7)

Volume 7 Number 5 September 1984 329

Hot wafer tank

Energy conversion subsystem

(~r'Ablorptlon

k ~c

S ;:i "rtmpemlure

Psychr0rnetric chort

. , r ; . . . .

II II I-o=1 Chiller A i .i....-- __

A Pc,. p..

F~fri~mHon ~ subsystem

Fig. 4 Solar absorption and desiccant cooling systems Fig. 4 S ystbmes frigorifiques so/aires ~ absorption et b d#shydratan t

Cir, the realistic cooling index is defined as the ratio of the net equivalent cooling output to thermal energy input; given by:

C,, = (Oc-~E~)/OH (8)

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

(SCPI ) ,= 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

of the auxiliary equipment of energy conversion sub- system (Ea). The value of E a for 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 tempera- ture 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

330 Revue Internationale du Froid

T a b l e 1. D e f i n i t i o n o f p a r a m e t e r s f o r a s s e s s m e n t o f s o l a r c o o l i n g s y s t e m s

Tableau 1. D~f in i t ion des paramdtres en vue de I '~valuat ion des syst#mes f r igor i f iques so/aires

Efficiency Electrical of energy energy

Converted conversion Cooling saving/ Parameters energy subsystem output generation apparent realistic apparent realistic

(kWht~ or (Dimensionless kWh~) or (kWh c ' (kWh~ (Dimensionless or

Solar m -2 d -~) kWhe/kWhso~) m -2 d -~) m -2 d -~) (kWhc/kWhe) (kWhc/kWhsoi) cool ing systems E c r/c Qc Es Cia Cir (SCPI)a (SCPI)r

Overall system Cooling index eff iciency

Absorpt ion cool ing system QH QH/Id Ecva Qc/~ -Ea Va

Desiccant cool ing system QH QH/1~ Ecv d Qc/~ -Ea Vd

Rankine powered vapour compression cool ing system Eg-E a Ec//d Ec(t, w or ~:d) Ec ~ or 8d

Photovoltaic powered vapour compression cool ing system Eg-E a Ec/I d Ec(,S w or 8d) Ec Sw or 8d

(Qc- ~Ea)ION rlcCia ~fcCir

( Q c - ~ a ) / Q H t~cCia ~cCir

or Sd qcCia rlcCir

e w o r 8 d ~cCia ~cCir

s w - 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; E a - parasitic power for absorption system=PHc+PHM+Pchw+Pw+PFw+Pcw; v a. Vd -- coefficients of performance of absorption chiller or desiccant cooler;/d -- solar insolation in kWhso I m -2 d -1

j 3O

20

§

Fig. 5

F-

i

i J

r-

I

I i

2.0~a---5.3 Photovoltaic Vopour absorption

~ SCPI )a

SCPI) r for Cw=O.O00 kWe/kW ~

SCPI) r for C w =0.017

SCP/)r for C w = 0,034

3olar energy = 7.64 kWh=o ~. m -z d -I

=Zd ~Ooling production = Qc

= Zd [ (scm)= or (scm),]

20---oP--5.3 Ronkine (mechanical drive)

Cooling production capability of different solar cooling systems Fig. 5 Production de froid de diffdrents syst#mes frigorifiques solaires

0.8 %

.¢ 0.6

I! =o ==

0.4

_g w

0.2

OH in kWhth m -z d -I

c, = 0.ooo kW,/kW for C w = 0Ol7

for C. : 0.034

Olor energy : 7.64 kWh=ol= r m -2 C

:Zd * Represents enerQy collection temperature in °C (thermal)

Photovoltaic 2.0---a--5.3

Vopour absorption

i

?_0-- a --5.3 Ronkine

3.0

2.0 -9

o E

1.0 I -

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

Vo lume 7 Numero 5 Septembre 1984 331

results, based on one square metre of col lector area for daily cooling output and electricity saving are shown in Figs. 5 and 6.

Conclusions

Solar cool ing systems are used to generate cooling effect and simultaneously, save conventional energy. The systems should therefore be able to either save electricity when compared wi th an equivalent cooling capacity standard system or else generate electr ical/mechanical energy for operating a conven- tional cool ing system. Criteria of cooling production and electrical energy saving proposed in the paper are a realistic and an engineering approach to evaluate solar cool ing systems. The proposed realistic cool ing index has a special signif icance for thermal systems (absorp- tion and desiccant) as it defines the breakeven point; [C ,= (SCP/ ) r=O] ; when the solar and the standard system consume the same amount of electrical energy in producing the cool ing output. Such situations can easily be expected in a number of commercial or demonstration plants wi th degraded operation condit ions. A similar situation can occur in electrical systems (Rankine) operating at low input thermal energy temperature condit ions:

[Ec = (SCPI)a = (SCPI ) ,= 0].

References

1 VanHaten, D., Dato, P. A. Description and performance of an active solar cooling system using 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 parameters on power saving /nt J Refrig 5 (1982) 274 279

4 Ayyash, S., Suri, R. K., Maheshwari , (3. P. An approach to solar cooling assessment study. KISR 1025, Kuwait Institute for Scientific Research, Kuwait (1983)

5 Suri, R. K., Ayyash, S., Maheshwari, G, P. Energy analysis of 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 losses on the performance of organic Rankine cycle plants /nt J Ambient Energy (accepted for publication)

7 Curran, H. M. Coefficient of performance of solar powered space cooling systems Solar Energy 19 (1977) 601-603

8 Barber, R. E. Solar Rankine air conditioning systems Ashrae 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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