Exergy Balance Research Articles

Computer-aided exergy analysis of microalgal biodiesel production using Chlorella vulgaris

Background: Exergy analysis has been recognized as a feasible approach to evaluate and improve industrial processes by identifying major irreversibilities in a system. Objectives: This work attempts to apply exergy analysis to a third-generation biodiesel production from Chlorella vulgaris microalgae. Methods/Analysis: Commercial industrial process simulation software was used to simulate this process. The specific exergy of many substances were found in literature and the others were calculated using Szargut, Morris & Steward’s equation. A global exergy balance around the system was carried out in order to determine total irreversibilities. The contribution of unit operations and equipment to total irreversibilities was also considered. In addition, exergy efficiency and exergy emission were calculated for each stage (pretreatment, reaction, separation, biodiesel purification, and glycerol treatment). Findings: The global exergy efficiency was calculated in 86% similar to the results reported in other researches. The equipment that contributes the most to total irreversibility was the separation column used to remove alcohol with 487.55 kJ/kg BD. In addition, the highest irreversibilities (5.22 MJ/kg BD) and exergy emission (2.71 MJ/ kg BD) per stage were reached during biodiesel purification. Novelty/Improvement: The application of exergy analysis allowed to identify potential improvements in this case of study, mainly in biodiesel purification stage and process modifications are suggested to reduce total irreversibilities as reutilizing methanol and glycerol streams. Keywords: Biodiesel, Exergy Analysis, Microalgae, Process Simulation, CAPE

A comparative advanced exergy analysis for a solid oxide fuel cell using the engineering and modified hybrid methods

A modified form of hybrid method for advanced exergy analysis is introduced and applied to an anode gas recirculation solid oxide fuel cell system. The results are compared with the corresponding values achieved from applying the well-known engineering method of advanced exergy analysis to the system. The modified hybrid method proved to be accurate and less time consuming and also, in contrast with the hybrid method introduced in literature, doesn’t require the violation of the conservation of mass or exergy balance. In addition the modified hybrid method is more appropriate for parametric studies and optimization of energy conversion systems. The results obtained from both the advanced exergy methods are found to agree with one another and differ from those obtained from conventional exergy analysis. The values of endogenous and unavoidable endogenous exergy destruction rates for modified hybrid method are approximately 4% higher than the corresponding values obtained by the engineering method. The unavoidable conditions required for the analyses in both methods are obtained by a micro-structure analysis showing that the energy and exergy efficiencies of the system, under unavoidable conditions can be higher by up to 26%, 24.8%, respectively, compared to the corresponding values under the real conditions.As the highest avoidable endogenous exergy destruction rates occur in the inverter, 6.6 kW, and the stack, 3.7 kW, more attention should be paid to these components when the system performance is to be optimized. A different order, however, is achieved by applying the conventional exergy analysis.

Energetic and Exergetic Optimization of a Combined Cycle Power Plant with Dual-Pressure HRSG, Compressed Air Cooling, Steam Injection and Vapor Extraction Systems

An energetic and exergetic optimization is conducted on a combined cycle power plant of net power of 400 MW. This power plant is equipped with dual-pressure heat recovery steam generator, compressed air cooling, steam injection and vapor extraction systems. Energy and exergy balances are established on the different components of the cycle. The selected objective function is to increase the energy efficiency and reduce the exergy losses. A code is established using the Engineering Equation Solver software in order to perform the required calculations. The effects of the main operating conditions, such as ambient temperature, pressure ratio, air excess ratio, steam injection ratio and extracted steam pressure, on the power plant performances are analyzed taking into consideration the local environmental conditions. Obtained results show that the ambient temperature affects significantly the overall thermal efficiency of the combined cycle. The maximum energy efficiency is obtained for an optimal pressure ratio of about 14, medium and low pressure of 45 and 4 bars, respectively. The air excess and the steam injection ratios should be carefully defined to better the control of cycle performances. The combustion chamber is a major contributor of exergy destruction followed by the heat recovery steam generator. Compared to a standard combined cycle, the selected design characteristics have enabled the overall energy and exergy efficiencies to be enhanced about 11.7 and 10%, respectively.

A novel solar-biomass based multi-generation energy system including water desalination and liquefaction of natural gas system: Thermodynamic and thermoeconomic optimization

Abstract In this study, an inventive multi-generation energy system utilizing solar and biomass energy as a complementary fuel are proposed and analyzed, by means of a thermodynamic and thermoeconomic investigation and multi-objective optimization. For supplying electricity, heating and cooling power, a Rankine cycle including a turbine, a heater and a double effect absorption chiller, for liquefaction of natural gas, a Linde-Hampson cycle, for desalination of sea water, a multi-effect desalination system, for solar energy exploitation, a parabolic Trough solar collector and for combustion of biomass, a burner is utilized. Results outline that, the studied system has potential to generate 16.11 kW electricity, 28.94 kW heating power, 23.41 kW cooling power, 8.8 kg/h fresh water and 0.02 m3/h liquefied natural gas with the energy and exergy efficiencies of 46.8%, 11.2%, and product cost rate of 15.16 $/h. A comprehensive modeling is accomplished by applying mass, energy, exergy and thermoeconomic balances to all component of the multi-generation energy system. The investigation becomes more comprehensive with a sensitivity analysis in order to survey the dependency of the thermodynamic and thermoeconomic performance upon the decision variables such as stack gasses temperature, temperature difference of evaporator1, evaporator2 temperature and Turbine inlet temperature. Finally, optimized performance of the system is determined using Genetic Algorithm and deriving Pareto front considering the exergy efficiency and product cost rate as objective functions. The optimized multi-generation energy system could yield the exergy efficiency of 9.9% and the product cost rate of 13.32 $/h.

Energy and exergy analyses of a mass-fired boiler for a proposed waste-to-energy power plant in Tehran

Thermodynamic analysis of a municipal solid waste-to-energy boiler was thoroughly conducted. For this purpose, mass, energy, and exergy balance equations were formed over the entire boiler and its components. This study aimed to report the first and second law efficiencies to identify the main sources of irreversibility within the boiler. Three districts of Tehran mega city were selected, and the fictive molecule C26.75H3.58O15.79N1.08S0.11 was obtained from the average chemical composition of the aggregated waste. Moreover, the zonal method was applied to design the furnace, following by a simpler approach to design heat recovery steam generator. The energy and exergy analysis was then performed for all components and the entire boiler, followed by a sensitivity analysis on the blow-down water ratio effects. Results indicated that the irreversibility in furnace, with 52,617.3 kJs-1, was the main source of the exergy destruction. In addition, the overall energetic and exergetic efficiency of the studied boiler were reported as 78.7 and 16.0%, respectively.It appears that as a consequence of relatively small lower heating value of the utilized waste and considering the heat loss due to the bottom ash and blow-down water, a considerable reduction in the boiler efficiency occurred.

Exergoeconomic and exergoenvironmental analysis of an integrated solar gas turbine/combined cycle power plant

Integration of solar power to Combined Cycle Power Plants is a solution attracting increasing interest, bridging solar thermal technology to a well-proven energy conversion solution. The integration is attractive for countries aiming to pass to natural gas as an energy feedstock and it could improve the environmental performance. In order to identify the performance and potential environmental benefits, a model of the plant was applied. It covered an annual operation period and included the effects of surroundings variables. The model allows to predict the power plant performance, and calculates a complete exergy balance for all the components of the complex plant. The calculations are repeated for referential CCGT and for the Integrated Solar CCGT.A complete exergoeconomic and exergoenvironmental model was applied at the design conditions after evaluating the cost of equipment and their environmental score using a detailed Life Cycle Assessment (LCA) modelling tool. The results, applied to a power plant in Southern Poland, show that the solution can be attractive for improving the environmental performance of a CCGT (CO2 emission factor decreased by 9%), and that the capital cost is only slightly increased so that the rate of return of the investment is only marginally affected.

Exergy analysis and performance of a tug boat power generator using kerosene fuel blended with aspire methly ester

Diesel engine is most commonly seen in applications demanding a large power output, such as ships and electric generation plants. Exergy analysis has been used in the design technology such as simulation and performance assessment of various types of engines, for identifying parameters. In this case; the energy and exergy analyses of an eight-cylinder, four-stroke power generator using the kerosene fuels with blended 20/50 aspire methly ester (AME) have been performed. 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. Conservation of energy equation and the exergy balance for each test steps were determined using measured data. Finally, various energetic and exergetic performance parameters of the engine were evaluated and compared with each other. CO emissions were dropped when running on AME and its blends. The AME was found to have the highest exhaust temperature due to its oxygen content 11.2% higher than marine fuel. The maximum NOx emission was indicated 100% AME blend. However, the NOx emissions were higher, using marine blends.

Thermodynamic assessment of solar photon-enhanced thermionic conversion

Photon-enhanced thermionic conversion, an innovative solar power technology, combines photovoltaic and thermionic effects into a single process, and has the potential to surpass the Shockley–Queisser limit and conventional photo-thermal limit. However, there is little understanding about the energy conversion process from a thermodynamic point of view. A detailed thermodynamic model is proposed, encompassing energy and exergy balance, and entropy analysis to evaluate a process for solar photon-enhanced thermionic conversion. The correlation of photons, phonons and electrons is presented, as well as the energy transfer pathway in solar thermionic conversion. The total solar-to-electricity efficiency of energy and exergy are 54.32% and 58.42%, respectively, for a photon-enhanced thermionic converter combined with a Carnot engine, at a 1.20 eV bandgap with an electron affinity of 1.20 eV when the concentrated solar flux is 500 kW/m2. The combination of photoexcitation and thermalization facilitates the overall thermionic emission exergy ratio up to 62.36%, higher than that of conventional thermionic conversion by 10.92%. Temperature-entropy diagrams with quantitative analysis are proposed for the thermodynamic processes of thermionic and photon-enhanced thermionic conversion. The electron fluid cycles from the Fermi level of the anode back to the valance band of the cathode with a reduced entropy, while being thermalized from the conduction band in photon-enhanced thermionic conversion, contributing to the entire conversion of photoexcited energy to electricity.

Exergy Analyses of Onion Drying by Convection: Influence of Dryer Parameters on Performance.

This research work is concerned in the exergy analysis of the continuous-convection drying of onion. The influence of temperature and air velocity was studied in terms of exergy parameters. The energy and exergy balances were carried out taking into account the onion drying chamber. Its behavior was analyzed based on exergy efficiency, exergy loss rate, exergetic improvement potential rate, and sustainability index. The exergy loss rates increase with the temperature and air velocity augmentation. Exergy loss rate is influenced by the drying air temperatures and velocities because the overall heat transfer coefficient varies with these operation conditions. On the other hand, the exergy efficiency increases with the air velocity augmentation. This behavior is due to the energy utilization was improved because the most amount of supplied energy was utilized for the moisture evaporation. However, the exergy efficiency decreases with the temperature augmentation due to the free moisture being lower, then, the moisture begins diffusing from the internal structure to the surface. The exergetic improvement potential rate values show that the exergy efficiency of onion drying process can be ameliorated. The sustainability index of the drying chamber varied from 1.9 to 5.1. To reduce the process environmental impact, the parameters must be modified in order to ameliorate the exergy efficiency of the process.

Studies on the Exergy Transfer Law for the Irreversible Process in the Waxy Crude Oil Pipeline Transportation.

With the increasing demand of oil products in China, the energy consumption of pipeline operation will continue to rise greatly, as well as the cost of oil transportation. In the field of practical engineering, saving energy, reducing energy consumption and adapting to the international oil situation are the development trends and represent difficult problems. Based on the basic principle of non-equilibrium thermodynamics, this paper derives the field equilibrium equations of non-equilibrium thermodynamic process for pipeline transportation. To seek the bilinear form of “force” and “flow” in the non-equilibrium thermodynamics of entropy generation rate, the oil pipeline exergy balance equation and the exergy transfer pipeline dynamic equation of the irreversibility were established. The exergy balance equation was applied to energy balance evaluation system, which makes the system more perfect. The exergy flow transfer law of the waxy oil pipeline were explored deeply from the directions of dynamic exergy, pressure exergy, thermal exergy and diffusion exergy. Taking an oil pipeline as an example, the influence factors of exergy transfer coefficient and exergy flow density were analyzed separately.

Exergy and exergoeconomic analyses with modeling for CO2 allocation of coal-fired CHP plants

The coal-fired power plant is confronted with severe challenges due to increasing coal price and negative impact on climate change. In this study, exergy and exergoeconomic analyses of a 330 MW coal-fired combined heat and power (CHP) plant are performed to determine the unit exergoeconomic cost and specific CO2 emission of the final products. The cycle is simulated by Aspen Plus software and thermodynamic properties under different conditions are extracted. Exergy balance equations are formed based on the exergy flow of coal, water/steam, ash, and flue gases. Cost formation process has been revealed by the exergoeconomic analysis combining the recent development of residue cost allocation. Different from previous studies, cost of flue gas cleaning is taken into account as a part of the unit exergoeconomic cost of the products. Moreover, CO2 allocation is conducted based on the exergoeconomic model and has been compared with other methods. Results show that the largest exergy destruction occurs in the boiler and the lowest exergy efficiency occurs in the low-pressure heater. There is an energy-saving potential of the residue exergy which accounts for 7.5% of the exergoeconomic cost in heating period and 10.4% in non-heating period.

Exergoeconomic optimization of a double effect desalination unit used in an industrial steam power plant

An exergoeconomic optimization of a double effect thermal vapor compression desalination system is performed. This unit is integrated in the thermal power plant of Phosphoric Acid factory owned by The Tunisian Chemical Group. The daily production of this unit is about 528 m3 of fresh water. A mathematical model based on energy, exergy and economic balances is established using EES software. Multi-objective optimization is carried out in the purpose to obtain the maximum exergy efficiency with minimum production cost. The effects of the main operating parameters on the desalination unit performances are analyzed.Obtained results show that, the maximum exergy efficiency is obtained for the evaporator II followed by the distillate and brine pumps. The condenser has the lowest exergy efficiency.The thermocompressor is the major contributor in exergy destruction. The mass flow rate and the pressure of the motive steam affect positively the GOR. The SPECO method is used to perform the economic optimization. The maximum exergoeconomic factor f is obtained for the evaporator II followed by the evaporator I, the thermocompressor, the distillate pump and the condenser. The optimum operating conditions leading to suitable overall exergy efficiency with low production cost of distillate water are determined.

ЕФЕКТИВНІСТЬ ПЛАСТИНЧАТИХ ТЕПЛОУТИЛІЗАТОРІВ ТЕПЛОУТИЛІЗАЦІЙНИХ СИСТЕМ

Розроблено методику розрахунку втрат ексергетичної потужності у процесах теплопровідності під час передачі теплоти через поперечний переріз пластини газоповітряного пластинчастого теплоутилізатора за граничних умов третього роду. Методику засновано на комплексному підході, що поєднує ексергетичні методи з методами термодинаміки незворотних процесів. Математична модель досліджуваних процесів включає рівняння ексергії, рівняння балансу ексергії та ентропії, рівняння нерозривності трифазної термодинамічної системи при зміні концентрації однієї з фаз, рівняння руху фаз, рівняння енергій, рівняння балансу ентальпій, рівняння Гіббса і рівняння теплопровідності за граничних умов третього роду. Для отримання формул для розрахунку втрат ексергетичної потужності використано локальне диференціальне рівняння балансу ексергії. У цьому рівнянні одна зі складових визначає втрати ексергетичної потужності, зумовлені незворотністю процесів і пов'язані з теплопровідністю, в'язкістю фаз, міжфазним теплообміном і тертям між фазами. На підставі цього рівняння і рішення рівняння теплопровідності за граничних умов третього роду для необмеженої пластини, якою моделювалася пластина газоповітряного пластинчастого теплоутилізатора, отримано формули для розрахунку втрат ексергетичної потужності. Виконано розрахунки загальних втрат ексергетичної потужності в газоповітряному пластинчастому теплоутилізаторі за різних режимів роботи котла і втрат ексергетичної потужності у процесах теплопровідності. Встановлено, що втрати ексергетичної потужності у процесах теплопровідності в газоповітряному пластинчатому теплоутилізаторі становлять 8,6-11,6 % від загальних втрат ексергетичної потужності і залежать від режиму роботи котла.

Exergy analysis of the operation of a solar dryer

The study considers the exergy analysis of rational ways to use and store thermal energy in a solar dryer in order to increase the exergy efficiency of the technological process of fruit drying at individual farms. We analyzed the application of solar energy for the technological process of fruit drying in individual farms based on an exergy analysis of a solar dryer operation. We carried out full-time tests of a solar dryer at the latitude of the location of Rivne oblast (Ukraine), which has an average annual solar power of 3.41 kW·h/m 2 per day. Therefore, this makes it possible to obtain from 74.8 to 123.1 MJ of solar energy per day from 1 m 2 of an air collector area. We improved the procedure of an exergy analysis of operation of a solar dryer, which is based on the simplified mechanism of calculation of an exergy balance of a plant depending on physical parameters of the environment relative specific territory of a farm. The performed complex of calculation-quantitative studies confirmed the possibility of significant intensification of fruit drying process in a solar dryer. We established that heat losses during fruit drying in a solar dryer make up 3...3.7 MJ/kg, and the degree of intensification increases by 3.3...12 times. The exergy efficiency of a solar dryer without the use of a thermal battery is 87.6 %; with it – 89.8 % with the use of a thermal battery. The exergy efficiency of a solar dryer increases by 1.02 times. We achieved such results by using a flat mirror concentrator and a thermal battery in a solar dryer. An applied aspect of using the results obtained is the possibility to increase the energy efficiency of fruit drying process based on exergy express-analysis of the efficiency of a solar dryer. It will reduce the cost of energy resources by using solar energy. It creates a prerequisite for the transfer of technological solutions for justification of exergy balances for various characteristics of fruit drying process in a solar dryer.

Exergy analysis of a lignocellulosic-based biorefinery annexed to a sugarcane mill for simultaneous lactic acid and electricity production

This study was aimed at exergetically analyzing a lignocellulosic biorefinery annexed to a sugar mill for simultaneous lactic acid and electricity production using sugarcane bagasse and brown leaves as feedstock. The main goal was to reveal the reasons and sources of thermodynamic inefficiencies associated with the biorefinery under investigation. After presenting the exergy balance equations for all the subunits of the biorefinery, their exergetic performance parameters were computed and consequently exergetic performance parameters were measured for the whole system. The exergetic values of the net electricity and steam were found to be 20.66 MW and 50.41 MW, respectively, while the chemical exergy content of the produced lactic acid was determined at 112.84 MW. The overall exergy destruction of the system stood at 207.69 MW. The contribution of the steam generation, as the main subunit wasting exergy, to the overall exergy destruction of the system was found to be 63.31%. Furthermore, lactic acid production subunit stood in the next rank in terms of exergy destruction, contributing 16.30% of the overall exergy destruction of the biorefinery. The universal and functional exergetic efficiencies of the system were determined as 52.71% and 44.73%, respectively, while the normalized exergy destruction was calculated as 1.13.

Application of exergetic methods in the analysis of different purifying techniques

This paper provides the rationale for applying an exergetic method when evaluating the thermodynamics of column-rectification apparatuses, while reviewing the operating profiles of typical purifying columns and purifying columns with stripping modules operating under atmospheric pressure, vacuum, and with mechanical vapour recompression. Graphical representations of the purifying column's exergy balance are provided as Grassmann-Szargut diagrams. The exergy balance data showed that the total value of exergy loss is comparable between conventional purifying columns and those equipped with a stripping module (30.94% and 30.78% efficiency, respectively); moreover, exergy loss is lower in systems where the components operate under a vacuum (31.33% efficiency). For purifying columns that operate using mechanical vapour recompression, their exergetic efficiency is highest (32.6%), which indicates that the thermodynamic precision of system being studied increases when a heat pump is used as the primary source of high-grade energy supply in the column apparatus.

Development of an exergy diagram for integrated energy systems illustrating the irreversibility along energy paths

This paper tackles the development of an exergy diagram, adapted from the heat (Q)-Carnot factor (θ) one, where energy quality factors are given in the ordinate axis while energy terms constitute the abscise axis. Energy and exergy balances are then shown and the nature of irreversibility sources (energy loss, energy potential degradation) is revealed, contrary to traditional Grassman diagrams. In this paper, the graphical construction is extended to integrated energy systems where energy paths are differentiated according to renewable and non-renewable sources. Thus, it permits the electricity use, which is really paid, to be emphasised, and the overall behaviour to be better understood. As well, the diagram is developed to illustrate the results of dynamic analyses by considering environment reference state variations. To this aim, the graphical construction has been applied to individual units and then, step by step, to a solar assisted heat pump using ice storage.

Application of exergetic methods in the analysis of different purifying techniques

This paper provides the rationale for applying an exergetic method when evaluating the thermodynamics of column-rectification apparatuses, while reviewing the operating profiles of typical purifying columns and purifying columns with stripping modules operating under atmospheric pressure, vacuum, and with mechanical vapour recompression. Graphical representations of the purifying column's exergy balance are provided as Grassmann-Szargut diagrams. The exergy balance data showed that the total value of exergy loss is comparable between conventional purifying columns and those equipped with a stripping module (30.94% and 30.78% efficiency, respectively); moreover, exergy loss is lower in systems where the components operate under a vacuum (31.33% efficiency). For purifying columns that operate using mechanical vapour recompression, their exergetic efficiency is highest (32.6%), which indicates that the thermodynamic precision of system being studied increases when a heat pump is used as the primary source of high-grade energy supply in the column apparatus.

Development of an exergy diagram for integrated energy systems illustrating the irreversibility along energy paths

This paper tackles the development of an exergy diagram, adapted from the heat (Q)-Carnot factor (θ) one, where energy quality factors are given in the ordinate axis while energy terms constitute the abscise axis. Energy and exergy balances are then shown and the nature of irreversibility sources (energy loss, energy potential degradation) is revealed, contrary to traditional Grassman diagrams. In this paper, the graphical construction is extended to integrated energy systems where energy paths are differentiated according to renewable and non-renewable sources. Thus, it permits the electricity use, which is really paid, to be emphasised, and the overall behaviour to be better understood. As well, the diagram is developed to illustrate the results of dynamic analyses by considering environment reference state variations. To this aim, the graphical construction has been applied to individual units and then, step by step, to a solar assisted heat pump using ice storage.

One-dimensional exergy analysis of an unglazed low-cost PhotoVoltaic/Thermal solar collector

In this paper, a one-dimensional finite-volume model of a low-cost unglazed flat-plate PhotoVoltaic/Thermal (PVT) solar collector is presented. Mass, energy and exergy balances are performed for each element of the computational domain, discretised along the flow direction. Experimental and numerical data are compared. Exergy destruction rate and exergetic efficiency are evaluated for each element of the computational domain. In addition, the effect of the main design/operation parameters on the collector exergetic performance is also investigated. The comparison between experimental and numerical data in terms of PVT outlet temperature shows an absolute error and a standard deviation of -1.06°C and 0.628°C, respectively. The calculated exergetic efficiency varies between 0.134 and 0.165 during the experimental measurements. For lower flow rates, the relationship between the total exergetic residual and inlet temperature is almost linear.