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This study presents the energetic and energetic analysis of the heat pump cycle, used for heating residential spaces. The heat pump is of the air-water type, taking heat at the level of the evaporator from the environment. The working fluids used are: R1233zd(E), R1234ze, R152a, R1234yf. Exergy analysis is used as a research tool. Based on the exergy analysis, the identification of malfunctions at the level of each device and process can be easily achieved. The study shows that the working fluid R1233zd(E) presents the highest coefficient of performance and the highest exergetic efficiency. [ABSTRACT FROM AUTHOR]

In this study, thermodynamic analysis for integration of a PEMFC with an organic Rankin cycle, and an ejector expansion vapor compression refrigeration system is presented. The input energy of the system is supplied by the waste heat of a PEMFC. Energy and exergy analysis is performed on each system component and compared with a simple ORC-VCR without an ejector. The results show that employing the ejector can improve the refrigeration capacity, energy efficiency, and exergy efficiency by 18.88%, 12.29 %, and 12.27%, respectively, compared to a simple ORC-VCR system. Moreover, the overall energy and exergy efficiency of the system is 33.43% and 5.46% higher than a standalone PEM fuel cell. Furthermore, in the parametric study, the effect of condenser temperature, evaporator temperature, ejector efficiency, PEMFC operating temperature, current density, and PEMFC operating pressure on the energy efficiency, exergy efficiency, and refrigeration capacity of the system is investigated. [ABSTRACT FROM AUTHOR]

This study focuses on a combined power generation and cooling system comprising an organic Rankine cycle and a dual evaporator cooling cycle. A parametric study based on exergy analysis was performed using different organic fluids: R134a, R290 and R600 in order to determine its optimal operation. The results show that the thermodynamic parameters that were studied, such as the operating temperature of the evaporator of the ORC cycle and the dual evaporator cycle on the performance of the system are of great importance and improve the performance of the combined system including energy efficiency and exergy efficiency using R600 as the working fluid. Furthermore, the results indicate that using R600 as the working fluid for both cycles improve the system performance. [ABSTRACT FROM AUTHOR]

• Zeotropic mixture of Hexane and R245fa is used as the working fluid. • Condenser and ORC evaporator contribute 75.3% to total exergy destruction. • Energy and exergetic performance of the system is maximum at mass fraction of 0.5. • The system can achieve the exergetic efficiency of 58% at optimal condition. In this study, a multi evaporator vapor-compression refrigeration (mVCR) system driven by Organic Rankine Cycle (ORC) is proposed as a refrigeration system for supermarkets, restaurants, etc. The system is analyzed for its thermodynamic performance in terms of coefficient of performance (COP s) and exergetic efficiency (η ex). A zeotropic mixture of hexane and R245fa is used as working fluid over pure fluid to assess its impact on the thermodynamic performance of the system. Moreover, the system is optimized using an evolutionary algorithm called genetic algorithm (GA) to determine the optimal values of the operating parameters for the maximum exergetic efficiency of the system. The results indicate that COP s and η ex are maximum at the mass fraction of 0.5 of the zeotropic mixture. The maximum values of COP s and η ex are 0.425 and 31.1% respectively. The system can achieve an exergetic efficiency of 58% at the optimal set of operating parameters. The corresponding optimal values of the temperature of the ORC evaporator, VCR evaporator 1, VCR evaporator 2, and the condenser are 388 K, 268 K, 268 K, and 303 K respectively. [ABSTRACT FROM AUTHOR]

Water scarcity and environmental impacts of blowdown within steam power plants are among the important growing concerns. In order to solve these problems, applying a zero liquid discharge (ZLD) system for treating the brine of the power plants and reusing this water is crucial. In this study, the process of a ZLD system is evaluated by using energy and exergy analyses. The ZLD system was designed to recover brines to demineralize water, which consists of four main parts including vacuum evaporator, roots pump, heat exchanger, and circulation pump. The effects of the dimensional and operating parameters on the freshwater flow rate, exergy efficiency, and the consumption power are investigated. When volume of the evaporator is 7 m3, with increase in total evaporation time from 1 to 3 h, total power consumption decreased from 106.16 to 99.52 kW h and freshwater production reduced from 5914.62 to 2048.52 L h−1. The amount of produced freshwater flow rate is independent of the recirculating flow rate and is a function of the evaporator's volume. Therefore, in volumes of 3, 5, and 7 m3, the produced freshwater flow rate is constant at about 1300, 2200, and 3070, respectively. Also, the results showed that when increasing the concentration of the brine in the range of 2000–30,000 ppm, the flow rate of the produced freshwater decreases from 3377 to 2911 L h−1 and the total power consumption reduced from 113.28 to 96.42 kW h. Moreover, by increasing the volume of vacuum evaporator, freshwater flow rate rises. Increasing the freshwater flow rate has a dramatic influence on the early working cycles. Since the evaporation is a cyclic process, the exergy efficiency of the roots pump and heat exchanger improves, while the exergy efficiency of the vacuum evaporator decreases versus increasing working cycles. [ABSTRACT FROM AUTHOR]

With the growth of energy-efficient building sector, ventilation systems are becoming increasingly important not only of fresh air supply but also in terms of energy consumption. The aim of this paper is to describe and analyse the operation of an air handling unit (AHU) with a heat pump using energy and exergy analyses under the variable environmental temperature (from –30 °C to 10 °C). The application of selected methods is illustrated in a case study of an AHU using environmental temperatures of Vilnius city during heating season (from the beginning of October to the end of April). An analytical method for determining distribution of the environmental (outdoor air) temperature is used. Energy and exergy analyses showed periods when the highest amounts of energy and exergy were consumed and the greatest exergy losses occurred. This allowed to reveal the component of the system with the highest exergy losses – the heat pump evaporator. Therefore, further research is needed for its design and application. At the end of the article, the seasonal indicators of the AHU with heat pump operation were calculated: coefficient of performance and exergy efficiency. The presented research procedure could be applied to the analysis of other energy systems and processes in them. [ABSTRACT FROM AUTHOR]

Featured Application: Novel combined ORC-VCR cycle and its comparison with usual ORC-VCR cycle are explained. Performance of different working fluids is investigated. Effects of various operational parameters on system performance are shown. This paper presents energy and exergy analysis of two vapor compression refrigeration cycles powered by organic Rankine cycle. Refrigeration cycle of combined system was designed with single and dual evaporators. R134a, R1234ze(E), R227ea, and R600a fluids were used as working fluids in combined systems. Influences of different parameters such as evaporator, condenser, boiler temperatures, and turbine and compressor isentropic efficiencies on COPsys and ηex,sys were analyzed. Second law efficiency, degree of thermodynamic perfection, exergy destruction rate, and exergy destruction ratio were detected for each component in systems. R600a was determined as the most efficient working fluid for proposed systems. Both COPsys and ηex,sys of combined ORC-single evaporator VCR cycle was detected to be higher than the system with dual evaporator. [ABSTRACT FROM AUTHOR]

• Novel condenser outlet split cycle with the presence of two ejectors working in different temperatures has been investigated. • Exergy and energy analysis has been performed. • COP enhancement of the proposed cycle at different condenser temperature has increased from 8.3 to 14.1%. In this paper, the possibility of performance enhancement of a new Condenser Outlet Split (COS) cycle with the presence of two ejectors working in different temperatures is investigated. In order to enhance the cycle performance, an auxiliary ejector-expansion refrigeration cycle equipped with a separator and a secondary ejector has been employed, resulting in further performance improvement on both exergy and energy analyses on the different condenser and evaporator temperatures. The thermodynamic equations of each component in the standard and proposed cycles have been solved in EES software. The auxiliary cycle boosted the pressure at the inlet of the secondary nozzle of the main ejector leads to a better performance of the proposed cycle. Through energy analysis, compressor work recovery increased by 12% and the coefficient of performance showed up to 14.1% enhancement. Through exergy analysis, our proposed cycle have been produced 6.8% reduction in exergy destruction compared to the standard cycle, and the total exergy performance of the given system can be increased from 8.2 to 14%. [ABSTRACT FROM AUTHOR]

In order to evaluate the performance of a solar photovoltaic thermal hybrid heat pump working with circular and triangular tube evaporator configurations, exergy analysis is performed in this study. The experiments were carried out under the meteorological conditions of Coimbatore city (latitude of 10.98°N and longitude of 76.96°E) in India. Exergy destruction and exergy efficiency of each components of the system were calculated with reference to four ambient conditions such as solar intensity, ambient temperature, ambient relative humidity and ambient wind velocity. The exergy performance parameters of the heat pump were simulated using artificial neural networks to have accurate exergy performance comparison. The results indicated that maximum exergy destruction for both the configurations occurred in PV-T evaporator. The exergy destruction in circular tube configuration is 2-7% higher than triangular tube configuration. It was also found out that exergy efficiency of the system using triangular tube configuration is 1.2-3.8% higher than circular tube configuration. [ABSTRACT FROM AUTHOR]

This paper presents an energy and exergy analysis of a domestic refrigeration system using R1234yf as a drop-in replacement for R134a. We base our analysis on a series of independent tests using two refrigerants, namely R1234yf and R134a. The test equipment consists of a fully instrumented domestic refrigeration system. A computational model is developed to work out the thermodynamic parameters such as coefficient of performance, exergy destruction ratio, exergy-based rational efficiency, component efficiency defect as well as the dimensionless exergy balance for both refrigerants. The parameters varied in the analysis were the evaporator and condenser temperatures as well as the amount of refrigerant used in the system. The results indicate that irreversibilities are mainly concentrated in the compressor, this is the case both for R1234yf and R134a. This exergy analysis proposed for evaluation of refrigerants in domestic refrigerators permitted to determine that R1234yf may not be a likely alternative to R134a. [ABSTRACT FROM AUTHOR]

A new organic Rankine cycle (ORC) architecture: partial evaporating organic Rankine cycle (PEORC) with the zeotropic mixture R245fa/R227ea as working fluid is presented in this paper. The thermodynamic first law analysis as well as the second law analysis were applied to evaluate the system performance. A techno-thermodynamic evaluation on heat exchangers and the expander was also performed in this research. The mass fraction of R227ea (corresponding to zeotropic mixture) and the expander inlet vapor quality (corresponding to PEORC) are chosen as the independent variables. The results showed that both the new cycle architecture: PEORC and zeotropic mixture working fluid can improve the thermodynamic performance and more importantly they can be combined together to further enhance the system. The optimized PEORC with R245fa/R227ea is able to generate about 24.7% more power than the traditional subcritical organic Rankine cycle (SCORC) with R227ea as working fluid. However, the heat exchanger economy may become worse, which should be considered. The results also revealed that the mixture working fluid mainly reduces the exergy loss in the condenser because of the temperature glide characteristic while the PEORC mainly reduces the exergy loss in the evaporator by improving the temperature matching between heat source and working fluid. [ABSTRACT FROM AUTHOR]

This study explores the implementation of two large micro-channel heat exchangers as evaporators in a refrigeration cycle intended for thermal management onboard future space vehicles. Examined are thermal performances of the two heat exchangers, thermodynamic performance of the refrigeration cycle, and system instabilities. Because of quality differences, different two-phase flow patterns and dryout effects are encountered in the two heat exchangers. A trade-off exists between optimum performances of the evaporators and compressor, with former favoring outlet quality values below unity to guard against dryout, and the compressor favoring a quality equal to or exceeding unity to preclude internal damage to the compressor from entrained droplets. Thermodynamic analysis of the refrigeration cycle shows exergy destruction is minimum when the avionics HX’s outlet quality is close to unity. The system is shown to be susceptible to instability resulting from interactions between compressible volumes both upstream and downstream of the condenser, and inertia of liquid in the condenser. This instability is manifest by pressure oscillations that are sensitive to both flow rate and evaporator heat input. [ABSTRACT FROM AUTHOR]

Energy and exergy analyses are carried out for hydrogen production via Gas Turbine Modular Helium Reactor/Organic Rankine Cycle (GTMHR/ORC) system coupled with a proton exchange membrane electrolyzer. A comprehensive parametric study is performed and the effects of some significant variables as compressor pressure ratio, turbine inlet temperature, evaporators’ temperature, pinch point temperature difference in the evaporators and degree of superheat at the ORC turbines inlet on the exergy efficiency, rate of produced hydrogen and sustainability index of the integrated system are investigated. Using direct search method by the Engineering Equation Solver (EES) software, the combined system is optimized to achieve the maximum exergy efficiency. Results show that the exergy efficiency difference between the combined system and GTMHR cycle increases with an increase in pressure ratio. Also, it is observed that the rate of produced hydrogen increases with increasing turbine inlet temperature and takes the maximum value with change in evaporators’ temperature. Under the optimized condition, exergy efficiency, rate of the produced hydrogen and sustainability index of the proposed combined system are calculated to be 49.21%, 56.2 kg/h and 1.972, respectively. [ABSTRACT FROM AUTHOR]

In this study, theoretical investigations of double evaporator refrigeration system and with adding an ejector to the current system were carried out using energy and momentum equations. The relative increase in COP with respect to the conventional double evaporator refrigeration system was analyzed in terms of condenser and evaporator temperatures. Also exergy analyses of systems were examined and results were showed graphically. In the study, R134a and its potential substitute R1234yf were used. The analyses showed that COP improvements of the proposed system by using ejector as an expansion device were approximately 15% for R134a and about 17% for R1234yf, respectively. [ABSTRACT FROM AUTHOR]

In this study, a detailed exergy evaluation of a commercial tomato paste plant with a double-effect evaporator was conducted in order to provide information on the system thermodynamic inefficiencies. Using energy and exergy balance equations, all components of the plant were analyzed individually and their exergetic parameters were calculated on the basis of actual operational data. The required data were obtained from Nazchin tomato paste factory located in Tehran, Iran. In addition, it was attempted to quantify the exergy utilized for processing a given amount of the tomato paste. The results showed that over 82% of the total destroyed exergy in the plant occurred in the boiler combination as the main component wasting exergy. Furthermore, exergy analysis introduced this combination as the main equipment rejecting exergy to the ambient where 4.79% of its total exergy input was lost. The rational exergy efficiency of the first- and second-effect evaporative units was found to be 65.33% and 56.60%, respectively. The specific exergy consumption of the tomato paste production was also determined as 16.83 MJ/kg. Generally, exergy concept and its extensions could be served as a powerful assessment technique to optimize the design and performance of multiple-effect evaporation systems employed in food industry. [ABSTRACT FROM AUTHOR]

The use of efficient refrigerator/freezers helps considerably to reduce the amount of the emitted greenhouse gas. A two-circuit refrigerator-freezer cycle (RF) reveals a higher energy saving potential than a conventional cycle with a single loop of serial evaporators, owing to pressure drop in each evaporator during refrigeration operation and low compression ratio. Therefore, several industrial applications and fish storage systems have been utilized by using multipurpose refrigeration cycle. That is why a theoretical performance analysis based on the exergetic performance coefficient, coefficient of performance (COP), exergy efficiency and exergy destruction ratio criteria, has been carried out for a multipurpose refrigeration system by using different refrigerants in serial and parallel operation conditions. The exergetic performance coefficient criterion is defined as the ratio of exergy output to the total exergy destruction rate (or loss rate of availability). According to the results of the study, the refrigerant R32 shows the best performance in terms of exergetic performance coefficient, COP, exergy efficiency, and exergy destruction ratio from among the other refrigerants (R1234yf, R1234ze, R404A, R407C, R410A, R143A and R502). The effects of the condenser, freezer-evaporator and refrigerator-evaporator temperatures on the exergetic performance coefficient, COP, exergy efficiency and exergy destruction ratios have been fully analyzed for the refrigerant R32. [ABSTRACT FROM AUTHOR]

In this study, the performance of a novel photovoltaic/thermal (PV/T) air dual-heat-source composite heat pump hot water system was investigated experimentally and analyzed theoretically. First, a system that simulates a solar PV/T collector by electric heating was built and tested to validate the superiority of the hot water system. The system was divided into dual-heat-source operating mode and single-heat-source operating mode. Results showed that the average COP and COP sys for dual-heat-source operating mode were 2.49 and 2.24, respectively, and for single-heat-source operating mode were 1.40 and 1.31, respectively, which indicate that the former had better system performance. Then, the actual PV/T collector was used in the system. The results showed that the average electrical gain and heat gain were 278 W and 664 W, respectively. The average electrical efficiency and heat efficiency were 14.5% and 36.02%, respectively. Meanwhile, the comprehensive efficiency was more than 50%. The average COP and COP sys of the heat pump were 4.08 and 3.07, respectively, along with the system exergy efficiency of 0.33. Therefore, the PV/T air dual-heat-source composite heat pump hot water system has excellent performance and is worth promoting and applying. [ABSTRACT FROM AUTHOR]

Highlights: [•] A full factorial analysis is conducted upon a triple absorption heat transformer. [•] The most influential variable settings are determined. [•] Condensation temperature and pinch heat transfer gradient have the greatest influence. [•] Points of optimum exist for the temperatures of the two absorber evaporators. [•] The generator causes the most irreversibility. [ABSTRACT FROM AUTHOR]

Abstract: In this study, we designed a multifunctional heat pump system using just one scroll compressor and which can be run in four different modes, namely air to air, air to water, water to water and water to air, in order to make an experimental energetic and exergetic performance comparison. Experimental system consists of two condensers and two evaporators and uses R22 as working fluid. One of the evaporators and condensers uses water and the others use air as heat source/sink. Heating capacities of four heat pump types are equal to each other. It is realized by adjusting the mass flow rate and temperature level of external fluid of condenser. Results show that the heat pump unit which has the maximum COP (coefficient of performance) value is water to air type with 3.94 and followed by water to water type with 3.73, air to air type with 3.54 and air to water type with 3.40. Ranking of four heat pump types with respect to their mean exergy efficiency is as follows; water to air type with 30.23%, air to air type with 30.22%, air to water type with 24.77% and water to water type with 24.01%. Exergy destruction rates of the systems were investigated in this study and the results revealed that the heat pump type which has the maximum exergy destruction is air to air type with 2.93 kW. The second highest one is air to water type with 2.84 kW. The third highest one is water to air type with 2.64 kW and last one is water to water type with 2.55 kW. It is understood that the temperature of the evaporator external fluid affects the exergetic efficiency of the system more than the mass flow rate. In contrast to the previous, the dominant parameter which has more important effect on the exergy destruction of the heat pump unit is the mass flow rate of evaporator external fluid. [Copyright &y& Elsevier]

Abstract: Higher efficiencies and optimal utilization of geothermal energy require a careful selection of the working fluid in organic Rankine cycles (ORC). The objectives of this study are to analyze and explain the effect of using alternative dry fluids on the efficiencies of the ORC and compare them with other refrigerants. In addition, the effect of the critical temperature on the thermal and exergetic efficiencies will also be determined. Results showed that iso-pentane is a good replacement for R-113, while neo-pentane outperformed C5F12. In addition, n-butane showed better efficiency than RC-318, R-236fa, and R-245fa. The best working fluid in the studied system was n-hexane, while R-227ea was the worst. It was also found that efficiencies correlate with the critical temperature of the working fluid where a strong functionality was noticed in the studied range. The contribution of the evaporator to the total exergy destruction was the most relevant, while the pump contribution was marginal. It is concluded that thermodynamically, hydrocarbons are superior to some refrigerants and could be the next generation working fluids for geothermal or waste heat recovery systems. [ABSTRACT FROM AUTHOR]

A computational model is developed for the parametric investigation of single-effect and series flow double-effect LiBr/H2O absorption refrigeration systems. The effects of generator, absorber, condenser, evaporator and dead state temperatures are examined on the performance of these systems. The parameters computed are coefficient of performance (COP), exergy destruction rates, thermal exergy loss rates, irreversibility and exergetic efficiency. The results indicate that COP and exergetic efficiency of both the systems increase with increase in the generator temperature. There exist different optimum values of generator temperature for maximum COP and maximum exergetic efficiency. The optimum generator temperature is lower corresponding to maximum exergetic efficiency as compared to optimum generator temperature corresponding to maximum COP. The effect of increase in absorber, condenser and evaporator temperatures is to decrease the exergetic efficiency of both the systems. The irreversibility is highest in absorber in both systems. Copyright © 2009 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

First and second law analysis have been conducted for three low temperature driven ammonia-water absorption cooling cycles with increasing internal heat recovery. Based on the results of exergy analysis the structural analysis has been achieved. The obtained Coefficients of Structural Bonds (CSB) consider how the irreversibility of the whole cycle is affected by a change in the irreversibility related to an efficiency improvement of a single component. Trends for the different configurations are similar, while quantitative differences among the main heat exchangers are considerable. The highest values of the CSB are found for the refrigerant heat exchanger. Also the evaporator, the condenser, the generator and the absorber show values higher than unity. The lowest CSB's are obtained in the solution heat exchanger. In general, CSB's decrease with increasing efficiency. That means that for very efficient heat exchangers, a further improvement looks less attractive. The dephlegmator is an exception as it shows a singularity of the CSB value due to its complex interactions with the other components. Once the CSB's are obtained for the main components, they can be used in the structural method of the thermoeconomic optimisation. This method enables us to find the optimum design of a component in a straightforward calculation. [ABSTRACT FROM AUTHOR]

• AEA was used to separate azeotropic mixture of ACN/EtOH/H2O via TCED process. • A superstructure TCED with four-parallel evaporator ORC system is built. • Four advantageous working fluids are selected to enhance the performance of ORC. • The intensification process shows better energy-saving and economic performance. The energy analysis and optimization of process system aiming to solve the problems of high consumption, low efficiency and unreasonable use of energy in the process of energy utilization has been widely researched and developed in recent decades. In this work, advanced exergy analysis was carried out for the triple-column extractive distillation (TCED) process separating ternary azeotropic mixture of ACN/EtOH/H2O. The total exergy destruction is 1097.69 KW. The avoidable exergy destruction, is 29.20%, mainly caused by the cooler and three condensers. Based on the thermodynamic analysis results, a superstructure TCED with four-parallel evaporator organic Rankine cycles (FPE-ORC) system is proposed, four working fluids were selected. An improved genetic algorithm is used to obtain the optimal operating parameters of the ORC system by using the exergy efficiency and annual net profit (ANP) of the ORC as two conflict objective functions. Compare with existing process, the FPE-ORC system with working fluid R600 provides the highest exergy efficiency of 12.27%, with working fluid R600a leads to the best economic benefit of 6.43 E + 4 dollar/year. [ABSTRACT FROM AUTHOR]

Abstract: Although the idea of using a solar collector as the evaporator in the traditional heat pump cycle was first proposed in the year 1955, studies on the subject began in the late 70s. One of the keystones for obtaining sustainable development is also the use of exergy analysis. In this regard, the main objectives in doing the present study are twofold, namely (i) to review studies on direct-expansion solar-assisted heat pump systems (DX-SAHPs) and (ii) to present a mathematical model along with an illustrative example, which is used for heating an office space in Solar Energy Institute of Ege University, Izmir, Turkey, by floor heating with a DX-SAHP system. The system uses a 4m2 bare flat-plate collector as the evaporator, while the working fluid is chosen to be R-22. Water is heated by the heat pump and heat is delivered to the office space by floor heating. Exergy equations for the system are derived, while exergy calculations are made. The exergy efficiency values for the individual components of the DX-SAHP system are found to range from 10.74% to 88.87%. It is expected that this study will be very beneficial to everyone involved or interested in the exergetic design, analysis and performance assessment of DX-SAHPs. [Copyright &y& Elsevier]

An optimization for the geometrical parameters of continuous fins on an array of tubes of a refrigeration evaporator is developed in this paper using the exergy method. The method is based on exergy, economic analysis, and optimization theory. As there are humid air and refrigerant single- and two-phase streams involved in the heat transfer process, then there are irreversibilities or exergy destruction, due to pressure losses İΔP, due to temperature difference İΔT and due to specific humidity gradient İΔω. These principal components of total irreversibility are not independent, and their relative contribution to total irreversibility of a cross-flow refrigeration evaporator is investigated. A change in geometry was obtained by varying the evaporator tube diameter for a selected evaporator capacity, and hence the evaporator tube length and total heat transfer area are calculated for a fixed evaporator face length. In this way, the effect of changes in the geometry on the total number of exergy destruction units of the heat exchange process is investigated. The optimum balance between the three components of irreversibility (İΔP, İΔT, and İΔω) is also determined, thereby giving the optimum solution for the heat exchanger area. The total cost function, which provides a measure of the contribution of the evaporator to the total cost of the refrigeration system, is expressed on the basis of annual capital and electrical energy costs. The total cost function is minimized with respect to the total heat transfer area and the total number of exergy destruction units (NI). The relationship between the operational variables, heat transfer area, refrigerant and air irreversibilities, and the total annual cost for this type of evaporator are developed, presented, and discussed. The pressure, temperature, and specific humidity irreversibilities are found to be 30.34%, 33.78%, and 35.88%, respectively, of the total irreversibility, which is 8.5% of the evaporator capacity. [ABSTRACT FROM AUTHOR]

This paper presents a computer-based first law and exergy analysis applied to vapour compression refrigeration systems for determining subcooling and superheating effects of environmentally safe new refrigerants. Three refrigerants are considered: R134a, R407c and R410a. It is found that subcooling and superheating temperatures directly influence the system performance as both condenser and evaporator temperatures are affected. The thermodynamic properties of the refrigerants are formulated using artificial neural network (ANN) methodology. Six ANNs were trained to predict various properties of the three refrigerants. The training and validation of the ANNs were performed with good accuracy. The correlation coefficient obtained when unknown data were used to the networks were found to be equal or very near to 1 which is very satisfactory. Additionally, the present methodology proved to be much better than the linear multiple regression analysis. From the analysis of the results it is found that condenser and evaporator temperatures have strong effects on coefficient of performance (COP) and system irreversibility. Also both subcooling and superheating affect the system performance. This effect is similar for R134a and R407c, and different for R410a. [ABSTRACT FROM AUTHOR]

This paper proposes a modified refrigeration cycle (MRC) with zeotropic mixture R290/R600a for refrigerator-freezer applications. In the MRC, the use of two parallel compressors enables the two evaporators to be operated at two proper evaporation temperatures, which reduces the irreversibility of evaporator heat transfer processes. Noted that a subcooler is installed between the two loops for improving the system performance. Moreover, the MRC applies a phase separator to ensure more low-boiling point refrigerant to go to the freezer evaporator for increasing the freezer evaporation pressure. Energy and exergy performances of MRC are investigated based on a thermodynamic model. The simulation results indicate that compared with the conventional refrigeration cycle (CRC), the COP and exergy efficiency of MRC obtain the same improvement of 30.4% at a fixed condensation temperature of 35 °C. In addition, from the viewpoint of exergy analysis, the high priority in need of improvement for the MRC is ordered as the compressors, condensers, expansion valves, two evaporators, subcooler and internal heat exchanger. Generally, applying the MRC could be an effective and practical method to enhance refrigerator-freezer energy efficiency. • A modified parallel compression refrigeration cycle with R290/R600a is proposed. • Influence of various operating parameters on performance of the cycle is investigated. • Irreversibility of each component and exergy efficiency of the cycle are examined. • The cycle outperforms the conventional cycle in both energetic and exergetic aspects. [ABSTRACT FROM AUTHOR]

• Medium-low grade waste heat recovery via ORC for stationary and on-board systems. • Irreversibility reduction through dual pressure heat recovery vapor generator. • Cycle optimum design for maximum energy and exergy cycle efficiency. • Heat transfer coefficients calculation and heat exchange surface assessment. • Benefit-to-cost analysis and heat exchanger cost assessment. The full exploitation of the upper thermal source is the key for enhanced energy performances of ORC-based units for medium and low-grade waste heat recovery. The adoption of a dual evaporation pressure cycle layout has the potential to reduce the heat exchange irreversibility at the evaporation section and to assure a higher net power available at the expander shaft, particularly in small scale units and in presence of upper thermal sources with a highly variable heat release characteristic. The adoption of the dual evaporation pressure technology to small scale recovery units represents a major technological breakthrough and an element of novelty, observing that, at present, the possibility to split the evaporation process in multiple pressure levels is considered mostly with reference to steam generators and boilers. The study investigates the potential energy and exergy advantage of a dual pressure heat recovery vapor generator, with respect to a base-single evaporation pressure layout, for a recovery unit with a mechanical power in the 1–15 kW range, for stationary (100 °C–150 °C hot source temperature) and on-board (350 °C–300 °C hot source temperature) applications. A dedicated optimization procedure allows the maximization of either the net power recovered or the cycle energy efficiency, dependently on the final application of the unit. The exergy efficiency of the heat recovery vapor generator is assessed and its dependence on the fluid characteristics and the main cycle variables discussed, along with the relationship between the energy and exergy gain for the enhanced heat exchange. A preliminary economic analysis provides a first indication of the financial merit of the dual evaporation pressure layout with respect to a base single evaporation pressure configuration. [ABSTRACT FROM AUTHOR]