Amélioration de la performance des machines frigorifiques á absorption par l'utilisation de cycles à absorption et désorption étagésPerformance improvement of absorption cooling systems by using staged absorption and desorption cycles

This study deals with staged absorption and desorption cooling systems which increase the performance of absorption cycles that are driven by only low-grade energy, particularly when the working fluids are NH₃---H₂0. Instead of working with only one absorber, these systems use a cascade of absorbers composed by one operating at the evaporator pressure, followed by a series of absorbers operating at staged pressures P_j, between P_ev and P_c In the same way, a cascade of generators is used for desorption. For the same operating parameters for other equipment and the same COP, the systems that we propose permit the generators to run at temperatures below those of all other systems offered up to now and using the same working fluids. When T_ev = −10°C, T_a = T_c = 30°C, the temperature of the generators can be as low as 65°C while the COP of the system is 0.258 and the COP_ex 0.317. By increasing the temperature of generators to 85°C while maintaining the other parameters at the same values, COP becomes 0.374 and the COP,, 0.336. These results improve the performance of absorption systems using only low-grade energy (T < 100°C). Particularly, they are better than the performance of two-stage absorption systems which consist of two single-stage absorption cycles coupled with each other through the evaporator of the first cycle and the absorber of the second cycle. With the same operating parameters indicated above for our system at the evaporator, the condenser, and the absorber, these coupled cycles need temperatures at generators of 80 and 100°C, whereas they give a COP of only 0.270     

Amélioration de la performance des machines frigorifiques á absorption par l'utilisation de cycles à absorption et désorption étagés

This study deals with staged absorption and desorption cooling systems which increase the performance of absorption cycles that are driven by only low-grade energy, particularly when the working fluids are NH₃H₂0. Instead of working with only one absorber, these systems use a cascade of absorbers composed by one operating at the evaporator pressure, followed by a series of absorbers operating at staged pressures P_j, between P_ev and P_c In the same way, a cascade of generators is used for desorption. For the same operating parameters for other equipment and the same COP, the systems that we propose permit the generators to run at temperatures below those of all other systems offered up to now and using the same working fluids. When T_ev = −10°C, T_a = T_c = 30°C, the temperature of the generators can be as low as 65°C while the COP of the system is 0.258 and the COP_ex 0.317. By increasing the temperature of generators to 85°C while maintaining the other parameters at the same values, COP becomes 0.374 and the COP,, 0.336. These results improve the performance of absorption systems using only low-grade energy (T < 100°C). Particularly, they are better than the performance of two-stage absorption systems which consist of two single-stage absorption cycles coupled with each other through the evaporator of the first cycle and the absorber of the second cycle. With the same operating parameters indicated above for our system at the evaporator, the condenser, and the absorber, these coupled cycles need temperatures at generators of 80 and 100°C, whereas they give a COP of only 0.270     

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.     

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.     

Performance of double effect absorption compression cycles for air-conditioning using methanol–TEGDME and TFE–TEGDME systems as working pairs: Performances de cycles à compression absorption à double effet pour le conditionnement d'air utilisant les couples méthanol–TEGDME ou TFE–TEGDME

The organic working pairs trifluoroethanol (TFE)–tetraethyleneglycol dimethyether (TEGDME or E181) and methanol–TEGDME have some advantages over classical water–LiBr and ammonia water working pairs in absorption cycles. One of the most important features is the wide working range caused by the absence of crystallization, the low freezing temperatures of the refrigerants and the thermal stability of the mixtures at high temperatures.The performance of a double effect absorption cycle for these organic mixtures can be improved if a compression stage is introduced between the evaporator and the absorber. The coefficient of performance (COP) and primary energy ratio (PER) values in the cooling mode are significantly increased over a wide working range: the cycle can work with temperature lifts of 50ºC at 5ºC in the evaporator or it can also be powered by low grade heat. For these conditions COP and PER values are higher than 1.0 and 0.7 respectively, and the power supplied to the compressor represents up to 15% of the thermal energy supplied to the generator. As it is possible to work at high temperatures lifts, the absorber and condenser can be air cooled.     

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.     

Economic biogas and cooling water rates in a lithium bromide—water absorption system

An economic analysis of the role of biogas and cooling water in a lithium bromide—water absorption system has been carried out to optimize the generator, condenser and absorber temperatures at a given evaporator temperature and solution pumping rate. The analysis has been repeated for different pumping rates (PR) to determine the optimum PR corresponding to the minimum over-all operating cost of the system. The study has also been carried out for the condition when biogas in the generator and cooling water in the absorber and condenser are supplied at equal flow-rates. It is found that the performance of the LiBr-H₂O system at equal biogas and cooling water flow-rates is about 5.988% higher than when operated at the minimum over-all operating cost, the latter being cheaper by only 2.71%. For low evaporation temperatures, use of a preheater in a LiBr-H₂O system creates a crystallization problem when operated at low pumping rates. The study has therefore been extended for a system without preheater. The parameters under study are illustrated graphically against the generator temperature. Equations to obtain the corresponding optimum condenser and absorber temperature are given. The functional relationship between crystallization limit and absorbent temperature has also been obtained. The optimum operating parameters are presented graphically.     

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.     

Thermodynamic analysis and optimization of a novel two-stage transcritical N2O cycle

Thermodynamic (energy and exergy) analyses and optimization studies of two-stage transcritical N₂O and CO₂ cycles, incorporating compressor intercooling, are presented based on cycle simulation employing simultaneous optimization of intercooler pressure and gas cooler pressure. Further, performance comparisons with the basic single-stage cycles are also presented. The N₂O cycle exhibits higher cooling COP, lower optimum gas cooler pressure and discharge temperature and higher second law efficiency as compared to an equivalent CO₂ cycle. However, two-stage compression with intercooling yields lesser COP improvement for N₂O compared to CO₂. Based on the cycle simulations, correlations of optimum gas cooler pressure and inter-stage pressure in terms of gas cooler exit temperature and evaporator temperature are obtained. This is expected to be of help as a guideline in optimal design and operation of such systems.     

Performance research of self regenerated absorption heat transformer cycle using TFE-NMP as working fluids

A heat transformer is proposed in order to upgrade low-temperature-level energy to a higher level and to recover more energy in low-temperature-level waste heat. It is difficult to achieve both purposes at the same time using a conventional heat transformer cycle and classical working pairs, such as H₂O–LiBr and HN₃–H₂O. The new organic working pair, 2,2,2-trifluoroethanol (TFE)-N-methylpyrolidone (NMP), has some advantages compared with H₂O–LiBr and NH₃–H₂O. One of the most important features is the wide working range as a result of the absence of crystallization, the low working pressure, the low freezing temperature of the refrigerant and the good thermal stability of the mixtures at high temperatures. Meanwhile, it has some negative features like NH₃–H₂O. For example, there is a lower boiling temperature difference between TFE and NMP, so a rectifier is needed in refrigeration and heat pump systems. Because TFE–NMP has a wide working range and does not cause crystallization, it can be used as the working pair in the self regenerated absorption heat transformer (SRAHT) cycle. In fact, the SRAHT cycle is the generator–absorber heat exchanger (GAX) cycle applied in a heat transformer cycle. In this paper, the SRAHT cycle and its flow diagram are shown and the computing models of the SRAHT cycle are presented. Thermal calculations of the SRAHT cycle under summer and winter season conditions have been worked out. From the results of the thermal calculations, it can be found that there is a larger temperature drop when the waste hot water flows through the generator and the evaporator in the SRAHT cycle but the heating temperature can be kept the same. That means more energy in the waste heat source can be recovered by the SRAHT cycle.     

Etude comparative de la performance des machines frigorifiques à absorption utilisant de l'energie thermique à très faible valeur exergétiquePerformance study of cooling systems using low grade heat energy for refrigeration

This study deals with cooling systems driven with low grade heat energy delivered by heat sources whose temperature do not exceed 100°C. Since we cannot neglect the irreversibilities associated to heat depreciation, we consider an approach temperature of 10°C for all the heat exchangers. So the temperature of the generators is <90°C to fulfill the approach conditions. For a heat sink temperature of 30°C and an evaporating temperature of −10°C, only two kinds of cycles are concerned: the cycles with staged absorption and desorption (CADE) and the cascade systems (CCAR) in which a series of two cycles is used, the evaporator of the former cooling the absorber of the later.The cycles with staged absorption and desorption are described. They permit a decrease of generators' temperature down to a level no system among those previously offered can run.To point out the importance of the CADE, we compare them to cascade systems. This comparative study shows that the CADE yields better performances. The gap of performance between the two systems increases with the decrease of the heat sink temperature and it is maximal for lower temperature at generators. The comparative study uses five criteria: the coefficient of performance, Σw_p/Q_ev, Q_t/Q_ev, Q_ref/Q_ev and the operating area in a (T_fr, T_g) plane. LiNO₃/NH₃ and H₂O/NH₃ are used as working fluids of the CADE while LiBr/H₂O + LiNO₃/NH₃ and LiBr/H₂O + H₂O/NH₃ are used for the CCAR.The CADE are very suitable for the use of low grade heat energy (70°C T_sc 100°C) in refrigeration (T_ev −10°C) with sink temperature as high as 30°C and a realistic approach of 10°C for all the heat exchangers.     

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.     

Review of advanced absorption cycles: performance improvement and temperature lift enhancement

The objective of this study is to propose and evaluate advanced absorption cycles for the coefficient of performance (COP) improvement and temperature lift enhancement applications. The characteristics of each cycle are assessed from the viewpoints of the ideal cycle COP and its applications. The advanced cycles for the COP improvement are categorized according to their heat recovery method: condensation heat recovery, absorption heat recovery, and condensation/absorption heat recovery. In H₂O–LiBr systems, the number of effects and the number of stages can be improved by adding a third or a fourth component to the solution pairs. The performance of NH₃–H₂O systems can be improved by internal heat recovery due to their thermal characteristics such as temperature gliding. NH₃–H₂O cycles can be combined with adsorption cycles and power generation cycles for waste heat utilization, performance improvement, panel heating and low temperature applications. The H₂O–LiBr cycle is better from the high COP viewpoints for the evaporation temperature over 0°C while the NH₃–H₂O cycle is better from the viewpoint of low temperature applications. This study suggests that the cycle performance would be significantly improved by combining the advanced H₂O–LiBr and NH₃–H₂O cycles.     

The natural fluid nitrous oxide—an option as substitute for low temperature synthetic refrigerants

A theoretical investigation was performed concerning the coefficient of performance (COP) of cascade refrigerating systems using N₂O as refrigerant for the low temperature cascade stage and various natural refrigerants like ammonia, propane, propene, carbon dioxide and nitrous oxide itself for the high temperature stage. The basis of the comparison was a conventional R23/R134a-cascade refrigerating system for heat rejection temperatures of +55, +35 and +25 °C for air cooling, cooling tower water cooling and city water cooling, respectively. It can be stated that such an application of N₂O at the primary stage and ammonia or hydrocarbons as refrigerants at the secondary stage in refrigerating systems achieves similar COP-values compared to the R23/R134a-cascade refrigerating system, whereas CO₂ and N₂O in a transcritical cycle in general perform worse.An application of N₂O in a two-stage compression cycle with interstage injection and city water cooling at low and high interstage temperatures has a nearly equal COP as a conventional R23/R134a-cascade refrigerating system and is an interesting alternative for small laboratory refrigerating systems.     

Performance assessment of multistage absorption cycles

The performance potential of 11 multistage, multi-effect absorption cycles is evaluated. They include water-lithium bromide, ammonia-water and cascade configurations. All evaluations are based on air-conditioning applications assuming a 4°C evaporator temperature and a 35°C condenser and absorber temperature. The sensitivity of the performance to the approach temperature in the heat exchanger was studied. Eight cycles were selected for a more detailed simulation. The highest COP at zero approach temperature was obtained for a three-stage water-lithium bromide cycle cascaded with two single-stage ammonia water cycles, while for approach temperatures of 5 K the best COP was obtained for the three-stage water-lithium bromide cycle.     

Etude comparative de la performance des machines frigorifiques à absorption utilisant de l'energie thermique à très faible valeur exergétique

This study deals with cooling systems driven with low grade heat energy delivered by heat sources whose temperature do not exceed 100°C. Since we cannot neglect the irreversibilities associated to heat depreciation, we consider an approach temperature of 10°C for all the heat exchangers. So the temperature of the generators is <90°C to fulfill the approach conditions. For a heat sink temperature of 30°C and an evaporating temperature of −10°C, only two kinds of cycles are concerned: the cycles with staged absorption and desorption (CADE) and the cascade systems (CCAR) in which a series of two cycles is used, the evaporator of the former cooling the absorber of the later.The cycles with staged absorption and desorption are described. They permit a decrease of generators' temperature down to a level no system among those previously offered can run.To point out the importance of the CADE, we compare them to cascade systems. This comparative study shows that the CADE yields better performances. The gap of performance between the two systems increases with the decrease of the heat sink temperature and it is maximal for lower temperature at generators. The comparative study uses five criteria: the coefficient of performance, , and the operating area in a (T_fr, T_g) plane. LiNO₃/NH₃ and H₂O/NH₃ are used as working fluids of the CADE while LiBr/H₂O + LiNO₃/NH₃ and LiBr/H₂O + H₂O/NH₃ are used for the CCAR.The CADE are very suitable for the use of low grade heat energy (70°C ? T_sc ? 100°C) in refrigeration (T_ev ? −10°C) with sink temperature as high as 30°C and a realistic approach of 10°C for all the heat exchangers.     

Experimental investigation on the performance of NH3/CO2 cascade refrigeration system with twin-screw compressor

This paper describes the experiment carried out to analyze the performance of a refrigeration system in cascade with ammonia and carbon dioxide as working fluids. The effect of operation parameters, such as the evaporating temperature of the low temperature cycle, the condensing temperature of low temperature cycle, temperature difference in cascade heat exchanger and superheat degree, on the system performance was investigated. Performance of the cascade system with NH₃/CO₂ was compared with that of two-stage NH₃ system and single-stage NH₃ system with or without economizer. It was found that the COP of the cascade system is the best among all the systems, when the evaporating temperature is below −40 °C. Also, the cascade system performance is greatly affected by evaporating temperature, condensing temperature of low temperature cycle, temperature difference in cascade heat exchanger and is only slightly sensitive to superheat degree. All the experimental results indicate that the NH₃/CO₂ cascade system is very competitive in low temperature applications.     

Experimental studies on air-cooled NH3-H2O based modified gax absorption cooling system

An experimental investigation on the performance of an air-cooled modified generator absorber heat exchange (GAX) absorption cooling system has been carried out and presented in this paper. The conventional system is modified by incorporating high pressure GAX, low pressure GAX, a solution cooler and an additional solution heat exchanger to reduce the heat input to the system. The system is designed for a cooling capacity of 10.5 kW using ammonia-water (NH₃-H₂O) as the working fluid. The performance of the system in terms of the circulation ratio, internal heat recovery and coefficient of performance (COP) has been obtained. The system is capable of producing a low evaporator temperature of −5 °C, at a sink temperature of 35 °C, under no load conditions. The results indicate that at a generator and evaporator temperature of 120 °C and 2 °C respectively, the system delivers a maximum cooling capacity of about 9.5 kW with a fuel and total COP of 0.61 and 0.57 respectively.     

Evaluation of a novel combined ejector-absorption refrigeration cycle — I: computer simulation

This paper describes a novel refrigeration cycle based on the combination of an absorption cycle with an ejector refrigeration cycle. The combination brings together the advantages of absorption and ejector refrigeration systems and provides high COP for refrigeration and air-conditioning. The combined cycle is particularly suitable for utilising waste thermal energy. A computer simulation program was developed for the combined cycle and used to determine the performance of the system using LiBrH₂O for various generator, condenser and evaporator temperatures. Optimum operating conditions and ejector design data are also provided.