Comparative study of irreversibilities in an aqua-ammonia absorption refrigeration system

Irreversibilities in components of an aqua-ammonia absorption refrigeratio system (ARS) have been determined by second law analysis. The components of the ARS are as follows: condenser, evaporator, absorber, generator, pump, expansion valves, mixture heat exchanger and refrigerant heat exchanger. It is assumed that the ammonia concentration at the generator exit is, independent of the other parameters, equal to 0.999 and at the evaporator exit the gas is saturated vapour. Pressrre losses between the generator and condenser, and the evaporator and absorber are taken into consideration. In the results the dimensionless exergy loss of each component, the exergetic coefficient of performance, the coefficient of performance and the circulation ratio are given graphically for each different generator, evaporator, condenser and absorber temperature.     

Performance of an HCFC22-based vapour absorption refrigeration system

A single-stage vapour absorption refrigeration system (VARS) is tested with monochlorodifluoromethane (HCF22) as refrigerant and different absorbents: dimethylether of tetraethylene glycol (DMETEG) and dimethyl acetamide (DMA). The influence of generator temperatures in the range 75–95°C, which represents low-grade heat sources, is studied. Cooling water temperatures were varied between 20 and 30°C. Two cases of cooling water flow paths are considered, i.e. water entering either absorber or condenser, which are connected in series. For HCFC22-DMETEG, COP values in the range 0.2–0.36 and evaporator temperatures between 0 and 10°C are obtained. For HCF22-DMA, COP values in the range 0.3–0.45 and evaporator temperatures between −10 and 10°C are obtained. It is observed that HCFC22-DMETEG can work at lower heat source temperatures than HCFC22-DMA. However, at the same operating conditions HCFC22-DMA is better from the viewpoints of circulation ratio and COP. Experiments also show that at low heat source temperature, cooling water temperature has strong influence on circulation ratio but does not affect COP significantly. Preferably, cooling water should first flow through the condenser and then through the absorber in order to achieve improved overall performance.     

Experimental investigation of a vapor absorption refrigeration system

An experimental investigation of the performance of a commercially available vapor absorption refrigeration (VAR) system is described. The natural gas-fired VAR system uses aqua-ammonia solution with ammonia as the refrigerant and water as the absorbent and has a rated cooling capacity of 10 kW. The unit was extensively modified to allow fluid pressures and temperatures to be measured at strategic points in the system. The mass flow rates of refrigerant, weak solution, and strong solution were also measured. The system as supplied incorporates air-cooled condenser and absorber units. Water-cooled absorber and condenser units were fitted to extend the VAR unit's range of operating conditions by varying the cooling water inlet temperature and/or flow rates to these units. The response of the refrigeration system to variations in chilled water inlet temperature, chilled water level in the evaporator drum, chilled water flow rate, and variable heat input are presented.     

Optimization of generator temperatures in two-stage dual-fluid absorption cycles operated by biogas

Generator temperatures in ammonia absorption systems at subfreezing evaporator conditions have been optimized to use the minimum volume of biogas required to operate two-stage dual-fluid cycles. In this dual-fluid cycle, a LiBr---H₂O absorption system is used at the first stage with ammonia absorption systems at the second stage. Three different refrigerant-absorbent combinations (NH₃---H₂O, NH₃---NaSCN, NH₃---LiNO₃) were selected for this study. The absorber at the second stage is cooled by the low temperature water-refrigerant from the evaporator at the first stage. Lowering the absorber temperature reduces the heat input to the generator, which lowers the generating temperature and improves the performance of the absorption systems at the second stage. The optimum generator temperatures and performance coefficients of the systems at the first and second stages and the overall two-stage dual-fluid cycles are presented graphically. A comparative study between the three ammonia systems used in the two-stage dual-fluid cycle has been carried out.     

Critical analysis of a biogas powered absorption system for climate change mitigation

The importance of biogas as a renewable alternative is being studied because of an increase in the cost of conventional fuels. The present article suggests a numerical study of a biogas powered NH₃–H₂O absorption refrigeration system where biogas is used to heat the water which serves as an energy input to generator of an absorption system. A computational model has been developed for the analysis which involves the determination of effect of generator temperature on various performance parameters, i.e., exergy losses in the different components, COP_cooling, COP_heating and the exergy efficiency. The results indicate that COP_cooling and COP_heating lies in the range of 0.159–0.33 and 1.16–1.33, respectively, whereas exergetic efficiency lies in the range of 0.29–0.80 for the same variation in generator temperature ranging from 50 to 70 °C. The highest exergy loss is found in the generator while the lowest is found in the condenser and it is also found that with an increase in the evaporator as well as absorber and condenser temperature, the COP increases and decreases, respectively. The effect of ambient temperature on exergy loss in the different components is also studied. Exergy analysis is an excellent tool to pin point the losses in the system due to irreversibility which are the basis for the further improvement in the system components.     

Experimental investigation on R134a vapour ejector refrigeration system

The experimental investigation of the performance of a vapour ejector refrigeration system is described. The system uses R134a as working fluid and has a rated cooling capacity of 0.5 kW. The influence of generator, evaporator and condenser temperatures on the system performance is studied. This kind of system can be operated with low grade thermal energy such as solar energy, waste heat, etc. The operating conditions are chosen accordingly as, generator temperature between 338 K and 363 K, condenser temperature between 299 K and 310.5 K, and evaporator temperature between 275 K and 285.5 K. Six configurations of ejectors of different geometrical dimensions are selected for the parametric study. The performance of the refrigeration system at different operating temperatures is presented.     

Optimization of operating temperatures in the gas operated single to triple effect vapour absorption refrigeration cycles

Thermodynamic analysis of LiBr–H₂O single, double and triple effect vapour absorption cycles has been carried out using LPG and CNG as sources of energy. Optimization of operating temperatures in single to triple effect cycles has been carried out for maximum COP of the system and minimum gas requirement in it at desired temperatures in evaporator, absorber and main condenser using iterative technique. In single effect cycle, optimum temperatures in main generator have been obtained, while in double effect cycle, low pressure generator, high pressure condenser and main generator temperatures have been optimized. In triple effect cycle having three condensers and three generators, condenser temperatures (T_c3 and T_c4) and generator temperatures (T_g2, T_g3 and T_g) have been optimized. The maximum COP of triple effect cycle goes up to 1.955 which is around 132% higher than single effect cycle with its gas requirement reduced to around 122% at the same conditions.     

Simulation d'une machine frigorifique à absorption fonctionnant avec des mélanges d'alcanes

We propose in this article an absorption chiller operating with binary alkane mixtures as an alternative to compression machines. It is an installation using low-level energy at a temperature below 150 °C (waste heat or solar energy) and operating with environmentally friendly fluids. Ten mixtures are considered and compared with two cooling mediums of the condenser and the absorber: the ambient air at 35 °C and the water at 25 °C. For an air-cooled chiller, the COP reaches 0.37 for the n-butane/octane system. This value remains 27% lower than that of an ammonia/water installation operating under the same conditions. For a water-cooling chiller, the n-butane/octane and propane/octane systems give a COP of about 0.63, which is comparable to that of the ammonia/water system. When n-butane is used as refrigerant, the machine works at a pressure under 5 bars, which is an advantage compared with machines working with ammonia/water mixtures.     

Modelling of the heat exchangers of a small capacity, hot water driven, air-cooled H2O–LiBr absorption cooling machine

General models for the design of the heat exchangers (absorber, generator, condenser and evaporator) of a prototype of an air-cooled absorption chiller of 2 kW for air-conditioning using the pair H₂O–LiBr have been developed. An absorption machine of such characteristics has been constructed to be used as a test facility for validating the results obtained from the mathematical models developed. The discrepancies considering the heat exchanged between numerical results and experimental data are under 15% in most cases for all these components except the condenser, where the discrepancies are higher. The conclusions reported will lead to: (i) future improvements of the mathematical simulation models and (ii) improvements in the experimental infrastructure.     

Development of type 2 solution transportation absorption system for utilizing LNG cold energy

The objective of this paper is to develop a new energy transport system for district cooling application by using type 2 absorption cycle. Cold energy from the LNG storage system is utilized as the cooling source of the condenser and the rectifier. The pressures of the system, UAs of the evaporator and the desorber, the inlet temperatures of the refrigerant for each component, transportation distance and the pumping power per unit length are considered as the key parameters. The results show that UA of the evaporator has more dominant effect on COP than that of the desorber. The optimum system pressure for the demand side is also determined. For the present system, it is recommended that the refrigerant inlet temperature of the evaporator be lower than 4.3 °C for long distance transportation. It is concluded that the cold energy from the LNG storage system can be effectively applied to the long distance transportation system for district cooling application with the type 2 absorption cycle. The maximum transportation distance and the pumping power per unit length are calculated. The optimum operation conditions are also predicted from the parametric analysis.     

Design,construction and operation of a solar powered ammonia–water absorption refrigeration system in Saudi Arabia

This study presents an experimental investigation of a solar thermal powered ammonia–water absorption refrigeration system. The focus of this study lies on the design of the components of the absorption chiller, the ice storages and the solar collector field as well as the integration of the data acquisition and control unit. An ammonia–water (NH₃/H₂O) absorption chiller was developed in the laboratory of the Institute of Thermodynamics & Thermal Engineering (ITW) at the University of Stuttgart (Germany). A demonstration plant was built in the laboratory of the CoRE-RE at King Fahd University of Petroleum & Minerals (KFUPM – Saudi Arabia). The whole system was tested successfully. The results of the experiments indicated a chiller coefficient of performance (COP) of 0.69 and a cooling capacity of 10.1 kW at 114/23/−2 (°C) representing the temperatures of the generator inlet, the condenser/absorber inlet and the evaporator outlet respectively. Even at 140/45/−4 (°C), the chiller was running with a cooling capacity of 4.5 kW and a COP of 0.42.     

Comparison of total energy systems using gas turbines and diesel engines for combined cooling

A gas turbine engine was used to drive the compressor of a vapour compression cycle so that the usually wasted energy in the exhaust gases was partially recovered and used in the generator of an absorption cycle. The cooling effect was therefore boosted. The degree of energy utilization was further enhanced when the energy released from the absorber and condenser of both cycles was recovered in the form of hot water, which could be used for different applications. The performance parameters for this combined system, such as the cooling effect, total heat recovered and performance effectiveness ratio, were calculated for various evaporator and condenser temperatures. It was found that a system driven by a gas turbine gives a better performance than a diesel engine system under similar operating conditions.     

Simulation of ternary ammonia–water–salt absorption refrigeration cyclesSimulation des cycles frigorifiques à absorption à ammoniac / eau / sel

This paper proposes a new working fluid for refrigeration cycles utilizing low temperature heat sources. The proposed working fluid consists of the ammonia–water working fluid mixture and a salt. The salt is used to aid the removal of ammonia from the liquid solution. This effect is a manifestation of the well known “salting-out” effect. While the addition of salt improves the generator performance, it also has a detrimental effect on the absorber. The overall effects on the performance of three absorption cycles using the NH₃–H₂O–NaOH working fluid have been investigated using computer simulations. The results indicated that salting out can lower the generator operating temperature while simultaneously improving the cycle performance. Furthermore, limiting the salt to the generator suggests potential for further improvement in cycle performance.     

Discussion of the feasibility of the Einstein refrigeration cycle

A careful modelling of the thermodynamic properties of the water–ammonia–butane system, the working fluid mixture used in the Einstein cycle, with the Patel–Teja cubic equation of state is performed. Numerical simulation is used to investigate the feasibility limits of this refrigeration cycle. Two modified configurations of the cycle are considered. A conflict between the evaporator and the condenser/absorber operating conditions is noted. The condenser/absorber operation needs a higher system pressure, which limits the refrigeration temperature in the case of air-cooling. On the other hand, the condensation of ammonia and the presence of a small quantity of water in the evaporator limit also the refrigeration temperature. In the case of a water-cooled machine, with a condenser/absorber temperature of 30 °C, the cycle COP reaches 0.19 which is still low.     

Condensation heat transfer of binary refrigerant mixtures of R22 and R114 inside a horizontal tube with internal spiral grooves

An experimental study of the condensation of pure and mixed refrigerants of R22 and R114 inside a spirally grooved horizontal copper tube has been carried out. A double-tube counterflow condenser in the pressure range 3–21 bar and at a mass flow-rate 26–70 kg h⁻¹ was used. The axial distributions of refrigerant, tube wall and cooling water temperatures, wall heat flux density and vapour quality are shown graphically. The variation of tube wall temperature around the circumference of the tube is also shown. The local Nusselt number depends on the molar fraction, whereas the average Nusselt number can be correlated by an equation which is modified from a previously established equation for pure refrigerants inside a horizontal smooth tube. The frictional pressure drop evaluated is correlated well by the Lockhart-Martinelli parameters and is independent of the concentration of the mixture.     

An absorption based miniature heat pump system for electronics cooling

The development of an absorption based miniature heat pump system is motivated by the need for removal of increasing rates of heat from high performance electronic chips such as microprocessors. The goal of the present study is to keep the chip temperature near ambient temperature, while removing 100 W of heat load. Water/LiBr pair is used as the working fluid. A novel dual micro-channel array evaporator is adopted, which reduces both the mass flux through each micro-channel, as well as the channel length, thus reducing the pressure drop. Micro-channel arrays for the desorber and condenser are placed in intimate communication with each other using a hydrophobic membrane. This acts as a common interface between the desorber and the condenser to separate the water vapor from LiBr solution. The escaped water vapor is immediately cooled and condensed at the condenser side. For direct air cooling of condenser and absorber, offset strip fin arrays are used. The performance of the components and the entire system is numerically evaluated and discussed.     

Cycle analysis of an air-cooled LiBr/H2O absorption heat pump of parallel-flow type

A gas-fired absorption heat pump with cooling capacity of 2 RT was analysed as an air-conditioner for domestic use during the summer. The absorption heat pump considered was an air-cooled, double-effect, LiBr/H₂O system of parallel-flow type. The performance of the absorption heat pump in the cooling mode of operation was investigated through cycle simulation to obtain the system characteristics depending on the inlet temperature of air to the absorber, the working solution concentrations, the solution distribution ratio of the mass of solution into the first generator to the total mass of solution from the absorber, and the LTDs (leaving temperature differences) of the heat-exchanging components. When the predicted results were compared with the measured data for similar design conditions, reasonable agreement was observed. The optimum design and operating conditions of an air-cooled absorption system are suggested based on this cycle simulation analysis.     

Mapping of optimum operating condition for LiBr–water refrigeration cycles

In this work, optimum operating condition maps are generated covering wide ranges of refrigeration and sink temperatures for single- and double-effect LiBr–water vapour absorption refrigeration cycle. These optimum condition maps will be useful to choose optimum operating conditions while designing LiBr–water cycle for desired applications. Methodology for generating such maps is discussed in detail, which can also be used for other absorption refrigeration cycles with various working fluids. Three configurations of LiBr–water absorption refrigeration cycles, single effect, double-effect series flow and double-effect parallel flow, are analysed with the most accurate thermodynamic property correlation available in the literature. Sensitivity of cycle performance to various operating variables such as generator, absorber and condenser temperatures is determined. Second law analysis shows that when a higher temperature heat source is available, double-effect cycles are more effective over single effect as they have higher coefficient of performance.     

A year-round dynamic simulation of a solar-driven ejector refrigeration system with iso-butane as a refrigerant

In this paper, the performance of the solar-driven ejector refrigeration system with iso-butane (R600a) as the refrigerant is studied. The effects that both the operating conditions and the solar collector types have on the system's performance are also examined by dynamic simulation. The TRNSYS and EES simulation tools are used to model and analyze the performance of a solar-driven ejector refrigeration system. The whole system is modelled under the TRNSYS environment, but the model of the ejector refrigeration subsystem is developed in the Engineering Equations Solver (EES) program. A solar fraction of 75% is obtained when using the evacuated tube solar collector. In the very hot environment, the system requires relatively high generator temperature, thus a flat plate solar collector is not economically competitive because the high amount of auxiliary heat needed to boost up the generator temperature. The results from the simulation indicate that an efficient ejector system can only work in a region with decent solar radiation and where a sufficiently low condenser temperature can be kept. The average yearly system thermal ratio (STR) is about 0.22, the COP of the cooling subsystem is about 0.48, and the solar collector efficiency is about 0.47 at T_e 15 °C, T_c 5 °C above the ambient temperature, evacuated collector area 50 m² and hot storage tank volume 2 m³.     

Theoretical analysis of low-temperature hot source driven two-stage LiBr/H2O absorption refrigeration system

A detailed theoretical analysis is presented for a two-stage LiBr/H₂O absorption refrigeration system, which consists of an evaporator, a low-pressure absorber, a low-pressure generator, a high-pressure absorber, a high-pressure generator, a condenser, a low-pressure heat exchanger and a high-pressure heat exchanger, driven by a low-temperature hot source. A comparison of results from the theoretical analysis and preliminary experiment indicates that the theoretical analysis developed can represent a real system with a reasonable accuracy, and is useful for future development.