Detailed Exergy Analysis Research Articles

The element contents in chickpeas grown under organic and conventional farming regimes using WDXRF analysis for human nutrition and health

In this study, exergy analysis of an integrated gasification combined cycle (IGCC) operating with 7 different types of low rank Turkish lignite was performed. The purpose was to determine and compare the performance of each type of lignite in the same gasifier. Exergy destruction of each component and exergetic performance of the overall plant was also investigated. In addition, CO2 emissions of each type of lignite were calculated. The results showed that Yenikoy lignite had the best performance among the different types of lignite, with 491 MW net power and 37.88% net exergy efficiency (without acid gas). It also had the minimum amount of CO2 emission at 726 kg/MWh. Therefore, Yenikoy lignite was chosen as a case study fuel and a detailed exergy analysis of its components was applied. The results showed that the major exergy destructions took place in the gasifier and the combustion chamber at 46.15% and 22.74%, respectively.

Techno-economic analysis of CO2 conditioning processes in a coal based oxy-combustion power plant

Oxy-combustion is a competitive technology to enable the capture of CO2 from coal based power plants. The CO2 conditioning process is an important contributor to the power penalty related to CO2 capture in such power plants. The two-stage flash process is commonly proposed in literature. This paper presents a study on the CO2 conditioning process in three cases: one-stage flash, two-stage flash and three-stage flash. The composite curves are applied to investigate the improvement potential. A detailed exergy analysis has been performed to compare the plant performance in the three cases. Considering the one-stage flash process as the base case, the specific power consumption is reduced by 7.3% in the two-stage flash process and 8.1% in the three-stage flash process. The investment cost will of course increase. The economic analysis shows that the two-stage flash process is the most cost effective alternative. However, a higher recovery rate can be achieved in the three-stage flash process when the CO2 purity is designed to be the same for the three cases.

Exergy efficiency enhancement of MSF desalination by heat recovery from hot distillate water stages

This detailed exergy analysis of a 3800 m3/h Multi-Stage Flash (MSF) desalination plant is based on the latest published thermodynamics properties of water and seawater. The parameters of the study were extracted from a validated model of MSF desalination using IPSEpro software. The results confirmed that the overall exergy efficiency of the unit is lower than would be desirable at only 5.8%. Exergy inputs were destroyed by 55%, 17%, 10%, 4.3%, and 14% respectively, in the heat recovery stages, brine heater, heat rejection stages, pumps and brine streams disposal. Moreover, the detail of the study showed that the lowest exergy destruction occurs in the first stage, increasing gradually in heat recovery stages and sharply in heat rejection stages. The study concludes that recovering the heat from the hot distillate water stages can improve unit exergy efficiency from its low 5.8% to a more economical 14%, with the hot water parameters suitable for powering other thermal systems such as absorption chiller and multi-effect desalination.

Exergy Analysis of Micro Gas Turbine Tri- Generation System

This paper presents exergy analysis of Micro Gas Turbine (MGT) system. It is proposed to use hot MGT exhaust gases heat in a heat recovery steam generator to produce steam. Absorption chillers can help to increase the performance of MGT tri generation plants. MGTs are fuelled with natural gas and their waste heat is used to drive absorption chillers and other thermal energy users. Based on a steady-state model of the processes, exergy flow rates are calculated for all components and a detailed exergy analysis is performed. The components with the highest proportion of irreversibility in these systems are identified and compared.

Performance Evaluation of an Automobile Air Conditioning System Using R134a

In this paper, a detailed energy and exergy analysis have been dealt with on experimental AAC (Automobile Air Conditioning) system with R134a as working refrigerant. For this aim, an experimental AAC system consisting of a laminated type evaporator, swash plate type compressor, a parallel flow type condenser, TXV(thermostatic expansion valve) and a receiver drier. The performance analysis of separate components of AAC system has been carried out under various compressor speeds and thermal loads. AAC system equipped with increasing compressor speed by three-phase electric motor controlled by frequency converter. Various thermal loads in the range of 1500 and 2850 W were applied to the system by means of electric heaters. The experiments were conducted at the condensing temperatures of 50-60 oC for each thermal load, and at the compressor speeds of 600, 800, 1000, 1200, 1400 rpm for each thermal load-condensing temperature combination. The refrigerant and air temperatures, refrigerant pressures, compressor speed, air velocity passing through the evaporator and thermal load were measured. Effects on system performance of such operational parameters as compressor speed, return air in the evaporator and condensing air temperatures have been experimentally evaluated and by means of energy and exergy analysis.

Exergy Analysis of 300MW Coal-Fired Power Plant

In order to find the real distribution of energy loss and weak links in electricity production, based on the second law of thermodynamics and “fuel” and “product” concept, a 300MW thermal power plant is taken for example, irreversible loss distribution is obtained through detailed exergy analysis. Then, the comparison of the performance of major equipments of plant under rated condition, 75%, 50% and 30% loads by sliding pressure regulation shows that load decrease makes the consumption of production uneconomically.

Exergy Analysis of a 600 MWe Oxy-combustion Pulverized-Coal-Fired Power Plant

In an oxy-combustion pulverized-coal-fired power plant (PC), a high CO2 content flue gas could be obtained, and this allows CO2 sequestration in an efficient and energy-saving way. To better understand the thermodynamic characteristics of the oxy-combustion process, a detailed exergy analysis of a 600 MWe oxy-combustion PC was carried out. The whole oxy-combustion PC system was divided into four models: boiler, turbines and feedwater heaters (FWHs), air separation unit (ASU), and flue gas treatment unit (FGU). The exergy (including physical exergy and chemical exergy) of each model was obtained, showing that the oxy-combustion boiler could reach higher exergy efficiency than the conventional combustion boiler. There is a significant difference between the exergy efficiencies of furnaces in the two boiler models, and the combustion exergy efficiency in the oxy-combustion furnace is about 4% higher. In each boiler model, the combustion process contributes nearly 60% exergy destruction. The exergy efficiency of the air heater in the oxy-combustion boiler is 10.3% higher than that in the conventional combustion boiler because of the flue gas recycle in the oxy-combustion boiler. The exergy destruction in the turbine and FWH model is just 9.01% of the total fuel exergy, and the turbines contribute about half. The exergy efficiencies of the ASU and FGU processes are 15.84 and 73.45%, respectively. The exergy efficiency of the oxy-combustion system is 37.13%, which is 4.08% lower than that of the conventional system.

Multi-objective thermoeconomic optimization of coupling MSF desalination with PWR nuclear power plant through evolutionary algorithms

This paper deals with the application of an evolutionary algorithm to multi-objective thermoeconomic optimization of coupling a multi stage flash desalination (MSF) plant with a pressurized water reactor (PWR) nuclear power plant. The thermodynamic simulation of this initial PWR plant has been performed in a Thermoflex simulator. An Excel add-in called Thermoflex Link has been developed to calculate the exergy of each stream from Thermoflex simulation results. Meanwhile, a computer code has been developed for thermoeconomic and improved combined pinch–exergy analysis in Matlab environment. Both the design configuration (feed water heater structure) and the process variables are optimized simultaneously. The optimization algorithm can choose among several design options included in a superstructure of the feed water heater in dual purpose plant. For the assumptions and simplifications made in this study, a 3000 MW (thermal) PWR power plant such as the Bushehr power plant has been considered. A detailed exergy and exergoeconomic analysis of selected final optimal design identifies the magnitude, location and causes of the thermodynamic inefficiencies. Improved combined pinch and exergy analysis has been applied to display the system information graphically for one to visualize the performance of the system in the initial and final case.

Exergy based performance analysis of a solid oxide fuel cell and steam injected gas turbine hybrid power system

This paper presents exergy analysis of a hybrid solid oxide fuel cell and gas turbine (SOFC/GT) system in comparison with retrofitted system with steam injection. It is proposed to use hot gas turbine exhaust gases heat in a heat recovery steam generator to produce steam and inject it into gas turbine. Based on a steady-state model of the processes, exergy flow rates are calculated for all components and a detailed exergy analysis is performed. The components with the highest proportion of irreversibility in the hybrid systems are identified and compared. It is shown that steam injection decreases the wasted exergy from the system exhaust and boosts the exergetic efficiency by 12.11%. Also, 17.87% and 12.31% increase in exergy output and the thermal efficiency, respectively, is demonstrated. A parametric study is also performed for different values of compression pressure ratio, current density and pinch point temperature difference.

Exergetic evaluation and improvement of biomass-to-synthetic natural gas conversion

Synthetic natural gas (SNG) is suggested as an important future energy carrier. The conventional route for SNG production is based on gasification of biomass to synthetic gas and the subsequent methanation of synthetic gas to SNG. This study is aimed to analyze the process units using the concept of exergy. Exergy analysis is a promising method, based on the 2nd law of thermodynamics, to analyze and improve chemical processes. In this work a detailed exergy analysis is performed for the SNG process based on woody biomass gasification. The main elements of the system are gasifier, gas cleaning, synthetic gas compression, methanation and final SNG condition. The above-mentioned process was simulated with a computer model using the flow-sheeting program Aspen Plus. Optimal values of the process conditions, particularly for the methanation reactors, are found. The internal exergy losses of different system units are evaluated. The largest internal exergy losses take place in the gasifier, methanation section and CO2 capture unit. The highest overall exergetic efficiency of 72.6% was found applying the following operating conditions: gasifier 700 °C and 1 bar; 1st methanation reactor 580 °C and 2nd methanation reactor 405 °C.

Exergoenvironmental analysis for evaluation of the environmental impact of energy conversion systems

To improve the ecological performance of energy conversion systems, it is essential to understand the formation of environmental impact at component level. A method has been developed that (a) reveals the extent to which each component of an energy conversion system is responsible for the overall environmental impact and (b) identifies the sources of the impact. The approach of exergoeconomic analysis is modified to deal with an evaluation of the ecological impact instead of an economic problem. The basic idea is that exergy represents a proper basis for allocating both costs and environmental impact to components of energy conversion processes. The proposed exergoenvironmental analysis consists of three steps. In the first step, a detailed exergy analysis of the system under consideration is conducted. In the second step, the required values of the environmental impact are determined by applying an appropriate method involving an appropriate quantifier of the environmental impact. Here the Eco-indicator 99 impact assessment method is applied in conjunction with the method of life cycle assessment. In the last step, the environmental impact associated with each component is assigned to the product exergy streams of the component; subsequently exergoenvironmental variables are calculated and an exergoenvironmental evaluation is conducted. As a case study, an energy conversion system consisting of a high-temperature solid oxide fuel cell integrated with an allothermal biomass gasification process has been analyzed. Exergoenvironmental analysis allows us to identify the environmentally most relevant system components and provides information about possibilities for design improvements.

Theoretical analysis of a vapour compression refrigeration system with R502, R404A and R507A

This paper presents a detailed exergy analysis of an actual vapour compression refrigeration (VCR) cycle. A computational model has been developed for computing coefficient of performance (COP), exergy destruction, exergetic efficiency and efficiency defects for R502, R404A and R507A. The present investigation has been done for evaporator and condenser temperatures in the range of −50 °C to 0 °C and 40 °C to 55 °C, respectively. The results indicate that R507A is a better substitute to R502 than R404A. The efficiency defect in condenser is highest, and lowest in liquid vapour heat exchanger for the refrigerants considered.

Efficiency analysis of hydrogen production methods from biomass

Hydrogen is considered as a universal energy carrier for the future, and biomass has the potential to become a sustainable source of hydrogen. This article presents an efficiency analysis of hydrogen production processes from a variety of biomass feedstocks by a thermochemical method – gasification as well as biochemical methods – fermentation and anaerobic digestion. The exergetic efficiency of H2 production by gasification of more dry biomass is comparable to that of the commonly used Steam Methane Reforming. The detailed exergy analysis of H2 production by biomass gasification shows that the largest exergy losses occur in the gasifier.

Optimization of combined cycle power plants using evolutionary algorithms

This paper deals with the application of an evolutionary algorithm to the minimization of the product cost of complex combined cycle power plants. Both the design configuration (process structure) and the process variables are optimized simultaneously. The optimization algorithm can choose among several design options included in a superstructure of the power plant such as different gas turbine systems available at the market, up to three pressure levels for steam generation in the heat-recovery steam generator, supplementary firing, steam reheat, parallel arrangement of heat exchangers in the gas path, and steam injection into the gas turbine. For the assumptions and simplifications made in this study, a 240 MW combined cycle power plant with a large gas turbine (150 MW), a two-pressure heat recovery steam generator with a reheater but without supplementary firing is favored from an economic viewpoint. A detailed exergy analysis of selected intermediate solutions and the final optimal design identifies the magnitude, location and causes of the thermodynamic inefficiencies.

Exergetic efficiency and options for improving sewage sludge gasification in supercritical water

The present article deals with an exergy analysis of a process under development for the gasification of biomass in supercritical water (supercritical water gasification, SCWG). This process is aimed at generating hydrogen out of the biogenic feedstock sewage sludge. The principle of the process is based on making use of the modifications of specific physical and chemical properties of water above the critical point (T=374°C, p=221 bar). These properties allow for a nearly complete conversion of the organic substance contained in the feed material into energy-rich fuel gases, containing hydrogen, carbon dioxide and methane. Based on a steady-state model of the process, exergy flow rates are calculated for all components and a detailed exergy analysis is performed. From the exergetic variables, options to improve the individual plant components as well as the overall plant are derived. The components with the highest proportion of exergy destruction in the complete process are identified and possibilities of improving them and the complete system in order to increase the overall efficiency are demonstrated. The combustion chamber necessary for heat supply is found to be the component with the highest proportion of exergy destruction of the complete plant. Moreover, the components of air preheater, reactor contribute significantly to the exergy destruction of the complete system. Copyright © 2006 John Wiley & Sons, Ltd.

05/02745 Experimental evaluation of the internal heat exchanger influence on a vapour compression plant energy efficiency working with R22, R134a and R407C

This paper presents a detailed exergy analysis of an actual vapour compression refrigeration (VCR) cycle. A computational model has been developed for computing coefficient of performance (COP), exergy destruction, exergetic efficiency and efficiency defects for R502, R404A and R507A. The present investigation has been done for evaporator and condenser temperatures in the range of −50 °C to 0 °C and 40 °C to 55 °C, respectively. The results indicate that R507A is a better substitute to R502 than R404A. The efficiency defect in condenser is highest, and lowest in liquid vapour heat exchanger for the refrigerants considered.

Exergy analysis of the solar cylindrical-parabolic cooker

For the first time the simple solar parabolic cooker (SPC), of the cylindrical trough shape, is analysed from the exergy viewpoint. The paper presents the methodology of detailed exergy analysis of the SPC, the distribution of the exergy losses, and, on the example of the cooker surfaces, explains the general problem of how the exergy loss on any radiating surface, should be determined, if the surface absorbs many radiation fluxes of different temperatures. An imagined surface was used in the considerations to close the system of the cooker surfaces. It was shown that optimization is needed, to increase the energy and exergy efficiencies of the cooker. Equations for heat transfer between the three surfaces: cooking pot, reflector and imagined surface making up the system, were derived. The model allowed for theoretical estimation of the energy and exergy losses: unabsorbed insolation, convective and radiative heat transfer to the ambient, and additionally, for the exergy losses: the radiative irreversibilities on the surfaces, and the irreversibility of the useful heat transferred to the water. The exergy efficiency of the SPC, was found to be relatively very low (∼1%), and to be about 10 times smaller than the respective energy efficiency which is in agreement with experimental data from the literature. The influence of the input parameters (geometrical configuration, emissivities of the surfaces, heat transfer coefficients and temperatures of water and ambience) was determined on the output parameters, the distribution of the energy and exergy losses and the respective efficiencies.

Exergy analysis of a two-stage refrigeration cycle using two natural substitutes of HCFC22

The aim of the present paper is to carry out a detailed exergy analysis of a two-stage vapour compression cycle by calculating its components exergetic losses. The exergy equations have been developed using refrigerant thermodynamic properties computed by means of a simple model of local equations of states. The results of the exergy analysis of a two-stage refrigeration system operating between a constant evaporating temperature of -30°C and condensation temperatures of 30, 40, 50 and 60°C with two natural substitutes of HCFC22, namely, propane (R290) and ammonia (R717) as working fluids, are presented. It is found that the most significant losses occur in the compressors, expansion valves and condenser. Furthermore, it is shown that the optimum inter-stage pressure for a two-stage refrigeration system is very close to the saturation pressure corresponding to the arithmetical mean of the refrigerant condensation and evaporation temperatures.

Exergy analysis of industrial ammonia synthesis

Exergy consumption of ammonia production plants depends strongly on the ammonia synthesis loop design. Due to the thermodynamically limited low degree of conversion of hydrogen–nitrogen mixture to ammonia, industrial ammonia synthesis is implemented as recycle process (so-called “ammonia synthesis loop”). Significant quantities of reactants are recycled back to reactor, after the removal of ammonia at low temperatures. Modern ammonia synthesis plants use well-developed heat- and cold recovery to improve the reaction heat utilisation and to reduce the refrigeration costs. In this work, the exergy method is applied to estimate the effect of the most important process parameters on the exergy efficiency of industrial ammonia synthesis. A specific approach, including suitable definitions of the system boundaries and process parameters, is proposed. Exergy efficiency indexes are discussed in order to make the results applicable to ammonia synthesis loops of various designs. The dependence of the exergy losses on properly selected independent process parameters is studied. Some results from detailed exergy analysis of the most commonly used ammonia synthesis loop design configurations at a wide range of selected parameters values are shown.

Simulation, exergy analysis and application of diabatic distillation to a tertiary amyl methyl ether production unit of a crude oil refinery

This paper presents the results of a detailed exergy analysis of a tertiary amyl methyl ether (TAME) unit of a crude oil refinery and the application of diabatic distillation to the depentanizer tower of the unit. Diabatic distillation is a separation process in which heat is not only supplied to the reboiler and extracted from the condenser [as in a conventional (adiabatic) distillation column], but is also transferred inside the column. The process enables operation to approach equilibrium conditions, thus reducing exergy losses and increasing exergy effectiveness. In a TAME unit of a refinery, isoamylenes are converted to TAME. Before transforming the isoamylenes in the reactors, it is necessary to recover them from a catalytic gasoline stream by a depentanization process. The exergy losses of this depentanization process represent about 70% of the total exergy losses of the unit. The results of the exergy analysis of the TAME unit are presented and a detailed exergy analysis of the conventional adiabatic depentanizer column is conducted for comparison purposes. Then, the application of diabatic distillation to the system is evaluated by using cooling water circulating in series from tray to tray in the rectification section and by making the steam emanating from the reboiler circulate in series from tray to tray in the stripping section. The results in terms of the reduction of exergy losses, heating and cooling media flow rates, and cost effectiveness of the diabatic option for the depentanizer section of the plant are compared to the original adiabatic system, and the effect of the diabatization on the overall exergy performance parameters of the depentanizer section and on the whole TAME unit, are presented in this paper.