heat from a space to be cooled to a high temperature sink (place to be heated). Low

Transcription

1 CHAPTER 2 Literature Review 2.1 Introduction A refrigeration system utilizes work supplied by an electric motor to transfer heat from a space to be cooled to a high temperature sink (place to be heated). Low temperature boiling fluids called refrigerants absorb thermal energy to get vaporized in the evaporator causing a cooling effect in the region being cooled. While comparing the advantages and disadvantages of various cooling systems, two most important parameters i.e the operating temperature and the coefficient of performance are of vital importance in these systems. These systems can be evaluated using energy and exergy analyses which are based on first and second law of thermodynamics, respectively and have been described in the previous chapter in detail. An extensive review of the literature has been done on different refrigeration and heat pump systems in present chapter. The main idea was to have possible future direction of research. The literature review has been classified as under: 1. Vapor Absorption Refrigeration Systems. 2. Vapor Compression Refrigeration Systems. 3. Vapor Compression-Absorption Refrigeration Systems. 2.2 Vapor Absorption System Vapor Absorption system is an attractive method for utilizing low grade energy directly for cooling. This is an important advantage as against the conventional vapor 17

2 compression system which operates on high grade energy. Another important feature of these systems is that it does not use any moving component except a very small liquid pump. Vapor absorption system consists of four basic components viz. an evaporator, an absorber (located on low pressure side), a generator and a compressor (located on high pressure side). A refrigerant flows from the condenser to the evaporator, then via absorber to the generator and back to condenser, while the absorbent passes from absorber to the generator and back to absorber. For maximum efficiency, the pressure difference between the low pressure side and high pressure side is maintained as small as possible. Although, the initial cost of these systems is at present higher but their operating expenses are often appreciably lower, which can further be reduced if efficient absorption and distillation can be achieved. Since, the efficiency of these processes is determined largely by thermodynamic properties of the refrigerant absorbent combination, an extensive study of these properties is of utmost importance in the development of an efficient absorption refrigeration cycle. A large number of researchers have carried out research in the field of vapor absorption refrigeration using different working pairs and the most common working pairs are LiBr-H 2 O and NH 3 -H 2 O. Alizadeh et al [1] carried out theoretical study on design and optimization of water lithium bromide refrigeration cycle. They concluded that for a given refrigerating capacity higher generator temperature causes high cooling ratio with smaller heat exchange surface and low cost. There is a limiting factor for water lithium bromide cycles because of the problem of crystallization. Anand and Kumar [2] carried out availability analysis and calculation of irreversibility in system components of single and double effect series flow water lithium bromide absorption 18

3 systems. The assumed parameters for computation of results were condenser and absorber temperature equal to 87.8 o C and o C for single effect and double effect systems respectively. Tyagi [3] carried out the detailed study on aqua-ammonia VAR system and plotted the coefficient of performance, mass flow rates as a function of operating parameters i.e. absorber, evaporator and generator temperatures. He showed that COP and work done are the function of evaporator, absorber, and condenser and generator temperature and also depends on the properties of binary solution. Ercan and Gogus [4] showed the irreversibility s in components of aqua-ammonia absorption refrigeration system by second law analysis. They calculated the dimensionless exergy loss of each component, exergetic coefficient of performance, coefficient of performance and circulation ratio for different generator, absorber evaporator and condenser temperature. They concluded that aqua-ammonia system needs a rectifier for high ammonia concentrations but it will lead to additional exergy loss in the system. They observed the highest exergy loss in evaporator followed by absorber. I was also concluded that the dimensionalless total exergy loss depends on generator temperature. Oh et al [5] investigated a gas fired, air cooled LiBr/H 2 0 double effect parallel flow type absorption heat pump of 2TR being used as an air conditioner. They investigated the performance of the absorption heat pump in the cooling mode through cycle simulation. They obtained the system characteristics depending on the inlet temperature of air to the absorber, the working solution concentration, the solution distribution ratio of the mass of the solution into the first generator to he total mass of 19

4 the solution from the absorber, and the leaving temperature differences of the heat exchanging components. They concluded that there exists a critical value of the solution distribution ratio that maximizes the cooling performance of the system. Aphornratana and Eames [6] investigated single effect water lithium bromide system using exergy analysis approach. It was shown that the irreversibility in generator was highest followed by absorber and evaporator. Bell et al [7] developed a LiBr-H 2 0 experimental absorption cooling system driven by heat generated by solar energy. The components of the system are housed in evacuated glass cylinders to observe all the processes. They determined the thermodynamic performance of the system by applying mass and energy balance for all the components. Their work was based on the assumption that the working fluids are in equilibrium and the temperature of the working fluid leaving the generator and absorber is equal to the temperature of generator and absorber respectively. They concluded that the COP of the system depends on generator temperature and there is optimum value of generator temperature at which COP is maximum. They also concluded that by operating the system at low condenser and absorber temperatures a satisfactory COP is obtained at a generator temp. as low as 68 o C. Horuz [8] explained the fundamental vapour absorption refrigeration system and carried out comparative study of such system based on ammonia-water and water lithium bromide working pairs. The comparison of two systems is presented in respect of COP, cooling capacity and maximum and minimum pressures. He concluded that VAR system based on water-lithium bromide is better than ammonia-water. However, problem of crystallization lies with water-lithium bromide system. 20

5 Kumar et al [9] studied in detail the exergy variation in solar assisted absorption system. They found that rise in first generator heat transfer, decreases the heat transfer in second stage generator. The increase in generator-ii temperature decreases the exergy and energy transfer rates at the condenser. They concluded that the availability at the devices varies with respect to quality of the device. Talbi and Agnew [10] carried our exergy analysis on single effect absorption refrigeration cycle with lithium bromide water as the working fluid pairs. They developed a computer simulation model based on heat and mass balance, heat transfer equations and thermodynamic properties. The cycle collects free energy from the exhaust of diesel engine. They calculated the dimensionless total exergy loss and exergy loss of each component. They found that the absorber has the highest exergy loss of 59.06% followed by generator. They concluded that the absorption refrigeration cycle is effective in demonstrating the advantages of exergy process which are other wise not accounted in the heat balance method. Lee and sheriff [11] carried out the second law analysis of a single effect water lithium bromide absorption refrigeration system. The effect of heat source temperature on COP and exergetic efficiency was evaluated. However, they did not analyzed effect of variation in absorber and condenser temperatures and also the effectiveness of solution heat exchanger was also not specified. Lee and sheriff [12] carried out the second law analysis of single effect and various double effect lithium bromide water absorption chillers for chilled water temperature of 7.22 o C and cooling water temperatures 29.4 o C and 35 o C and computed COP and exergetic efficiency. The effect of heat source temperature on COP and exergetic efficiency was investigated. In 21

6 this study, the effectiveness values of solution heat exchangers considered for analysis has not been specified and their results are only valid for water cooled systems. Sozen [13] studied the effect of heat exchangers on the system performance in an ammonia water absorption refrigeration system. Thermodynamic performance of the system is analyzed and the irreversibility s in the system components have been determined for three different cases. The COP, ECOP, circulation ratio, and non dimensional exergy loss of each component of the system is calculated. They concluded that the evaporator, absorber, generator, mixture heat exchanger and condenser show high non-dimensional exergy losses. They also concluded that using refrigerant exchanger in addition to mixture heat exchanger does not increase the system performance. Fernandez-Seara and Vazquez [14] studied the optimal generator temperature in single stage ammonia water absorption refrigeration system. They studied the behavior of this temperature on thermal operating conditions and system design parameters. They carried out study based on parametric analysis by developing a computer program and based on the results designed a control system. The control system developed maintains a constant temperature for the space to be refrigerated and also control the optimal temperature in the system generator. De Francisco et al [15] developed and tested the prototype of a 2kW capacity water ammonia absorption system operating on solar energy for rural applications. The system also suffered from leakages in different components and need further improvements. They concluded that the efficiency of the system is very low. The new and improved prototype has to be developed. Horuz and Callander [16] described the 22

7 experimental investigation of the performance of a commercially available absorption refrigeration system. The system is natural gas fired with a capacity of 10 kw. They studied the response of the refrigeration system to variations in chilled water temperature, chilled water level in evaporator drum; chilled water level flow rate and variable heat input are presented. They concluded that lower the energy input, lower will be the cooling effect. De Lucas et al [17] studied the use of alternative absorbent used in absorption refrigeration cycles to replace the existing absorbent. New absorbent used is a mixture of lithium bromide and potassium formate in a 2:1 w/w. The performance of the system is compared by developing a program. They concluded that less energy is required in the generator and due to this; waste heat with a temperature of K is required. The efficiency of the system is increased and the new absorbent is less corrosive and less expensive to manufacture. Sencan et al [18] carried out the exergy analysis of a single effect water lithium bromide absorption refrigeration system and calculated the exergy losses in the system components. The effect of heat source temperature on COP and exergetic efficiency was computed. They did not analyze the effect of variation in absorber and condenser temperature. They concluded that the cooling and heating COP of the system increases slightly when increasing the heat source temperature but the exergetic efficiency of the system decreases when increasing the heat source temperature for both cooling and heating applications. Kilic and Kaynakli [19] investigated single effect and series flow double effect vapour absorption systems using energy analysis approach. The effect of different parameters such as generator temperature, absorber temperature, condenser 23

8 temperature, solution circulation ratio and solution concentration etc had been investigated by these researchers on COP. Their results revealed that COP of double effect absorption refrigeration system is higher in comparison to single effect system. Kilic and Kaynakli [20] used the first and second law of thermodynamics to analyze the performance of a single stage water lithium bromide absorption refrigeration system by varying some working parameters. They introduced a mathematical model based on exergy method. They found that the performance of the ARS increases with increasing generator and evaporator temperatures but decreases with increasing condenser and absorber temperatures. They concluded that the highest exergy loss occurs in generator regardless of operating conditions and therefore it is most important component of the system. Gong et al [21] presented the method of product exergy cost for scheme selection optimization of cooling and heating source system of air conditioning system. They developed the optimization algorithm which adopts an integrative; multiple objective decision method with the analysis of the product exergy cost and concluded that the method is scientific and reliable. Kaynakli and Yamankaradeniz [22] studied the single effect VA system on the basis of entropy generation method. Kaynakli and Yamankaradeniz [22] performed calculations for a 10kW cooling load system. The evaporator and condenser temperature was taken as 4 o C and 38 o C respectively. The generator temperature was taken as 90 o C. Effectiveness of solution heat exchanger was assumed as 0.5 and efficiency of pump was assumed equal to 0.9. They concluded that entropy generation of the generator is an important fraction of the total entropy generation in the system basically due to the temperature difference between 24

9 the heat source and the working fluid and in order to decrease the total entropy generation of the system, the generator should be developed Morosuk and Tsatsaronis [23] used an absorption refrigeration machine to represent splitting the exergy destruction into endogenous/exogenous and unavoidable/avoidable parts and this is new development in the exergy analysis of energy conversion systems. They concluded that advanced exergetic evaluation of an ARM supplies useful additional information which is not provided by exergy analysis. The avoidable exergy destruction identifies the potential for improving each system component. Gomri and Hakimi [24] carried out exergy analysis of double effect lithium bromide/water absorption refrigeration system. They showed that the performance of the system increases with increasing LP generator temperature, but decreases with increasing HP generator temperature. They concluded that the highest exergy loss occurs in the absorber and in the HP generator and therefore the absorber and HP generator is the most important component of the double effect refrigeration system. Gomri [25] carried out the comparative study between single effect and double effect absorption refrigeration systems. They developed the computer program based on energy balances, thermodynamic properties to carry out thermodynamic analysis. They concluded that for each condenser and evaporator temperature, there is an optimum generator temperature where change in exergy of single effect and double effect absorption refrigeration system is minimum. Their study showed that the COP of double effect system is approximately twice the COP of single effect system but there is marginal difference between the exergetic efficiency of the system. Kaushik and Arora [26] presented the energy and exergy analysis of single effect and series flow 25

10 double effect water lithium bromide absorption system. They developed the computational model for parametric investigation. Their analysis involves the effect of generator, absorber and evaporator temperatures on the energetic and exergetic performance. They concluded that the irreversibility is highest in the absorber in both systems as compared to other systems. Zhu and Gu [27] used the first and second law of thermodynamic to analyze the performance of ammonia sodium thiocyanate absorption system for cooling and heating applications. A mathematical model based on exergy analysis was developed. The performance of the system is analyzed using different operating conditions. They concluded that the cooling and heating COP increases with increasing generator and evaporator temperatures but it decreases with increasing condenser and absorber temperatures. Garousi Farshi et al [28] developed a computational model to study and compare the effects of operating parameters on crystallization phenomena in three classes of double effect lithium bromide water absorption refrigeration systems (series, parallel and reverse parallel) with identical refrigeration capacities. They concluded that the range of operating conditions without crystallization risks in the parallel and the reverse parallel configurations is wider than those of the series flow system. Behrooz and Ziapour [29] carried out thermodynamic analysis of a diffusion absorption refrigeration heat pipe (DARHP) cycle. A computer code was developed for an ammonia water DARHP cycle with helium as the auxiliary inert gas using EES software. The second law efficiency was examined parametrically by the computer simulation. They validated the model by comparison with previously published experimental data for DARHP system. The cycle performance results under different conditions indicated that the best performance was obtained for the 26

11 concentration rich solution of 0.35 ammonia mass fraction and the concentration of weak solution about 0.1. They concluded that the exergy losses in the evaporator, condenser and dephlegmator were small. Also the second law efficiency increases with increasing evaporator temperature; and decreases with increasing thermosyphon temperature. Khaliq et al [30] carried out first and second law investigation of waste heat based combined power and ejector-absorption refrigeration cycle using R141b as a working fluid. Estimates for irreversibilities of individual components of the cycle lead to possible measures for performance improvement. Results show that around 53.6% of the total input exergy is destroyed due to irreversibilities in the components, 22.7% is available as a useful exergy output, and 23.7% is exhaust exergy lost to the environment, whereas energy distribution shows 44% is exhaust energy and 19.7% is useful energy output. They concluded that proposed cogeneration cycle yields much better thermal and exergy efficiencies than the previously investigated cycles and the current investigation clearly show that the second law analysis is quantitatively visualizes losses within a cycle and gives clear trends for optimization. 2.3 Vapour Compression System In vapor compression system there are four major components: evaporator, compressor, condenser and expansion device. Power is supplied to the compressor and heat is added to the system in the evaporator, whereas in the condenser heat rejection occurs. Heat rejection and heat addition are dissimilar to different refrigerants. A standard vapor compression cycle consists of four processes viz. a 27

12 reversible adiabatic compression from the saturated vapor to the compressor pressure followed by a reversible heat rejection at constant pressure causing de-superheating and condensation. This is further extended to an irreversible expansion at constant enthalpy from saturated liquid to evaporator pressure and there after a reversible heat addition at constant pressure causing evaporation to saturated vapor. Keshwani and Rastogi [31] determined the optimum interstage pressure in a two stage VCR system for refrigerant CFC12. They concentrated their research on minimization of overall compressor work. Arora and Dhar [32] used the discrete maximum principle, discusses by Katz (1962), to solve the problem of optimum interstage pressure allocation in multistage compression systems for R-12, with and without intercooling between the stages. They concluded that the optimum interstage pressure approximately equals the geometric means of the condensation and evaporation pressure but when the flash inter cooler was incorporated, they found a considerable difference between the geometric means and the optimal pressure values. Prasad [33] determined the optimum interstage pressure in a two stage vapour compression refrigeration system for the refrigerant R-12 with a view to maximize the COP. They concluded that the inter-stage temperature of a two-stage refrigeration cycle is given by the geometric mean of the condensation and evaporation temperatures. Kumar et al [34] explained a method of carrying out exergy analysis on a vapour compression refrigeration system using R-11 and R-12 as refrigerants. They presented the exergy-enthalpy diagrams to facilitate the analysis. They explained the procedure to calculate various losses in different components of the system. 28

13 McGovern and Harte [35] presented an exergy method for compressor analysis. This is used to find and quantify defects in the use of compressor shaft power and will lead to the improvement in design of the compressor. The exergy destruction and its location are identified. They analyzed the refrigerant compressor using R-12 as refrigerant. They presented the graphs of the instantaneous rates of exergy destruction. They concluded that it is particularly suitable for applications in computer simulation of compressors and provides a sound basis for design optimization. Zubair and Khan [36] showed that the optimum interstage pressure for a two stage refrigeration system can be approximated by the saturation pressure corresponding to the arithmetic mean of the condensation and evaporation temperatures. Zubair et al [37] found that optimum interstage pressure for refrigerant R-134a for maximum COP of the system was close to the saturation pressure corresponding to the arithmetic mean of the refrigerant condensation and evaporation temperatures. They showed that the system irreversible losses are lowest at an intermediate saturation temperature near to arithmetic mean of the condensation and evaporation temperatures. Aprea et al [38] reported that vapour compression refrigeration systems are widely used for cold storage and super market refrigeration. The suitable working fluid for these applications is the refrigerant R-502 which is an azeotropic mixture of refrigerants R-22 and R-115. Aprea et al [39] experimentally evaluated the general characteristics and system performances of substitutes for R-502 in a refrigeration plant. They examined different refrigerants such as R-402A, R-402B, R-403B, R-408A, R-404A, R-407A and FX-40. All the refrigerants showed performances very close to those of R-502 except 29

14 R-403B whose COP was found to be about 8% lower than that of R-502. They concluded that the above refrigerants can be used as substitutes to R-502. Doring et al [40] carried out an experimental study of R-507 to measure thermodynamic data. They presented the data in the form of mathematical correlations. Their theoretical results show that the compressor discharge temperature for R-507 was approximately 8K below in comparison to R-402. Camporese et al [41] experimentally investigated the mixtures such as HC290/HFC134a, HFC125/HC290/HFC134a, FC125/HFC143a/HC290, HFC125/HFC143a/HCC270 and HFC32/HFC125/HFC143a for their influence on the solubility of various lubricant oils by measuring critical solubility temperatures. The experiments were conducted to compare refrigerating capacity, COP, discharge temperature and mass flow rates. The mixtures selected for new units were the mixture of HFC143a/HC290/R-22 showed the best performance and its COP and cooling capacity were found to be higher in comparison to R-502 Nikolaidis and Probert [42] used exergy analysis to investigate the behaviour of two stage compound compression cycle with flash intercooling using R-22 as refrigerant by varying the condenser saturation temperature and evaporator saturation temperature from 298 to 308 K and 238 to 228 K respectively. They determined the effect of temperature change in condenser and evaporator on plants irreversibility rate. They concluded that the changes in the temperatures of condenser and evaporator significantly effect the plants overall irreversibility and therefore the system needs optimization. Sami and Desjardins [43] carried out performance evaluation of R-407B, R-507, R-408A and R-404A as substitutes to R-502. Their results revealed that R- 408A blend has a superior performance than R-502 but it is characterized by high 30

15 discharge pressure. Ratts and Brown [44] used entropy generation minimization method to compute the optimum reduced intermediate temperature for R-134a in a two stage VCR system. They concluded that this method gives better results than geometric means method for evaluation of interstage temperature charge pressure compared to R-502. Rakehesh et al [45] carried out experimental study on a heat pump with different refrigerant R-22, R-407C and R-407A. They concluded that R- 407C heat pump chiller systems offered higher exergy efficiency than those operating with R-22 and in the case of R-407A systems; the exergy efficiency was higher than that of HCFC at condensing temperatures less than 50 o C. Aprea et al [46] studied the performance of a VCR experimental plant both as water chiller and as a heat pump using R-22 and its substitute R-417A. The use of R-417A does not require lubricant change and equipment redesign. The results showed that the COP and exergetic efficiency of the plant is higher for R-22 than R-417A. Aprea and Renno [47] experimentally investigated the energetic and exergetic performance of a VCR plant for cold storage application using both R-22 and its substitute R-417A. The results showed that COP was 15% greater than R-417A where as exergy destruction for R-417 was greater than R-22. Areaklioglu et al [48] numerically calculated the rational efficiency and components based irreversibility ratios of a cooling system based on the second law of thermodynamics using HFC and HC based pure refrigerants such as HFC32, HFC125. HFC134a, HFC143a, HFC152a, HC290, HC600a and there binary and turnery mixtures, along with CFC12, R-22 and R-502. The effect of temperature glide, occurring at the condenser and evaporator, on the rational efficiency of the cooling system was evaluated. The irreversibility in 31

16 condenser was found to varying between 40-55% of the total irreversibility. The results also suggest that for both binary and ternary mixtures the rational efficiency increases against temperature glide. Xuan and Chen [49] carried out an experimental study of a ternary near azeotropic mixture of HFC161 as an alternative to R-502. Without any modification to system components, experimental tests were performed on a vapour compression refrigeration plant with a reciprocating compressor which was originally designed to use R-404A, a major substitute for R-502. The experimental results under two different rated working conditions indicated that the pressure ratios of this new refrigerant were nearly equal to those of R-404A. Under lower evaporative temperature, its COP was almost equal to that of R-404A and its discharge temperature was found to be slightly higher than that of R-404A, while under higher evaporative temperature, its COP was found was found to greater than that of R-404A and its discharge temperature was lower than that of the latter. This new refrigerant achieved a high level of COP and hence was considered as a promising retrofit refrigerant to R-502. Park and Jung [50] studied two pure hydrocarbon refrigerants, R-1270 (Propylene) and R-290(Propane) and three binary mixturescompared to R-1270, R- 290 anf R-152a were tested in a refrigerating bench tester with a scroll compressor I an attempt to substitute R-502. The results showed that all refrigerants tested had 9.6 to 18.7% higher capacity and 17.1 to 27.3% higher COP than R-502. There was problem with mineral oil. They concluded that these alternative refrigerants offer better system performance and reliability than R-502. They studied the thermodynamic performance of two pure hydrocarbons and seven mixtures composed of propylene 32

17 (R-1270), Propane (R-290), HFC152a and dimethyl ether (RE170,DME) in an attempt to substitute R-22 in residential air conditioners. The mixtures are all near azeotropic having the gliding temperature difference of less than 0.6 o C. Test results revealed that the COP of these mixtures was up to 5.7% higher than that of R-22. The compressor discharge temperatures were reduced by o C with these fluids. There was no problem found with mineral oil since the mixtures were mainly composed of hydrocarbons. These fluids provide good thermodynamic performance with reasonable energy saving. Arora et al [51] carried out parametric investigation of actual vapour compression refrigeration cycle in terms of COP, exergy destruction and exergy efficiency for R-22, R-407C and R-410A by developing a computational model. The results showed that COP and exergy efficiency for R-22 are higher in comparison to R-407C and R-410A. It was concluded that R-410A is better alternate as compared to R-407C with high coefficient of performance and low exergy destruction ratio when considering for refrigeration applications. For air conditioning application R-407A is better alternative than R-410A. Park et al [52] experimentally tested the thermodynamic performance of R-433A, R-432A for possible replacement to R-22 in a heat pump bench tester under air conditioning and heat pumping conditions. The test results showed that the COP of R-433A was % higher than that of R-22 while the capacity of R-433A was found to be % lower than that of R-22 under both test conditions. The COP of R-432A was found to be % higher than HCFC and its refrigerating capacity was % higher than that of R-22 under both test conditions. The compressor discharge temperatures of R-432A and R-433A were 33

18 lower than that of R-22. The amount of charge required for both of these refrigerants were 50-57% lower than that of R-22 due to their low density. They concluded that these refrigerants are good long term environmental friendly alternatives to replace R-22 in residential air conditioners and heat pumps due to their excellent thermodynamic and environmental properties with minor adjustments. However, they did not comment on the compatibility of these refrigerants with lubricating oil. Palm [53] reported tat vapor pressure curves of the propane and propene are quite similar to those of R-22 and ammonia, indicating that the application areas would be same. Recently, air conditioning provided by ammonia refrigeration systems has found application on college campuses and office parks, small scale building such as convenience stores, and larger office buildings. These applications have been achieved by using water chillers, ice thermal storage units and district cooling systems. Pearson [54] specified that ammonia is widely used in industrial systems for food refrigeration, cold storage, distribution ware housing and process cooling. It has more recently been proposed for use in applications such as water chilling for air conditioning systems. Bhattacharyya et al [55] carried out analysis of an endoreversible two-stage cascade cycle and obtained an optimum intermediate temperature for maximum exergy and refrigeration effect. They developed a comprehensive numerical model of a transcritical CO 2 -C 3 H 8 cascade system. The cycle was optimized with the operating temperatures and the results obtained were in line with the simulation results. Arora and Kaushik [56] presented a detailed exergy analysis of an actual vapour compression refrigeration cycle. They developed a computational model for 34

19 computing coefficient of performance, exergy destruction, exergetic efficiency and efficiency defect for different refrigerants. They concluded that R-507A is a better substitute to R-502 than R-404A and the efficiency defect in condenser is highest and lowest in liquid vapour heat exchanger for the refrigerant considered. Mafi et al [57] carried out exergy analysis for multistage cascade low temperature refrigeration systems used in olefin plants. They developed the equations of exergy destruction and exergetic efficiency for heat exchanger, compressors and expansion valves. The relations for total exergy destruction in the system and the system overall exergetic efficiency are obtained. They also developed the expression for minimum work requirement for cascade low temperature refrigeration used in olefins plants. They determined the overall exergetic efficiency to be 30.88% Dopazo et al [58] reported the analysis of the parameters in design and operation of a CO 2 /NH 3 cascade cooling system and their effect on system COP and exergetic efficiency. They carried out the analysis based on general mathematical model which was validated using experimental results. They concluded that for specific installation, the isentropic efficiency for each compressor in cascade system should be determined as accurately as possible from the manufacturer or experimental data in order to obtain reliable values for the optimum CO 2 condensing temperature and maximum COP. Miguel Padilla et al [59] did exergy analysis of the impact of direct replacement of R-12 with zeotropic mixture R-413A on the performance of a domestic vapour compression refrigeration system originally designed to work with R-12 using a simulated analysis model. They concluded that the overall energy and exergy performance of system working with R-413A is better than R

20 Qureshi and Zubair [60] investigated the performance degradation due to fouling in a vapor compression cycle for various applications. The investigation was carried out using refrigerants R-134a, R-410A and R-407C. The first law analysis indicates that R-134a always performs better unless only the evaporator is being fouled. However, the second law shows that R-134a performs the best in all cases. While considering the second set of refrigerants i.e. R-717, R-404A and R-290. The first law shows that R-717 always performs better unless only the evaporator is being fouled. In contrast to this, from a second-law standpoint, the second-law efficiency indicates that R-717 performs the best in all cases. Volumetric efficiency of R-410A and R-717 remained the highest under the respective conditions studied. Furthermore, performance degradation of the evaporator often has a larger effect on compressor power requirement while that of the condenser has an overall larger effect on the COP. Sayyaadi and Nejatolahi [61] considered a cooling tower assisted vapor compression refrigeration machine has for optimization with multiple criteria. Two objective functions, the total exergy destruction of the system and the total product cost of the system are considered as thermodynamic and economic criterion respectively, have been considered simultaneously. They developed the thermodynamic model based on energy and exergy analyses and an economic model according to the Total Revenue Requirement (TRR) method. The exergetic and economic results obtained for three optimized systems have been compared and discussed. They concluded that the multi-objective design more acceptably satisfies generalized engineering criteria than other two single-objective optimized designs. 36

21 Zhu and Jiang [62] developed a refrigeration cycle which combines a basic vapor compression refrigeration cycle with an ejector cooling cycle. The ejector cooling cycle is driven by the waste heat from the condenser in the vapor compression refrigeration cycle. The additional cooling capacity from the ejector cycle is directly input into the evaporator of the vapor compression refrigeration cycle. The governing equations are derived based on energy and mass conservation in each component including the compressor, ejector, generator, booster and heat exchangers. They concluded that the COP is improved by 9.1% for R-22 system. Qureshi and Zubair [63] investigated the performance characteristics due to use of different refrigerant combinations in vapor compression cycles with dedicated mechanical sub-cooling. For basic designs, R-134a used in both cycles produced the best results in terms of COP, COP gain and relative compressor sizing. In retrofit cases, considering the high sensitivity of COP to the relative size of heat exchangers in the sub-cooler cycle and the low gain in COP obtained due to installation of a dedicated sub-cooling cycle when R-717 is the main cycle refrigerant, it seems that dedicated mechanical sub-cooling may be more suited to cycles using R-134a as the main cycle refrigerant rather than R-717. With R-134a as the main cycle refrigerant, no major difference was noted, by changing the sub-cooler cycle refrigerant, in the degradation of the performance parameters such as COP and cooling capacity, due to equal fouling of the heat exchangers. A cascade system consists of two independently operated single-stage refrigeration systems: a lower system that maintains a lower evaporating temperature and produces a refrigeration effect and a higher system that operates at a higher 37

22 evaporating temperature. These two separate systems are connected by a cascade condenser in which the heat released by the condenser in the lower system is extracted by the evaporator in the higher system. Wang et al. [64] examined the potential of a double-stage coupled heat pumps heating system, whereby an air source heat pump was coupled to a water source heat pump. Comparatively, they found that such a coupling process improved energy efficiency ratio by 20% compared to a purely air source heat pump. Satoru Okamoto [65] carried out the analysis of a heat pump system with a latent heat storage utilizing sea water installed in an aquarium. In this study a heat pump installed in an aquarium with latent heat storage potential using sea water is analyzed. This installation is quite helpful in maintaining the indoor conditions at constant temperature and humidity. In this study the comparison of the actual operating characteristics and efficiency of sea water source heat pump is carried out with two assumed conventional systems that are, an air source heat pump without ice storage and an oil-fired absorption refrigeration system. The results indicate that cost of generation of heat energy with sea water heat pump is significantly lower than that of air-source heat pump and the oil-fired heat pump and. The actual operating costs of sea water heat pump is 42% lesser than the air fired and oil fired heat pumps and the energy consumption for the generation of heat is also 19% lesser for the sea water heat pump. Also the emission of the harmful gases like CO 2 is also lesser for sea water heat pump when compared to air fired and oil fired heat pumps. Zhen et al. [66] carried out a study on the use of the ocean energy or sea water as heat source as well as sink for district cooling and heating in Dalin city of China. They 38

23 suggested that coastal areas are the best suitable location for the use of sea water source heat pump technology for both cooling as well as heating. The government helped in the commissioning of the plants both for heating as well as cooling with the capacity of 68 MW and 76 MW respectively for this study. In this study the economic, energy and environment impacts of the sea water source heat pump technology are analyzed. In this study, the system is compared with coal fired heating system and the conventional air-conditioning system in terms of the economic, energy and environment impacts. The economic impacts include the analysis of money through series of sensitivity cases which needs to be invested in different forms like annual cost, net present value used for the calculation of coal price, electricity price and interest rate of loans. The energy effects include the analysis of change in the sea water temperature which has been done with the simulation study of sea water temperature field with a two dimensional convection-diffusion equation. The results of the study indicates that the sea water source heat pump can be used for both heating as well cooling applications and has a great potential if used in other locations depending on the geographical conditions and local environment. Shu Haiwen et al. [67] carried out the study of the energy saving judgment of electricdriven sea water source heat pump district heating system over boiler house district heating system. They concluded that for renewable energy utilization system, the electricity driven heat pumps are gaining popularity, but the energy saving condition is still not clear. In this study, an expression of the critical COP of the heat pump system for energy saving is derived through the comparison of the system and conventional boiler house district heating system in the energy consumption aspects. Also, the 39

24 actual COP values of the heat pump unit are calculated by the experimental data regression model based on the details from the supplier of the heat pump. The comparison of the values of both COP s critical as well as actual brings out a judgment on the energy saving aspect of an electric driven sea water heat source pump for district heating. The results also indicate that both the heating radius and the natural conditions of sea water are the most important factors to determine the energy efficiency of the system. The results indicate that selection of sea water area should be done in a way that it should have the largest sea water temperature difference that could be utilized thereby decreasing the water head of the sea water pump. The type of fuel used in the boiler greatly influences the critical COP value of the heat pump. 2.4 Vapor Compression-Absorption System Vapor Compression-absorption heat pump/refrigeration cycle represents a cycle in which vapor is mechanically compressed, absorbed and then desorbed using a liquid solution cycle. These systems may be considered as hybrid systems between conventional vapor compression and vapor absorption systems. The hybrid vapor compression/absorption heat pump cycle combines two well known heat pump concepts, the compression heat pump and the absorption heat pump. It uses a mixture of refrigerants as the working fluid, one as the absorbent and the other as the desorbent. A key advantage of the hybrid heat pump is the extended range of temperatures available for a mixture compared to pure refrigerants. This is the effect of the reduced vapor pressure obtained for a refrigerant in a mixture with less volatile component. Another advantage is the gliding temperature obtained in the absorber 40

25 and desorber. It reduces irreversibility during heat exchange process between working fluids and results in improved system performance. Pourreza-Djourshari and Radermacher [68] presented the performance calculation of two vapour compression heat pump cycles, one with single stage solution circuit and the other with two stage solution circuit. The working fluid chosen was R-22-DEGDME. They found that both cycles show a significant increase in COP as compared to R-22. The results indicate that there is potential of control capacity by a ratio of 7:1, energy saving up to 50% and significant reduction in pressure ratio compared to conventional R-22 cycle. Radermacher [69] examined the performance of vapour compression heat pump cycle with desorber/absorber heat exchange working on R-22-R-113 mixture using successive substitution method. The results showed an improvement in the cooling COP by 57% and a reduction in pressure ratio by 69% compared to a conventional R-22 cycle. Stroker and Trepp [70] presented the first simulation model which includes the calculation of the overall heat transfer resistance. The heat transfer resistance has been calculated as a function of the mass flow rate for the working pair NH 3 -H 2 O from the experimental data. They presented design and experimental results of a compression heat pump with solution circuit. The test plant heats water from 40 to 70 o C and cools water from 40 to 15 o C. A COP of 4.3 was measured and an energy saving of 23% was achieved. George et al [71] studied the performance of compression-absorption heat pump working on R-22-Dimethyl formamide (DMF) through thermodynamic analysis. The heating COP, concentration difference and the circulation ratio are calculated by varying compression ratio and operating temperatures at the absorber and desorber. 41

26 The assumptions taken are that the absorbent does not evaporate in the considered temperature range to necessitate rectification; equilibrium conditions exist at the exit of absorber and desorber; effectiveness of heat exchanger is 100%; isentropic compression in compressor; isenthalpic expansion in pressure reducing valve and no heat losses and pressure drops in various components etc. They concluded that at certain operating conditions COP as high as 6 and temperature lift as high as 60 o C can be achieved. Amrane et al [72] developed two simulation models, one for vapour compression cycle with single stage solution circuit and another for vapour compression cycle with two stage solution circuit utilizing NH 3 -H 2 O mixture. The analysis of heat exchangers has been carried out by using UA values as input to program. Herold et al [73] analyzed a hybrid refrigeration cycle which combines a mechanical compressor and an absorption cycle using single evaporator. The analysis involves using the output from internal combustion engine efficiently. LiBr-H 2 O is used as working fluid and the cycle has been analyzed assuming oil free compressor. Although, these types of compressors are available, they are rarely used due to their high capital cost and low isentropic efficiency. High initial cost and low performance results in poor economics for the hybrid cycles. Some other assumptions taken in the analysis are no pressure drops in heat exchangers and pipes; all phases are in thermodynamic equilibrium. In this analysis also, only internal behavior of the cycle has been studied and external temperatures are not taken into account. Ahlby et al [74] carried out optimization study on the compression-absorption cycle operating on NH 3 -H 2 O mixture. The improvement in cycle performance which is 42

27 gained by optimizing the temperature gradient in the absorber is considerable particularly for situations with small external temperature gradient. The assumptions taken in the analysis are: saturated conditions at the desorber and absorber outlets; adiabatic absorption and desorption in the first part of the absorber and desorber, respectively, until equilibrium is reached; constant UA values for heat exchanger; no pressure drops and heat losses. The optimum point of operation is found by studying the changes in the compressor and pump and the heat loss obtained in the solution heat exchanger with the working conditions. They concluded that for each external situation, an optimum working condition can be found. The improvement in cycle performance gained by optimizing the temperature gradient in the absorber is considerable. A comparison of performance with the vapour compression cycle shows that compression-absorption cycle is better or equally good. Ahlby et al [75] studied the performance of compression-absorption heat pump with the ternary working fluid NH 3 -H 2 O-LiBr. At 60% by mass salt concentration, ternary mixture showed 10% better cycle performance than binary fluid NH 3 -H 2 O. The calculations are uncertain since the properties of such mixtures are estimated from properties for NH 3 -H 2 O and NH 3 -H 2 O-60%LiBr and have not been experimentally validated. Results indicate that the best mixture would be a solution with a salt content of about 40-50% by mass. Riffat and Shankland [76] described the integration of different types of absorption systems and vapour compression system. They analyzed the performance of such systems using various refrigerant/absorber pairs. Their study is concerned with the intermittent absorption system, intermittent absorption/vapour compression system and combined intermittent absorption/vapour compression 43

28 system. They concluded that integrated compression absorption systems could provide higher COP than individual systems Rane et al [77] compared the performances of four versions of two stage vapour compression heat pump with solution circuits. This represents a cascade system. They developed a computer simulation model based on heat and mass balances of each component. For heat exchangers, calculations, UA values have been taken as input parameter. Performance of the cycles has been compared and it has been found that the cycle with bleed line and desuperheater has 40-50% higher COP than the cycle with rectifier. Various parameters, viz., cooling COP, solution heat exchanger effectiveness, pressure ratio, temperature glides in the absorber and desorber, and low temperature desorber load have been studied as a function of weak solution concentration. The results indicate that the above system can work at temperature above 100 o C and achieve a temperature lift of more than 100K. Groll and Radernacher [78] presented the simulation model for the vapour compression cycle with single stage solution circuit and cycle with desorber/absorber heat exchange. The working pair used was CO 2 -acetone and R-23-DEGDME. It has been found that the mixtures CO2-acetone and R-23-DEGDME are not suitable for higher absorber temperatures in heat pump applications due to low COP and high absolute pressures compared to that for NH 3 -H 2 O mixture. Contrary to this, high COP and low absolute pressures were obtained with CO 2 -acetone and R-23-DEGDME compared to NH 3 -H 2 O mixture for low desorber temperature in refrigeration applications. It has also been found that for temperature lifts below 70K, the vapour compression cycle with single solution circuit is better compared to the cycle with 44