Exergy analysis of lithium bromide/water absorption systems

Exergy analysis of a single-effect lithium bromide/water absorption system for cooling and heating applications is presented in this paper. Exergy loss, enthalpy, entropy, temperature, mass flow rate and heat rate in each component of the system are evaluated. From the results obtained it can be concluded that the condenser and evaporator heat loads and exergy losses are less than those of the generator and absorber. This is due to the heat of mixing in the solution, which is not present in pure fluids. Furthermore, a simulation program is written and used for the determination of the coefficient of performance (COP) and exergetic efficiency of the absorption system under different operating conditions. The results show that the cooling and heating COP of the system increase slightly when increasing the heat source temperature. However, the exergetic efficiency of the system decreases when increasing the heat source temperature for both cooling and heating applications.     

一种新型第二类吸收式热泵的原理及性能分析

提出一种包含吸收溶液冷却结晶过程的新型第二类吸收式热泵循环,并对其工作过程及性能特性进行理论分析与实验研究。结果表明,该循环可在吸收器吸收溶液质量分数显著高于发生器吸收溶液质量分数的条件下工作,其热泵温升能力明显优于现有AHT循环。当冷却结晶终温和冷凝器温度为35℃、发生器温度和蒸发器温度为92℃时,其热泵温升理论上可达97℃。     

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.     

Analysis of the process characteristics of an absorption heat transformer with compact heat exchangers and the mixture TFE–E181

In the project described in this paper an experimental rig for a one-stage absorption heat transformer was designed and constructed. One aim of the project was to reduce the investment costs for the apparatus. This incorporates new and less expensive compact brazed plate heat exchangers for generator, evaporator, condenser and solution heat exchanger. The absorber was designed as a helical coil pipe absorber, where the weak solution trickles down as a falling film outside of the coil. The tests of the equipment involved measurements using a mixture of trifluorethanol (TFE) and tetraethyleneglycoldimethylether (E181). The process characteristics were investigated for different temperatures of the rich solution leaving the absorber. Experimental results are presented and compared with the results of a computer simulation model. Additionally the model was used to compare the COP of the heat transformation process with the mixtures lithium bromide–water (LiBr–H₂O) and ammonia–water (NH₃–H₂O). Furthermore, the overall heat and mass transfer coefficients for the plate heat exchangers and the falling film absorber were evaluated and compared with those of shell and tube heat exchangers.     

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.     

A new concept in gas-fired heat pump reversed rectification heat pump

The reversed rectification heat pump (RRHP) is a novel type of absorption heat pump developed at Laboratoire des sciences du génie chimique (LSGC) in the research group of Professor Le Goff. The distinctive characteristic of the RRHP is that both the separator and the absorber are multi-stage gas contacting units of a type commonly used in the process industry and each composed of the multi-stage contactor, an evaporator and a condenser. The separator is a conventional rectification unit in which the working fluid is separated into a rich and a lean phase which are recombined in the absorber. The absorption is an exact reversal of the rectification, both units having inverted flow diagrams; for this reason the absorption is refered to as reversed rectification. This heat pump can produce simultanious heat and cold and provides significant primary energy savings and reduction of harmful environmental effects.     

Energy and exergy analysis of a double absorption heat transformer operating with water/lithium bromide

In the present study, the first and second law of thermodynamics have been used to analyze in detail the performance of a double absorption (lift) heat transformer operating with the water–lithium bromide mixture. A mathematical model was developed to estimate the coefficient of performance (COP), the exergy coefficient of performance (ECOP), the total exergy destruction in the system (Ψ_TD) and the exergy destruction (Ψ_D) in each one of the main components, as a function of the system temperatures, the efficiency of the economizer (EF_EC), the gross temperature lift and flow ratio (FR). The results showed that the generator is the component with the highest irreversibilities or exergy destruction contributing to about 40% of the total exergy destruction in the whole system, reason why this component should be carefully designed and optimized. The results also showed that the COP and ECOP increase with increase in the generator, the evaporator and the absorber–evaporator temperatures and decrease with the absorber and condenser temperatures. Finally, it was observed that the COP and ECOP are very dependent of the FR and the economizer efficiency (EF_EC) values. Also the optimum operating region of the analyzed system is shown in the present study. Copyright © 2009 John Wiley & Sons, Ltd.     

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

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

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]     

Performance of a second generation solar cooling unit

This paper describes the performance of a 1.5 ton solar-operated absorption refrigeration unit operating with a 14 m² flat-plate solar collector system and containing five heat exchangers: the generator, the absorber, the condenser, the solution heat exchanger (all of these being of shell-and-tube type) and finally the evaporator, which is of the fin-and-tube type. One circulation pump is used for solution flow and another for the hot water flow. The condenser and the absorber are both cooled by city mains water. This particular unit, called the second generation unit, is compared with an earlier, first-generation unit (FGU), i.e. a low-cost, locally manufactured unit of 0.5 ton capacity. The results are based on the observed operation of the unit during hours of sufficient solar irradiance in April and May, the beginning of the air-conditioning season in Jordan. The variation of both the generator and evaporator temperatures during the test period are reported. Also reported is the performance of the unit as measured by the actual and theoretical (ideal) coefficients of performance, both of these being functions of the temperatures and solar irradiance. The maximum values obtained for both actual and theoretical coefficients of performance were 0.85 and 2.7, respectively. These values are within the range of values published in literature, and higher than those obtained by the FGU.     

Heat pump assisted distillation. V: A feasibility study on absorption heat pump assisted distillation systems

The problems in matching a heat driven absorption heat pump to a distillation process in heat pump assisted distillation are discussed. The performance of an absorption system is a function of the temperatures in the evaporator, the condenser, the absorber and the generator and the ratio of the mass flow rate in the secondary circuit to the mass flow rate in the primary circuit. In absorption systems design choices are limited by the Gibbs phase rule. Plots are given of the coefficient of performance against the temperatures of the top and bottom products and also against the energy saved.     

Effect of component pressure drops in two-fluid pumpless continuous vapour absorption refrigerator

Miniaturisation of the vapour absorption refrigerator requires replacement of the solution pump by a heat operated bubble pump and air cooled condenser and absorber. The replacement necessitates the selection of working media restricted to vacuum operation, and the air cooling poses the problem of high pressure drops in the refrigerator. Thermodynamic analysis of the absorption refrigerator with such a suitable working medium is performed considering the pressure drops in the system as parameters. The analysis shows that the effect of the evaporator to absorber pressure drop on the system performance is more significant than that of the generator to condenser pressure drop, and it becomes more predominant at the low generator temperature normally encountered in solar operated systems.     

A comparative study of the performance characteristics of double‐effect absorption refrigeration systems

Three classes of double‐effect lithium bromide–water absorption refrigeration systems (series, parallel and reverse parallel) with identical refrigeration capacities are studied and compared thermodynamically. In order to simulate the performances of the systems, a new set of computationally efficient formulations is used for thermodynamic properties of Li‐Br solutions at equilibrium. The simulation results are used to examine the influence of various operating parameters on the first and second law performance characteristics of the systems. In addition, the dependences are investigated of system performance on the effectivenesses of the solution heat exchangers, the pressure drops between the evaporator and the absorber and between the low‐pressure generator and the condenser, and the low‐grade heat externally supplied to the low‐pressure generator. The results reveal the advantages and disadvantages of different configurations of double‐effect lithium bromide–water absorption refrigeration systems, and are expected to be useful in the design and control of such systems. Copyright © 2010 John Wiley & Sons, Ltd.     

Temperature–enthalpy/entropy charts of combined ejector–absorption cycle using NH3–H2O

This paper describes the performance of an ammonia–water combine ejector–absorption cycle as refrigerator and heat pump. This combination brings together the advantages of absorption and ejector systems. Also, thermodynamic cycles on the temperature–enthalpy and temperature–entropy charts are shown. The thermodynamics of the combined ejector–absorption cycles are simulated by a suitable method and a corresponding computer code, based on analytic functions describing the behaviour of the binary mixture NH₃–H₂O. It is found that in the case of the refrigerator and heat pump, the theoretical coefficient of performance (COP) or the theoretical heat gain factor (HGF) vary from 1.6 to 90.4 per cent and 0.7 to 37.6 per cent, greater than those of the conventional absorption system, respectively. The operation conditions were: generator temperature (205.5 to 237.1°C), condenser temperature (25.9 to 37.4°C) and evaporator temperature (−8.4 to 5°C). Copyright © 2000 John Wiley & Sons, Ltd.     

A comparative study of an absorber heat recovery cycle for solar refrigeration using NH3-refrigerant with liquid/solid absorbents

This paper presents an investigation on using an ammonia refrigerant with liquid/solid absorbents in an absorber heat recovery cycle where heat released during the absorption process is used to heat up the strong solution coming out of the absorber, thereby reducing the generator heat input and hence improving the coefficient of performance. A comparative thermodynamic study is made with NH₃-H₂O and NH₃-LiNO₃ pairs as working fluids for both conventional absorption and absorber heat recovery systems. It is found that an improvement of about 10 per cent in COP for the absorber heat recovery cycle is achieved over the conventional absorption cycle and the NH₃-LiNO₃ system yields a higher COP than for NH₃-H₂O over a wide range of generator temperatures and condenser/absorber temperatures. A detailed parametric study is also presented in this paper.     

Equilibrium low pressure generator temperatures for double-effect series flow absorption refrigeration systems

The equilibrium temperatures at the low pressure (LP) generator for double-effect series flow lithium bromide–water vapour absorption chiller are evaluated and the system performance is estimated at these temperatures. Influence of temperatures at high pressure (HP) generator, evaporator, condenser and absorber, and the effectiveness of heat exchangers on equilibrium temperature and internal heat transfer at LP generator, circulation ratio, and coefficient of performance are studied. Dual-heat mode of operation of the system is also investigated utilising low grade waste heat at the LP generator. Correlations are presented for equilibrium LP generator temperature, internal heat transfer at the low pressure generator, circulation ratio, coefficient of performance, optimum HP generator temperature and optimum circulation ratio for maximum coefficient of performance in terms of operating temperatures, which are useful in the design and control of absorption system even at the off-design conditions.     

Heat and mass transfer intensification at steam absorption by surfactant additives

The mechanisms of surfactant admixture effect on heat and mass transfer were studied experimentally at steam absorption by a salt solution at operation of the heat pump absorber. Steam absorption by initially immobile water solution of lithium bromide with additives of n-octanol with concentrations of 25–400 ppm (both lower and higher than the solubility limit) was studied. Thermograms of a layer surface, moving during absorption, were obtained by thermal imaging equipment. Data on surface tension of lithium bromide solution with small additives of n-octanol was analyzed.     

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.     

HCFC22-based absorption cooling systems,part III: Effects of different absorbers and condenser temperatures

Generally in a vapour absorption refrigeration system (VARS) heat rejection temperatures at absorber (T_a) and condenser (T_c) are taken to be equal. However, different temperatures can exist when the cooling water flows in series through the two components. Under such situations, it is essential to know which of T_a and T_c has greater influence on the performance of the VARS. Here the influence of different T_a and T_c on the performance of a single-stage VARS working with HCFC22 as a refrigerant and three organic solvents, namely DMA, DMF and DEMTEG, as absorbents is studied. Results are obtained over a wide range of operating temperatures. To improve the performance of HCFC22-based VARS, results reveal that (i) the cooling water in parallel pipe connections should be used at low values of temperatures at evaporator, cooling water and heat source, and (ii) cooling water should first flow through condenser and then through the absorber when evaporator and heat source temperatures are high over the complete range of cooling water temperatures. COP_th is more sensitive to T_c than to T_a.     

Performance characteristics of an ammonia–water absorption heat pump system

The influences of the performance parameters and the heat transfer characteristics of the absorption heat pump using ammonia–water mixture are theoretically carried out. There is a pronounced effect of the ammonia concentration ξ after rectifier on the temperature glides that has been investigated. At ξ = 0.9000 and saturation pressures of 75 and 0.5 bar, the temperature glides are 64.4°C and 81.21°C, respectively, whereas these glides are 0°C and 16.1°C at ξ = 0.9999 and at the same pressures. This mixture property considerably affects the absorption system performance and the design of the rectifier as well as other absorption components. A correlation of the Nusselt number, Nu, is developed and compared with some published work in the literature for plate type heat exchanger. The effects of ammonia concentration ξ, mass fraction spread Δξ, specific solution circulation ratio f, and pressure ratio R_p on the refrigerant mass flow rate, the pressure drop, and the heat transfer coefficients during the condensation, the evaporation, and the absorption processes are investigated. It was found that increasing ammonia mass fraction spread Δξ results in both specific circulation ratio f and R_p that have insignificant effects on the refrigerant mass flow rate. Mounting Δξ at constant f reduces the pressure drop gradually and subsequently starts to increase as Δξ escalates. The ammonia concentration ξ has insignificant effect on the evaporation heat transfer coefficient but has a little effect on the condensation and the absorber heat transfer coefficients. The ammonia mass fraction spread Δξ and f have considerable effects on the heat transfer coefficient for different absorption heat pump components. R_p has a pronounced effect on the evaporation heat transfer coefficient, although it has a slight effect on the condensation and the absorber heat transfer coefficients. The effect of R_p on the heat transfer coefficient may be eliminated in the absorber for Δξ > 0.18. Copyright © 2013 John Wiley & Sons, Ltd.