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Solar refrigeration for rural applications
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Pergamon
Renewable Energy, Vol. 12, No. 2, pp. 157-167, 1997 ~ 1997 Published by Elsevier Science Ltd All rights reserved. Printed in Great Britain
PIh S0960-1481(97)00036-0
SOLAR REFRIGERATION
0960-1481/97$17.00+0.00
FOR RURAL
APPLICATIONS S. O. ENIBE National Centre for Energy Research and Development, University of Nigeria, Nsukka, Nigeria
(Received 12 December 1994; accepted 24 April 1997) Abstract--A large proportion of people in developing countries live in rural or remote locations where grid electricity is presently unavailable and is not envisaged in the foreseeable future. Since conventional, electrically powered vapour compression refrigeration systems may not be of much use in such areas, for essential applications such as food and drug preservation, alternative refrigeration systems are required. Such systems are presented and discussed in this paper. These include photovoltaic (PV) powered vapour compression systems ; continuous and intermittent liquid or solid absorption systems; and adsorption systems. Typical application examples are drawn from recent experiences worldwide. Technical and financial constraints which limit their widespread application are reviewed, and strategies for overcoming them are discussed. © 1997 Published by Elsevier Science Ltd.
1. NEED FOR SOLAR REFRIGERATION The need for refrigeration and air conditioning has long been recognised, and many systems have been developed commercially to provide them, the most commonly used being the vapour compression system. This is well described in basic textbooks of thermodynamics [1, 2]. The coefficient of performance of a vapour compression refrigeration cycle, COP, is usually expressed as Energy removed at the evaporator COP = Energy supplied at the compressor'
(1)
The maximum possible COP is that of a Carnot cycle, and is given as COP .....
t
Tev -
Ted -- Tev
(2)
where Tev = evaporator temperature, and Too = condenser temperature. Practical vapour compression systems have COPs in the range 75-85% that of (COP) ..... t. Vapour compression refrigeration systems require mechanical power for driving the compressor, which is usually provided electrically. In areas where electrical energy is available, this refrigeration system is usually adequate to satisfy most refrigeration require157
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ments. There are several areas, however, where grid electricity is not available at the moment, and is unlikely to be available in the next few decades due to the huge financial outlays involved. These include villages and rural areas in developing countries, mountainous and island communities, and other remote locations. Cooling in such areas, therefore, requires alternative refrigeration systems, which, fortunately, are available. These include photovoltaic-powered vapour-compression systems, absorption systems, adsorption systems, desiccant cooling systems, and passive cooling systems. A schematic diagram of a solar refrigeration system, adapted from Garg [3], is shown in Fig. 1. It consists essentially of five sub-systems for solar energy collection, heat storage, solar cooling, heat rejection and cold storage. Solar energy is attractive as an energy resource for cooling in that it is inexhaustible and is most abundant in areas where the need for cooling is greatest. In addition, though it is diffuse and periodic, the peak of hourly solar radiation intensity usually occurs in symphony with the peak cooling demand. Further, since solar radiation is a free natural resource, the running costs of well-developed solar cooling systems can be expected to be low once the initial capital costs for their installation have been met. For this reason, a lot of work has been done on the development of solar cooling systems, some of which are described below. 2. SOLAR REFRIGERATION
2.1. PV, vapour-eompression refrigeration With the exception of the energy-supply sub-system, photovoltaic (PV) powered refrigeration is similar in most respects to any other vapour compression refrigeration system. The energy supply in the system is provided by an array of solar cells which convert the incident solar radiation to electricity. The latter is used to drive the compressor and the excess charges a battery system for use at periods of low or no insolation. Between 1981 and 1986, over 800 PV-powered refrigerators were reportedly built and adapted specifically for the
Solar Collector
Heat ~ Storage Device
Solar Cooling Device
Cold Storage Device
tSRejection/ Heat I y~em~ Fig. l. Schematic diagram ofasolarreffigerationsystem.
1
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storage of medicines and vaccines, mainly for demonstration purposes [4]. Most of these were sponsored by the World Health Organisation (WHO), U.S. Agency for International Development (USAID), and the European Community (EC), and were motivated by the need to maintain a sustainable cold chain for the WHO's Expanded Programme on Immunisation (EPI). Derrick and Durand [4] reported that the percentage operational time obtained for nine installations fully tested in Mali varied between 27 and 79%, the average mean time between failures varied between 4 and 20 months, and the average system reliability was about 59%. The major problem with PV-powered refrigeration is the system cost. While solar cell efficiencies have risen in recent times from about 7 to 13% [5], and the unit costs have fallen [6], the overall system cost for any moderately sized unit remains several times above the costs of ordinary vapour-compression refrigerators. The utilisation of PV-powered refrigeration on a larger scale will therefore await further increases in cell efficiencies with a fall in their prices and rises in the costs of conventional energy sources. 2.2. Absorption refrigeration Absorption refrigeration is based on the principle that some absorbents can absorb large quantities of refrigerants which can be regenerated upon the application of heat. The refrigerant absorption process is exothermic, while the regeneration process is endothermic. The heat energy required for the regeneration process may be supplied from any suitable heat sources, such as waste heat from a heat engine or process plant, heat from a compression system, biofuels, or solar energy. There are two types of absorbents, namely liquid and solid absorbents, and refrigerating machines have been built using each type. Table 1 gives a list of absorbenbrefrigerant pairs that have been utilised or proposed for solar refrigeration. Figure 2 shows the basic schematic diagram of an intermittent absorption refrigerator. It operates in two modes, namely the absorption and generation modes. During the absorption mode, the vessel A acts as the absorber, and absorbs the refrigerant released from the evaporator, B. Heat Ql is absorbed in the evaporator and heat Q2 is rejected at the absorber. During the generation mode, the vessel A, now acts as the generator, while vessel B acts as the condenser. Heat Q4 is supplied to the generator from a suitable heat source, while heat Q3 is rejected at the condenser. The valve C is used to control the direction of flow of the refrigerant. Thus, vessels A and B double as the absorber/generator and condenser/evaporator respectively. Vessel A contains the liquid or solid absorbent prepared in a suitable form. For absorption systems, the coefficient of performance, COP, is defined as the ratio of the energy intake at the evaporator, Q1, to the Table 1. Refrigerant absorbent combinations Refrigerant
Liquid absorbents
H20 NH3
LiBr, LiC1, LiSCN, CsF, RbF or their multiple salt solutions [7] CaC12 [7, 28] H20 [12] NaSCN [7] SrClz [7] LiNO3 [7] NaC1 [17]
Note : Numbers in brackets indicate references.
Solid absorbents
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C
j A b so rber
Q Evaporator
Q3 Q4
C,o n denser Generator
Fig. 2. Schematicdiagram of an absorption refrigerator. energy input at the generator, Q4. Thus, COP = QI/Q4. The actual implementation of absorption refrigeration varies depending on the refrigerant-absorbent pair under consideration. In general, for liquid systems, the absorbent and refrigerant may both move through the system, while for solid absorbents, it is only the refrigerant that moves. A comparison of the performance of solid and liquid absorption systems using the absorbent-refrigerant pairs of CaC12/NH3, SrCI2/NH3 and LiNO3/NH3, H 2 0 / N H 3 is reported in [7]. For similar operating conditions, it is indicated that the solid absorption systems perform better than the liquid absorption systems, with the CaC12 as absorbent being slightly better (and cheaper) than SrC12. In the case of the liquid systems, LiNO3 gave a superior performance, followed by H20. Liquid absorption systems can be adapted to intermittent or continuous operation, but the solid absorption systems can only operate intermittently, since the solid absorbent cannot move through the system. Liquid absorption systems may require a small pump for circulating the absorbent solution and a rectifier to prevent droplets of the liquid absorbent from entering the condenser, but a solid absorption system does not require any of these. It can therefore operate quietly without maintenance over some period of time. 2.2.1. Liquid absorption systems. Following the pioneer work of Trombe and Foex [8] who built and tested a H20/NH3 solar refrigerator, many reports on the performance of solid and liquid absorption systems have appeared in the literature. Chinapa [9] constructed an intermittent HzO/NH3 plant with three glazing covers which produced 1.43 kg of ice per m 2 per day. Other intermittent systems based on H20/NH3 and NaSCN/NH3 are reported by Swartman [10], and these gave coefficients of performance in the range 0.054).14. A continuous H20/NH3 plant which used two pumps for circulating the absorbing solution and chilled water is reported by Farber [l 1]. The system was single glazed with 1.49 m 2 collector area, and gave a coefficient of performance of 0.1 and an ice production capacity of 12.5 kg m -2 per day. An intermittent ammonia-water system was also built by Staicovici [12] in Romania. The system COP varied between 0.09 and 0.152. Figure 3 shows the schematic diagram of the intermittent aqua-ammonia liquid absorption refrigeration system due to Excell [14], which is typical of intermittent liquid absorption systems.
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B
A
Evaporator ~=~y
/
Condenser
Receiver
1
@
c
_
Fig. 3. Schematic diagram of an intermittent aqua-ammonia refrigerator. During the day, ammonia is vaporised from the combined collector/absorber/generator with valve A open and valves B and C closed. The ammonia is condensed and stored in a water cooled receiver. At night, the collector/generator is allowed to cool, valve A is closed and valves B and C are opened to enable the liquid ammonia to evaporate, thereby producing the refrigerating effect. The ammonia vapour goes through the lower header into the collector where it is reabsorbed. The cycle is completed once per day. A summary of the performance of liquid absorption systems is given in Table 2. 2.2.2. Solid absorption systems. In comparison with the liquid absorption systems, relatively fewer solid absorption plants have been reported. Plank's [15] CaC12/NH3 system used 100"C hot water as heat source, had a cooling capacity of 3768-4187 k J/cycle, and gave 3 cycles per day. Andrews" [16] CaC16/NH 3 plant used 100°C steam as heat source, and operated commercially on refrigerated food transport vans on London Liverpool trains during the Second World War. Muradov and Shadiev [17, 18] built a NaCI2/NH3 system which produced 1 kg of ice per m 2 per day, while Eggers-Lura et al. [19] designed a plant capable of producing 4.6 kg of ice per m 2 per day with CaCI2 or SrCI 2 as absorbent Table 2. Performance of liquid absorption systems Designer and reference Trombe and Foex [8] Chinappa [9] Swartman [10] Farber [11] Staicovici [12] Sloetjes et al. [13]
Refrigerant Absorbent NH3
H20
NH3 NH3 NH3 NH 3 NH3 NH3
H20 H20 NaSCN H20 H20 H20
COP range (kg/m2-day)
Ice production capacity (kJ/m2-day) Comments Intermittent 1.43
0.05-0.14 0.05-0.14 0.1 0.09-0.152
12.5
Intermittent Intermittent Intermittent Continous Intermittent Continous
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and NH3 as refrigerant. Iloeje [7] built an intermittent CaC12/NH3 system with a double glazed collector of 1.41 m 2 surface area. This gave a cooling capacity of 714 kJ/m 2 with an effective ice production of 1.5 kg m -2 per day. For solar operation, the first reported investigation of the system is by Nielson et al. [21]. The unit, which was tested in the Sudan, had a total cooling capacity of 2 MJ/m 2 of the collector surface, and a useful average COP of about 0.096. A field unit was subsequently built and operated autonomously in the Sudan for 6 years [19]. Iloeje [7] built and tested an experimental system at Nsukka, Nigeria in 1983. Figure 4 shows the schematic diagram of the CaC12/NH3 refrigeration system due to Iloeje [7], which is typical of solid absorption systems. Operation of solid absorption systems is necessarily intermittent, since the solid absorbents cannot move within the system. One cycle of operation is completed in a 24-h day. During the day, valve A is open and valves B and C are closed. Ammonia, generated from the collector/absorber/generator, is condensed and stored in the liquid receiver. Net ammonia generation ceases in the evening as solar intensity falls. The collector is then covered with an opaque screen to start the collector cool down phase. The generator/absorber pressure falls below the evaporation valve of about 3 bar, making possible the evaporation/reabsorption phase. The screen is removed, valve A is closed and valves B and C opened to start the ammonia evaporation and night-time exothermic reabsorption phase. The cycle is completed before morning and the cold produced is stored as ice. Table 3 gives a comparison of the performance of solid absorption systems which appeared recently. 2.3. Adsorption refriyeration Many solar refrigeration systems have been proposed based on the principle of adsorption. Adsorption differs from absorption in that while the latter is a bulk phenomenon, adsorption is essentially a surface phenomenon, and is similar in many respects to all those phenomena which depend upon the properties of surfaces, such as electron emission, bleaching with charcoal, catalysis, chromatographic analysis, etc. The theory of adsorption has been extensively discussed in the literature, and a brief account may be found in Levenspiel [23]. In essence, during adsorption, a molecular species of a fluid is attached to the surface of a solid, resulting in an increased concentration of the substance at the interface. G o o d adsorbents for refrigerants include zeolites 5A and 13X, as well as the activated charcoals, some of which may be made from coconut shells. The organic refrigerants, and in some cases ammonia, have been preferred in adsorption systems. The organic
,
A
®
Condenser ~Th Receiver otfle Valve
i
[
I
Evaporator
Fig. 4. Schematic diagram of the CaC12/NH3 refrigerator.
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Table 3. Performance of solid absorption systems Designer and reference
Refrigerant Absorbent
COP range
Ice production Cooling capacity capacity (kg/m2-day) (kJ/mZ-day)
Plank [15]* Andrews [16]:~ Muradov and Shadiev [17, 18] Eggers-Lura et al. [19]
NH3 NH3 NH3
CaC12 CaCI2 NaCI
11,9oo? 1
NH3
CaC12orSrCl2
4.6
Iloeje [7] Nielson et al. [21]
NH3 NH3
SrCI2 CaCI2 CaCl2
0.008 0.053 0.096
1.5
714 2000
* Used 100°C steam as heat source. ?MJ day 1 Used 100rC hot water as heat source.
refrigerants have the advantage of being compatible with copper tubing systems, low cost and low operating pressures. Of the various adsorbent/refrigerant pairs tested by Critoph [24], R22 with activated charcoal appeared to be the best, with a maximum COP estimated at about half that of the ammonia/water system. More recently, however, the same author [25] built an activated carbon refrigerator using ammonia as refrigerant. The collector has an area of 1.4 m 2 and contained 17 kg of active carbon. Under simulated radiation conditions in the laboratory, the unit gave an equivalent COP of 0.05-0.09, with a cooling capacity of511-1007 kJ/m2-day. Ethanol has also been investigated as a possible refrigerant, with activated charcoal as the adsorbent [26]. 3. P R O B L E M S A N D P R O S P E C T S
Up to the present time, two major problems have limited the emergence of solar cooling technologies in the market to any significant extent. These are mainly technical and economic, and are considered below. 3.1. Technical p r o b l e m s
Each solar cooling technology has its own unique technical problems. For example, in the case of the liquid absorption systems, the need for a solution pump (in continuous systems) and a rectifier for the vapour leaving the generator introduce unpleasant complications in the system design, and also result in performance reductions. For the solid absorption systems, the major problems are the expansion of the absorbent upon the absorption of the refrigerant and its low thermal conductivity. Adsorption systems also share this limitation. The reduction of the absorbent expansion has been considered by many investigators. For example, in the case of the CaC12/NH3 system, upon the absorption of ammonia, the anhydrous calcium chloride swells up, cracks, splits in all directions and at last falls to a white powder. The initial absorption is rapid, and slows down towards the end, the product of the action occupying at least 3 times the original volume of the solid [27, 28].
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To avoid these difficulties, a number of treatments have been proposed to pelletize the absorbent into hard porous granules which can withstand multiple cycling of the absorption/generation processes. A number of these treatments have been reviewed recently by Iloeje [28]. In general, the treatments involve mixing the absorbent with a cementatious material or binder, processing the mixture into a porous matrix, and pelletizing the matrix into a suitable size range. Some of the binders reported in the literature include LiNO3, NaSiO3, portland cement and CaSO4, but the latter mixed in the proportion of 80% CaC12 + 20% CaSO4 was found to yield the best results when both the strength and ammonia absorption capacity of the resultant pellet were taken into account [29]. The recommended procedure for the absorbent preparation involves grinding the available CaCI~ and CaSO4 granules separately to a fine powder, mixing in the ratio of 4 : 1 by mass, adding water and mixing to a thick paste. The latter is then heated at 90°C for 24 h, 90-150°C for another 24 h, and finally at 200-250r'C until fully dried. This produces a porous matrix which is then broken down to the required size range. This treatment appears effective since no detectable reduction in the absorption/generation characteristics of the absorbent occurred after undergoing 40 successful refrigeration cycles during test operations over a 1 yr period [32]. Meunier [30] attributes another successful approach to the absorbent stabilization to Spinner [31], which involves the use of a graphite binder. The other major technical problem with solid adsorption and absorption systems, namely the low thermal conductivity of the absorbent (or adsorbent), has not been adequately solved. Iloeje [33] mixed the absorbent with aluminium filings to improve its thermal conductivity, but this did not yield successful results. Moreover, the aluminium appeared to react with the absorbent in some way. Another attempt in the same direction by Enibe [34] to suspend the absorbent in an inert fluid did not yield successful results. A possible solution to the problem has been proposed recently by the Critoph group [35] who prepared the adsorbent (active carbon in this case) as flat, porous, thin discs packed in a tube with interior fins. If this solution is effective, it will also apply to solid absorption systems, and will introduce significant improvements in the performance of solar refrigeration systems. 3.2. Economic problems The major economic problems militating against the widespread utilisation of solar cooling technologies are the costs of the units, which, in most cases, are several times those of comparable units using conventional cooling technology. For example, Clerx and Trezek [36] indicate that the price of their aqua-ammonia solar assisted refrigerator was 8-10 times more than that of a comparable conventional unit. Similarly, the cost of the 13 kW peak cooling power, 50 m 2 capacity aqua-ammonia system of Sloetjes et al. [13] proved to be far too expensive for the intended application in the Sudan (the total system cost was estimated at U.S.$ 95,000 at 1987 prices). For this reason, several companies which produced and marketed solar powered refrigerators have been forced to stop production because of purely economic reasons. A typical example is Sun-Ice of Denmark which produced CaC12 refrigerators which were technically successful but expensive. The company has since ceased production. For the same reason, BLM of France [25] produced a range of refrigerators using active carbon and methanol, but has now ceased production. 3.3. Prospects In spite of the foregoing problems limiting the widespread application of solar cooling technologies, solar refrigeration and air-conditioning can be expected to contribute sig-
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nificantly to the welfare of future generations. This expectation is based on the fact that more technical progress is being made in the field which, when translated to commercial success, may bring down the price of the units considerably. For example, recent simulation studies on the CaCIz/NH3 system at Nsukka, Nigeria [37-42] have shown that the system cooling capacity can be doubled with the use of high efficiency collectors. In addition, it is expected that energy prices may rise in future, thereby making solar and renewable energy technologies competitive with the conventional systems. 4. CONCLUSIONS It may be seen from the foregoing that solar refrigeration can be expected to play a significant role in meeting the needs of people in the rural areas of developing countries for refrigeration. Many solar cooling technologies have proved to be technically feasible, and have scopes for further improvements. The improvements required are mainly in the areas of increasing system performance and lowering costs. With probable increases in the costs of conventional energy sources, it is expected that solar cooling technologies will become competitive with the conventional systems in future.
Acknowledgements--The author acknowledges, with gratitude, the financial sponsorship provided, at various times, by the International Foundation for Science, Stockholm, Sweden through grant G/1503-1 ; the Energy Commission of Nigeria, Lagos, Nigeria; and the National Centre for Energy Research and Development, University of Nigeria, Nsukka, Nigeria. The author is also grateful to Professor O. C. Iloeje of the Department of Mechanical Engineering, University of Nigeria, Nsukka, Nigeria for several helpful comments and encouragement.
1. 2. 3. 4.
5. 6. 7. 8.
REFERENCES Rogers, G. F. C. and Mayhew, Y. R., Engineering Thermodynamics, Work and Heat Transfer, (SI Units). Longmans, London (1973). Eastop, T. D. and McConkey, A., Applied Thermodynamics for Enyineering Technologists, 3rd Ed. Longman, London (1978). Garg, H. P., Solar cooling technologies and techniques, paper No. SMR/4000. Workshop on the Interaction between Physics and Architecture in Environment Conscious Design, International Centre for Theoretical Physics, Trieste, Italy, 25-29 Sept. 1989. Derrick, A. and Durand, J. M., Photovoltaic refrigerators for rural health care-experiences and conclusions. Conf. Proc. C44, Solar Energy for Developing Countries : Power for Villages, U.K.-section of the International Solar Energy Society, 15 May 1986. Schmidt, W., Recent progress in crystalline silicon solar cells, paper No H4-SMR/21510. Workshop on Material Science and the Physics of Non-conventional Energy Sources, International Centre for Theoretical Physics, Trieste, Italy, 1987. Start, M. R., Rural electrification--solar versus grid extention-updating the economics. Conf. Proc. C44, Solar Energy for Developing Countries: Power for Villages, U.K.section of the Internal Solar Energy Society, 15 May 1986. Iloeje, O. C., Design, construction and test run of a CaC12 solar refrigerator, Solar Energy 35, 447-455 (1985). Trombe, T. and Foex, M., The production of cold by means of solar radiation, Solar Energy 1(1), 51 (1957).
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9. Chinappa, J. V. C., Performance of an intermittent refrigerator operated by solar energy, Solar Energy 6(4), 143 (1962). 10. Swartman, R. K., Ha, V. and Swaminathean, C., Comparison of ammonia-water, and ammonia-sodium thiocyanate in a solar refrigeration systems, Solar Energy 12, 123127 (1975). 11. Farber, E. A., Design and performance of a compact refrigeration system, Engineering Progress at the University of Florida xxiv(2), 17 (1970). 12. Staicovici, M. D., An autonomous solar ammonia-water refrigeration system, Solar Energy 36(2), 115 124 (1986). 13. Sloetjes, W., Haverhals, J., Kerdijk, K., Ahmed, I. O., Saber, H., Eldin, S. S., Persius, R., Stolk, A. and Hassan, H. W., Operational results of the 13 kW/50 m 3 solar driven cold store in Khartoum, the Sudan, Solar Energy 41(4), 341-347 (1988). 14. Excel, R. H. B., Refrigeration and airconditioning. Proc. Solar Energy Applications in the Tropics, B.B.P.L.M. (Ed) D. Reidel Publishing Company, Holland (1983). 15. Plank, R., Sorptions-kaltemaschinen. Handbuch der Caltetechinite, Vol. 7, p. 239. Springer Verlag, Berlin (1959) (in German). 16. Andrews, H. I., A solid absorption machine for refrigerated railway vehicles. 8th Int. Congr. of Refrigeration, pp. 663 665, London, (1951). 17. Muradov, D. and Shadiev, O., Experimental investigation of the operation of an intermittent solar refrigerator. Proc. All. Union Conf. on Utilization of Solar Energy. VNITT, Moscow (1969) (in Russian) (as reported in 7). 18. Muradov, D. and Shadiev, O., Intermittent solar refrigerator with solid absorbent. Problems of Natural Sciences. Izd. Tash. G.V.TI.B.G.P.I., Tash (1969) (in Russian) (as reported in 7). 19. Eggers-Lura, A., Bechtoft Nielson, P., Stubkier, Bo and Worsoe-Schmidt, P., Potential use of solar-powered refrigeration by an intermittent solid absorption system. Comples International Meeting, HELIOTECHNIQUE AND DEVELOPMENT, University of Petroleum and Minerals, Dharan, Saudi Arabia, 2-6 Nov. 1975 (as reported in 7). 20. Blytas, G. C. and Daniels, F., Concentrated solutions of NaSCN in liquid ammonia: solubility, density, vapour pressure, viscosity, thermal conductance, heat of solution and heat capacity, Journal of the American Chemical Society 84, 1075 (1962) (as reported in 10). 21. Nielson, P. B. and Worsoe-Schmidt, P., Development of solar-powered solid absorption refrigeration s y s t e ~ P a r t 1 : Experimental investigation of the generation and absorption processes, Report No. F30-77, The Technical University of Denmark, Refrigeration Laboratory (1977). 22. Duffle, J. A. and Beckman, W. A, Solar Engineering of Thermal Processes. Wiley, New York (1980). 23. Levenspiel, O., Chemical Reaction Engineering, 2nd Edn. Wiley, New York (1972). 24. Critoph, R. E. and Vogel, R., Possible Absorption Pairs for Use in Solar Cooling, Department of Engineering, University of Warwick, Coventry, England (1986). 25. Critoph, R. E., An ammonia carbon solar refrigerator for vaccine cooling, Renewable Energy 5(Part I), 502-508 (1994). 26. Kimambo, C. Z. M., Ethanol-activated carbon adsorption solar refrigeration, M. Sc. Dissertation, Department of Engineering, University of Reading, Reading (1986). 27. Mellor, J. W. (Ed.), A Comprehensive Treatise on Inorganic and Theoretical Chemistry, Vol. III (Cu, Ag, Ca, Sr, Ba), p. 716. Longmans, London (1960). 28. Iloeje, O. C., Treatments of calcium chloride for use as a solid absorber in solar powered refrigeration. Technical Report No. SE-2, Solar Energy Project, University of Nigeria, Nsukka, (October 1982).
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29. Iloeje, O. C., Quantitative comparisons of treated calcium chloride pellets for solar refrigeration, Solar Energy 37(4), 253 260 (1986). 30. Meunier, F., Sorption solar cooling, Renewable Energy 5(Part I), 422~429 (1994). 31. Spinner, B., Ammonia-based thermochemical transformers, J. Heat Rec. Systems 301308 (1993). 32. Iloeje, O. C., Extended performance tests of a solid absorption solar powered refrigerator, Technical Report No. SE-3, Solar Energy Project, University of Nigeria, Nsukka, Nigeria (April 1984). 33. Iloeje, O, C., Thermophysical properties of Nsukkanut, Nigerian J. Solar Energy 8, 319 335 (1989). 34. Enibe, S. O., Solid absorbent dispersion in an inert fluid for solar refrigeration applications, Solar & Wind Techn. 7(5), 591-595 (1990). 35. Munyebvu, E., Heat transfer in monolithic charcoals for use in adsorption refrigeration systems, M. Sc. Dissertation, Department of Engineering, University of Warwick, Coventry (1994). 36. Clerx, M, and Trezek, G. J., Performance of an aqua-ammonia absorption refrigerator at sub-freezing evaporator conditions, Solar Energy 5, 379 389 (1987). 37. Iloeje, O. C., Ndili, A. N. and Enibe, S. O., Computer simulation of a CaCI2 solid absorption solar refrigerator, Proceedings of the 1st Worm Renewable Energy Congress, Vol. 2, pp. 1159-1168, Reading, England, (1990). 38. Iloeje, O. C. and Enibe, S. O., Computer prediction of the seasonal performance of a CaC12-NH3 solar refrigerator, Nigerian J. Solar Energy 9, 1-12 (1990). 39. Iloeje, O. C. and Enibe, S. O., Extended computer prediction of the seasonal performance of a CaCI2 NH3 solar refrigerator in Nigerian climates, Nigerian J. Solar Energy 10, 101-112 (1991). 40. Enibe, S. O. and Iloeje, O. C., Computer prediction of the parametric effects on the performance of a solid absorption solar refrigerator, Proceedings of the Congress of the International Solar Energy Society, Vol. 4, pp. 365-370, Budapest, Hungary, 20-26 August 1993. 4l. Enibe, S. O., Iloeje, O. C. and Ezekwe, C. I., Prediction of heat and mass transfer in the collector/absorber/generator of a solid absorption solar refrigerator, Nigerian J. Solar Energy 12, 5 18 (1993). 42. Enibe, S. O., Numerical prediction of heat and mass transfer in a solid absorption solar refrigerator, Ph.D. Thesis, Department of Mechanical Engineering, University of Nigeria, Nsukka, Nigeria (1995).
Pergamon
Renewable Energy, Vol. 12, No. 2, pp. 157-167, 1997 ~ 1997 Published by Elsevier Science Ltd All rights reserved. Printed in Great Britain
PIh S0960-1481(97)00036-0
SOLAR REFRIGERATION
0960-1481/97$17.00+0.00
FOR RURAL
APPLICATIONS S. O. ENIBE National Centre for Energy Research and Development, University of Nigeria, Nsukka, Nigeria
(Received 12 December 1994; accepted 24 April 1997) Abstract--A large proportion of people in developing countries live in rural or remote locations where grid electricity is presently unavailable and is not envisaged in the foreseeable future. Since conventional, electrically powered vapour compression refrigeration systems may not be of much use in such areas, for essential applications such as food and drug preservation, alternative refrigeration systems are required. Such systems are presented and discussed in this paper. These include photovoltaic (PV) powered vapour compression systems ; continuous and intermittent liquid or solid absorption systems; and adsorption systems. Typical application examples are drawn from recent experiences worldwide. Technical and financial constraints which limit their widespread application are reviewed, and strategies for overcoming them are discussed. © 1997 Published by Elsevier Science Ltd.
1. NEED FOR SOLAR REFRIGERATION The need for refrigeration and air conditioning has long been recognised, and many systems have been developed commercially to provide them, the most commonly used being the vapour compression system. This is well described in basic textbooks of thermodynamics [1, 2]. The coefficient of performance of a vapour compression refrigeration cycle, COP, is usually expressed as Energy removed at the evaporator COP = Energy supplied at the compressor'
(1)
The maximum possible COP is that of a Carnot cycle, and is given as COP .....
t
Tev -
Ted -- Tev
(2)
where Tev = evaporator temperature, and Too = condenser temperature. Practical vapour compression systems have COPs in the range 75-85% that of (COP) ..... t. Vapour compression refrigeration systems require mechanical power for driving the compressor, which is usually provided electrically. In areas where electrical energy is available, this refrigeration system is usually adequate to satisfy most refrigeration require157
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ments. There are several areas, however, where grid electricity is not available at the moment, and is unlikely to be available in the next few decades due to the huge financial outlays involved. These include villages and rural areas in developing countries, mountainous and island communities, and other remote locations. Cooling in such areas, therefore, requires alternative refrigeration systems, which, fortunately, are available. These include photovoltaic-powered vapour-compression systems, absorption systems, adsorption systems, desiccant cooling systems, and passive cooling systems. A schematic diagram of a solar refrigeration system, adapted from Garg [3], is shown in Fig. 1. It consists essentially of five sub-systems for solar energy collection, heat storage, solar cooling, heat rejection and cold storage. Solar energy is attractive as an energy resource for cooling in that it is inexhaustible and is most abundant in areas where the need for cooling is greatest. In addition, though it is diffuse and periodic, the peak of hourly solar radiation intensity usually occurs in symphony with the peak cooling demand. Further, since solar radiation is a free natural resource, the running costs of well-developed solar cooling systems can be expected to be low once the initial capital costs for their installation have been met. For this reason, a lot of work has been done on the development of solar cooling systems, some of which are described below. 2. SOLAR REFRIGERATION
2.1. PV, vapour-eompression refrigeration With the exception of the energy-supply sub-system, photovoltaic (PV) powered refrigeration is similar in most respects to any other vapour compression refrigeration system. The energy supply in the system is provided by an array of solar cells which convert the incident solar radiation to electricity. The latter is used to drive the compressor and the excess charges a battery system for use at periods of low or no insolation. Between 1981 and 1986, over 800 PV-powered refrigerators were reportedly built and adapted specifically for the
Solar Collector
Heat ~ Storage Device
Solar Cooling Device
Cold Storage Device
tSRejection/ Heat I y~em~ Fig. l. Schematic diagram ofasolarreffigerationsystem.
1
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storage of medicines and vaccines, mainly for demonstration purposes [4]. Most of these were sponsored by the World Health Organisation (WHO), U.S. Agency for International Development (USAID), and the European Community (EC), and were motivated by the need to maintain a sustainable cold chain for the WHO's Expanded Programme on Immunisation (EPI). Derrick and Durand [4] reported that the percentage operational time obtained for nine installations fully tested in Mali varied between 27 and 79%, the average mean time between failures varied between 4 and 20 months, and the average system reliability was about 59%. The major problem with PV-powered refrigeration is the system cost. While solar cell efficiencies have risen in recent times from about 7 to 13% [5], and the unit costs have fallen [6], the overall system cost for any moderately sized unit remains several times above the costs of ordinary vapour-compression refrigerators. The utilisation of PV-powered refrigeration on a larger scale will therefore await further increases in cell efficiencies with a fall in their prices and rises in the costs of conventional energy sources. 2.2. Absorption refrigeration Absorption refrigeration is based on the principle that some absorbents can absorb large quantities of refrigerants which can be regenerated upon the application of heat. The refrigerant absorption process is exothermic, while the regeneration process is endothermic. The heat energy required for the regeneration process may be supplied from any suitable heat sources, such as waste heat from a heat engine or process plant, heat from a compression system, biofuels, or solar energy. There are two types of absorbents, namely liquid and solid absorbents, and refrigerating machines have been built using each type. Table 1 gives a list of absorbenbrefrigerant pairs that have been utilised or proposed for solar refrigeration. Figure 2 shows the basic schematic diagram of an intermittent absorption refrigerator. It operates in two modes, namely the absorption and generation modes. During the absorption mode, the vessel A acts as the absorber, and absorbs the refrigerant released from the evaporator, B. Heat Ql is absorbed in the evaporator and heat Q2 is rejected at the absorber. During the generation mode, the vessel A, now acts as the generator, while vessel B acts as the condenser. Heat Q4 is supplied to the generator from a suitable heat source, while heat Q3 is rejected at the condenser. The valve C is used to control the direction of flow of the refrigerant. Thus, vessels A and B double as the absorber/generator and condenser/evaporator respectively. Vessel A contains the liquid or solid absorbent prepared in a suitable form. For absorption systems, the coefficient of performance, COP, is defined as the ratio of the energy intake at the evaporator, Q1, to the Table 1. Refrigerant absorbent combinations Refrigerant
Liquid absorbents
H20 NH3
LiBr, LiC1, LiSCN, CsF, RbF or their multiple salt solutions [7] CaC12 [7, 28] H20 [12] NaSCN [7] SrClz [7] LiNO3 [7] NaC1 [17]
Note : Numbers in brackets indicate references.
Solid absorbents
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C
j A b so rber
Q Evaporator
Q3 Q4
C,o n denser Generator
Fig. 2. Schematicdiagram of an absorption refrigerator. energy input at the generator, Q4. Thus, COP = QI/Q4. The actual implementation of absorption refrigeration varies depending on the refrigerant-absorbent pair under consideration. In general, for liquid systems, the absorbent and refrigerant may both move through the system, while for solid absorbents, it is only the refrigerant that moves. A comparison of the performance of solid and liquid absorption systems using the absorbent-refrigerant pairs of CaC12/NH3, SrCI2/NH3 and LiNO3/NH3, H 2 0 / N H 3 is reported in [7]. For similar operating conditions, it is indicated that the solid absorption systems perform better than the liquid absorption systems, with the CaC12 as absorbent being slightly better (and cheaper) than SrC12. In the case of the liquid systems, LiNO3 gave a superior performance, followed by H20. Liquid absorption systems can be adapted to intermittent or continuous operation, but the solid absorption systems can only operate intermittently, since the solid absorbent cannot move through the system. Liquid absorption systems may require a small pump for circulating the absorbent solution and a rectifier to prevent droplets of the liquid absorbent from entering the condenser, but a solid absorption system does not require any of these. It can therefore operate quietly without maintenance over some period of time. 2.2.1. Liquid absorption systems. Following the pioneer work of Trombe and Foex [8] who built and tested a H20/NH3 solar refrigerator, many reports on the performance of solid and liquid absorption systems have appeared in the literature. Chinapa [9] constructed an intermittent HzO/NH3 plant with three glazing covers which produced 1.43 kg of ice per m 2 per day. Other intermittent systems based on H20/NH3 and NaSCN/NH3 are reported by Swartman [10], and these gave coefficients of performance in the range 0.054).14. A continuous H20/NH3 plant which used two pumps for circulating the absorbing solution and chilled water is reported by Farber [l 1]. The system was single glazed with 1.49 m 2 collector area, and gave a coefficient of performance of 0.1 and an ice production capacity of 12.5 kg m -2 per day. An intermittent ammonia-water system was also built by Staicovici [12] in Romania. The system COP varied between 0.09 and 0.152. Figure 3 shows the schematic diagram of the intermittent aqua-ammonia liquid absorption refrigeration system due to Excell [14], which is typical of intermittent liquid absorption systems.
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B
A
Evaporator ~=~y
/
Condenser
Receiver
1
@
c
_
Fig. 3. Schematic diagram of an intermittent aqua-ammonia refrigerator. During the day, ammonia is vaporised from the combined collector/absorber/generator with valve A open and valves B and C closed. The ammonia is condensed and stored in a water cooled receiver. At night, the collector/generator is allowed to cool, valve A is closed and valves B and C are opened to enable the liquid ammonia to evaporate, thereby producing the refrigerating effect. The ammonia vapour goes through the lower header into the collector where it is reabsorbed. The cycle is completed once per day. A summary of the performance of liquid absorption systems is given in Table 2. 2.2.2. Solid absorption systems. In comparison with the liquid absorption systems, relatively fewer solid absorption plants have been reported. Plank's [15] CaC12/NH3 system used 100"C hot water as heat source, had a cooling capacity of 3768-4187 k J/cycle, and gave 3 cycles per day. Andrews" [16] CaC16/NH 3 plant used 100°C steam as heat source, and operated commercially on refrigerated food transport vans on London Liverpool trains during the Second World War. Muradov and Shadiev [17, 18] built a NaCI2/NH3 system which produced 1 kg of ice per m 2 per day, while Eggers-Lura et al. [19] designed a plant capable of producing 4.6 kg of ice per m 2 per day with CaCI2 or SrCI 2 as absorbent Table 2. Performance of liquid absorption systems Designer and reference Trombe and Foex [8] Chinappa [9] Swartman [10] Farber [11] Staicovici [12] Sloetjes et al. [13]
Refrigerant Absorbent NH3
H20
NH3 NH3 NH3 NH 3 NH3 NH3
H20 H20 NaSCN H20 H20 H20
COP range (kg/m2-day)
Ice production capacity (kJ/m2-day) Comments Intermittent 1.43
0.05-0.14 0.05-0.14 0.1 0.09-0.152
12.5
Intermittent Intermittent Intermittent Continous Intermittent Continous
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and NH3 as refrigerant. Iloeje [7] built an intermittent CaC12/NH3 system with a double glazed collector of 1.41 m 2 surface area. This gave a cooling capacity of 714 kJ/m 2 with an effective ice production of 1.5 kg m -2 per day. For solar operation, the first reported investigation of the system is by Nielson et al. [21]. The unit, which was tested in the Sudan, had a total cooling capacity of 2 MJ/m 2 of the collector surface, and a useful average COP of about 0.096. A field unit was subsequently built and operated autonomously in the Sudan for 6 years [19]. Iloeje [7] built and tested an experimental system at Nsukka, Nigeria in 1983. Figure 4 shows the schematic diagram of the CaC12/NH3 refrigeration system due to Iloeje [7], which is typical of solid absorption systems. Operation of solid absorption systems is necessarily intermittent, since the solid absorbents cannot move within the system. One cycle of operation is completed in a 24-h day. During the day, valve A is open and valves B and C are closed. Ammonia, generated from the collector/absorber/generator, is condensed and stored in the liquid receiver. Net ammonia generation ceases in the evening as solar intensity falls. The collector is then covered with an opaque screen to start the collector cool down phase. The generator/absorber pressure falls below the evaporation valve of about 3 bar, making possible the evaporation/reabsorption phase. The screen is removed, valve A is closed and valves B and C opened to start the ammonia evaporation and night-time exothermic reabsorption phase. The cycle is completed before morning and the cold produced is stored as ice. Table 3 gives a comparison of the performance of solid absorption systems which appeared recently. 2.3. Adsorption refriyeration Many solar refrigeration systems have been proposed based on the principle of adsorption. Adsorption differs from absorption in that while the latter is a bulk phenomenon, adsorption is essentially a surface phenomenon, and is similar in many respects to all those phenomena which depend upon the properties of surfaces, such as electron emission, bleaching with charcoal, catalysis, chromatographic analysis, etc. The theory of adsorption has been extensively discussed in the literature, and a brief account may be found in Levenspiel [23]. In essence, during adsorption, a molecular species of a fluid is attached to the surface of a solid, resulting in an increased concentration of the substance at the interface. G o o d adsorbents for refrigerants include zeolites 5A and 13X, as well as the activated charcoals, some of which may be made from coconut shells. The organic refrigerants, and in some cases ammonia, have been preferred in adsorption systems. The organic
,
A
®
Condenser ~Th Receiver otfle Valve
i
[
I
Evaporator
Fig. 4. Schematic diagram of the CaC12/NH3 refrigerator.
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163
Table 3. Performance of solid absorption systems Designer and reference
Refrigerant Absorbent
COP range
Ice production Cooling capacity capacity (kg/m2-day) (kJ/mZ-day)
Plank [15]* Andrews [16]:~ Muradov and Shadiev [17, 18] Eggers-Lura et al. [19]
NH3 NH3 NH3
CaC12 CaCI2 NaCI
11,9oo? 1
NH3
CaC12orSrCl2
4.6
Iloeje [7] Nielson et al. [21]
NH3 NH3
SrCI2 CaCI2 CaCl2
0.008 0.053 0.096
1.5
714 2000
* Used 100°C steam as heat source. ?MJ day 1 Used 100rC hot water as heat source.
refrigerants have the advantage of being compatible with copper tubing systems, low cost and low operating pressures. Of the various adsorbent/refrigerant pairs tested by Critoph [24], R22 with activated charcoal appeared to be the best, with a maximum COP estimated at about half that of the ammonia/water system. More recently, however, the same author [25] built an activated carbon refrigerator using ammonia as refrigerant. The collector has an area of 1.4 m 2 and contained 17 kg of active carbon. Under simulated radiation conditions in the laboratory, the unit gave an equivalent COP of 0.05-0.09, with a cooling capacity of511-1007 kJ/m2-day. Ethanol has also been investigated as a possible refrigerant, with activated charcoal as the adsorbent [26]. 3. P R O B L E M S A N D P R O S P E C T S
Up to the present time, two major problems have limited the emergence of solar cooling technologies in the market to any significant extent. These are mainly technical and economic, and are considered below. 3.1. Technical p r o b l e m s
Each solar cooling technology has its own unique technical problems. For example, in the case of the liquid absorption systems, the need for a solution pump (in continuous systems) and a rectifier for the vapour leaving the generator introduce unpleasant complications in the system design, and also result in performance reductions. For the solid absorption systems, the major problems are the expansion of the absorbent upon the absorption of the refrigerant and its low thermal conductivity. Adsorption systems also share this limitation. The reduction of the absorbent expansion has been considered by many investigators. For example, in the case of the CaC12/NH3 system, upon the absorption of ammonia, the anhydrous calcium chloride swells up, cracks, splits in all directions and at last falls to a white powder. The initial absorption is rapid, and slows down towards the end, the product of the action occupying at least 3 times the original volume of the solid [27, 28].
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To avoid these difficulties, a number of treatments have been proposed to pelletize the absorbent into hard porous granules which can withstand multiple cycling of the absorption/generation processes. A number of these treatments have been reviewed recently by Iloeje [28]. In general, the treatments involve mixing the absorbent with a cementatious material or binder, processing the mixture into a porous matrix, and pelletizing the matrix into a suitable size range. Some of the binders reported in the literature include LiNO3, NaSiO3, portland cement and CaSO4, but the latter mixed in the proportion of 80% CaC12 + 20% CaSO4 was found to yield the best results when both the strength and ammonia absorption capacity of the resultant pellet were taken into account [29]. The recommended procedure for the absorbent preparation involves grinding the available CaCI~ and CaSO4 granules separately to a fine powder, mixing in the ratio of 4 : 1 by mass, adding water and mixing to a thick paste. The latter is then heated at 90°C for 24 h, 90-150°C for another 24 h, and finally at 200-250r'C until fully dried. This produces a porous matrix which is then broken down to the required size range. This treatment appears effective since no detectable reduction in the absorption/generation characteristics of the absorbent occurred after undergoing 40 successful refrigeration cycles during test operations over a 1 yr period [32]. Meunier [30] attributes another successful approach to the absorbent stabilization to Spinner [31], which involves the use of a graphite binder. The other major technical problem with solid adsorption and absorption systems, namely the low thermal conductivity of the absorbent (or adsorbent), has not been adequately solved. Iloeje [33] mixed the absorbent with aluminium filings to improve its thermal conductivity, but this did not yield successful results. Moreover, the aluminium appeared to react with the absorbent in some way. Another attempt in the same direction by Enibe [34] to suspend the absorbent in an inert fluid did not yield successful results. A possible solution to the problem has been proposed recently by the Critoph group [35] who prepared the adsorbent (active carbon in this case) as flat, porous, thin discs packed in a tube with interior fins. If this solution is effective, it will also apply to solid absorption systems, and will introduce significant improvements in the performance of solar refrigeration systems. 3.2. Economic problems The major economic problems militating against the widespread utilisation of solar cooling technologies are the costs of the units, which, in most cases, are several times those of comparable units using conventional cooling technology. For example, Clerx and Trezek [36] indicate that the price of their aqua-ammonia solar assisted refrigerator was 8-10 times more than that of a comparable conventional unit. Similarly, the cost of the 13 kW peak cooling power, 50 m 2 capacity aqua-ammonia system of Sloetjes et al. [13] proved to be far too expensive for the intended application in the Sudan (the total system cost was estimated at U.S.$ 95,000 at 1987 prices). For this reason, several companies which produced and marketed solar powered refrigerators have been forced to stop production because of purely economic reasons. A typical example is Sun-Ice of Denmark which produced CaC12 refrigerators which were technically successful but expensive. The company has since ceased production. For the same reason, BLM of France [25] produced a range of refrigerators using active carbon and methanol, but has now ceased production. 3.3. Prospects In spite of the foregoing problems limiting the widespread application of solar cooling technologies, solar refrigeration and air-conditioning can be expected to contribute sig-
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nificantly to the welfare of future generations. This expectation is based on the fact that more technical progress is being made in the field which, when translated to commercial success, may bring down the price of the units considerably. For example, recent simulation studies on the CaCIz/NH3 system at Nsukka, Nigeria [37-42] have shown that the system cooling capacity can be doubled with the use of high efficiency collectors. In addition, it is expected that energy prices may rise in future, thereby making solar and renewable energy technologies competitive with the conventional systems. 4. CONCLUSIONS It may be seen from the foregoing that solar refrigeration can be expected to play a significant role in meeting the needs of people in the rural areas of developing countries for refrigeration. Many solar cooling technologies have proved to be technically feasible, and have scopes for further improvements. The improvements required are mainly in the areas of increasing system performance and lowering costs. With probable increases in the costs of conventional energy sources, it is expected that solar cooling technologies will become competitive with the conventional systems in future.
Acknowledgements--The author acknowledges, with gratitude, the financial sponsorship provided, at various times, by the International Foundation for Science, Stockholm, Sweden through grant G/1503-1 ; the Energy Commission of Nigeria, Lagos, Nigeria; and the National Centre for Energy Research and Development, University of Nigeria, Nsukka, Nigeria. The author is also grateful to Professor O. C. Iloeje of the Department of Mechanical Engineering, University of Nigeria, Nsukka, Nigeria for several helpful comments and encouragement.
1. 2. 3. 4.
5. 6. 7. 8.
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29. Iloeje, O. C., Quantitative comparisons of treated calcium chloride pellets for solar refrigeration, Solar Energy 37(4), 253 260 (1986). 30. Meunier, F., Sorption solar cooling, Renewable Energy 5(Part I), 422~429 (1994). 31. Spinner, B., Ammonia-based thermochemical transformers, J. Heat Rec. Systems 301308 (1993). 32. Iloeje, O. C., Extended performance tests of a solid absorption solar powered refrigerator, Technical Report No. SE-3, Solar Energy Project, University of Nigeria, Nsukka, Nigeria (April 1984). 33. Iloeje, O, C., Thermophysical properties of Nsukkanut, Nigerian J. Solar Energy 8, 319 335 (1989). 34. Enibe, S. O., Solid absorbent dispersion in an inert fluid for solar refrigeration applications, Solar & Wind Techn. 7(5), 591-595 (1990). 35. Munyebvu, E., Heat transfer in monolithic charcoals for use in adsorption refrigeration systems, M. Sc. Dissertation, Department of Engineering, University of Warwick, Coventry (1994). 36. Clerx, M, and Trezek, G. J., Performance of an aqua-ammonia absorption refrigerator at sub-freezing evaporator conditions, Solar Energy 5, 379 389 (1987). 37. Iloeje, O. C., Ndili, A. N. and Enibe, S. O., Computer simulation of a CaCI2 solid absorption solar refrigerator, Proceedings of the 1st Worm Renewable Energy Congress, Vol. 2, pp. 1159-1168, Reading, England, (1990). 38. Iloeje, O. C. and Enibe, S. O., Computer prediction of the seasonal performance of a CaC12-NH3 solar refrigerator, Nigerian J. Solar Energy 9, 1-12 (1990). 39. Iloeje, O. C. and Enibe, S. O., Extended computer prediction of the seasonal performance of a CaCI2 NH3 solar refrigerator in Nigerian climates, Nigerian J. Solar Energy 10, 101-112 (1991). 40. Enibe, S. O. and Iloeje, O. C., Computer prediction of the parametric effects on the performance of a solid absorption solar refrigerator, Proceedings of the Congress of the International Solar Energy Society, Vol. 4, pp. 365-370, Budapest, Hungary, 20-26 August 1993. 4l. Enibe, S. O., Iloeje, O. C. and Ezekwe, C. I., Prediction of heat and mass transfer in the collector/absorber/generator of a solid absorption solar refrigerator, Nigerian J. Solar Energy 12, 5 18 (1993). 42. Enibe, S. O., Numerical prediction of heat and mass transfer in a solid absorption solar refrigerator, Ph.D. Thesis, Department of Mechanical Engineering, University of Nigeria, Nsukka, Nigeria (1995).