Exergetic Coefficient Of Performance Research Articles

Comparison of Single-stage and Two-stage Tubular SOFC-GT Hybrid Cycles: Energy and Exergy Viewpoints

In this work, single-stage and two-stage tubular solid oxide fuel cell (SOFC)/gas turbine hybrid cycles are comparatively examined from the energy and exergy viewpoints. For this purpose, mass, energy and exergy balances are applied to all components of the cycles. The behavior of tubular solid oxide fuel cell modeled in this study is validated with the experimental test data of tubular SOFC developed by Siemens Westinghouse. The results of simulation show that two-stage SOFC-GT hybrid cycle not only generate more power,but also it has high efficiency in comparison to single-stage SOFC-GT. The values of first law efficiency and Exergetic Performance Coefficient (EPC) are increased from 60.69% and 1.405 in single-stage SOFC-GT to 63.93% and 1.725 in two-stage SOFC-GT respectively. This means that for equivalent generated power of single and two-stage SOFC-GT hybrid cycles, the amount of exergy destruction for two-stage SOFC-GT cycle is less than single-stage SOFC-GT. Also exergy destruction of all components of the hybrid cycles is calculated separately and results are compared. Finally a parametric study is performed to find out optimal values for solid oxide fuel cell design parameters. Effects of these parameters are evaluated on efficiency, generated power and total exergy destruction of the hybrid cycles.

Performance investigation of a diffusion absorption refrigeration system using nano-size alumina particles in the refrigerant

In this study, the effects of the passive heat transfer improvement method of coupling ammonia/water with nano-size alumina (Al2O3) particles were examined in regard to the heat performance of a diffusion absorption refrigeration system (DARS). Adding nanoparticles into the fluid leads to significant improvement in heat transfer since the surface area and heat capacity of the fluid increase due to the high surface area of the nanoparticles. In this study, cooling/absorption fluid mixtures with Al2O3 nanoparticles and their impact on system energy and exergy performance were assessed. The results of experiments indicated that the system with nanoparticles provided better absorption of heat from the generator and faster evaporation of the cooler from the cooling/absorption fluid. Addition of alumina nanoparticles to DARS improved the system's coefficient of performance (COP) and exergetic coefficient of performance (ECOP) by 55.56% and 22.8%, respectively, and reduced the circulation ratio (f) by 51.72%.

Hourly Performance Prediction of Solar Ejector-Absorption Refrigeration Based on Exergy and Exergoeconomic Concept

Solar cooling technology is being increasingly studied due to its environmental compatibility and cost saving capability. In this article, application of solar absorption refrigeration for cooling of an office building is investigated through modeling and simulation. Solar radiation and cooling load demand have hourly modeled during summer days using Tehran’s climate data as a typical weather scenario. The simulation results show that from 9 AM to 5 PM, product cost per exergy unit ( c P,tot ) for the entire system decreases 87%. During this time, thermodynamic coefficient of performance ( COP th ) increases from 0.16 to 0.48 (auxiliary heat needed reduces from 4.36 to 1.23 kW), suggesting that the performance of the system increases until 5 PM. This is also confirmed by studying exergetic coefficient of performance ( COP exe ) which reaches to the maximum point at the same time. Furthermore summer days analysis shows that at maximum air temperature the system has optimal COPs while the minimum of c P,tot occurs at maximum radiation. The results show the exact times at which the system performs less efficiently by solar energy and therefore the opportunity of using other available renewable energy resources at these times.

Analysis of a Cascade Refrigeration System (CRS) by Using Different Refrigerant Couples Based on the Exergetic Performance Coefficient (EPC) Criterion

A theoretical performance analysis based on the exergetic performance coefficient (EPC) criterion has been carried out for a cascade refrigeration system for different refrigerant couples. The EPC 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 couples R23-R717 show the best performance in terms of EPC and coefficient of performance from among the other refrigerant couples (R23-R290, R23-R404A, R23-R507A and R23-R717). In the analysis, R23 was used for the lower temperature cycle and the other refrigerants are used for the higher temperature cycle. The effects of the temperature difference in the cascade condenser and the evaporator and condenser temperature on the EPC, coefficient of performance and exergy destructions have been thoroughly investigated for the refrigerant couple R23-R717.

Exergy analysis of an ionic-liquid absorption refrigeration system utilizing waste-heat from datacenters

Ionic-liquid (IL) was introduced as an absorbent of an absorption refrigeration system designed for high power electronics cooling. IL is a salt in liquid-state, which is nonvolatile, thermally-stable, nonflammable, and environmentally-benign. It provides an alternative to the normally toxic working fluids, such as ammonia, also eliminates crystallization and metal-compatibility issues of the water/LiBr system. The performance of IL absorption refrigeration system was theoretically examined using exergy analysis. Various combinations of refrigerant and imidazolium-based ILs were chosen as working fluid pairs. The thermodynamic properties of ILs were evaluated using the correlations based on group contribution methods. A non-random two-liquid (NRTL) model was built and used to predict the solubility of the mixtures. Both the coefficient of performance (COP) and the exergetic coefficient of performance (ECOP) were evaluated. The effects of operating conditions on ECOP were explored. Also, the exergy destruction of each component was evaluated and discussed as a means to identify the critical component(s) of the system that would require optimization.

Energy and exergy analysis of a combined ground source heat pump system

The main focus of this paper is to analyze the energy and exergy efficiencies of a combined ground source heat pump (GSHP) system whose evaporator component works as a photovoltaic-thermal (PV/T) collector, and to determine the locations of the inefficiencies. This combined GSHP system consists of a heat pump cycle, a radiator thermal cycle, a ground heat exchanger cycle and a cooling tower cycle. The performance effects of the cooling tower have been examined, and optimal operation parameters have been selected to maximize the system efficiency. Exergy destruction rates, exergy efficiencies, exergy loss ratios of the system components and the whole system, as well as the energetic and exergetic coefficient of performance (COP) for the heat pump cycle have been evaluated for investigation of the potential improvement of the system components. From the results, exergy efficiency, COPsys and COPex of the system have been found as 74.72%, 2.895% and 0.39%, respectively. Additionally, parametric studies, including the thermodynamic performance of the system and its components, have been conducted by the change in some operating conditions, as variation of the ambient temperature changes from 0 °C to 30 °C, and the intensity of solar radiation that comes on the PV/T evaporator surface ranges from 400 W/m2 to 900 W/m2.

3E Analysis and GA-Based Multi Objective Optimization of an Ejector-Flash Tank-Absorption Refrigeration System Fuelled by Solar Energy

This paper proposes and investigates an improved ejector-absorption refrigeration system for providing the cooling load of an office building. Performance hourly modeling of the system is performed based on modeling the hourly required cooling load and hourly solar insolation rate on July 21 with maximum solar insolation. The results indicate that during the given day thermodynamic coefficient of performance (COP ) and th exergetic coefficient of performance (COP ) increase 150 and 300%, respectively. While, the product cost per exe exergy unit of the whole system (c ) decreases 90%. The results of thermodynamic and thermoeconomic Ptot analysis determine effective design parameters on COP , COP and c . Also, single and multiple objectives th exe Ptot optimizations are applied to maximize COPs and minimize c . According to the results, the single objective Ptot optimization for maximizing COPs increases COPs by about 1.1% and decreases c by about 7.4%. Moreover, Ptot single objective optimization for minimizing c results in 2.2 and 11.5% decrease in COPs and c , respectively. Ptot Ptot Finally, multiple objectives optimization for maximizing COPs and minimizing c simultaneously causes about Ptot 1.9% decline in COPs while c decreases 10.9%. Ptot

First and second law multidimensional analysis of a triple absorption heat transformer (TAHT)

In this paper, a rigorous multi-dimensional analysis is conducted upon a triple absorption heat transformer (TAHT) using the working fluids water and lithium bromide (LiBr). A full factorial design is created which determines the most influential factors affecting the system’s coefficient of performance (COP), exergetic coefficient of performance (ECOP), flow ratio (FR) and total exergy destruction (ED). The aim is to draw general conclusions which may be adopted into any such TAHT cycle and not simply be specific to any one scenario. Accordingly the paper analyses the position of each variable across its thermodynamically available range instead of the traditional selection of arbitrary temperatures. It is found that in general the condensation temperature and the pinch heat transfer gradient selected have the greatest effect, and that these should be minimised in all situations. There exist points of optimum for the temperatures of the two absorber–evaporators within the cycle, however the evaporation temperature has conflicting effects for different dependent variables, and must therefore be selected based on an economic analysis. The results of this study also show that the generator is the source of the largest exergy destruction in the cycle, followed by the two absorber–evaporators.

Performance analysis and evaluation of a triple-effect ammonia-water absorption-refrigeration system

In this paper, a comprehensive thermodynamic analysis of the triple-effect absorption-refrigeration system using ammonia-water solution is carried out in order to investigate the effect of different operating conditions on the coefficient of performance (COPs) and rate of heat input to the system. It is found that both energetic and exergetic COPs vary from 0.31 to 1.867 and 0.165 to 0.964, respectively, as the inlet temperature to the high-temperature generator (HTG), cooling load, and concentration of the strong solution are increased, while the rate of heat input varies from 65.5 to 71.5%. Moreover, increase in the mixture exit temperature from the HTG, concentration of the weak solution, and mass flow rate result in the variation of energetic and exergetic COPs from 2.49 to 0.68 and 1.0 to 0.23, respectively, which results in the change of rate of heat input from 7.1 to 28.5 kW. When the outdoor temperature is increased, the exergetic COP varies from 0.43 to 1.23. Copyright © 2013 John Wiley & Sons, Ltd.

Exergetic and environmental assessment of room air conditioners in Turkish market

This study utilizes two factors, EEF (exergy efficiency factor) or exergetic COP (coefficient of performance) and MTEWI (modified total equivalent warming impact), which was proposed to evaluate exergetic and environmental performance of RAC (room air conditioners) sold in the Turkish market. In the study, vapour compression cooling cycle used whole RAC units is taken as model for the analyses. The results are shown that average EEF value of units using R-22 and R-410A gas are 74.53% and 74.64% respectively. Besides, R-410A gas, which is used in many split systems and marketed as an environmental friendly gas, has an effect that is approximately 23.18% higher than the R-22 gas which is no more in use. The study finally emphasizes the reasons why EEF and MTEWI factors should be given priority in terms of efficiency and environmental effects in the RAC units.

Exergoeconomic, enviroeconomic and sustainability analyses of a novel air cooler

This study presents the energy, exergy, environmental, exergoeconomic, enviroeconomic and sustainability analyses of the Maisotsenko cycle based novel air cooler considering the nine different dead state temperatures, while the environment temperature is kept constant. In the energy analysis, the wet bulb and dew point effectivenesses, cooling capacity, energetic coefficient of performance and primary energy ratio rates are calculated. Also, in the exergy analysis, exergy input, output, loss and destruction rates as well as exergetic coefficient of performance and primary exergy ratio and exergy efficiency values are determined. Furthermore, sustainability analysis of the system is conducted through a sustainability index method. The electrical energy consumption cost of this novel air cooler shows that, it consumes only 59.85$/year, when it is operated 8h a day and 125 days in a year. The maximum exergetic cost rate is found to be 0.0228kWh/$-year at a dead state temperature of 37.77°C. Also, according to the enviroeconomic (environmental cost) analysis, this novel air cooler has very CO2 emissions cost as 6.96$/year. Consequently, these results show the originality of the Maisotsenko cycle based novel air cooler.

Exergy analysis of a TMS (thermal management system) for range-extended EVs (electric vehicles)

This paper examines a TMS (thermal management system) of a range-extended EV (electric vehicle), which operates on its battery before the gasoline engine engages. In this period of independence of the ICE (internal combustion engine), the vehicle performance characteristics such as range, power for acceleration, fuel economy and emissions depend solely on the battery performance. Since the magnitude and uniformity of temperature in battery packs has significant impact on their performance, it is imperative to keep the battery temperature within optimal performance levels through a capable TMS that can efficiently transfer heat within the vehicle. The objective of this paper is to analyze the refrigerant and coolant circuits in the vehicle for cooling during high ambient temperatures and determine the irreversibilities and areas for improvement within these circuits. A second law analysis is used to examine the areas of low exergy efficiency in the system and minimize the entropy generation. Based on the analysis, the energetic COP (coefficient of performance) ranges of 1.8–2.4 and exergetic COP ranges of 0.26–0.39 are calculated for the system. Various parametric studies have been conducted and the most significant impact on the overall exergetic COP is found to be the ambient temperature.

A comparative study on energetic, exergetic and environmental performance assessments of novel M-Cycle based air coolers for buildings

In this study, three various novel air coolers based on M-Cycle are evaluated using energy and exergy analyses based efficiency assessments along with environmental impact and sustainability parameters. The M-Cycle systems are considered to cool a building room air while their inlet air parameters are same, but outlet cooled air parameters are different. Systems I and III draw electricity directly taken from an electric grid in the building while System II, which is stand alone system, produces and draws electricity from its solar PV panels. In the energy analysis, wet bulb effectiveness, cooling capacity, Coefficient of Performance (energetic COP) and Primary Energy Ratio ( PER) are found. In the exergy analysis, exergy input and output rates, exergy loss rate, exergy destruction rate, Exergetic Coefficient of Performance ( COP ex ), Primary Exergy Ratio ( PE xR ) and exergy efficiency are obtained for six different dead state temperatures changing between 10 °C and 35 °C. Also, sustainability assessments of the systems are obtained using sustainability index ( SI) tool for these various dead state temperatures. Finally, environmental assessments of the systems are calculated from their greenhouse gas (GHG) emissions (gCO 2/kW h) due to their electricity consumptions. Maximum exergy efficiencies and sustainability assessments are found to be 35.13% and 1.5415 for System III and 34.94% and 1.5372 for System II, respectively. GHG emissions of the systems are calculated to be 2119.68 gCO 2/day, 153.6 gCO 2/day and 3840 gCO 2/day for Systems I, II and III respectively. So, System II becomes a good choose to prevent the global warming and to attain sustainable future.

Exergy analysis of industrial waste heat recovery based ejector vapour compression refrigeration system

AbstractThis communication employed a thermodynamic analysis through energy and exergy for the performance evaluation of waste heat recovery based ejector compression refrigeration in which internal heat exchanger and cooler are used to enhance the performance of the cycle. The system is an integration of the Rankine power cycle and the compression refrigeration cycle. A comprehensive parametric study is performed to investigate the effects of exhaust gas inlet temperature, pinch point (PP) and evaporator temperature on the energetic and exergetic coefficient of performance (COP) of the system as well on the exergy destruction in each component. It is shown that the exergetic COP of the system decreases significantly with the increase in exhaust gas temperature and PP, while the energetic COP increases slightly. The effect of evaporator temperature also shows a similar behaviour. It is further shown that the exergetic coefficient of performance of the system is ∼75% lower than the COP of the same system. ...

Analysis of a vapour compression refrigeration system via exergetic performance coefficient criterion

A theoretical performance analysis based on exergetic performance coefficient (EPC) criterion was carried out for a vapour compression refrigeration system for different refrigerants. The EPC criterion is defined as the ratio of exergy output to the total exergy destruction rate (or loss rate of availability). According to the results, the refrigerant R32 shows the best performance in terms of EPC among the other refrigerants (R410A, R143A, R404A and R125). The effects of temperature and pressure drops in the evaporator and condenser and of environment temperature on the exergy destructions and EPCs for the components of the vapour compression refrigeration system have been investigated in detail for the refrigerant R32.

An Exergetic Performance Evaluation of Multiple Source Heat Pump Systems

In this study, some heat pumps having multiple heat source combinations were investigated experimentally. Multiple systems including mentioned heat pump systems, such as solar assisted-ground source, solar assisted-air source, and ground source-air source heat pumps, were tested. All the heat pump systems were designed and constructed in a test room with 60 m2 floor area at Firat University, Elazig (38.41°N, 39.14°E), Turkey. In evaluating the efficiency of the heat pump systems, the most commonly used measure is the energy or the first law efficiency, which is modified to a coefficient of performance for heat pump systems. However, for indicating the possibilities for thermodynamic improvement, energy analysis inadequate and exergy analysis is needed. The coefficient of performance (COP) of solar assisted-ground source, solar assisted-air source, and ground-air source heat pumps as multiple source heat pump systems were also determined to be 3.36, 2.90, and 2.14, while the exergy losses of the refrigerant subsystems were approximately 2.13, 2.996, and 3.113 kW, respectively. Consequently, the energy and exergy loss analysis results show that the COP increases when the exergy loss of the condenser decreases. Exergetic performance coefficient is introduced and is applied to the heat pump systems having various heat sources. The results imply that the functional forms of the exergetic performance coefficient and first law efficiency are different. Results show that Exloss,total becomes a minimum value where exergetic performance coefficient is the maximum value.

Performance analysis of solar powered absorption refrigeration system

The present work provides a detailed thermodynamic analysis of a 10 kW solar absorption refrigeration system using ammonia-water mixtures as a working medium. This analysis includes both first law and second law of thermodynamics. The coefficient of performance (COP), exergetic coefficient of performance (ECOP) and the exergy losses (ΔE) through each component of the system at different operating conditions are obtained. The minimum and maximum values of COP and ECOP were found to be at 110 and 200°C generator temperatures respectively. About 40% of the system exergy losses were found to be in the generator. The maximum exergy losses in the absorber occur at generator temperature of 130°C for all evaporator temperatures. A computer simulation model is developed to carry out the calculations and to obtain the results of the present study.

An experimental study of a direct expansion ground-coupled heat pump system in heating mode

In this paper, an experimental performance evaluation of a direct expansion ground-coupled heat pump (DX-GCHP) system in heating mode is presented. The DX-GCHP uses R134a as the refrigerant, and consists of three single U-tube copper ground heat exchangers (GHEs) placed in three 30 m vertical boreholes. During the on–off operations from December 25, 2007, to February 6, 2008, the heat pump supplied hot water to fan-coil at around 50.4°C, and its heating capacity was about 6.43 kW. The energy-based heating coefficient of performance (COP) values of the heat pump and the whole system were found to be on average 3.55 and 3.28 at an evaporating temperature of 3.14°C and a condensing temperature of 53.4°C, respectively. The second law efficiency on the DX-GCHP unit basis was around 0.36. The exergetic COP values of the heat pump and the whole system were obtained to be 0.599 and 0.553 (the reference state temperature was set equal to the average outdoor temperature of −1.66°C during the tests), respectively. The authors also discussed some practical points such as the heat extraction rate from the ground, refrigerant charge and two possible new configurations to simultaneously deal with maldistribution and instability of parallel GHE evaporators. This paper may reveal insights that will aid more efficient design and improvement for potential investigators, designers and operators of such DX-GCHP systems. Copyright © 2009 John Wiley & Sons, Ltd.

Comparative energetic and exergetic performance analyses for coal-fired thermal power plants in Turkey

The purpose of this study is to analyze comparatively the performance of nine thermal power plants under control governmental bodies in Turkey, from energetic and exergetic viewpoint. The considered power plants are mostly conventional reheat steam power plant fed by low quality coal. Firstly, thermodynamic models of the plants are developed based on first and second law of thermodynamics. Secondly, some energetic simulation results of the developed models are compared with the design values of the power plants in order to demonstrate the reliability. Thirdly, design point performance analyses based on energetic and exergetic performance criteria such as thermal efficiency, exergy efficiency, exergy loss, exergetic performance coefficient are performed for all considered plants in order to make comprehensive evaluations. Finally, by means of these analyses, the main sources of thermodynamic inefficiencies as well as reasonable comparison of each plant to others are identified and discussed. As a result, the outcomes of this study can provide a basis used for plant performance improvement for the considered coal-fired thermal power plants.

Thermodynamic model for exergetic performance of a tubular SOFC module

A tubular solid oxide fuel cell (TSOFC) module fed by methane is modelled and analyzed thermodynamically from the exergy point of view in this paper. The model of TSOFC module consists of mixer, pre-reformer, internal reforming fuel cell group, afterburner and internal pre-heater components. The model of the components forming module is given based on mass, energy and exergy balance equations. The developed thermodynamic model is simulated, and the obtained performance characteristics are compared and validated with the experimental data taken from the literature concerning TSOFC module. For exergetic performance analysis, the effects of operating variables such as current density, pressure, and fuel utilization factor on exergetic performances (module exergy efficiency, module exergetic performance coefficient, module exergy output and total exergy destruction rate, and components' exergy efficiencies, exergy destruction rates) are investigated. From the analysis, it is determined that the biggest exergy loss stems from exhaust gasses. Other important sources of exergy destruction involve fuel cell group and afterburner. Consequently, the developed thermodynamic model is expected to provide not only a convenient tool to determine the module exergetic performances and component irreversibility but also an appropriate basis to design complex hybrid power generation plants.