Exergy Balance Research Articles

Analysis of entropy generation and exergy losses of iso-octane and n-butanol adiabatic constant-volume combustion process

Butanol is a promising bio-fuel. Using iso-octane as gasoline surrogate, the entropy production and exergy balance of n-butanol and iso-octane constant volume combustion process were analysed with the CHEMKIN software by considering detailed n-butanol and iso-octane chemical reaction mechanisms. The results of exergy balance analysis showed that, during the adiabatic constant-volume combustion process, there is a trade-off between exergy destruction and exergy loss due to incomplete combustion. The exergy destruction of n-butanol is obviously higher than that of iso-octane. The results of entropy generation analysis showed that, with high initial temperature, two apparent peaks of heat release rate and exergy loss rate are observed both for iso-octane and n-butanol. During combustion process, the internal energy exchange between high temperature product and the unburned reactant via a series of chemical reactions, especially the reactions of small molecule intermediate products, is the major reason for the exergy destruction.

Thermal and Exergetic Analysis of a Solar Still

In this work an energy and exergy analysis of solar distillation process is presented. The analysis is based on the experimental observation of the simple basin type solar stills for 4 different initial volumes (5.5, 6.5, 7.5 and 8.5 L). Energy and exergy balance equations have been written for all components of the solar still including glass cover, brine and absorber plate. The thermodynamic models for the energy and exergy analysis are presented on the critical heat transfer correlations in literatures for the simple basin type solar still. The results show that maximum values are reached in the energy efficiency of 45.6, 41.5, 35.7 and 31.8%, however exergetic efficiency for maximum values are 7.5, 7.2, 7 and 5.4%, corresponding to volumes 5.5, 6.5, 7.5 and 8.5 L respectively.

Performance analysis of power, desalination and cooling poly-generation system using parabolic trough solar collectors

This work presents a thermodynamic analysis of a poly-generation system powered by solar energy using parabolic trough solar collectors. The system is composed of an organic Rankine cycle (ORC), a multiple effect distillation and an absorption cooling unit. The analysis is based on the solution of mass, energy and exergy balances of the set of equations of all the components of the system. It is also based on the technical specifications of sub-systems and working fluid properties. The validation of the computer program is achieved systematically. The performance of the poly-generation plant is investigated under Riyadh weather conditions and for several conditions of the operating parameters. The variation of the energy rates required for desalination, cooling and electricity generation has been obtained for two representative days in summer and winter of 2013 in Riyadh. Results expressing the plant performance using the energy and exergy efficiencies are presented and discussed. Specifically, energy utilization factor, artificial thermal efficiency, fuel energy saving factor and exergy efficiency were introduced and used as plant performance indicators. The fresh water production rate and the power to water ratio were also evaluated for two representative days in June and January of 2013. The main results of the study show that although there is a clear dispersion of the obtained values for the performance of the solar poly-generation plant, the exergy efficiency and the fuel energy saving factor give close values.

Second-law efficiency and performance of a diesel generator using marine fuel blended with aspire methyl ester

Nowadays, exergy analysis has been used in the design technology such as simulation and performance assessment of various types of engines, for identifying parameters. The energy and exergy analyses of a four-cylinder, four-stroke diesel generator using the AME/MF fuels of 50 have been performed. Balances of energy and exergy rates for the engine were determined and then various performance parameters and energy and exergy efficiencies were calculated for each fuel operation and compared with each other. The results of tested biodiesel offer similar energetic performance as petroleum diesel fuel. The engine was operated under different loads to determine the performance, emissions under 500 hours testing. Each fuel type was tested under steady engine speed-torque. Afterwards, the energy and the exergy balances for each test step were determined using measured data. Finally, various energetic and exergetic performance parameters of the engine were evaluated and compared with each other.

A systematic approach: combining process optimisation exergy analysis and energy recovery for a better efficiency of industrial processes

Exergy analysis has been shown to be a powerful tool for assessing and improving the energy efficiency of thermal and chemical processes. However, the lack of systematic procedures in examining and interpreting the results of the exergy balance or the exergy efficiency makes this kind of analysis more or less limited to the academic world. This paper intends to promote an efficient energy usage through pinch, exergy analyses and energetic optimisation and by providing the engineer with specific improvement means by using a case-based reasoning (CBR) system. To make exergy analysis more understandable, it was decided to develop two methodological tools: a supporting graphical representation of the exergy analysis and a CBR system to help the user in developing alternatives. Both tools are grouped together into a general methodology for the analysis and retrofit design of industrial sites. This new approach is illustrated through an ammonia processing plant case study.

A systematic approach: combining process optimisation exergy analysis and energy recovery for a better efficiency of industrial processes

Exergy analysis has been shown to be a powerful tool for assessing and improving the energy efficiency of thermal and chemical processes. However, the lack of systematic procedures in examining and interpreting the results of the exergy balance or the exergy efficiency makes this kind of analysis more or less limited to the academic world. This paper intends to promote an efficient energy usage through pinch, exergy analyses and energetic optimisation and by providing the engineer with specific improvement means by using a case-based reasoning (CBR) system. To make exergy analysis more understandable, it was decided to develop two methodological tools: a supporting graphical representation of the exergy analysis and a CBR system to help the user in developing alternatives. Both tools are grouped together into a general methodology for the analysis and retrofit design of industrial sites. This new approach is illustrated through an ammonia processing plant case study

Exergy analysis of the turbine system in a 1000 MW double reheat ultra-supercritical power plant

This study aims to achieve exergy distribution in a turbine system for a 1000 MW double reheat ultra-supercritical power plant. An exergy balance equation is used to identify the real points of energy loss in the turbine system, which can provide a reference for optimizing operation and conserving energy. Results show that irreversibilities yield the highest exergy loss in the turbine, particularly in the very high pressure (VHP) and low pressure (LP) cylinders. Compared with that in the turbine of a single reheat unit, exergy loss in the studied turbine is higher and exergy efficiency is lower. For the regenerative system, exergy loss is lower and exergy efficiency is higher in the double reheat unit than in the single reheat unit. Thus, the layout of the regenerative heaters in the double reheat unit is more reasonable. However, exergy loss in high pressure regenerative heater Nos. 3 and 6 and low pressure regenerative heater Nos. 7 and 10 in the heat recovery system is extraordinary because of different reasons. Exergy loss in the condenser of the double reheat unit is relatively smaller than that in the condenser of the single reheat unit.

Exergy analysis: A CO2 removal plant using a-MDEA as the solvent

An exergy analysis was conducted for CO2 removal process from syngas using methyldiethanolamine activated by piperazine (a-MDEA). The process was simulated with Aspen HYSYS and the validation of the results was performed using the field data of Fanavaran Petrochemical Company. The process was divided into five separate sections. In order to determine the exergy loss, exergy balance equation was established around each section. The results showed that the highest exergy loss amongst five sections returns to the flasher (contributes 31.5% of total exergy loss); followed by the absorber column, stripper column, heat recovery section and the pump. In order to decrease the exergy loss in the process, piperazine concentration in a-MDEA solvent was increased which led to reduction in amine solvent flow rate. According to the results, the total exergy loss is decreased from 3.25 MW to 2.63 MW. When the amine flow rate decreased by 27%, the exergy loss, total energy consumption and the total utilities cost decreased by 19%, 18% and 25%, respectively.

Thermodynamic performance assessment of an integrated geothermal powered supercritical regenerative organic Rankine cycle and parabolic trough solar collectors

In this study, the thermodynamic performance of an integrated geothermal powered supercritical regenerative organic Rankine cycle (ORC) and parabolic trough solar collectors (PTSC) is assessed. A thermal model based on the principles of thermodynamics (mass, energy, and exergy balances) and heat transfer is first developed for the components of this integrated system. This model gives the performance assessment parameters of the system such as the electrical and exergetic efficiencies, total exergy destruction and loss, productivity lack, fuel depletion ratio, and improvement potential rate. To validate this model, the data of an existing geothermal power plant based on a supercritical ORC and literature data for the PTSC are used. After validation, parametric studies are conducted to assess the effect of some of the important design and operating parameters on the performance of the system. As a result of these studies, it is found that the integration of ORC and PTSC systems increases the net power output but decreases the electrical and exergetic efficiencies of the integrated system. It is also shown that R134a is the most suitable working fluid type for this system; and the PTSCs and air cooled condenser are the main sources of the exergy destructions.

Exergy and exergoeconomic analyses of a supercritical CO2 cycle for a cogeneration application

Detailed exergy and exergoeconomic analyses are performed for a combined cogeneration cycle in which the waste heat from a recompression supercritical CO2 Brayton cycle (sCO2) is recovered by a transcritical CO2 cycle (tCO2) for generating electricity. Thermodynamic and exergoeconomic models are developed on the basis of mass and energy conservations, exergy balance and exergy cost equations. Parametric investigations are then conducted to evaluate the influence of key decision variables on the sCO2/tCO2 performance. Finally, the combined cycle is optimized from the viewpoint of exergoeconomics. It is found that, combining the sCO2 with a tCO2 cycle not only enhances the energy and exergy efficiencies of the sCO2, but also improves the cycle exergoeconomic performance. The results show that the most exergy destruction rate takes place in the reactor, and the components of the tCO2 bottoming cycle have less exergy destruction. When the optimization is conducted based on the exergoeconomics, the overall exergoeconomic factor, the total cost rate and the exergy destruction cost rate are 53.52%, 11243.15 $/h and 5225.17 $/h, respectively. The optimization study reveals that an increase in reactor outlet temperature leads to a decrease in total cost rate and total exergy destruction cost rate of the system.

APROVECHAMIENTO DEL RECURSO BIOMASA A PARTIR DE LOS DESECHOS DE MADERA PARA UNA CALDERA DE VAPOR

<p>Desde el auge del petróleo y la consecuente quema de combustibles fósiles, las emisiones de gases que provocan el efecto invernadero han aumentado. Teniendo en cuenta este punto de vista, la producción de energía renovable es necesario; por ejemplo, podemos aprovechar el potencial de la energía térmica de la biomasa residual utilizando equipos tales como una caldera de vapor, dispositivos de gran demanda en muchos procesos dentro de la industria.</p><p> </p><p>El uso de la biomasa residual, en este caso, los de residuos de madera, son de vital importancia en el medio ambiente; ya que, se trata de una contaminación neutral frente a la contaminación por hidrocarburos. Por lo tanto, la operación de una caldera de vapor con biomasa tiene como objetivo ofrecer las mismas características que un equipo convencional.</p><p> </p><p>El lector sabrá sobre: el origen de la biomasa a partir de residuos de madera, las fuentes más comerciales de la biomasa de madera, el uso final de estos desechos, y los niveles de contaminación que produce la biomasa comparables a los derivados del petróleo utilizados en la caldera. A través de pruebas experimentales de laboratorio se demuestran las propiedades físicas y energéticas que posee este recurso, con el que vamos a trabajar en los cálculos pertinentes para ser generado en la caldera de vapor.</p>

The exergy analysis of energy efficiency of the technology of granulated phosphorus-potassium fertilizers

The need for creating of complex mineral fertilizers was demonstrated, namely, granulated phosphorus and potassium fertilizers, which are a certain alternative to basic superphosphate. The advantages of the semi­dry method of granulation of phosphorus and potassium fertilizers were defined, which consist in the agglomeration of phosphorite powder with the help of the plasticizer based on the compounds of potassium at humidity of the charge not exceeding 12 %. The structural scheme of the technology of obtaining phosphorus and potassium fertilizers was presented with indication of the amounts and temperatures of material flows.It was demonstrated that the technology of phosphate mineral fertilizers by the method of granulation of pulp and drying of granules possesses increased energy expenditures for their production. It is indicated that the technology of phosphorus and potassium fertilizers, obtained by the semi­dry method, is based on the fabrication of granules by the method of agglomeration, that is why we may expect that this technology will have lower energy consumption, and, therefore, the important question is the analysis of completeness of the use of supplied thermal energy to the main technological equipment, i. e., the exergy analysis.We proposed the method of the exergy analysis of technology flows, which makes it possible, by the represented analytical dependences, to determine the exergy expenses and losses by each technological stage in the technology of phosphorus and potassium fertilizers. The summary exergy balance of the technology of phosphorus and potassium fertilizers is presented and analyzed. The comparison of energy costs for the production of phosphorus and potassium fertilizers and ammoniated superphosphate was carried out, as a result of which we demonstrated the decrease in the expenses when obtaining phosphorus and potassium fertilizers by 30 % in comparison to the widely used ammoniated superphosphate.

Thermoeconomic Analysis and Optimization of a New Combined Supercritical Carbon Dioxide Recompression Brayton/Kalina Cycle

A new combined supercritical CO2 recompression Brayton/Kalina cycle (SCRB/KC) is proposed. In the proposed system, waste heat from a supercritical CO2 recompression Brayton cycle (SCRBC) is recovered by a Kalina cycle (KC) to generate additional electrical power. The performances of the two cycles are simulated and compared using mass, energy and exergy balances of the overall systems and their components. Using the SPECO (Specific Exergy Costing) approach and employing selected cost balance equations for the components of each system, the total product unit costs of the cycles are obtained. Parametric studies are performed to investigate the effects on the SCRB/KC and SCRBC thermodynamic and thermoeconomic performances of key decision parameters. In addition, considering the exergy efficiency and total product unit cost as criteria, optimization is performed for the SCRBC and SCRB/KC using Engineering Equation Solver software. The results indicate that the maximum exergy efficiency of the SCRB/KC is higher than that of the SCRBC by up to 10%, and that the minimum total product unit cost of the SCRB/KC is lower than that of the SCRBC by up to 4.9%.

On the exergy balance equation and the exergy destruction

The exergy balance equation can be written expressing the useful work for a general process as depending on the useful work for the corresponding reversible process and the exergy destruction term. However, this can be made only if the terms (other than the exergy destruction term) on the exergy balance equation for the general process lead to the same result as the terms on the exergy balance equation for the corresponding reversible process. It is shown that this is not generally the case for unsteady processes, and that this can be the case for steady processes, for both closed and open systems. It is also shown that this is not generally the case for unsteady processes in terms of instantaneous powers, but that this can be the case for the whole unsteady processes. This is an important result, for: (i) Expressing the useful work for a general process as depending on the useful work for the corresponding reversible process and the exergy destruction; (ii) Exergy analysis; and (iii) The teaching/learning process of exergy and exergy analysis, and a better understanding and application of the exergy balance equation.

Energy and exergy analysis of solar PV powered reverse osmosis desalination

In the current paper presented short review of the state of the art of solar powered reverse osmosis (RO) desalination systems, basic parameters, energy demand of RO process and also exergy analysis. In order to identify the main energy losses in desalination processes, to improve their performances from a thermodynamic point of view, there is a need for exergy analysis. The exergy analysis or second law analysis involves a comparison of exergy input and exergy destruction along various desalination processes. Using the exergy analysis methodology, presented in the current paper, the specific exergy and exergy flow rates at various points of a RO system can be evaluated. Once exergy flow rates are available, exergy destroyed within any component can be determined from exergy balance.

Exergetic Analysis of a Novel Solar Cooling System for Combined Cycle Power Plants

This paper presents a detailed exergetic analysis of a novel high-temperature Solar Assisted Combined Cycle (SACC) power plant. The system includes a solar field consisting of innovative high-temperature flat plate evacuated solar thermal collectors, a double stage LiBr-H2O absorption chiller, pumps, heat exchangers, storage tanks, mixers, diverters, controllers and a simple single-pressure Combined Cycle (CC) power plant. Here, a high temperature solar cooling system is coupled with a conventional combined cycle, in order to pre-cool gas turbine inlet air in order to enhance system efficiency and electrical capacity. In this paper, the system is analyzed from an exergetic point of view, on the basis of an energy-economic model presented in a recent work, where the obtained main results show that SACC exhibits a higher electrical production and efficiency with respect to the conventional CC. The system performance is evaluated by a dynamic simulation, where detailed simulation models are implemented for all the components included in the system. In addition, for all the components and for the system as whole, energy and exergy balances are implemented in order to calculate the magnitude of the irreversibilities within the system. In fact, exergy analysis is used in order to assess: exergy destructions and exergetic efficiencies. Such parameters are used in order to evaluate the magnitude of the irreversibilities in the system and to identify the sources of such irreversibilities. Exergetic efficiencies and exergy destructions are dynamically calculated for the 1-year operation of the system. Similarly, exergetic results are also integrated on weekly and yearly bases in order to evaluate the corresponding irreversibilities. The results showed that the components of the Joule cycle (combustor, turbine and compressor) are the major sources of irreversibilities. System overall exergetic efficiency was around 48%. Average weekly solar collector exergetic efficiency ranged from 6.5% to 14.5%, significantly increasing during the summer season. Conversely, absorption chiller exergy efficiency varies from 7.7% to 20.2%, being higher during the winter season. Combustor exergy efficiency is stably close to 68%, whereas the exergy efficiencies of the remaining components are higher than 80%.

A model of human thermal comfort for analysing the energy performance of buildings

Despite numerous studies devoted to a comprehensive analysis of buildings as complex energy systems and, in particular, human thermal comfort, an attempt to combine these two aspects to solve the problems of energy conservation and comfort has been made for the first time. The designed comfort model serves to determine the structure of the human body exergy balance and calculate the minimum exergy consumption and comfortable air temperature. The paper presents a structure of the exergy balance in winter and summer, the dependence of the human body exergy consumption on the mean radiant temperature and room air temperature in winter. We have analyzed various models of human thermal comfort and devised a model of a comprehensive analysis of the system “heat source – human being – building envelope”. We have calculated the standard deviation of comfortable room air temperature for different values of the mean radiant temperature, for the exergy model of thermal comfort and the model for which the Predicted Mean Vote (PMV) index of human sensation equals to zero. It is found that the standard deviation equals to 1.4 °C. Using thermal comfort models in a comprehensive analysis allows constructing buildings with low energy or exergy consumption and with high­quality thermal comfort.

A comparative study of two SOFC based cogeneration systems fed by municipal solid waste by means of either the gasifier or digester

Two new cogeneration systems (producing power and heating) based on solid oxide fuel cell fed by either the syngas or biogas are proposed. The performance of systems is analyzed and compared with one another from the thermodynamic and economic viewpoints. Applying the conservation of mass and energy as well as the exergy and cost balance equations for each system component and using the engineering equation solver software, the systems are modeled. Through a parametric study, effects of some key variables such as the current density and the stack temperature difference on the systems' performance are investigated. It is found that for power generation, digester based solid oxide fuel cell shows better performance (with a first law efficiency of 40.14% vs 20.31%); however, considering the combined power and heating system, the total efficiency is obtained 51.05% and 58.75% for digester and gasifier based systems, respectively. In addition, it is found that the digester based SOFC is more cost-efficient and has 54% less unit product cost compared to that of the gasifier based system.

Exergy-based sustainability assessment of continuous photobiological hydrogen production using anaerobic bacterium Rhodospirillum rubrum

The present research work was aimed at analyzing a continuous photobioreactor for hydrogen production from syngas via Rhodospirillum rubrum from exergetic point of view. Exergy balance equation was developed for the investigated photobioreactor based on both conventional exergy and eco-exergy concepts in order to determine its exergetic performance parameters. Two new exergetic criteria called the process exergetic efficiency and normalized exergy destruction were also introduced in order to maximize biohydrogen production and minimize resources destruction. The results obtained showed that the process exergy efficiency, a decision making parameter, varied between 14.71–22.90% and 14.71–22.84% using the conventional exergy and eco-exergy approaches, respectively. The minimum normalized exergy destruction was found to be 0.97 using both approaches. In general, no noticeable changes were observed in the conventional exergetic and eco-exergetic performance parameters of the bioreactor over 540 h of continuous operation. Nevertheless, eco-exergetic analysis owing to the inclusion of the work of information embedded in the genomes of living organisms is still recommended for improving the design features of photobioreactors for hydrogen production. The outcomes of the present study could provide practical insights to engineers and researchers dealing with analysis, design, development, optimization, and improvement of large-scale photobioreactors for hydrogen production.

Exergoeconomic evaluation and optimization of a novel combined augmented Kalina cycle/gas turbine-modular helium reactor

A new combined system including Gas Turbine-Modular Helium Reactor (GT-MHR) and an augmented Kalina cycle (AKC) is proposed, analyzed and optimized thermodynamically and economically. The simulation is performed using the conservation of energy, exergy balance and cost equations for each system component. For comparison purposes the previously published data for the combined cycle consisting of the GT-MHR and a conventional Kalina cycle (GT-MHR/KCS34), are also presented. Parametric studies are carried out to show the influences on exergy efficiency and total product unit cost of such decision parameters as compressor pressure ratio, pump pressure ratio, ammonia concentrations at different state points and separator temperature. The results indicate that the maximum exergy efficiency of the proposed system is 8.7% and 0.64% higher compared to the corresponding values for the GT-MHR and GT-MHR/KCS34, respectively. The results also show that the minimum total product unit cost for GT-MHR/AKC is 11.3% and 2.53% lower than the corresponding values for the GT-MHR and GT-MHR/KCS34, respectively. It is observed that, under optimized condition, the helium mass flow rate in GT-MHR is reduced as the system is combined with the AKC. This is significant in reducing the size of system and consequently having more economically efficient system.