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.     

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

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

Modeling and simulation studies on single/double-effect absorption cycle using water-multicomponent salt (MCS) mixture

This communication presents an investigation on modeling and simulation studies of single/double-effect absorption cooling systems using a water-multicomponent salt mixture vis-à-vis water---LiCl---CaCl₂---Zn(NO₃)₂ solution as a working fluid. The computer modeling/simulation is based on mass, material, and heat balance equations for each component of the system. Effects of different input variables used for the computer modeling and simulation studies are investigated and a detailed parametric study of the double-effect cycle is carried out. A comparative study of the water-multicomponent salt (MCS) mixture and the conventional water---LiBr mixture is also presented for both single-stage and double-effect absorption cycles. It is found that the cooling COP of the double-effect cycle is nearly twice that of the single-stage cycle for both of the working fluids under the same operating conditions. The COP is higher for the water-multicomponent salt (MCS) mixture than for the water---LiBr case. The system feasibility and constraints are also discussed.     

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.     

Simple LiBr/Water absorption cycle limitations

The aim of this paper is to determine the optimal design temperatures and pressures of a simple LiBr/water absorption cycle. To achieve this goal, a computer program, based on the characteristics of lithium bromide water combinations, has been designed. The output results were the generator temperature operating range which is defined relative to the other component temperatures. The limitations of the cycle operation, based on the crystallization of lithium bromide at high generator temperatures, are also shown. The variation of the theoretical COP and the circulation ratio of the cycle are calculated.     

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

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

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.     

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.     

Space-conditioning using triple-effect absorption heat pumps

A triple-effect cycle absorption heat pump was investigated in this study using OSU-ABSIM, a modular steady state simulation program. Ammonia-water solution was used as the working fluid for the upper, high pressure cycle, while ammonia-water and ammonia-sodium thiocyanate solutions were investigated for the lower pressure cycle. The overall system is so configured that the absorber, condenser, and rectifier heat duty from the high pressure cycle is rejected to the generator of the low pressure cycle. The complete absorption system was analyzed, including air-to-hydronic heat exchangers and the natural gas-fired generator. Cycle performance was modeled over a wide range of cooling and heating mode ambients to determine the overall benefits of using this cycle. The effect of high ambient temperatures on the rise in the highest cycle pressures and temperatures (to maintain coupling between the two cycles) was investigated over the range of expected ambients. Parametric studies were conducted on several significant variables that affect cycle performance, such as heat exchanger sizes and different control schemes. The results of this study can be used to determine the merits of this cycle versus other absorption heat pump alternatives, specifically in terms of potential COP benefits for additional system complexity.     

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 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.     

Mass and energy storage analysis of an absorption heat pump with simulated time dependent generator heat input

This communication presents an assessment of the feasibility of energy storage via refrigerant mass storage within an absorption cycle heat pump with simulated time dependent generator heat input. The system consists of storage volumes with the condenser and absorber of the conventional absorption cycle heat pump to store liquid refrigerant, weak and strong solutions during the generation period, which are required for the heat pump operation during the generation off period. A time dependent mass and energy storage analysis based on mass and energy balance equations for various components of the heat pump system has been carried out to evaluate energy storage concentration and storage efficiency for combined and separate storage schemes for the weak and strong solutions. Two possible performance modes, viz constant pumping ratio or the constant flow of the strong solution from the absorber to the generator have been analysed: the latter is preferable over the former from a practical point of view. Numerical computer simulation has been made for a typical winter day in Melbourne (Australia) with the desired heating load specified. It is found that the concept of refrigerant storage within the absorption cycle heat pump is technically feasible for efficient space heating. The energy storage concentration in the condenser store is slighly higher while that in absorber store is slightly lower for the separate storage mode as compared to the combined storage. However, the combined storage has an advantage of less storage volume and hence is more cost effective than separate storage and the disadvantage of limited system operation due to the decrease of solution concentrations.     

Single stage and double absorption heat transformers in an industrial application

An industrial application of the single stage and double absorption heat transformer (AHT) systems using water–lithium bromide solutions with water as the refrigerant was analyzed. First, a basic single stage AHT system was described, the operating sequence was explained and thermodynamic system analysis was presented. Next, an application of the single stage AHT system to an industrial company was analyzed. A computer code was prepared to determine the effect of different parameters on the AHT system performance and the results were presented in graphical form. Additionally, the series and parallel double absorption AHT systems were introduced, the operating sequences were explained and thermodynamic system analysis was included. All results were presented in tabular form for comparison. It is concluded that about 50% of the waste heat can be utilized and the hot process water and vapor could be produced by applying single stage and double AHT systems, respectively. The parallel double AHT system could generate more vapor than the series double AHT system. Copyright © 2009 John Wiley & Sons, Ltd.     

从热工学角度探讨国产吸收式制冷机的发展方向

该文从热工学角度探讨了目前我国溴化锂吸收式制冷机整机热力循环及各热质交换设备性能、结构等方面存在的问题;在整机循环方面,指出实现各设备的合匹配、发展新的结构流程及增加新品种、新规格的机组为进一步的发展方向;在各设备性能、结构方面提出应加强吸收机理研究以及表面活化剂、高效强化管的应用研究。     

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.     

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.     

Simulation research on an EAX (Evaporator-Absorber-Exchange) absorption refrigeration cycle

There are some heat sources whose temperature is much higher than the limited generation temperature of conventional single effect absorption refrigeration cycle but lower than that of conventional double effect absorption refrigeration cycle. These heat sources can not be utilized efficiently by prior cycles. To make efficient use of these heat sources, this paper proposed an EAX (Evaporator-Absorber-Exchange) absorption refrigeration cycle. The proposed cycle can make use of the condensing heat of the vapor separated from high temperature generator to make additional refrigeration. Therefore, the COP (Coefficient of Performance) of the proposed cycle is much higher than that of the conventional single effect cycle. Simulation results show that the COP of the EAX cycle can be 40% higher than that of the conventional single effect cycle at some simulated conditions.     

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.     

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.