- Email: [email protected]
Multi-pressure absorption cycles in solar refrigeration:
Pergamon
PII: S0038 – 092X( 00 )00007 – 4
Solar Energy Vol. 69, No. 1, pp. 37–44, 2000 2000 Elsevier Science Ltd All rights reserved. Printed in Great Britain 0038-092X / 00 / $ - see front matter
www.elsevier.com / locate / solener
MULTI-PRESSURE ABSORPTION CYCLES IN SOLAR REFRIGERATION: A TECHNICAL AND ECONOMICAL STUDY SHAHAB ALIZADEH† School of Mechanical and Manufacturing Engineering, University of New South Wales, Sydney 2052, Australia Received 15 April 1998; revised version accepted 11 November 1999 Communicated by BYARD WOOD
Abstract—A technical and economical study of regenerative absorption chillers with multi-pressure cycle has been undertaken as solar operated refrigeration systems. Referred to as advanced absorption chillers they represent one of the new technology options that are under development. Advanced absorption cooling technology offers the possibility of chillers with thermal COPs of 1.5 or greater at driving temperatures of 1408C, which reduces the collector area and the heat rejection requirements compared to current absorption cooling technology. Two different absorption systems have been considered. The first is an advanced, double-effect regenerative absorption cooling system, driven at 1408C, whose efficiency is about 55% of the Carnot efficiency. The second is an ideal, single-effect regenerative absorption system that achieves 70% of the Carnot efficiency driven at 1408C or 2008C. To evaluate the solar performance of a thermally driven chiller requires a separate analysis of the solar availability for a given location compared to the required monthly average solar input. In this analysis different systems, including the vapour compression chillers, have been compared in terms of the thermal and electrical energy input. An effective electrical COP may be computed assuming that the ratio of electrical energy cost to thermal energy cost is four, which is typical of today’s fossil fuel costs. The effective electrical COPs of different technical options can then be compared. Those systems with higher electrical COPs will have lower energy costs. If solar is to be competitive, then the cost of delivered solar thermal energy should be less than the cost of delivered fossil thermal energy. 2000 Elsevier Science Ltd. All rights reserved.
broken down into the same number as the generator sections. As a result of these features, an essentially constant boiling temperature and an overall cycle that looks more like the Carnot cycle than the conventional vapour compression cycle are maintained. There are several sources of energy for production of refrigeration, the most important of which are gas, electricity and solar energy. With increasing gas and electricity tariffs, free solar energy becomes attractive. Compared with other energy transfer devices, relatively inexpensive flat plate solar collectors can be used to transfer this energy to flowing water or air which could be used for heating and cooling purposes. Solar is an attractive source of energy because it is renewable, has low energy cost, and cooling load and availability of solar radiation are approximately in phase. At low temperatures the efficiency of solar collectors can be higher than the efficiency of high temperature collectors (Duffie and Beckman, 1980) or much higher than the photovoltaic conversion to electricity (Mitchell, 1983). For solar energy applications, present practice
1. INTRODUCTION
While absorption technology isn’t new, its poor efficiency for refrigeration prevented it from competing with vapour compression systems except where cheap heat was available. For high efficiency in these applications, the working fluid needs to retain a high temperature in its boiler. However, in conventional absorption system boilers, where constant pressure is maintained, the working fluid temperature decreases sharply. To remedy that, the pressure can be varied while keeping fluid temperature high. This results in a multi-pressure regenerative cycle ranging from condenser pressure to evaporator pressure. To do this the condenser and evaporator sections are kept essentially the same as in a conventional system but the generator and absorber are modified. The boiler or generator consists of several sections each comprising a boiling stage. After the fluid goes through one stage, it expands through a valve into the next stage, therefore the absorber is †
Tel.: 161-8-8302-3794; e-mail: shahab [email protected] ] 37
38
S. Alizadeh
Fig. 1. Variation of collector efficiency with r (Yazaki, 1978).
is to use the water cooled water–lithium bromide absorption chiller (Anderson, 1976). In comparison to the ammonia–water cycle the water– lithium bromide cycle has a number of advantages, the principal advantages being, a higher coefficient of performance (COP) and its generator temperature operating range is within the output temperature operating range of high quality flat plate solar collectors. High quality correctly installed flat plate solar collectors using selective surfaces will deliver hot water at temperatures up to 1008C. The performance characteristics of such a collector (Yazaki, 1978) is depicted in Fig. 1 where the instantaneous efficiency is plotted versus r, defined as TC 2 T0 r 5 ]]]. I
(1)
Evacuated tube solar collectors have also been used for powering the water–lithium bromide absorption cooling systems (Ward et al., 1976). 2. PERFORMANCE OF THE COMMERCIAL SYSTEMS
2.1. Basic solar absorption cycles A number of authors have investigated the performance of absorption refrigeration cycles
where the heat source has been supplied by solar energy. Phillips (1976) discusses the ammonia– water absorption cycle with respect to its suitability for air cooling. Whitlow (1976) analysed the ammonia–water cycle with respect to variations in heat source and heat sink temperatures and coefficient of performance. His findings were that if flat plate collectors are to be considered, then the source temperature is limited to 90–1008C which necessitates sink temperatures of less than 328C to give a COP of 0.4. This limitation negates the possibility of an air cooled system with source temperatures of 1008C or less. To obtain condenser and absorber temperatures below 328C requires cooling water at a temperature of about 288C, thus, cooling towers are ruled out where wet-bulb design temperature is 248C or greater. Whitlow recommends that concentrating collectors be used for driving NH 3 –H 2 O systems thereby eliminating the necessity for water cooling. Figs. 2 and 3 show the performance of air cooled and water cooled NH 3 –H 2 O systems. For both cases the COP drops sharply as the concentration of the strong solution approaches that of the weak solution. Ammonia–water absorption cycles with coefficients of performance greater than one have been postulated by Chinnappa (1974). Two cycles, the double-effect condensing cycle and the doubleeffect resorption cycle have calculated COPs of
Multi-pressure absorption cycles in solar refrigeration
39
Fig. 2. COP values for the air cooled and water cooled conditions (Whitlow, 1976).
1.2 and 1.3 for mean generator temperatures of 848C and 908C, respectively. Since these are calculated values of COP, in practice they will be lower. For the double-effect condensing cycle estimating evaporator temperature is 2108C. The condensing temperature for these two cycles was
assumed to be 348C. Chinnappa concludes that these non-conventional cycles require smaller collector area than the present conventional units and estimates their costs to be 1.6 to 1.7 times that of conventional units. Absorption chillers have been incorporated into solar heating and
Fig. 3. Generator temperatures for the air cooled and water cooled conditions (Whitlow, 1976).
40
S. Alizadeh
cooling systems and experimental results published by a number of authors (Ward, 1979; Sheridan, 1972; Lof and Ward, 1975; Mumma and Sepsy, 1976). These results indicate suitability of the water cooled H 2 O–LiBr system for refrigeration using solar energy as heat input.
2.2. Double-effect absorption cycles As was discussed previously, in a single-effect absorption cycle a considerable amount of heat is rejected from the condenser and absorber during the condensation and absorption of the refrigerant vapour. A double-effect absorption cycle which is made up of a coupling of two single-effect cycles makes possible the recovery of this heat that otherwise would be lost, thus improving the efficiency of the system. In the conventional double-effect cycle, the condensation of the refrigerant vapour generated from the first effect generator releases heat to boil out refrigerant in the second-effect generator. In the common condenser cycle the refrigerant vapours generated by both generators are at the same pressure; they are subsequently rectified and condensed in the same condenser. The common condenser cycle is essentially the coupling of two conventional single-effect cycles. The first single-effect cycle operates at high generator temperatures to boil out refrigerant vapour at the common condenser pressure and absorbs refrigerant vapour at the common evaporator pressure (the evaporator is common to both coupled cycles). This absorption of vapour releases heat at a temperature high enough to boil out refrigerant in the generator of the second single-effect cycle. The conventional double-effect cycle is also essentially the coupling of two single-effect cycles. The difference between the conventional and common condenser cycles consists in the mode of coupling: for the conventional double-effect cycle, the coupling is between the first-effect condensing process and the second-effect generating process; for the common condenser cycle the coupling is between the first-effect absorption and the secondeffect generating process. The common condenser double-effect cycle cannot be used with NH 3 – H 2 O mixtures because the pressure in the generator of the first-effect would be too high (about 61 bars).
2.3. Multi-pressure absorption cycles; single and double-effect regenerative (1 R and 2 R) cycles As already mentioned, basic absorption cycles have two undesirable characteristics, namely
sharp cut-off input temperatures below which the cycle ceases to operate and almost constant coefficient of performance with increasing input temperature. Because of these deficiencies the basic cycle cannot benefit from the development of efficient high temperature collectors and cannot be used during considerable periods when the insolation is low. The 1R cycle’s much higher COP at high input temperatures and workable COP at very low input temperatures largely eliminate the shortcomings of the basic cycles. The double-effect regenerative absorption refrigeration cycle (or 2R cycle) is an effort to design an efficient cooling system for solar applications. As already mentioned, present solar activated cooling systems have either a low efficiency or a sharp cut-off temperature that prevents the effective use of highly available solar radiation. Conventional single-effect absorption systems have a low COP of the order of 0.70 and a sharp cut-off input temperature of about 938C. Conventional double-effect absorption systems have a good COP of about 1.10 but unfortunately, they also have a sharp cut-off input temperature of about 1508C. The 2R cycle described in this paper has a continuously increasing COP at higher temperatures, approaching 63% of the Carnot COP. This cycle works efficiently with input temperatures from 708C (with the condenser and the evaporator temperatures being 438C and 48C, respectively). Calculated COPs are plotted in Figs. 4 and 5 versus some important parameters. As a comparison, the COP values for the basic single-effect and conventional double-effect cycles have also been shown in Fig. 6. The figures show that the 2R cycle performs as well as the single-effect cycle at low boiler temperatures and as well as the double-effect at high temperatures. The COP of the 2R cycle increases continuously with increasing boiler temperature. While this is slightly less than the COP of the 1R cycle, the configuration of the 2R cycle is less complicated and only a careful cost analysis of the two systems can determine which one is better. The absence of cut-off temperatures is a feature of the 2R cycle that makes it superior to double-effect cycles. Figs. 4 and 5 show that at low condenser– absorber temperatures, T 0 , and high boiler temperatures, the COP of the 2R cycle can be substantially higher than the basic double-effect cycle and for the 1R cycle it can be even much higher. These figures also show that 2R and 1R cycles can be used as heat pump for heating purposes since they have an acceptable COP at T E of about 48C.
Multi-pressure absorption cycles in solar refrigeration
41
Fig. 4. Calculated performance of 1R cycle using NH 3 –H 2 O mixtures (Dao, 1978a,b).
3. ECONOMIC CONSIDERATIONS
3.1. Cost and performance goals Since energy costs will continue to rise in the future, higher efficiency energy conversion equipment is being developed and used. The competition of non-solar devices is as smart as the solar advocates are, consequently the main competition will be advanced, high-efficiency, electric-driven chillers for cooling. It is therefore necessary to develop advanced, high-efficiency, gas-driven
chillers for refrigeration which have the following characteristics: (a) to be able to compete in the marketplace with electric-driven units; (b) to provide heat driven chillers to which solar-collected thermal energy can couple. In fact the solar-collected heat must be delivered to the energy conversion device (chiller) for the same cost as gas-produced heat is delivered. Ideally, the solar cooling research has a twofold objective: firstly, to develop an efficient chiller that will operate on gas input (variable
Fig. 5. Calculated performance of 2R cycle using NH 3 –H 2 O mixtures (Dao, 1978a,b).
42
S. Alizadeh
Fig. 6. Basic single effect (NH 3 –H 2 O) and conventional double-effect (H 2 O–LiBr) absorption cycles.
temperature). Secondly, to develop solar collector subsystems (collectors, piping, controls) that can drive these chillers on an economically competitive basis with gas heat sources.
3.2. Electric power from solar Solar energy may be converted to electrical energy using photovoltaic cells. The conversion efficiency is normally around 15%, which can vary up to 30%, and the cell efficiency is strongly temperature dependent. It is technically feasible to produce electricity using photovoltaic cells. The technology to manufacture cells and collectors exists, and the transmission of DC electricity and its conversion to AC electricity are well known. The barrier to implementation is economic. The cost of solar cells with 15% efficiency and mass produced is expected to be about $220 / m 2 of cell area. As an example, for a system using a flat plate array, the total cost for photovoltaic cells for a power plant with a capacity of 20,000 kW would be $460310 6 . If a concentrating system were used with a concentration ratio of 50:1, the cell cost would be $444 / m 2 . However, collector costs will probably be $110–$220 / m 2 , and so the system costs would be in the range of $230310 6 to $460310 6 . Photovoltaic conversion to electricity can be
cost effective if cell and collector prices are as low as expected. Also the technical problems of dissipating heat from the cells and efficiently transmitting electricity from the cells to the distribution system still remain. The power produced is in phase with refrigeration demands which reduces the need for conventional refrigeration systems. It appears that photovoltaic systems will become a competitive electricity generating system.
3.3. Status of high efficiency solar cooling and outlook for success In order to compare the different systems in terms of the thermal and electrical energy input, an effective electrical COP (EFFECOP) can be computed for each technical option assuming that the ratio of electrical energy cost to thermal energy cost is four. Those systems with higher effective electrical COPs will have lower energy costs (see Table 1). This comparison indicates that the ideal 1R cycle has the highest effective electrical COP and consequently the lowest cost (Warren and Wahlig, 1991). 4. CONCLUSIONS
In this study regenerative absorption chillers
Multi-pressure absorption cycles in solar refrigeration
43
Table 1. Comparison of effective electrical COPs of different technical options (Warren and Wahlig, 1991) Chiller type
(a) Electrical performance
Reciprocating chiller Centrifugal chiller Advanced absorption Ideal absorption Ideal absorption
TG 8C
Etotal kW
ECOP
140 140 200
271.4 173.1 87.1 85.6 82.6
2.04 3.59 10.75 11.12 11.91
with multi-pressure cycle were analysed for solar operation and compared with conventional absorption chillers. Basic absorption refrigeration cycles have two undesirable characteristics; namely the low coefficient of performance and the high cut-off input temperature below which the cycle ceases to operate. Two solutions to the problem of low COP and cut-off phenomena have been finalised. The first solution is the double-effect regenerative absorption refrigeration cycle (2R cycle). The second solution is the single-effect regenerative absorption cycle (1R cycle). The performance characteristics of the two cycles are about equal. The approach adopted for development of the advanced absorption chillers is to make use of the 2R cycle system first, because it is relatively simpler and therefore easier to investigate technically. Since the 1R cycle system uses processes similar to those of the 2R cycle, the success of the 2R cycle system should greatly enhance the success of the 1R cycle. Based on an economical analysis a comparison has been made amongst different systems in terms of thermal and electrical energy input. The effective electrical COP is computed for each technical option. Those systems with higher effective electrical COPs will have lower energy costs. This comparison shows the highest effective electrical COP for the ideal single-effect regenerative absorption chiller and therefore the lowest energy cost. Photovoltaic conversion to electricity can be cost effective in solar refrigeration if collector prices are as low as expected. Costs are being reduced and the price is nearly competitive with current conventionally generated electricity. It appears that photovoltaic systems will become a competitive electricity generating system. NOMENCLATURE COP ECOP
(b) Thermal performance
coefficient of performance electrical coefficient of performance
QG kW
390 330 241
TCOP
EFFECOP
1.19 1.41 1.93
2.04 3.59 3.30 3.74 4.68
EFFECOP Etotal I r TCOP T a ´ hC
effective electrical coefficient of performance total electrical use (kW) 2 hourly solar insolation (kJ / h m ) ratio defined by Eq. (1) (8C h m 2 / kJ) thermal coefficient of performance temperature (8C) absorptivity of the collector surface emittance of glass cover of the collector collector efficiency
Subscripts A C E G 0
absorber condenser evaporator generator condenser, absorber or ambient
Acknowledgements—The author wishes to thank Dr. I.L. Maclaine-Cross for many valuable suggestions and discussions during the period of this research.
REFERENCES Anderson P. P. (1976) Solar operation of ammonia–water chiller, a comparison of aqueous lithium-bromide and aqua ammonia cycles. ASHRAE Trans. 82(Part 1), 959–965. Chinnappa J. C. V. (1974) Solar operation of ammonia–water multi-stage air-conditioning cycles in the tropics. Solar Energy 16, 165–170. Dao K. (1978a). A New Absorption Cycle: The Single-Effect Regenerative Absorption Refrigeration Cycle, Lawrence Berkeley Laboratory, University of California, Berkeley, CA, USA. Dao K. (1978b). Conceptual Design of an Advanced Absorption Refrigeration Cycle: The Double-Effect Regenerative Absorption Refrigeration Cycle, Lawrence Berkeley Laboratory, University of California, Berkeley, CA, USA. Duffie J. A. and Beckman W. A. (1980). Solar Engineering of Thermal Processes, Wiley, New York, Chapters 6 and 8. Lof G. O. G. and Ward D. S. (1975). Performance of a Residential Solar Heating and Cooling System, Energy Research and Development Administration, Report No. C. 00-2577-9. Mitchell J. W. (1983). Energy Engineering, Wiley, New York, Chapter 12. Mumma S. A. and Sepsy C. F. (1976) Solar absorption air conditioning performance in central Ohio. ASHRAE Trans. 82(Part 1), 825–833. Phillips B. A. (1976) Absorption cycles for air-cooled solar air conditioning. ASHRAE Trans. 82(Part 1), 966–974. Sheridan N. R. (1972) Performance of the Brisbane solar house. Solar Energy 13, 395–401. Ward D. S. et al. (1976) Cooling subsystem design in C.S.U. solar house III. In Joint Conference, American Section, I.S.E.S., Winnipeg, Canada, pp. 68–89.
44
S. Alizadeh
Ward D. S. (1979) Solar absorption coding feasibility. Solar Energy 22(3), 259–268. Warren M. L. and Wahlig M. (1991). Analysis and Comparison of Active Solar Dessiccant and Absorption Systems ( Part I), Lawrence Berkeley Laboratory, Berkeley, CA, USA.
Whitlow E. P. (1976) Relationship between heat source temperature, heat sink temperature and coefficient of performance for solar powered absorption air conditioners. ASHRAE Trans. 82(Part 1), 950–958. Yazaki T. J. (1978) Solar air conditioning system — a case study. In I.S.E.S. Symposium, Melbourne.
PII: S0038 – 092X( 00 )00007 – 4
Solar Energy Vol. 69, No. 1, pp. 37–44, 2000 2000 Elsevier Science Ltd All rights reserved. Printed in Great Britain 0038-092X / 00 / $ - see front matter
www.elsevier.com / locate / solener
MULTI-PRESSURE ABSORPTION CYCLES IN SOLAR REFRIGERATION: A TECHNICAL AND ECONOMICAL STUDY SHAHAB ALIZADEH† School of Mechanical and Manufacturing Engineering, University of New South Wales, Sydney 2052, Australia Received 15 April 1998; revised version accepted 11 November 1999 Communicated by BYARD WOOD
Abstract—A technical and economical study of regenerative absorption chillers with multi-pressure cycle has been undertaken as solar operated refrigeration systems. Referred to as advanced absorption chillers they represent one of the new technology options that are under development. Advanced absorption cooling technology offers the possibility of chillers with thermal COPs of 1.5 or greater at driving temperatures of 1408C, which reduces the collector area and the heat rejection requirements compared to current absorption cooling technology. Two different absorption systems have been considered. The first is an advanced, double-effect regenerative absorption cooling system, driven at 1408C, whose efficiency is about 55% of the Carnot efficiency. The second is an ideal, single-effect regenerative absorption system that achieves 70% of the Carnot efficiency driven at 1408C or 2008C. To evaluate the solar performance of a thermally driven chiller requires a separate analysis of the solar availability for a given location compared to the required monthly average solar input. In this analysis different systems, including the vapour compression chillers, have been compared in terms of the thermal and electrical energy input. An effective electrical COP may be computed assuming that the ratio of electrical energy cost to thermal energy cost is four, which is typical of today’s fossil fuel costs. The effective electrical COPs of different technical options can then be compared. Those systems with higher electrical COPs will have lower energy costs. If solar is to be competitive, then the cost of delivered solar thermal energy should be less than the cost of delivered fossil thermal energy. 2000 Elsevier Science Ltd. All rights reserved.
broken down into the same number as the generator sections. As a result of these features, an essentially constant boiling temperature and an overall cycle that looks more like the Carnot cycle than the conventional vapour compression cycle are maintained. There are several sources of energy for production of refrigeration, the most important of which are gas, electricity and solar energy. With increasing gas and electricity tariffs, free solar energy becomes attractive. Compared with other energy transfer devices, relatively inexpensive flat plate solar collectors can be used to transfer this energy to flowing water or air which could be used for heating and cooling purposes. Solar is an attractive source of energy because it is renewable, has low energy cost, and cooling load and availability of solar radiation are approximately in phase. At low temperatures the efficiency of solar collectors can be higher than the efficiency of high temperature collectors (Duffie and Beckman, 1980) or much higher than the photovoltaic conversion to electricity (Mitchell, 1983). For solar energy applications, present practice
1. INTRODUCTION
While absorption technology isn’t new, its poor efficiency for refrigeration prevented it from competing with vapour compression systems except where cheap heat was available. For high efficiency in these applications, the working fluid needs to retain a high temperature in its boiler. However, in conventional absorption system boilers, where constant pressure is maintained, the working fluid temperature decreases sharply. To remedy that, the pressure can be varied while keeping fluid temperature high. This results in a multi-pressure regenerative cycle ranging from condenser pressure to evaporator pressure. To do this the condenser and evaporator sections are kept essentially the same as in a conventional system but the generator and absorber are modified. The boiler or generator consists of several sections each comprising a boiling stage. After the fluid goes through one stage, it expands through a valve into the next stage, therefore the absorber is †
Tel.: 161-8-8302-3794; e-mail: shahab [email protected] ] 37
38
S. Alizadeh
Fig. 1. Variation of collector efficiency with r (Yazaki, 1978).
is to use the water cooled water–lithium bromide absorption chiller (Anderson, 1976). In comparison to the ammonia–water cycle the water– lithium bromide cycle has a number of advantages, the principal advantages being, a higher coefficient of performance (COP) and its generator temperature operating range is within the output temperature operating range of high quality flat plate solar collectors. High quality correctly installed flat plate solar collectors using selective surfaces will deliver hot water at temperatures up to 1008C. The performance characteristics of such a collector (Yazaki, 1978) is depicted in Fig. 1 where the instantaneous efficiency is plotted versus r, defined as TC 2 T0 r 5 ]]]. I
(1)
Evacuated tube solar collectors have also been used for powering the water–lithium bromide absorption cooling systems (Ward et al., 1976). 2. PERFORMANCE OF THE COMMERCIAL SYSTEMS
2.1. Basic solar absorption cycles A number of authors have investigated the performance of absorption refrigeration cycles
where the heat source has been supplied by solar energy. Phillips (1976) discusses the ammonia– water absorption cycle with respect to its suitability for air cooling. Whitlow (1976) analysed the ammonia–water cycle with respect to variations in heat source and heat sink temperatures and coefficient of performance. His findings were that if flat plate collectors are to be considered, then the source temperature is limited to 90–1008C which necessitates sink temperatures of less than 328C to give a COP of 0.4. This limitation negates the possibility of an air cooled system with source temperatures of 1008C or less. To obtain condenser and absorber temperatures below 328C requires cooling water at a temperature of about 288C, thus, cooling towers are ruled out where wet-bulb design temperature is 248C or greater. Whitlow recommends that concentrating collectors be used for driving NH 3 –H 2 O systems thereby eliminating the necessity for water cooling. Figs. 2 and 3 show the performance of air cooled and water cooled NH 3 –H 2 O systems. For both cases the COP drops sharply as the concentration of the strong solution approaches that of the weak solution. Ammonia–water absorption cycles with coefficients of performance greater than one have been postulated by Chinnappa (1974). Two cycles, the double-effect condensing cycle and the doubleeffect resorption cycle have calculated COPs of
Multi-pressure absorption cycles in solar refrigeration
39
Fig. 2. COP values for the air cooled and water cooled conditions (Whitlow, 1976).
1.2 and 1.3 for mean generator temperatures of 848C and 908C, respectively. Since these are calculated values of COP, in practice they will be lower. For the double-effect condensing cycle estimating evaporator temperature is 2108C. The condensing temperature for these two cycles was
assumed to be 348C. Chinnappa concludes that these non-conventional cycles require smaller collector area than the present conventional units and estimates their costs to be 1.6 to 1.7 times that of conventional units. Absorption chillers have been incorporated into solar heating and
Fig. 3. Generator temperatures for the air cooled and water cooled conditions (Whitlow, 1976).
40
S. Alizadeh
cooling systems and experimental results published by a number of authors (Ward, 1979; Sheridan, 1972; Lof and Ward, 1975; Mumma and Sepsy, 1976). These results indicate suitability of the water cooled H 2 O–LiBr system for refrigeration using solar energy as heat input.
2.2. Double-effect absorption cycles As was discussed previously, in a single-effect absorption cycle a considerable amount of heat is rejected from the condenser and absorber during the condensation and absorption of the refrigerant vapour. A double-effect absorption cycle which is made up of a coupling of two single-effect cycles makes possible the recovery of this heat that otherwise would be lost, thus improving the efficiency of the system. In the conventional double-effect cycle, the condensation of the refrigerant vapour generated from the first effect generator releases heat to boil out refrigerant in the second-effect generator. In the common condenser cycle the refrigerant vapours generated by both generators are at the same pressure; they are subsequently rectified and condensed in the same condenser. The common condenser cycle is essentially the coupling of two conventional single-effect cycles. The first single-effect cycle operates at high generator temperatures to boil out refrigerant vapour at the common condenser pressure and absorbs refrigerant vapour at the common evaporator pressure (the evaporator is common to both coupled cycles). This absorption of vapour releases heat at a temperature high enough to boil out refrigerant in the generator of the second single-effect cycle. The conventional double-effect cycle is also essentially the coupling of two single-effect cycles. The difference between the conventional and common condenser cycles consists in the mode of coupling: for the conventional double-effect cycle, the coupling is between the first-effect condensing process and the second-effect generating process; for the common condenser cycle the coupling is between the first-effect absorption and the secondeffect generating process. The common condenser double-effect cycle cannot be used with NH 3 – H 2 O mixtures because the pressure in the generator of the first-effect would be too high (about 61 bars).
2.3. Multi-pressure absorption cycles; single and double-effect regenerative (1 R and 2 R) cycles As already mentioned, basic absorption cycles have two undesirable characteristics, namely
sharp cut-off input temperatures below which the cycle ceases to operate and almost constant coefficient of performance with increasing input temperature. Because of these deficiencies the basic cycle cannot benefit from the development of efficient high temperature collectors and cannot be used during considerable periods when the insolation is low. The 1R cycle’s much higher COP at high input temperatures and workable COP at very low input temperatures largely eliminate the shortcomings of the basic cycles. The double-effect regenerative absorption refrigeration cycle (or 2R cycle) is an effort to design an efficient cooling system for solar applications. As already mentioned, present solar activated cooling systems have either a low efficiency or a sharp cut-off temperature that prevents the effective use of highly available solar radiation. Conventional single-effect absorption systems have a low COP of the order of 0.70 and a sharp cut-off input temperature of about 938C. Conventional double-effect absorption systems have a good COP of about 1.10 but unfortunately, they also have a sharp cut-off input temperature of about 1508C. The 2R cycle described in this paper has a continuously increasing COP at higher temperatures, approaching 63% of the Carnot COP. This cycle works efficiently with input temperatures from 708C (with the condenser and the evaporator temperatures being 438C and 48C, respectively). Calculated COPs are plotted in Figs. 4 and 5 versus some important parameters. As a comparison, the COP values for the basic single-effect and conventional double-effect cycles have also been shown in Fig. 6. The figures show that the 2R cycle performs as well as the single-effect cycle at low boiler temperatures and as well as the double-effect at high temperatures. The COP of the 2R cycle increases continuously with increasing boiler temperature. While this is slightly less than the COP of the 1R cycle, the configuration of the 2R cycle is less complicated and only a careful cost analysis of the two systems can determine which one is better. The absence of cut-off temperatures is a feature of the 2R cycle that makes it superior to double-effect cycles. Figs. 4 and 5 show that at low condenser– absorber temperatures, T 0 , and high boiler temperatures, the COP of the 2R cycle can be substantially higher than the basic double-effect cycle and for the 1R cycle it can be even much higher. These figures also show that 2R and 1R cycles can be used as heat pump for heating purposes since they have an acceptable COP at T E of about 48C.
Multi-pressure absorption cycles in solar refrigeration
41
Fig. 4. Calculated performance of 1R cycle using NH 3 –H 2 O mixtures (Dao, 1978a,b).
3. ECONOMIC CONSIDERATIONS
3.1. Cost and performance goals Since energy costs will continue to rise in the future, higher efficiency energy conversion equipment is being developed and used. The competition of non-solar devices is as smart as the solar advocates are, consequently the main competition will be advanced, high-efficiency, electric-driven chillers for cooling. It is therefore necessary to develop advanced, high-efficiency, gas-driven
chillers for refrigeration which have the following characteristics: (a) to be able to compete in the marketplace with electric-driven units; (b) to provide heat driven chillers to which solar-collected thermal energy can couple. In fact the solar-collected heat must be delivered to the energy conversion device (chiller) for the same cost as gas-produced heat is delivered. Ideally, the solar cooling research has a twofold objective: firstly, to develop an efficient chiller that will operate on gas input (variable
Fig. 5. Calculated performance of 2R cycle using NH 3 –H 2 O mixtures (Dao, 1978a,b).
42
S. Alizadeh
Fig. 6. Basic single effect (NH 3 –H 2 O) and conventional double-effect (H 2 O–LiBr) absorption cycles.
temperature). Secondly, to develop solar collector subsystems (collectors, piping, controls) that can drive these chillers on an economically competitive basis with gas heat sources.
3.2. Electric power from solar Solar energy may be converted to electrical energy using photovoltaic cells. The conversion efficiency is normally around 15%, which can vary up to 30%, and the cell efficiency is strongly temperature dependent. It is technically feasible to produce electricity using photovoltaic cells. The technology to manufacture cells and collectors exists, and the transmission of DC electricity and its conversion to AC electricity are well known. The barrier to implementation is economic. The cost of solar cells with 15% efficiency and mass produced is expected to be about $220 / m 2 of cell area. As an example, for a system using a flat plate array, the total cost for photovoltaic cells for a power plant with a capacity of 20,000 kW would be $460310 6 . If a concentrating system were used with a concentration ratio of 50:1, the cell cost would be $444 / m 2 . However, collector costs will probably be $110–$220 / m 2 , and so the system costs would be in the range of $230310 6 to $460310 6 . Photovoltaic conversion to electricity can be
cost effective if cell and collector prices are as low as expected. Also the technical problems of dissipating heat from the cells and efficiently transmitting electricity from the cells to the distribution system still remain. The power produced is in phase with refrigeration demands which reduces the need for conventional refrigeration systems. It appears that photovoltaic systems will become a competitive electricity generating system.
3.3. Status of high efficiency solar cooling and outlook for success In order to compare the different systems in terms of the thermal and electrical energy input, an effective electrical COP (EFFECOP) can be computed for each technical option assuming that the ratio of electrical energy cost to thermal energy cost is four. Those systems with higher effective electrical COPs will have lower energy costs (see Table 1). This comparison indicates that the ideal 1R cycle has the highest effective electrical COP and consequently the lowest cost (Warren and Wahlig, 1991). 4. CONCLUSIONS
In this study regenerative absorption chillers
Multi-pressure absorption cycles in solar refrigeration
43
Table 1. Comparison of effective electrical COPs of different technical options (Warren and Wahlig, 1991) Chiller type
(a) Electrical performance
Reciprocating chiller Centrifugal chiller Advanced absorption Ideal absorption Ideal absorption
TG 8C
Etotal kW
ECOP
140 140 200
271.4 173.1 87.1 85.6 82.6
2.04 3.59 10.75 11.12 11.91
with multi-pressure cycle were analysed for solar operation and compared with conventional absorption chillers. Basic absorption refrigeration cycles have two undesirable characteristics; namely the low coefficient of performance and the high cut-off input temperature below which the cycle ceases to operate. Two solutions to the problem of low COP and cut-off phenomena have been finalised. The first solution is the double-effect regenerative absorption refrigeration cycle (2R cycle). The second solution is the single-effect regenerative absorption cycle (1R cycle). The performance characteristics of the two cycles are about equal. The approach adopted for development of the advanced absorption chillers is to make use of the 2R cycle system first, because it is relatively simpler and therefore easier to investigate technically. Since the 1R cycle system uses processes similar to those of the 2R cycle, the success of the 2R cycle system should greatly enhance the success of the 1R cycle. Based on an economical analysis a comparison has been made amongst different systems in terms of thermal and electrical energy input. The effective electrical COP is computed for each technical option. Those systems with higher effective electrical COPs will have lower energy costs. This comparison shows the highest effective electrical COP for the ideal single-effect regenerative absorption chiller and therefore the lowest energy cost. Photovoltaic conversion to electricity can be cost effective in solar refrigeration if collector prices are as low as expected. Costs are being reduced and the price is nearly competitive with current conventionally generated electricity. It appears that photovoltaic systems will become a competitive electricity generating system. NOMENCLATURE COP ECOP
(b) Thermal performance
coefficient of performance electrical coefficient of performance
QG kW
390 330 241
TCOP
EFFECOP
1.19 1.41 1.93
2.04 3.59 3.30 3.74 4.68
EFFECOP Etotal I r TCOP T a ´ hC
effective electrical coefficient of performance total electrical use (kW) 2 hourly solar insolation (kJ / h m ) ratio defined by Eq. (1) (8C h m 2 / kJ) thermal coefficient of performance temperature (8C) absorptivity of the collector surface emittance of glass cover of the collector collector efficiency
Subscripts A C E G 0
absorber condenser evaporator generator condenser, absorber or ambient
Acknowledgements—The author wishes to thank Dr. I.L. Maclaine-Cross for many valuable suggestions and discussions during the period of this research.
REFERENCES Anderson P. P. (1976) Solar operation of ammonia–water chiller, a comparison of aqueous lithium-bromide and aqua ammonia cycles. ASHRAE Trans. 82(Part 1), 959–965. Chinnappa J. C. V. (1974) Solar operation of ammonia–water multi-stage air-conditioning cycles in the tropics. Solar Energy 16, 165–170. Dao K. (1978a). A New Absorption Cycle: The Single-Effect Regenerative Absorption Refrigeration Cycle, Lawrence Berkeley Laboratory, University of California, Berkeley, CA, USA. Dao K. (1978b). Conceptual Design of an Advanced Absorption Refrigeration Cycle: The Double-Effect Regenerative Absorption Refrigeration Cycle, Lawrence Berkeley Laboratory, University of California, Berkeley, CA, USA. Duffie J. A. and Beckman W. A. (1980). Solar Engineering of Thermal Processes, Wiley, New York, Chapters 6 and 8. Lof G. O. G. and Ward D. S. (1975). Performance of a Residential Solar Heating and Cooling System, Energy Research and Development Administration, Report No. C. 00-2577-9. Mitchell J. W. (1983). Energy Engineering, Wiley, New York, Chapter 12. Mumma S. A. and Sepsy C. F. (1976) Solar absorption air conditioning performance in central Ohio. ASHRAE Trans. 82(Part 1), 825–833. Phillips B. A. (1976) Absorption cycles for air-cooled solar air conditioning. ASHRAE Trans. 82(Part 1), 966–974. Sheridan N. R. (1972) Performance of the Brisbane solar house. Solar Energy 13, 395–401. Ward D. S. et al. (1976) Cooling subsystem design in C.S.U. solar house III. In Joint Conference, American Section, I.S.E.S., Winnipeg, Canada, pp. 68–89.
44
S. Alizadeh
Ward D. S. (1979) Solar absorption coding feasibility. Solar Energy 22(3), 259–268. Warren M. L. and Wahlig M. (1991). Analysis and Comparison of Active Solar Dessiccant and Absorption Systems ( Part I), Lawrence Berkeley Laboratory, Berkeley, CA, USA.
Whitlow E. P. (1976) Relationship between heat source temperature, heat sink temperature and coefficient of performance for solar powered absorption air conditioners. ASHRAE Trans. 82(Part 1), 950–958. Yazaki T. J. (1978) Solar air conditioning system — a case study. In I.S.E.S. Symposium, Melbourne.