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.     

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.     

Computer-aided conceptual thermodynamic design of a two-stage dual fluid absorption cycle for solar refrigeration

This communication presents thermodynamic assessment of a two-stage dual fluid absorption refrigeration system using H₂O---LiBr and NH₃---H₂O as working fluids at the first and second stage, respectively. Both stages are assumed to be operated with hot water available from separate solar collectors. In the cascading of two-stage absorption systems, the evaporator of the first stage produces cooling water which is circulated in the absorber of the second stage. It is found that two-stage systems can be used for the production of very low temperatures using moderate generator temperatures at the first stage. The effects of generator temperature, absorber temperature and condenser temperature on the coefficient of performance, minimum evaporator temperature and effective refrigeration produced are also presented.     

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.     

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.     

溴化锂吸收式制冷技术研究进展

回顾了近十年来有关溴化锂吸收式制冷技术的发展及主要研究成果。H₂O/LiBr作为一种广泛应用的吸收式制冷工质对,具有优良的热力学性能与环保特性,但存在结晶、腐蚀和循环性能低等问题。文章简述了醇类、盐混合物、离子液体及纳米颗粒等添加剂对H₂O/LiBr溶液传热传质、防结晶及防腐蚀等性能的提升;介绍了关键部件吸收器和发生器的理论及实验研究现状;回顾了吸收式制冷系统循环优化的研究成果。通过归纳分析,总结溴化锂吸收式制冷技术存在的一些问题及未来发展趋势,为后续的研究提供参考。     

Thermal modelling and parametric study of two stage absorption refrigeration and air-conditioning systems

This paper presents a thoermodynamic assessment of two stage absorption refrigeration and air-conditioning systems. Two working fluids, namely water-LiBr and NH₃-H₂O as refrigerant-absorbent combinations have been considered for the production of different cooling temperatures. Both the systems are assumed to be operated with hot water available from solar collectors. Thermal modelling and a parametric study of a two stage absorption system, based on thermodynamic analysis aided by computer, have been carried out in detail. It is found that the cascading of two stage absorption systems (in which the first stage evaporator produces cooling water to be circulated in the absorber of the second stage) can be easily operated to produce much lower temperatures suitable for refrigeration and air-conditioning applications. The COP of a two stage absorption system is lower than that of a single stage; however, the second stage can be operated with lower generator temperatures than the first stage. However, there is an advantage for a two stage system because of the slow fall-off in COP at higher generator temperatures.     

Thermodynamic basis for the choice of working fluids for solar absorption cooling systems

Through the application of the first and second laws of thermodynamics upper and lower limits for the coefficient of performance (COP) of absorption cooling cycles are derived. These upper and lower limits, besides being dependent on the environmental temperatures of components of the cycle, are also dependent on the thermodynamic properties of refrigerants, absorbents, and their mixtures. With the use of these upper and lower limits of COP it is now possible to make a quantitative comparative study of different refrigerant-absorbent combinations. The technique developed is applied for the comparative evaluation of NH₃ + H₂O, NH₃ + NaSCN and H₂O + LiBr combinations which are the favorable candidates used in solar absorption cooling cycles.     

Research on performance of mixed absorption refrigeration for solar air-conditioning

A novel lithium bromide/water mixed absorption refrigeration cycle that is suitable for the utilization of solar air-conditioning and can overcome the drawbacks of low system overall efficiency of traditional solar absorption refrigeration air-condition systems is presented. The accessorial high pressure generator was added in the cycle. The lithium bromide solution flowing out from the high pressure generator was mixed with the solution from the low pressure absorber to increase lithium bromide solution concentration and decrease pressure in the high pressure absorber. The performance of a mixed absorption refrigeration cycle was analyzed. The theoretical analysis shows that the highest COP is 0.61, while the highest available temperature difference of heat resource is 33.2°C. The whole coefficient of performance of the solar air-conditioning using mixed absorption cycle is 94.5% higher than that of two-stage absorption. The advantages of solar air-conditioning can be markedly made use of by the cycle.     

Theoretical study of a solar powered absorption/MED combined system

In this article, a theoretical study is presented for a solar powered combined system comprising a LiBr---H₂O absorption cooling machine and a multiple-effect distillator (MED). The MED has 8 VTE of the falling film type, and it replaces the condenser in conventional absorption machines. Steam released at the generator pressure is supplied to the effect which matches its conditions, and the condensate follows its usual route towards the evaporator of an A/C unit. Thus, the MED is powered by the waste heat of the absorption machine which improves the overall gain and the thermodynamic characteristics significantly.

Governing equations for the combined system are given and are numerically solved. Medium parabolic concentrators are used to power the system, and a transient simulation for the combined arrangement is presented.

Results are given for a typical design summer day in Jeddah, Saudi Arabia, for a range of firing temperatures 150–190°C with a storage temperature amplitude of 10–20°C over a daily working period of 12 h. For a given cooling load of 100 ton refrigeration, the system can produce up to 40m³ of fresh water at a specific collector area of 12.41. H₂O plus 0.03 TR/m². The overall COP_o reaches 1.44, which is more than twice that of a conventional absorption machine at the same temperature levels.     

Energy and Entropy-Based Optimization of a Single-Stage Water–Lithium Bromide Absorption Refrigeration System

Thermodynamic optimization of single-stage water–LiBr absorption system was performed earlier both from first- and second-law points of view. However, a realistic comparison between the two approaches is essential to identify the superior one. Hence a theoretical model is developed to optimize the absorption chiller for identifying conditions corresponding to maximum system coefficient of performance (COP) and minimum entropy generation rate. Optimum generator temperature for minimum entropy generation is found to be lower than that corresponding to maximum COP, with absorber having a dominant role in enhancing irreversibility in system. Optimal generator temperature decreases with increasing evaporator temperature and decreasing condenser temperature. Thus, it is possible to identify optimum value for any combination of condenser and evaporator temperature from both the energy and entropy points of view. Concerned contour maps are presented. Adoption of an entropy-optimized map is suggested, owing to higher sensitivity of entropy generation to the changes in condition compared to COP.     

Experimental research on a new solar pump-free lithium bromide absorption refrigeration system with a second generator

This paper is concerned with experimental research on a new solar pump-free lithium bromide absorption refrigeration system with a second generator. By using the second generator together with a lunate thermosiphon elevation tube, the required minimum driving temperature of the heat source is only 68 °C compared to above 100 °C in traditional absorption refrigeration systems. Based on the horizontal-tube falling-film method, the performance of the absorber can be enhanced by the second generator due to an increase in the differential concentration of the solution between the inlet and the outlet of the absorber and an increase in the temperature difference between the inlet and the outlet of the cooling water in the absorber. The yield of condensate with the second generator open is increased by 68% compared to that with the second generator closed. The performance of the evaporator is significantly improved due to the increase in temperature drop of the chilled water and the decrease in the outlet temperature of the chilled water. This leads to an improvement of the performance of the overall refrigeration system. The maximum coefficient of performance (COP) approaches 0.787.     

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.     

Increase of COP for heat transformer in water purification systems. Part I – Increasing heat source temperature

The integration of a water purification system in a heat transformer allows a fraction of heat obtained by the heat transformer to be recycled, increasing the heat source temperature. Consequently, the evaporator and generator temperatures are also increased. For any operating conditions, keeping the condenser and absorber temperatures and also the heat load to the evaporator and generator, a higher value of COP is obtained when only the evaporator and generator temperatures are increased. Simulation with proven software compares the performance of the modeling of an absorption heat transformer for water purification (AHTWP) operating with water/lithium bromide, as the working fluid–absorbent pair. Plots of enthalpy-based coefficients of performance (COP_ET) and the increase in the coefficient of performance (COP) are shown against absorber temperature for several thermodynamic operating conditions. The results showed that proposed (AHTWP) system is capable of increasing the original value of COP_ET more than 120%, by recycling part of the energy from a water purification system. The proposed system allows to increase COP values from any experimental data for water purification or any other distillation system integrated to a heat transformer, regardless of the actual COP value and any working fluid–absorbent pair.     

New refrigeration system using CO vapor-solid as refrigerant

A refrigerant must be in the vapor-liquid phase in a vapor-compression refrigeration system, therefore, CO₂ cannot be used as a refrigerant for temperatures lower than -56°C because solid CO₂ will form under the triple point temperature of -56°C. A refrigeration system with CO₂ vapor-solid particles as refrigerant is put forward, by which a temperature lower than the triple point is achieved. An adjustable nozzle, a sublimator, a high-pressure regulating valve and a low-pressure regulating valve are used to replace the conventional evaporator. Theoretical cycle analysis of the refrigeration system shows that its COP can be 50% higher than that of the conventional one.     

Theoretical analysis of ammonia-water absorption cycles for refrigeration and space conditioning systems

This paper presents an investigation of an ammonia-water absorption cycle for solar refrigeration, airconditioning and heat pump operations at higher heat supply temperatures. The system consists of a solar driven generator, rectifier, condenser, evaporator, absorber and heat exchangers for preheating and subcooling within the system. A steady state thermodynamic cycle analysis based on mass and heat balances along with the state equations for the thermodynamic properties of the ammonia-water mixture has been carried out. A numerical computer simulation of the system with input component temperatures, refrigerant concentration/mass flow rate and effectiveness of the heat exchangers has been made to evaluate the relative heat transfer rates (i.e. coefficients of performance) and the mass flow rates for the cooling/heating modes. It is found that unlike the low generator temperature behaviour the coefficients of performance for both cooling and heating modes are reduced at higher generator temperatures. However, an increase of condenser temperature for each mode of operation improves the performance of the systems at higher generator temperatures. A choice for keeping the absorber temperature equal to/lower than that of the condenser is also predicted at lower/higher generator temperatures, respectively. In general the results are more pronounced for the refrigeration mode than for the heat pump mode and are least effective for the airconditioning mode.     

A high-efficiency, compound NH3/H2O-H2O/LiBr absorption-refrigeration system

A high-efficiency, compound absorption-refrigeration system is considered, which is composed of two cooperating absorption units using NH₃/H₂O and H₂O/LiBr solutions, respectively. The heat output from the NH₃/H₂O unit is employed to drive the H₂O/LiBr unit. The thermodynamics of the new system are simulated by using a procedure which showed that very high theoretical coefficients of performance may be obtained (up to 230%) compared to the corresponding theoretical values for the usual single absorption units, which do not exceed 100%.     

Choice of absorbent-refrigerant mixtures

Twenty-six absorbent—refrigerant combinations, holding good promise as fluid systems, have been considered for single stage absorption air conditioning system. These fluids have been compared on the basis of solution characteristics, life expectancy characteristics and refrigeration cycle characteristics. The mass flow rates of rich and poor solutions per ton of refrigeration capacity and the coefficient of performance (CP) were compared for an evaporator temperature of 5°C, absorber and condenser temperatures of 35°C and a generator temperature of 120°C (low grade energy sources). More than half of the waste energy available in industry happens to be at a temperatures below 200°C. Other types of low grade thermal energy such as solar energy and geothermal energy can be used in operating vapour absorption refrigeration and air-conditioning systems.     

Performance improvement of absorption refrigeration system using triple-pressure-level

In the absorption refrigeration system (ARS) working with aqua–ammonia, the ejector is commonly located at the condenser inlet. In this study, the ejector was located at the absorber inlet. Therefore, the absorber pressure becomes higher than the evaporator pressure and the system works with triple-pressure-level. The ejector has two main functions: (i) aiding pressure recovery from the evaporator, (ii) upgrading the mixing process and the pre-absorption by the weak solution of the ammonia coming from the evaporator. In addition to these functions, it can also act to lower the refrigeration and heat-source temperatures. Energy analyses show that the system’s coefficient of performance (COP) and exergetic coefficient of performance (ECOP) were improved by 49% and 56%, respectively and the circulation ratio (f) was reduced by 57% when ARS is initiated at lower generator temperatures. Due to the reduced circulation ratio, the system dimensions can be reduced; consequently, this decreases overall cost. The heat source and refrigeration temperatures decreased in the range of 5–15 °C and 1–3 °C, respectively. Exergy analyses show that the exergy loss of the absorber of ARS with ejector had a higher exergy loss than those of the other components. Therefore, a multiple compartment absorber can be proposed to reduce the exergy loss of the absorber of ARS with ejector.     

Simulation studies on R134a—DMAC based half effect absorption cold storage systems

This paper presents simulation studies conducted on a half effect vapour absorption cycle using R134a-DMAC as the refrigerant-absorbent pair with low temperature heat sources for cold storage applications. The intermediate pressure of the cycle has been optimized for maximum COP. The effects of the temperatures of the evaporator, condenser, absorber and generator on the COP of the cycle have also been studied. It is found that the effect of the temperature of the low absorber on the performance is more pronounced than that of the high absorber. The COP for the baseline system is found to vary from 0.35 for low evaporating and high condensing temperatures to 0.46 for high evaporating and low condensing temperatures. The use of a condensate pre-cooler has resulted in an improvement of 5–15% in COP. The performance of this working fluid pair is better than that of ammonia–water for low heat source temperatures in the half effect configuration.