Solution Heat Exchanger Effectiveness Research Articles

Space cooling using geothermal single‐effect water/lithium bromide absorption chiller

AbstractThis research is proposed to fully investigate the performance of a single‐effect water/lithium bromide absorption chiller driven by geothermal energy. Since absorption cycles are considered as low‐grade energy cycles, this innovative idea of rejecting fluid from a single‐flash geothermal power plant with low‐grade energy would serve as efficient, economical, and promising technology. In order to examine the feasibility of this approach, a residential building which is located in Sharjah, UAE, considered to evaluate its cooling capacity of 39 kW which is calculated using MATLAB software. Based on the obtained cooling load, modeling of the required water/lithium bromide single‐effect absorption chiller machine is implemented and discussed. A detailed performance analysis of the proposed model under different conditions is performed using Engineering Equation Solver software (EES). Based on the obtained results, the major factors in the design of the proposed system are the size of the heat exchangers and the input heat source temperature. The results are presented graphically to find out the geofluid temperature and mass flow and solution heat exchanger effectiveness effects on the chiller thermal performance. Moreover, the effects of the size of all components of the absorption chiller on the cooling load to meet the space heating are presented. The thermal efficiency of the single‐flash geothermal power plant is about 13% when the power plant is at production well temperature 250℃, separator pressure 0.24 MPa, and condenser pressure 7.5 kPa. The results show that the coefficient of performance (COP) reaches about 0.87 at solution heat exchanger effectiveness of 0.9, when the geofluid temperature is 120℃.

Cycle performance analysis and experimental validation of a novel diffusion absorption refrigeration system using R600a/n-octane

Diffusion absorption refrigeration (DAR) is a promising cycle with regard to renewable energy utilization, as the cycle can operate using only a thermal energy input. This study focuses on the application of a low-global warming potential (GWP) refrigerant as a substitution for conventional ammonia refrigerant. In this study, R600a/n-octane is chosen as a novel refrigerant/absorbent pair for DAR. The cycle operation characteristics are investigated through numerical analysis, in terms of the coefficient of performance (COP) and minimum temperature. The influences of five key parameters (driving temperature, total pressure, evaporator temperature, absorber effectiveness, and solution heat exchanger effectiveness) on the COP are also analyzed. Furthermore, additional experiments are performed to validate the simulation results through comparisons with the experimental data. The variation trend of the COP shows good agreement between the simulations and experiments, and the mathematical model provides a reasonable estimation of the maximum COP. It is concluded that R600a/n-octane has significant potential for DAR application, as a maximum COP of 0.162 is achieved at a driving temperature of approximately 100 °C, making it more favorable than other low-GWP refrigerant working fluids pairs and applicable for solar cooling.

Hybrid absorption-compression heat pump with two-stage rectification and subcooler

Conventional hybrid absorption-compression heat pumps are difficult to efficiently upgrade waste heat to a heating temperature of above 90 °C. In this paper, to solve this problem, two novel approaches, namely a two-stage rectification and a subcooler, are specially proposed for the hybrid heat pump. By designing a two-stage rectification to reduce the suction temperature of compressor, thereby lowering its discharge temperature, commercially available normal-temperature compressors can be employed in the hybrid heat pump for high-temperature heating. By utilizing a subcooler proposed to reduce the irreversible loss during the throttling process, the energy efficiency of the hybrid heat pump is improved. Moreover, the hybrid heat pump with multi-stage subcoolers is also proposed and analyzed. Thermodynamic analysis exhibits that, at a heating temperature of 120 °C and a solution heat exchanger effectiveness of 0.8, the hybrid heat pump with a two-stage rectification and a subcooler brings about an energy efficiency improvement of 17.4% in comparison to the conventional one.

Thermodynamic Modeling and Performance Optimization of a Solar-Assisted Vapor Absorption Refrigeration System (SAVARS)

A thermodynamic steady-state model for a single-effect lithium bromide–water (LiBr-H2O)-based vapor absorption refrigeration system of 17.5[Formula: see text]kW capacities has been presented using the first and second laws of thermodynamics. The mass, energy and exergy balance equations in each component of the vapor absorption cycle have been fitted into a computer program to carry out the calculation using the thermo-physical properties of the working fluid. The performance parameters such as coefficient of performance (COP), exergy coefficient of performance (ECOP), total exergy destruction (TED), etc. have been evaluated considering different temperatures in generator and evaporator, different LiBr concentrations in the weak and strong LiBr-H2O solution and different solution heat exchanger effectiveness. The model evaluated the optimum performance parameters like COP, ECOP, TED, etc. of the vapor absorption system by using Design Expert-12 software for an application like on-farm cooling or transit storage of fruits and vegetables.

Energy and exergy analysis of an air-cooled waste heat-driven absorption refrigeration cycle using R290/oil as working fluid

This paper presents an air-cooled waste heat-driven absorption refrigeration cycle with a non-adiabatic absorber using R290/oil as working fluid. The effect of various operating parameters (generator temperature, absorber outlet solution temperature, evaporator and condenser temperature, absorption efficiency and solution heat exchanger effectiveness) on the system performance has been discussed based on the first and second law of thermodynamics. In order to reveal which components have a poor utilization of the input exergy, theexergy destruction coefficient has been defined and calculated under considered working conditions. The simulation results show that the system can run steadily under air-cooled conditions with an appreciable COP of 0.4696 and exergetic efficiency of 0.1293. So R290/oil mixture is a highly potential working fluid for absorption refrigeration. The application of the air-cooled non-adiabatic absorber improves both the coefficient of performance and the exergetic efficiency of the system, which also contributes to the miniaturization and cost reduction of air-cooled systems. And the solution heat exchanger and generator are the major contributors to the total exergy destruction of the system. More efforts should be made to optimize these components for higher exergetic efficiency of the system.

Performance investigation of a combined heat pump transformer operating with water/lithium bromide

In the present study, we propose a combined heat pump transformer (CHPT) system operating with a H2O/LiBr working pair, driven by waste heat at medium temperature to supply two types of useful heat. Based on the energy and mass conservation law and phase equilibrium, a computational model is developed in Engineering Equation Solver to investigate performance of the CHPT against the system temperatures, the solution heat exchanger effectiveness, the flow ratio, and the gross temperature lift. The results show that the system efficiency is nearly 100%, and the exergy coefficient of performance (ECOP) is 71.1%, which are 33.2% and 33.1% higher than those in the reference system, respectively. The high-pressure absorber exhibits the highest irreversibility, which accounts for 26% of the total system irreversibility, followed by the low-pressure absorber, the low-pressure generator and the medium-pressure generator; hence, special attention should be paid to these four components. Moreover, an optimal flow ratio or optimal gross temperature lift exists, which results in the largest ECOP for larger energy ratio values. The payback period is approximately 2years, which is suitable for workshop or industrial park where there is a great requirement for different levels of heat.

NLP model based thermoeconomic optimization of vapor compression–absorption cascaded refrigeration system

This paper addresses the size and cost estimation of vapor compression–absorption cascaded refrigeration system (VCACRS) for water chilling application taking R410a and water–LiBr as refrigerants in compression and absorption section respectively which can help the design engineers in manufacturing and experimenting on such kind of systems. The main limitation in the practical implementation of VCACRS is its size and cost which are optimized in the present work by implementing Direct Search Method in non-linear programming (NLP) mathematical model of VCACRS. The main objective of optimization is to minimize the total annual cost of system which comprises of costs of exergy input and capital costs in monetary units. The appropriate set of decision variables (temperature of evaporator, condenser, generator, absorber, cascade condenser, degree of overlap and effectiveness of solution heat exchanger) minimizes the total annual cost of VCACRS by 11.9% with 22.4% reduction in investment cost at the base case whereas the same is reduced by 7.5% with 11.7% reduction in investment cost with reduced rate of interest and increased life span and period of operation. Optimization results show that the more investment cost in later case is well compensated through the performance and operational cost of the system. In the present analysis, optimum cascade condensing temperature is a strong function of period of operation and capital recovery factor. The cascading of compression and absorption systems becomes attractive for lower rate of interest and increase life span and operational period.

Thermo-economic analysis of steady state waste heat recovery in data centers using absorption refrigeration

This paper addresses the technical and economic issues associated with waste heat recovery in data centers through the use of absorption cooling machines. The theoretical possibility of utilizing the heat dissipated by a server, or a number of servers, to power an absorption system, which in turn produces cooling for other servers in the data center, is investigated. For this purpose, a steady-state thermodynamic model is developed to perform energy balance and exergy analyses for a novel configuration of an on-chip two-phase cooling system and an absorption refrigeration system. This combination is created by replacing the condenser in the on-chip cooling circuit with the generator of an absorption refrigeration cycle. The performance of the developed model in simulating both LiBr–water and water–ammonia absorption cooling systems is examined through verification of the model results against the reference data available in the literature. The verification indicates the superiority of LiBr–water absorption system for data center/server operating conditions. Therefore, a LiBr–water absorption refrigeration system is modeled in the novel combined heat recovery system. For these systems it is shown that the traditional definition for the coefficient of performance (COP) is not appropriate to evaluate the performance and, in its place, introduce a new figure of merit. Through a sensitivity analysis, the effects of server waste heat quality, server coolant type, solution peak concentration, solution heat exchanger effectiveness, evaporator temperature, and operating pressures on the performance of the novel system are investigated. Finally, using the thermodynamic model and cost information provided by the absorption refrigeration industry, an economic analysis is carried out to calculate the payback period when this technology is used for data center waste heat recovery.

Thermoeconomic optimization of LiBr/H2O-R134a compression-absorption cascade refrigeration cycle

This study presents the thermoeconomic optimization of LiBr/H2O-R134a compression-absorption cascade refrigeration cycle. The detailed exergy-based thermoeconomic analyses, thermoeconomic evaluation with exergoeconomic variables, and thermoeconomic optimization by using non-linear simplex direct search method have been performed for the cascade refrigeration cycle. In the sample application, the thermoeconomic optimization reveals the optimum generator temperature, condenser temperature, absorber temperature, condenser temperature of vapour compression section, effectiveness of solution heat exchanger and compressor isentropic efficiency. This analysis points out that the evaporator equipage and solution heat exchanger should be designed carefully according to the exergoeconomic factor values. The exergetic efficiency and minimum cost of objective function are determined as 7.30% and 4.05 ($/h) for the optimum case of sample application. The minimum cost of objective function is reduced about 3.3%, the coefficient of performance (COPcyclegen) and exergetic efficiency of cascade cycle are improved about 7% and 3.1% respectively, according to the base case.

Thermodynamic Analysis of Heat and Mass Transfer Enhancement for R134a-DMAC/CNT Vapour Absorption Refrigeration System

The objective of this paper is to study thermodynamic analysis of R134a-DMAC and R134a-DMAC/CNT vapour absorption system and compare this both system performance. This investigation for R134a-DMAC is carried out based on the correlations available in the literature. The change of thermophysical properties for R134a-DMAC/CNT is predicted and is incorporated in the pure R134a-DMAC correlations. The analysis is carried for 1 kW evaporator capacity and effectiveness of solution heat exchanger is taken as 0.8. Variations in the performance parameters of the system with and without nano fluids are studied against various operating temperatures of generator, absorber and different volume concentration. The result of this theoretical study shows that co-efficient of performance (COP) of system with CNT nano fluid is improved as well as the absorber capacity also increased due to enhanced heat transfer of nano fluids. It also observed that the performance achieved by the pure R134a-DMAC at the elevated generator temperature could be achieved at low generator temperature using R134a-DMAC/CNT.

EXERGY ANALYSIS OF A VAPOR COMPRESSION–VAPOR ABSORPTION CASCADE SYSTEM

In this paper, second law analysis has been done for the vapor compression–vapor absorption (VC–VA) cascade system. Ammonia–water is considered as the working pair in absorption section and R407C is dealt as the working fluid in VC section. Exergy destruction or the irreversibility rate is determined in each components of VC–VA cascade system, for a wide range of cooling capacity by considering a variable speed reciprocating compressor. Further in this, Coefficient of structural bond (CSB) analysis is carried out to quantify the effect of varying the generator temperature, effectiveness of solution heat exchanger, inlet temperature of external fluids in evaporator/condenser and some other variables. Solution heat exchanger and the condenser are reported to have high CSB value, so have a great scope of improvement to reduce the irreversibility rate of the whole system.

Performance evaluation of a small capacity compression–absorption refrigeration system

Simulation of a 100 kW ammonia-water compression–absorption system has been carried out for water chilling application by incorporating detailed heat and mass transfer calculations in the absorber and desorber of the system. Warner's technique has been used for the convergence of iterations. The effect of relative solution heat exchanger area on the COP, cooling capacity, absorber heat load and effectiveness of solution heat exchanger has been studied. With increase in the relative solution heat exchanger area from 17 to 50%, COP increases initially, becomes maximum at 39% and then decreases. However, capacity of the system decreases considerably as a result of increase in relative solution heat exchanger area. Due to pressure drop in the solution heat exchanger on weak solution side, pumping power increased by 30% but COP decreased by only 0.92%. The performance of this system has been compared with that of a conventional vapor compression type chiller. ► Simulation of a 100 kW ammonia-water compression–absorption system for cooling application. ► Actual heat and mass transfer calculations in the absorber and desorber. ► The effect of relative solution heat exchanger area and various pressure drops on the performance. ► An optimum SHX area exists at which COP of the system becomes maximum. ► Comparison of compression–absorption system with vapor compression system.

Exergetic performance of eco friendly absorption heat transformer for seawater desalination

Vapour Absorption Heat Transformers (VAHT) have shown good potential for upgrading waste heat to useful temperature levels. VAHT can be effectively coupled with Multi Effect Desalination (MED) systems for water purification. In this study an exergetic analysis is carried out for VAHT integrated with MED. A simulation model has been developed to evaluate the exergetic efficiency, exergy losses in the components and exergy balance of the system. The effects of operating parameters such as heat source temperature, sink temperature, effectiveness of solution heat exchanger are evaluated on the performance of the system. Exergy efficiency varies from 11.14% to 35.35% depending upon the temperatures of heat source and sink. It is found that the highest exergy loss is in the condenser of VAHT, followed by second effect of MED.

Thermodynamic modelling of a single-effect LiBr-H2O absorption refrigeration cycle

The objective of this paper is to develop a computer program for design and thermodynamic analysis of a single effect absorption chiller using LiBr-H2O solution as working fluid. The conditions of hot water entering and leaving the desorber, cooling water entering the absorber and leaving the condenser, chilled water entering and leaving the evaporator, as well as the approach temperatures in condenser, evaporator, desorber, and absorber, the effectiveness of solution heat exchanger, the chiller refrigeration power, and the ambient temperature are used as input data. The program then gives the thermodynamic properties of all state points, the design information of all heat exchangers in the cycle and the overall cycle performance. The results deduced from the computer program are used to study the effect of design parameters on cycle performance. For example, increasing the evaporator and generator temperatures or decreasing the condenser and desorber temperatures can improve the second-law efficiency of the cycle. It is also noticed that the temperatures of hot water, cooling water, and chilled water, respectively, at the inlet of the desorber, condenser, and evaporator have a great effect on cycle coefficient of performance. The results of this program can be used either for sizing a new refrigeration cycle or rating an existing system. It can also be used for optimization purposes. The predictions of the present program are compared with other simulating programs and qualitative agreement is achieved.

HCFC22‐based vapour absorption refrigeration systems. Part II: Influence of component effectiveness

A thermodynamic analysis was performed on a single-stage vapour absorption refrigeration system (VARS) operating with monochlorodifluouromethane (HCFC22) as a refrigerant and dimethylether of tetraethyleneglycol (DMETEG) or dimethyl acitamide (DMA) as a working fluid. Influence of solution heat exchanger effectiveness (E h ) and mass transfer effectivenesses of absorber (E a ) and generator (E g ) on the performance of VARS were studied. The variations in heat quantities at solution heat exchanger, generator and absorber as well as performance characteristics, namely CR, COP th and second law efficiency (e) at various operating temperatures are reported. As expected, low CR and high COP th and e can be obtained at high values of E h , E a and E g . The effects of E h on the performance is more pronounced when compared to that of E a and E g . Also, the change in the circulation ratio (CR) due to a given change in E a is higher than that due to the same change in E g . CR is not a function of E h . A comparison of the influence of E h , E a , and E g on the VARS performance for HCFC22-DMA and HCFC22-DMETEG pairs was made.

An entropic model of liquid vapour absorption cooling system: Second Law analysis

ABSTRACT This communication presents an entropic analysis based on combined first and second laws of thermodynamics for a vapour liquid absorption (VLAB) cooling system. The model provides an insight understanding of entropy production mechanisms of various irreversible processes inherent in absorption systems and also gives an explicit explanation of actual COP degradation with respect to the ideal reversible Coefficient of Performance (COP) in terms of entropy generation associated with irreversible processes. Numerical results for water-LiBr absorption system and given cold production, source and sink temperatures are presented for different effectiveness of solution heat exchanger and the components/parameters which affect most the system performance are visualised. It is ascertained that the solution circuit components are most sensitive and the individual entropy productions are inter-dependent on each other as well as on the solution recirculation ratio and the effectiveness of the solution heat ex...

Thermodynamic study of advanced absorption heat transformers—I. Single and two stage configurations with heat exchangers

A thermodynamic analysis was carried out to study the effect of heat exchanger effectiveness ( EF) on the performance of single stage heat transformers (SSHT). Moreover, an analysis of three different arrangements of two stage heat transformers was performed using a mathematical model assuming water/lithium bromide as the working fluid. An increase in the solution heat exchanger effectiveness ( EF) greatly improved the performance of absorption heat transformers when the absorber temperature was at least 40°C higher than the temperature of the heat supplied to the system. In two stage heat transformers (TSHT), higher absorber temperatures were obtained by coupling the absorber of the first stage to the evaporator of the second. However, higher performance coefficients were obtained in general by coupling the absorber of the first stage to the generator of the second.

Performance enhancement of a two-stage vapour compression heat pump with solution circuits by eliminating the rectifier

Performances of four versions of a two-stage vapour compression heat pump with solution circuits (VCHSC) are compared. The VCHSC represents a cascade system that requires only one compressor. Performance curves for the best cycle, the modified version with a bleed line and a desuperheater are obtained and studied in detail. The four cycles are compared for the same total UA value (product of overall heat transfer coefficient and area) including all heat exchangers while pumping heat from an average desorber temperature of −2.5°C to an average absorber temperature of 105°C. Simulation results show that the cycle with a bleed line and a desuperheater has 40 to 50% higher cooling COPs and 30 to 40% higher cooling loads as compared to the cycle with a rectifier. Both the rectifier and the bleed line cycles show an increase of up 20% in the cooling COP and capacity by adding a desuperheater. Performance curves are obtained by simulating the best cycle at various weak solution concentrations in the low temperature loop and different solution pump flow rates. The parameters studied are the cooling COP, the solution heat exchanger effectiveness, the pressure ratio, the solution temperature glides in the absorbers and desorbers, the low temperature desorber load and the distribution of the UA value. Cooling COPs up to 1.05 and pressure as low as 7.05 are obtained for the modified VCHSC. The cooling COPs are twice as much and the pressure ratios are two thirds less of what would be found in a conventional single-stage vapour compression system. Changing the weak solution concentration in the low temperature loop from 40 to 80 wt% ammonia increased the cooling load by four times. The results indicate that the two-stage VCHSC with a bleed line and a desuperheater can work at temperatures above 100°C and achieve temperature lifts of more than 100 K with well known and environmentally safe refrigerants.