Performance improvement of absorption heat transformer

In this study, a mathematical model of absorption heat transformer (AHT) operating with the aqua/ammonia was developed to simulate the performance of these systems coupled to a solar pond in order to increase the temperature of the useful heat produced by solar ponds and used a special ejector located at the absorber inlet. By the use of the ejector, the obtained absorber pressure becomes higher than the evaporator pressure and thus the system works with triple-pressure-level. The ejector has two functions: (i) aids pressure recovery from the evaporator and (ii) upgrades the mixing process and the pre-absorption by the weak solution of the ammonia coming from the evaporator. The other advantage of the system with ejector is increased absorber temperature. Therefore, pressure recovery and pre-absorption in the ejector improves the efficiency of the AHT. Under the same circumstances, when compared to an AHT with and without an ejector, the system's COP and exergetic coefficient of performance (ECOP) were improved by 14% and 30%, respectively and the circulation ratio (f) was reduced by 57% at the maximum efficiency condition. Due to the reduced circulation ratio, the system dimensions can be reduced; consequently, this decreases overall cost. The maximum upgrading of the solar pond's temperature by the AHT was obtained at 57.5 °C and gross temperature lift at 97.5 °C with coefficients of performance of about 0.5. The maximum temperature of the useful heat produced by the AHT was 150 °C. In addition, exergy losses for each component in the system were calculated at different working temperatures and the results of both systems with and without an ejector were compared. Exergy analysis emphasised that both the losses and irreversibilities have an impact on the system performance and exergy analysis can be used to identify the less efficient components of the system. Exergy analyses also showed that the exergy loss of the absorber of AHT with ejector was higher than those of other components.     

Performance analysis of a waste‐heat‐powered thermodynamic cycle for multieffect refrigeration

A multieffect refrigeration system that is based on a waste‐heat‐driven organic Rankine cycle that could produce refrigeration output of different magnitudes at different levels of temperature is presented. The proposed system is integration of combined ejector–absorption refrigeration cycle and ejector expansion Joule–Thomson (EJT) cooling cycle that can meet the requirements of air‐conditioning, refrigeration, and cryogenic cooling simultaneously at the expense of industrial waste heat. The variation of the parameters that affect the system performance such as industrial waste heat temperature, refrigerant turbine inlet pressure, and the evaporator temperature of ejector refrigeration cycle (ERC) and EJT cycles was examined, respectively. It was found that refrigeration output and thermal efficiency of the multieffect cycle decrease considerably with the increase in industrial waste heat temperature, while its exergy efficiency varies marginally. A thermal efficiency value of 22.5% and exergy efficiency value of 8.6% were obtained at an industrial waste heat temperature of 210°C, a turbine inlet pressure of 1.3 MPa, and ejector evaporator temperature of 268 K. Both refrigeration output and thermal efficiency increase with the increase in turbine inlet pressure and ERC evaporator temperature. Change in EJT cycle evaporator temperature shows a little impact on both thermal and exergy efficiency values of the multieffect cycle. Analysis of the results clearly shows that the proposed cycle has an effective potential for cooling production through exploitation of lost energy from the industry. Copyright © 2014 John Wiley & Sons, Ltd.     

Parametric analysis for a new combined power and ejector–absorption refrigeration cycle

A new combined power and ejector–absorption refrigeration cycle is proposed, which combines the Rankine cycle and the ejector–absorption refrigeration cycle, and could produce both power output and refrigeration output simultaneously. This combined cycle, which originates from the cycle proposed by authors previously, introduces an ejector between the rectifier and the condenser, and provides a performance improvement without greatly increasing the complexity of the system. A parametric analysis is conducted to evaluate the effects of the key thermodynamic parameters on the cycle performance. It is shown that heat source temperature, condenser temperature, evaporator temperature, turbine inlet pressure, turbine inlet temperature, and basic solution ammonia concentration have significant effects on the net power output, refrigeration output and exergy efficiency of the combined cycle. It is evident that the ejector can improve the performance of the combined cycle proposed by authors previously.     

Exergy analysis,parametric analysis and optimization for a novel combined power and ejector refrigeration cycle

A new combined power and refrigeration cycle is proposed, which combines the Rankine cycle and the ejector refrigeration cycle. This combined cycle produces both power output and refrigeration output simultaneously. It can be driven by the flue gas of gas turbine or engine, solar energy, geothermal energy and industrial waste heats. An exergy analysis is performed to guide the thermodynamic improvement for this cycle. And a parametric analysis is conducted to evaluate the effects of the key thermodynamic parameters on the performance of the combined cycle. In addition, a parameter optimization is achieved by means of genetic algorithm to reach the maximum exergy efficiency. The results show that the biggest exergy loss due to the irreversibility occurs in heat addition processes, and the ejector causes the next largest exergy loss. It is also shown that the turbine inlet pressure, the turbine back pressure, the condenser temperature and the evaporator temperature have significant effects on the turbine power output, refrigeration output and exergy efficiency of the combined cycle. The optimized exergy efficiency is 27.10% under the given condition.     

Exergy analysis of ejector‐refrigeration cycle using water as working fluid

Exergy is based on the second law of thermodynamics and is the only rational basis for evaluating the system performance. The aim of this paper is to study in detail the irreversibilities in the steam‐ejector refrigeration system. The influence of the cycle parameters is analysed on the basis of the first and second law and the results indicated the components with the greater irreversibility. A better quality of the ejector has more effect on the system performance than the better quality of other components, because the ejector at first and the condenser at second have the greater exergy loss of the system. For the refrigeration system the maximum coefficient of performance varying between 0.4 to 0.6 and the second law efficiency remains close to 0.17 for generator pressure 6 bar, condenser temperature 44–50°C and evaporator temperature 4–8°C. Also the study showed that the second law analysis quantitatively visualizes losses within a system and gives clear trends for optimization. Copyright © 2005 John Wiley & Sons, Ltd.     

Residential solar air conditioning: Energy and exergy analyses of an ammonia–water absorption cooling system

Large scale heat-driven absorption cooling systems are available in the marketplace for industrial applications but the concept of a solar driven absorption chiller for air-conditioning applications is relatively new. Absorption chillers have a lower efficiency than compression refrigeration systems, when used for small scale applications and this restrains the absorption cooling system from air conditioning applications in residential buildings. The potential of a solar driven ammonia–water absorption chiller for residential air conditioning application is discussed and analyzed in this paper. A thermodynamic model has been developed based on a 10 kW air cooled ammonia–water absorption chiller driven by solar thermal energy. Both energy and exergy analyses have been conducted to evaluate the performance of this residential scale cooling system. The analyses uncovered that the absorber is where the most exergy loss occurs (63%) followed by the generator (13%) and the condenser (11%). Furthermore, the exergy loss of the condenser and absorber greatly increase with temperature, the generator less so, and the exergy loss in the evaporator is the least sensitive to increasing temperature.     

First and second law analysis of an ejector expansion Joule–Thomson cryogenic refrigeration cycle

In this paper, a combined first and second law approach is applied to study an ejector expansion Joule–Thomson cryogenic refrigeration cycle. The effects of the evaporator temperature, ejector pressure ratio and compressor function on the coefficient of performance (COP), exergy destruction and the exergetic efficiency have been investigated. The present study has been conducted for the evaporator and compressor temperature in the range of 75–135 and 270–330 K, respectively. The ejector pressure ratio is varied from 1.5 to 5.5. Simulation results show that COP and exergy efficiency increase with increasing evaporator temperature and ejector pressure ratio. In addition, it was found that the increase in the compressor temperature leads to the reduction in the first and second law efficiencies. Copyright © 2010 John Wiley & Sons, Ltd.     

Comprehensive performance analysis and optimization of 1,3-dimethylimidazolylium dimethylphosphate-water binary mixture for a single effect absorption refrigeration system

The energy and exergy analyses of the absorption refrigeration system (ARS) using H₂O-[mmim][DMP] mixture were investigated for a wide range of temperature. The equilibrium Dühring (P-T-X_IL) and enthalpy (h-T-X_IL) of mixture were assessed using the excess Gibbs free non-random two liquid (NRTL) model for a temperature range of 20°C to 140°C and X_IL from 0.1 to 0.9. The performance validation of the ARS cycle showed a better coefficient of performance (COP) of 0.834 for H₂O-[mmim][DMP] in comparison to NH₃-H₂O, H₂O-LiBr, H₂O-[emim][DMP], and H₂O-[emim][BF4]. Further, ARS performances with various operating temperatures of the absorber (T_a), condenser (T_c), generator (T_g), and evaporator (T_e) were simulated and optimized for a maximum COP and exergetic COP (ECOP). The effects of T_g from 50°C to 150°C and T_a and T_c from 30°C to 50°C on COP and ECOP, the X_a, X_g, and circulation ratio (CR) of the ARS were evaluated and optimized for T_e from 5°C to 15°C. The optimization revealed that T_g needed to achieve a maximum COP which was more than that for a maximum ECOP. Therefore, this investigation provides criteria to select low grade heat source temperature. Most of the series flow of the cases of cooling water from the condenser to the absorber was found to be better than the absorber to the condenser.     

Performance characteristics of two combined ejector–absorption cycles

The paper describes the performance of an ammonia–water combined ejector–absorption cycle as refrigerator using two simple models. In the first an ejector draws vapour from an evaporator and discharges to a condenser. In the second, an ejector draws vapour from an evaporator and discharges to an absorber. The thermodynamics cycles and ejector operation on the temperature–entropy charts are shown. The thermodynamics of the combined ejector–absorption cycle are simulated by a suitable method and a corresponding computer code, based on analytic functions, describes the behaviour of the binary mixture NH₃–H₂O. It was found from the first model that the refrigerator (theoretical) coefficient of performance (COP) varied from 1.099 to 1.355 when the operation conditions were: generation temperature (237°C), condenser temperature (25.9–30.6°C), absorber temperature (48.6–59.1°C) and evaporator temperature (−1.1–7.7°C). In the second the theoretical COP vary from 0.274 to 0.382 when the operation conditions were: generation temperature (237°C), condenser temperature (91°C), absorber temperature (76.7–81°C) and evaporator temperature (−1.1–7.7°C).     

Design and parametric investigation of an ejector in an air-conditioning system

This paper discusses the behavior of ammonia (R-717) through an ejector, operating in an air-conditioning system with a low temperature thermal source. For the detailed calculation of the proposed system a method has been developed, which employs analytical functions describing the thermodynamic properties of the ammonia. The proposed cycle has been compared with the Carnot cycle working at the same temperature levels. The influence of three major parameters: generator, condenser and evaporator temperature, on ejector efficiency and coefficient of performance is discussed. Also the maximum value of COP was estimated by correlation of the three temperatures for constant superheated temperature (100°C). The design conditions were generator temperature (76.11–79.57°C), condenser temperature (34–42°C) and evaporator temperature (4–12°C).     

Experimental investigations on ejector refrigeration system with ammonia

A vapor ejector refrigeration system has been designed and developed to operate with ammonia. In this paper, performance of ejector refrigeration system has been experimentally studied with three different area ratio ejectors by varying operational parameters namely generator, condenser and evaporator temperatures. Effect of non-dimensional parameters like compression ratio, expansion ratio and area ratio on the system performance is studied. Entrainment ratio and coefficient of performance of the system increase with increase in ejector area ratio and expansion ratio and they increase with decrease in compression ratio.     

Experimental studies on an ammonia ejector refrigeration system

An ejector refrigeration system has been designed and developed to operate with a simulated (electric) heat source, which can be realized in practical applications by renewable energy sources like solar energy, geothermal energy, etc., or waste heat. In this paper, an experimental study on an ejector refrigeration system working with ammonia is presented. The influence of the generator, condenser, and evaporator temperatures on the ejector refrigeration system performance is presented. The entrainment ratio and COP of the system increase with increasing generator and evaporator temperatures and decrease with increasing condenser temperature.     

Performance Evaluation of R134a/DMF-Based Vapor Absorption Refrigeration System

This article describes an experimental investigation to measure performances of a vapor absorption refrigeration system of 1 ton of refrigeration capacity employing tetrafluoro ethane (R134a)/dimethyl formamide (DMF). Plate heat exchangers are used as system components for evaporator, condenser, absorber, generator, and solution heat exchanger. The bubble absorption principle is employed in the absorber. Hot water is used as a heat source to supply heat to the generator. Effects of operating parameters such as generator, condenser, and evaporator temperatures on system performance are investigated. System performance was compared with theoretically simulated performance. It was found that circulation ratio is lower at high generator and evaporator temperatures, whereas it is higher at higher condenser temperatures. The coefficient of performance is higher at high generator and evaporator temperatures, whereas it is lower at higher condenser temperatures. Experimental results indicate that with addition of a rectifier as well as improvement of vapor separation in the generator storage tank, the R134a/DMF-based vapor absorption refrigeration system with plate heat exchangers could be very competitive for applications ranging from –10°C to 10°C, with heat source temperature in the range of 80°C to 90°C and with cooling water as coolant for the absorber and condenser in a temperature range of 20°C to 35°C.     

Energy and exergy analysis of novel solar bi‐ejector refrigeration system with injector

Energy and exergy balances were done on a novel solar bi‐ejector refrigeration system with R123, whose circulation pump is replaced by an injector. The analysis result of the novel system was compared with that of the original one. The effect of operation condition on system energy efficiency, exergy efficiency and exergy loss was analyzed, and the dynamic performance of a designed solar bi‐ejector refrigeration system was also studied. The comparative results indicate that under the same operating condition, the novel system and the original system have equal energy efficiency, exergy efficiency and exergy loss, and the only difference between them is the exergy losses of the generators and the added injector. The other conclusions mainly include: the solar collector has the largest exergy loss rate of over 90% and for the bi‐ejector refrigeration subcycle, the ejector has the largest exergy loss rate of about 5%; the total exergy loss changes inversely proportional to the evaporation temperature and positively proportional to the condensation temperature; when the other parameters are fixed, there exists an optimum generation temperature, at which the overall energy and exergy efficiencies are both the maximum and the total exergy loss is the minimum. The study points out the direction for optimizing the novel solar bi‐ejector refrigeration system. Copyright © 2009 John Wiley & Sons, Ltd.     

Exergy analysis of double effect vapor absorption refrigeration system

Energy and exergy analyses previously performed by the authors for a single effect absorption refrigeration system have been extended to double effect vapor absorption refrigeration system with the expectation of reducing energy supply as well as an interest in the diversification of the motive power employed by HVAC technologies. The total exergy destruction in the system as a percentage of the exergy input from a generator heating water over a range of operating temperatures is examined for a system operating on LiBr–H₂O solution. The exergy destruction in each component, the coefficient of performance (COP) and the exergetic COP of the system are determined. It is shown that exergy destructions occur significantly in generators, absorbers, evaporator2 and heat exchangers while the exergy destructions in condenser1, evaporator1, throttling valves, and expansion valves are relatively smaller within the range of 1–5%. The results further indicate that with an increase in the generator1 temperature the COP and ECOP increase, but there is a significant reduction in total exergy destruction of the system for the same. On the other hand, the COP and ECOP decrease with an increase in the absorber1 temperature while the total exergy destruction of the system increases significantly with a small increase in the absorber1 temperature. The results show that the exergy method can be used as an effective criterion in designing an irreversible double effect absorption refrigeration system and may be a good tool for the determination of the optimum working conditions of such systems. Copyright © 2007 John Wiley & Sons, Ltd.     

Performance characteristics of the ejector refrigeration system based on the constant area ejector flow model

A theoretical analysis of the ejector refrigeration system based on the constant area ejector flow model is performed. Optimised results for R-123 are presented. It is determined that the variations in condenser and evaporator temperature have a greater effect on the optimum coefficient of performance (COP) than the variation in generator temperature. At the same operating temperatures of the ejector refrigeration system, the optimum COP and area ratio determined in this study using the constant area flow model are greater than the values given in the literature for the constant pressure flow model. For the same area ratio, the COP for the system with the constant pressure ejector is relatively higher than that with the constant area ejector. In this case, however, the condenser temperature should be lowered. In addition, the refrigeration systems have almost the same COP values at lower evaporator or higher condenser temperatures.     

First and second laws analysis of the heat pipe/ejector refrigeration cycle

Integration of the heat pipe with an ejector will result in a compact and high performance system. The concept of the heat pipe/ejector refrigeration cycle is discussed, in this paper. The needed driving capillary forces are firmly established. The basic characteristics of the system, such as entrainment ratio, coefficient of performance, exergy efficiency and thermal efficiency of the system are evaluated. Also, the zero-dimensional constant pressure mixing theory is applied to ejector. In this study, water is used as the working fluid. Whenever the mixed flow is supersonic, a normal shockwave is assumed to occur upstream of diffuser inlet. The simulation results indicate that, the coefficient of performance can reach about 0.30 at T_e = 10 °C, T_c = 30 °C and T_g = 100 °C. Also, the second law efficiency of the heat pipe/ejector refrigeration cycle increases with increasing evaporator temperature and decreasing condenser temperature. It is seen that, the maximum heat pipe cooling capacity obtains for large heat pipe diameters, near the small heat pipe lengths. It has proven that, this refrigeration system can be widely used in many areas, especially in renewable energy utilization such as solar energy.     

Second law-based thermodynamic analysis of water-lithium bromide absorption refrigeration system

In this study, the first and the second law of thermodynamics are used to analyze the performance of a single-stage water-lithium bromide absorption refrigeration system (ARS) when some working parameters are varied. A mathematical model based on the exergy method is introduced to evaluate the system performance, exergy loss of each component and total exergy loss of all the system components. Parameters connected with performance of the cycle–circulation ratio (CR), coefficient of performance (COP), Carnot coefficient of performance (COP_c), exergetic efficiency (ξ) and efficiency ratio (τ)–are calculated from the thermodynamic properties of the working fluids at various operating conditions. Using the developed model, the effect of main system temperatures on the performance parameters of the system, irreversibilities in the thermal process and non-dimensional exergy loss of each component are analyzed in detail. The results show that the performance of the ARS increases with increasing generator and evaporator temperatures, but decreases with increasing condenser and absorber temperatures. Exergy losses in the expansion valves, pump and heat exchangers, especially refrigerant heat exchanger, are small compared to other components. The highest exergy loss occurs in the generator regardless of operating conditions, which therefore makes the generator the most important component of the cycle.     

Particular characteristics of transcritical CO2 refrigeration cycle with an ejector

The present study describes a theoretical analysis of a transcritical CO₂ ejector expansion refrigeration cycle (EERC) which uses an ejector as the main expansion device instead of an expansion valve. The system performance is strongly coupled to the ejector entrainment ratio which must produce the proper CO₂ quality at the ejector exit. If the exit quality is not correct, either the liquid will enter the compressor or the evaporator will be filled with vapor. Thus, the ejector entrainment ratio significantly influences the refrigeration effect with an optimum ratio giving the ideal system performance. For the working conditions studied in this paper, the ejector expansion system maximum cooling COP is up to 18.6% better than the internal heat exchanger cycle (IHEC) cooling COP and 22.0% better than the conventional vapor compression refrigeration cycle (VCRC) cooling COP. At the conditions for the maximum cooling COP, the ejector expansion cycle refrigeration output is 8.2% better than the internal heat exchanger cycle refrigeration output and 11.5% better than the conventional cycle refrigeration output. An exergy analysis showed that the ejector expansion cycle greatly reduces the throttling losses. The analysis was also used to study the variations of the ejector expansion cycle cooling COP for various heat rejection pressures, refrigerant temperatures at the gas cooler exit, nozzle efficiencies and diffuser efficiencies.     

Theoretical analysis of LiBr/H2O absorption refrigeration systems

A computational model is developed for the parametric investigation of single‐effect and series flow double‐effect LiBr/H₂O absorption refrigeration systems. The effects of generator, absorber, condenser, evaporator and dead state temperatures are examined on the performance of these systems. The parameters computed are coefficient of performance (COP), exergy destruction rates, thermal exergy loss rates, irreversibility and exergetic efficiency. The results indicate that COP and exergetic efficiency of both the systems increase with increase in the generator temperature. There exist different optimum values of generator temperature for maximum COP and maximum exergetic efficiency. The optimum generator temperature is lower corresponding to maximum exergetic efficiency as compared to optimum generator temperature corresponding to maximum COP. The effect of increase in absorber, condenser and evaporator temperatures is to decrease the exergetic efficiency of both the systems. The irreversibility is highest in absorber in both systems. Copyright © 2009 John Wiley & Sons, Ltd.