Thermodynamic study of a two-stage vapour absorption refrigeration system using NH3 refrigerant with liquid/solid absorbents

A systematic investigation is made of the two-stage vapour absorption refrigeration system employing the refrigerant absorbent combinations of NH₃---H₂O and NH₃---LiNO₃. The system consists of coupling two conventional absorption cycles so that the first-stage evaporator produces cooling water to circulate in the absorber of the second stage. The effect of operating variables such as generator temperature, evaporator temperature, absorber temperature and condenser temperature on the coefficient of performance (COP), heat transfer rates and relative circulation have been studied for both single-stage and two-stage absorption refrigeration systems. It is found that the COP is higher for NH₃---LiNO₃ than for NH₃---H₂O, in both single-stage and two-stage absorption systems, especially at higher generator temperatures. Furthermore, the minimum evaporator temperature achieved is lower for NH₃---LiNO₃, and the system can be operated at lower generator temperatures.     

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.     

Computer modeling and parametric study of a double effect generation absorption refrigeration cycle

This paper presents as assessment based on steady-state thermodynamic analysis and computer modeling of a double effect generation absorption refrigeration cycle for solar air-conditioning. The system consists of a second effect generator between the generator and condenser of the single effect absorption cycle and two solution heat exchangers between the absorber and the two generators. A numerical computer modeling of a water LiBr system based on the solution of simultaneous heat, mass and material balance equations for various components of the system has been carried out. The influences of component temperatures and heat exchanger effectiveness on the cooling coefficients of performance and component heat transfer rates have been investigated to obtain optimum operating conditions for the proposed air-conditioning system. Further, the single and double effect absorption cycles are compared with each other as well as with an ideal absorption cycle operating over the same range of temperatures.     

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.     

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.     

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 draw-backs 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. __________ Translated from Journal of Huazhong University of Science and Technology, 2006, 34(8): 62–64 [译自: 华中科技大学学报]     

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.     

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.     

Development and comparison of advanced cascading cycles coupling a solid/gas thermochemical process and a liquid/gas absorption process

Currently marketed double-effect absorption machines attain coefficients of performance (COP) of the order of 1.2 and, therefore, do not enable standard vapour compression air-conditioning systems to compete. The improvement of the COP requires increasing the high driving temperature level of the system in order to make possible additional stages and further refrigeration effects. But at high temperatures, working couples currently used in absorption systems (H₂O/LiBr, NH₃/H₂O) pose corrosion problems for exchangers or decomposition of the working fluid. The implementation at these high temperatures of a solid/gas thermochemical reaction system enables bypassing these restrictions. The coupling of a chemical reaction process thermally cascaded with a liquid/gas absorption process enables leading to triple-effect machines, indeed quadruple effect, the COP of which are from 30% to 60% higher than commercialised double-effect absorption chillers. Numerous coupling configurations are presented in this paper. A method of evaluation of the COP of the global machine is also developed. A comparison of these different configurations is carried out through value criteria characterising the quality of the obtained coupling. In this way, a first selection of combinations of interest can be performed. As part of a Franco–German cooperation, a triple-effect machine based on this approach is currently being realised. This new concept of coupling must lead to a new generation of thermal machines which will be capable in the near future of competing with vapor compression machines by the complementary use of the potentialities appropriate to each of the sorption processes.     

Thermodynamic analysis of a lithium bromide/water absorption system for cooling and heating applications

Absorption systems have the potential of employing thermal energy such as waste heat to produce both chilled water and hot water for building cooling and heating applications. In the present study, a lithium bromide/water (LiBr/H₂O) absorption system for cooling and heating applications was analysed on the basis of the first and second laws of thermodynamics. Simulation was employed to determine the coefficient of performance (COP) and the exergetic efficiency of the absorption system under different operating conditions such as the heat source, cooling water, chilled water, and supply hot water temperatures. Copyright © 2001 John Wiley & Sons, Ltd.     

Multi-pressure absorption cycles in solar refrigeration:: A technical and economical study

A technical and economical study of regenerative absorption chillers with multi-pressure cycle has been undertaken as solar operated refrigeration systems. Referred to as advanced absorption chillers they represent one of the new technology options that are under development. Advanced absorption cooling technology offers the possibility of chillers with thermal COPs of 1.5 or greater at driving temperatures of 140°C, which reduces the collector area and the heat rejection requirements compared to current absorption cooling technology. Two different absorption systems have been considered. The first is an advanced, double-effect regenerative absorption cooling system, driven at 140°C, whose efficiency is about 55% of the Carnot efficiency. The second is an ideal, single-effect regenerative absorption system that achieves 70% of the Carnot efficiency driven at 140°C or 200°C. To evaluate the solar performance of a thermally driven chiller requires a separate analysis of the solar availability for a given location compared to the required monthly average solar input. In this analysis different systems, including the vapour compression chillers, have been compared in terms of the thermal and electrical energy input. An effective electrical COP may be computed assuming that the ratio of electrical energy cost to thermal energy cost is four, which is typical of today’s fossil fuel costs. The effective electrical COPs of different technical options can then be compared. Those systems with higher electrical COPs will have lower energy costs. If solar is to be competitive, then the cost of delivered solar thermal energy should be less than the cost of delivered fossil thermal energy.     

An experimental investigation of a different type vapor compression cascade refrigeration system

Experimental investigation and theoretical study of a different type of two-stage vapor compression cascade refrigeration system using R-134 as the refrigerant are presented. Performance evaluations of two single stage vapor compression systems and two-stage vapor compression refrigeration cascade system are performed with respect to theoretical model developed. In the first section of the experiments, one refrigeration system, namely RU2, is operated. During the experiments, rate of the water flow connecting both systems is kept constant at various values and the voltage across evaporator heaters is increased from 100 to 200 V with intervals of 20 V. In the second part of the first category, experiments are repeated by using different mass flow rates of water. In the second section, two separate refrigeration systems, namely RU1 and RU2 are connected to each other by using the water loop. This system is also called cascade refrigeration system. It is observed that the change in water mass flow rate has little effect on the coefficient of performance for single stage and cascade stage refrigeration systems. It is also observed that the coefficient of performance is mainly a function of evaporator temperature and pressure. When RU2 operating in the single stage refrigeration system is compared with RU2 operating in the two-stage cascade refrigeration system at the same refrigeration load interval (360–460 W), the average percentage values of the decrease in the condensing pressure, the decrease in the compressor power and the increase in the COP are 21.9, 31.7 and 32.7, respectively.     

Solar absorption heat transformer applications to absorption refrigerating machines

This paper concerns the study of a two-stage vapour absorption system (heat transformer and refrigerating machine) employing the refrigerant absorbent combinations of LiBr-H₂O and NH₃-H₂O successively. The system consists of coupling the previous absorption cycles so that the first-stage absorber produces heating water to circulate in the generator of the second stage. The performances of a solar installation (two stage system plus solar collector) had been tested in Rabat (Morocco). It is found that the system can be operated at lower hot source temperatures and, thus, it can be supplied either from flat plate collectors or from thermal effluents.     

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.     

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.     

Integration of the solar thermal energy in the construction: Analysis of the solar-assisted air-conditioning system installed in CIESOL building

In the last few years the solar-powered air-conditioning systems have been in intensive development and more often are considered as the viable application for the thermal solar systems in the regions of southern Europe. This is mainly due to the increasing higher electrical consumption in many countries (especially in Spain) which is moving from the winter to summer months due to expanding usage of cooling systems in this period of the year.In this paper we will analyze the behaviour of the solar-assisted air-conditioning system installed in the CIESOL building. This system consists mainly of flat-plate solar collectors and the simple effect LiBr–H₂O absorption chiller. Different operation modes were analyzed. Coefficient of performance (COP) at various generator, absorber, condenser and evaporator temperatures is investigated and experimental results show that in practice it is easy to obtain values of about 0.6. The main goal of this paper is to describe the characteristics of the developing building and the solar-assisted air-conditioning system. Another useful purpose is to find the optimum conditions and operation parameters for the solar system through analyzing various systems' operation strategies.     

Modelling and simulation of a solar absorption cooling system

A detailed numerical simulation model is developed for a commercially available solar absorption chiller. The model incorporates the performance data of a Yazaki-manufactured water-cooled system. We take into consideration the variation of the COP and cooling water temperature. Using a summer season's meteorological data for an arid location in the Sahara desert, the system performance is computed for different collector types, areas and storage volumes. The results show that an optimum storage volume/collector area ratio exists. Also a high solar fraction can be obtained with relatively small areas of collectors, even when the collectors are of the inexpensive type. The interesting feature was that the system operated at design load conditions with generator temperatures as low as 80°C owing to the fact that very low cooling water temperatures are available in the dry conditions of the Sahara. The study establishes the high potential of solar operated, water-cooled absorption coolers especially for arid conditions.     

Computer simulation studies of A double effect generation absorption air-conditioning system using water-salt and alcohol-salt mixtures

This paper presents a computer-simulation based thermodynamic assessment of a double effect generation absorption air-conditioning system. The proposed system consists of a second effect generator between the first effect generator and the condenser, along with two solution heat exchangers between the generators and the absorber. Input variables to the computer simulation based on heat, mass and material balance equations are the input generator heat supply, the solution flow rate and the ambient conditions. A detailed parametric study using water-LiBr and Methanol-LiBr. ZnBr₂ as working fluids for the double effect generation cycle has been carried out for a wide range of ambient conditions under optimum design and off-design conditions. It is found that the COP of this cycle approaches twice that of the single effect cycle for all operating conditions. It is concluded that a double effect cycle is thermodynamically feasible for solar air-conditioning.     

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.     

Experimental investigations of a small-scale solar-assisted absorption cooling system

The present study deals with a small-scale solar-assisted absorption cooling system having a cooling capacity of 3.52 kW and was investigated experimentally under the climatic conditions of Taxila, Pakistan. Initially, a mathematical model was developed for LiBr/H₂O vapor absorption system alongside flat-plate solar thermal collectors to achieve the required operating temperature range of 75°C. Following this, a parametric analysis of the whole system was performed, including various design and climate parameters, such as the working temperatures of the generator, evaporator, condenser, absorber, mass flow rate, and coefficient of performance (COP) of the system. An experimental setup was coupled with solar collectors and instruments to get hot water using solar energy and measurements of main parameters for real-time performance assessment. From the results obtained, it was revealed that the maximum average COP of the system achieved was 0.70, and the maximum outlet temperature from solar thermal collectors was 75°C. A sensitivity analysis was performed to validate the potential of the absorption machine in the seasonal cooling demand. An economic valuation was accomplished based on the current cost of conventional cooling systems. It was established that the solar cooling system is economical only when shared with domestic water heating.