A simulation study of performance evaluation of single-stage absorption refrigeration system using conventional working fluids and alternatives

This study presents a literature review, especially in recent years, on the absorption refrigeration systems (ARSs), the currently used refrigerant–absorbent pairs and their alternatives. Additionally, in this study, the thermodynamic analysis of ARS using commonly encountered solution pairs in the literature was carried out, and a user-friendly software package including visual components was developed. The effects of operating temperatures, the effectivenesses of solution, refrigerant and solution–refrigerant heat exchangers (SHE, RHE, SRHE), and selection of working fluid pair on the system performance were examined by using the developed package. It was concluded that performances of the cycles improve with increasing generator and evaporator temperatures, but reduce with increasing condenser and absorber temperatures. The performance of system was affected from the SHE more than the RHE and SRHE. While the use of SHE improves the system COP up to 66%, RHE and SRHE have an effect of only 14% and 6%, respectively. For that reason, the SRHE may not be considered practically significant. Suitable solutions depending on operating conditions showed variations with respect to generator and evaporator temperatures.     

Energy and exergy analysis of an experimental single‐stage heat transformer operating with the water/lithium bromide mixture

The first and second law of thermodynamics have been used to analyze the performance of an experimental single‐stage heat transformer operating with the water/lithium bromide mixture. Enthalpy coefficients of performance (COP), external coefficients of performance (COP_EXT), exergy coefficient of performance (ECOP), exergy destruction or irreversibility in the system and components (I) and the improvement potential (Pot) have been calculated against the gross temperature lift and the main operating temperatures of the system. The results showed that the highest COP, COP_EXT and ECOP values are obtained at the highest solution concentrations meanwhile the Pot and the I of the cycle remain almost constant against these parameters. Also it was shown that the COP, COP_EXT and ECOP decrease with an increase with the absorber temperature, meanwhile the Pot and the I increase. Moreover, it was observed that in all the cases independently of the operating temperatures of the system, the absorber accounts with most of the half of the total irreversibility in the system. Finally, it was shown that the improvement potential is considerable for the system. Copyright © 2009 John Wiley & Sons, Ltd.     

HCFC22-based vapour absorption refrigeration system: Part I: Parametric studies

Influence of operating temperatures at generator, absorber, condenser and evaporator on the performance of a single-stage vapour absorption refrigeration system (VARS) working with HCFC22-DMA, HCFC22-DMF and HCFC22-DMETEG pairs as working fluids is studied by performing thermodynamic analysis. Variations of circulation ratio (CR), heat load at each component, Carnot and thermodynamic coefficients of performance (COP_cr and COP_th) and second law efficiency (ϵ) at various operating parameters are reported. It is deduced that the CR, COP_th and ϵ decrease with T_g while COP_th decreases and CR and ϵ increases with T_a and T_c. Also, CR values are lower whereas COP_th and ϵ values are higher at high T_e. HCFC22-DMETEG yields the lowest value of CR at low heat source temperature while the change in CR between the considered pairs is negligible at high heat source temperature. HCFC22-DMA stands out when both COP_th and ϵ are included.     

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.     

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.     

Performance of a multi-bed adsorption heat pump using SWS-1L composite adsorbent and water as the working pair

An analysis of the coefficient of performance (COP), specific cooling power (Q_scp) and exergy losses for a four-bed adsorption heat pump is presented. A composite adsorbent (SWS-1L) and water are the adsorption pair. An optimum cycle time, corresponding to a maximum specific cooling power, was found. This maximum specific cooling power increases almost linearly with the regeneration temperature. For the operation corresponding to the maximum specific cooling power at the regeneration temperature of 120 °C, using the SWS-1L composite adsorbent to substitute a regular-density silica gel in the adsorbers, the COP and Q_scp values can be increased by 51% and 38.4%, respectively. At the regeneration temperature of 100 °C and the mode operating time of 360 s, the second-law efficiency of the adsorption heat pump is 20.4%. The cycle exergy loss mainly occurs in the adsorbers. The exergy losses in the condenser and evaporator are small. Among the four processes in the adsorbers, the precooling and preheating processes result in 41.55% and 28.96% of the cycle exergy loss, respectively, while the adsorption and regeneration processes cause 8.44% and 18.97%, respectively. The exergy losses in the precooling and preheating processes mainly result from heat transfer through a significant temperature difference.     

Performance prediction of a solar driven ejector-absorption cycle using fuzzy logic

Theoretical performance analysis of the absorption systems is very complex because of analytic functions used for calculating the properties of fluid couples and simulation programs. To simplify this complex process, this paper proposes a new approach to performance analysis of solar driven ejector-absorption refrigeration system (EARS) operated aqua/ammonia. Performance of EARS was predicted using fuzzy logic controller at different working conditions instead of complex rules and mathematical routines. Fuzzy logic’s linguistic terms provide a feasible method for defining the operational characteristics of EARSs. Input data for fuzzy logic are experimental results performed in the climate condition of Ankara in Turkey. In the comparison of performance analysis results between analytic equations and by means of fuzzy logic controller, deviations coefficient of performance (COP), exergetic coefficient of performance (ECOP) and circulation ratio (F) for all working temperatures are less than 2, 5 and 0.2%, respectively. The statistical coefficient of multiple determinations (R² value) equals to 1, 0.9996, 1 for the COP, ECOP and F, respectively. These accuracy degrees are acceptable in design of EARS. This study is considered to be helpful in predicting the performance of an EARS prior to its setting up in an environment where the temperatures are known. Also, this study provides a fast and accurate means of determining the performance under transient operating regimes without the need to resort to classical physical modeling.     

Performance improvement of absorption refrigeration system using triple-pressure-level

In the absorption refrigeration system (ARS) working with aqua–ammonia, the ejector is commonly located at the condenser inlet. In this study, the ejector was located at the absorber inlet. Therefore, the absorber pressure becomes higher than the evaporator pressure and the system works with triple-pressure-level. The ejector has two main functions: (i) aiding pressure recovery from the evaporator, (ii) upgrading the mixing process and the pre-absorption by the weak solution of the ammonia coming from the evaporator. In addition to these functions, it can also act to lower the refrigeration and heat-source temperatures. Energy analyses show that the system’s coefficient of performance (COP) and exergetic coefficient of performance (ECOP) were improved by 49% and 56%, respectively and the circulation ratio (f) was reduced by 57% when ARS is initiated at lower generator temperatures. Due to the reduced circulation ratio, the system dimensions can be reduced; consequently, this decreases overall cost. The heat source and refrigeration temperatures decreased in the range of 5–15 °C and 1–3 °C, respectively. Exergy analyses show that the exergy loss of the absorber of ARS with ejector had a higher exergy loss than those of the other components. Therefore, a multiple compartment absorber can be proposed to reduce the exergy loss of the absorber of ARS with ejector.     

Exergetic analysis and evaluation of a new application of gas engine heat pumps (GEHPs) for food drying processes

In this study, three medicinal and aromatic plants (Foeniculum vulgare, Malva sylvestris L. and Thymus vulgaris) were dried in a pilot scale gas engine driven heat pump drier, which was designed, constructed and installed in Ege University, Izmir, Turkey. Drying experiments were performed at an air temperature of 45 °C with an air velocity of 1 m/s. In this work, the performance of the drier along with its main components is evaluated using exergy analysis method. The most important component for improving the system efficiency is found to be the gas engine, followed by the exhaust air heat exchanger for the drying system. An exergy loss and flow diagram (the so-called Grassmann diagram) of the whole drying system is also presented to give quantitative information regarding the proportion of the exergy input dissipated in the various system components, while the sustainability index values for the system components are calculated to indicate how sustainability is affected by changing the exergy efficiency of a process. Gas engine, expansion valve and drying ducts account for more than 60% amount of exergy in the system. The exergetic efficiency values are in the range of 77.68–79.21% for the heat pump unit, 39.26–43.24% for the gas engine driven heat pump unit, 81.29–81.56% for the drying chamber and 48.24–51.28% for the overall drying system.     

Exergy analysis of a thermal power plant with measured boiler and turbine losses

In this paper, a thermodynamic analysis of a subcritical boiler–turbine generator is performed for a 32 MW coal-fired power plant. Both energy and exergy formulations are developed for the system. A parametric study is conducted for the plant under various operating conditions, including different operating pressures, temperatures and flow rates, in order to determine the parameters that maximize plant performance. The exergy loss distribution indicates that boiler and turbine irreversibilities yield the highest exergy losses in the power plant. In addition, an environmental impact and sustainability analysis are performed and presented, with respect to exergy losses within the system.     

Increase of COP for heat transformer in water purification systems. Part II – Without increasing heat source temperature

The integration of a water purification system allows a heat transformer to increase the actual coefficient of performance, by the reduction of the amount of heat supplied by unit of heat. A new defined COP called COP_WP is proposed for the present system, which considers the fraction of heat recycled. 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 working fluid–absorbent pair. Plots of enthalpy-based coefficients of performance (COP_ET) and water purification coefficient of performance (COP_WP) are shown against absorber temperature for several thermodynamic operating conditions. The results showed that the proposed (AHTWP) system is capable of increasing the original value of COP_ET up to 1.6 times its original value by recycling energy from a water purification system. The proposed COP_WP allows increments for COP values from any experimental data for water purification or for any other distillation system integrated to a heat transformer, regardless of actual COP_A value or working fluid–absorbent pair.     

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.     

A numerical investigation of a diffusion-absorption refrigeration cycle based on R124-DMAC mixture for solar cooling

Research on new working fluid for uses in absorption systems has been continued. The feasibility of a solar driven DAR using the mixture R124/DMAC as working fluid is investigated by numerical simulation. The cycle is simulated for two cooling medium temperatures, 27 °C and 35 °C, and four driving heat temperatures in the range [90 °C–180 °C]. The performance characteristics of this system is analyzed parametrically by computer simulation for a design cooling capacity of 1 kW. The results show that the system performance and the lowest (minimum) evaporation temperature reached are largely dependent upon the absorber efficiency and the driving temperature. It is shown that for solar applications this fluid mixture has a higher COP and may constitute an alternative to the conventional ammonia–water system.     

Simulation of double-stage double-effect metal hydride heat pump

This paper presents the operational characteristics of a double-stage double-effect metal hydride heat pump (DSDE-MHHP) working with LaNi_4.1Al_0.52Mn_0.38/LmNi_4.91Sn_0.15/Ti_0.99Zr_0.01V_0.43Fe_0.09Cr_0.05Mn_1.5 as high/medium/low temperature alloys. The performances of the DSDE-MHHP are predicted by solving the transient, two-dimensional, conjugate heat and mass transfer characteristics between the paired metal hydride reactors of cylindrical configuration using the finite volume method (FVM). The designed operating temperatures chosen for the present analysis are 568, 361, 296, and 289 K as heat driven (T_D), heat rejection (T_H), heat sink (T_M) and refrigeration (T_C) temperatures, respectively. The variations in hydrogen concentrations, hydride equilibrium pressures, and temperatures within the hydride beds, and the heat exchange between the hydride beds with the heat transfer fluids are presented for a complete cycle. The operating cycle of a DSDE-MHHP is explained on dynamic pressure–concentration–temperature (PCT) plot. The variation of temperatures in the reactors during hydriding and dehydriding processes is compared with experimental data and a good agreement was observed between them. For given operating temperatures of 568/361/296/289 K, the average coefficient of performance (COP) and the specific cooling power (SCP) of the system are found to be 0.471 and 28.4 W/kg of total hydride mass, respectively.     

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 (ϵ) at various operating temperatures are reported. As expected, low CR and high COP_th and ϵ 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.     

Optimal performance of an irreversible solar-assisted heat pump

The thermodynamic optimization of a mechanically driven solar heat pump is presented. A new expression to describe the optimal thermal performance under finite operating conditions considering the internal and external irreversibilities during actual operation is derived. The optimum ratio between the condenser and collector–evaporator conductances (UA) determines the coefficient of performance (COP) for the maximum heating load of the system. An experimental air-R22 heat pump was used to determine the traditional performance parameters (COP and second law efficiency) which are compared with those obtained using the expressions derived in this work. Results show that the new model very closely represents the performance of real systems.     

Second law-based thermodynamic analysis of ammonia/sodium thiocyanate absorption system

In this study, the first and second law of thermodynamics are used to analyze the performance of a novel absorption system for cooling and heating applications. The active component of the sorbent used in this study is sodium thiocyanate (NaSCN). Ammonia (NH₃) is chosen as sorptive. A mathematic model based on exergy analysis is introduced to analyze the system performance. Enthalpy, entropy, temperature, mass flow rate and exergy loss of each component and the total exergy loss of the system are evaluated. Furthermore, the coefficient of performance (COP) and exergetic efficiency of the absorption system for cooling and heating processes are calculated from the thermodynamic properties of the working fluids under different operating conditions. The results show that the COP of cooling and heating increases with the heat source temperature and decreases with the cooling water inlet temperature, but the system exergetic efficiency does not show the same trends for both cooling and heating applications. The simulation results can be used for the thermodynamic optimization of the current system.     

Performance of ejector cooling system with thermal pumping effect using R141b and R365mfc

The ejector cooling system (ECS) is suitable for solar cooling application due to its simple design and low cost. An ECS using a multi-function generator (ECS/MFG) as a thermal pumping device without rotating machines for refrigerant circulation has been designed and tested. The experiment of an ECS/MFG operating at full-cycle while using R141b has shown that the COP_o can reach 0.225 and cooling capacity of 0.75 kW at generator temperature 90 °C, condenser temperature 37 °C, and evaporator temperature 8.5 °C. The present study also redesigned the ejector for working fluid R365mfc in order to replace R141b. This study has shown that R365mfc can replace R141b as the working fluid of ECS/MFG at no payoff of system performance as long as the ejector design is optimized.