An exergetically-sustainable operational condition of a photo-biohydrogen production system optimized using conventional and innovative fuzzy techniques

The aim of the present study was to perform an exergy-based multi-objective fuzzy optimization of a continuous photobioreactor applied for biohydrogen production from syngas via the water-gas shift reaction by Rhodospirillum rubrum. For this purpose, the conventional and innovative fuzzy optimization techniques coupled with multilayer perceptron (MLP) neural model to optimize the main exergetic performance parameters of the photobioreactor. The MLP neural model was applied to correlate three dependent variables (rational and process exergy efficiencies and normalized exergy destruction) with two independent variables (syngas flow rate and agitation speed). The developed MLP model was then interfaced with three different multi-objective fuzzy optimization systems with independent, interdependent, and locally modified interdependent objectives. The optimization process was aimed at maximizing the rational exergy and process efficiencies, while minimizing the normalized exergy destruction, simultaneously. Generally, the innovative locally modified interdependent objectives fuzzy system showed a better optimization capabilities compared with the other two fuzzy systems. Accordingly, the optimal syngas photo-fermentation for biohydrogen production in the continuous bioreactor corresponded to the agitation speed of 383.34 rpm and syngas flow rate of 13.35 mL/min in order to achieve the normalized exergy destruction of 1.56, rational exergy efficiency of 85.65%, and process exergy efficiency of 21.66%.     

Multi-objective optimization of a joule cycle for re-liquefaction of the Liquefied Natural Gas

A LNG re-liquefaction plant is optimized with a multi-objective approach which simultaneously considers exergetic and exergoeconomic objectives. In this regard, optimization is performed in order to maximize the exergetic efficiency of plant and minimize the unit cost of the system product (refrigeration effect), simultaneously. Thermodynamic modeling is performed based on energy and exergy analyses, while an exergoeconomic model based on the total revenue requirement (TRR) are developed. Optimization programming in MATLAB is performed using one of the most powerful and robust multi-objective optimization algorithms namely NSGA-II. This approach which is based on the Genetic Algorithm is applied to find a set of Pareto optimal solutions. Pareto optimal frontier is obtained and a final optimal solution is selected in a decision-making process. An example of decision-making process for selection of the final solution from the available optimal points of the Pareto frontier is presented here. The feature of selected final optimal system is compared with corresponding features of the base case and exergoeconomic single-objective optimized systems and discussed.     

Implementing of the multi-objective particle swarm optimizer and fuzzy decision-maker in exergetic, exergoeconomic and environmental optimization of a benchmark cogeneration system

Multi-objective optimization for design of a benchmark cogeneration system namely as the CGAM cogeneration system is performed. In optimization approach, Exergetic, Exergoeconomic and Environmental objectives are considered, simultaneously. In this regard, the set of Pareto optimal solutions known as the Pareto frontier is obtained using the MOPSO (multi-objective particle swarm optimizer). The exergetic efficiency as an exergetic objective is maximized while the unit cost of the system product and the cost of the environmental impact respectively as exergoeconomic and environmental objectives are minimized. Economic model which is utilized in the exergoeconomic analysis is built based on both simple model (used in original researches of the CGAM system) and the comprehensive modeling namely as TTR (total revenue requirement) method (used in sophisticated exergoeconomic analysis). Finally, a final optimal solution from optimal set of the Pareto frontier is selected using a fuzzy decision-making process based on the Bellman-Zadeh approach and results are compared with corresponding results obtained in a traditional decision-making process. Further, results are compared with the corresponding performance of the base case CGAM system and optimal designs of previous works and discussed.     

Performance assessment of a biogas fuelled molten carbonate fuel cell-thermophotovoltaic cell-thermally regenerative electrochemical cycle-absorption refrigerator-alkaline electrolyzer for multigenerational applications

In recent years, there has been increasing interest in fuel cell hybrid systems. In this paper, a novel multi-generation combined energy system is proposed. The system consists of a molten carbonate fuel cell (MCFC), a thermally regenerative electro-chemical cycle (TREC), a thermo photovoltaic cell (TPV), an alkaline electrolyzer (AE) and an absorption refrigerator (AR). It has four useful outputs, namely electricity, hydrogen, cooling and heating. The overall system is thermodynamically modeled in a detailed manner while its simulation and modeling are done through the TRNSYS software tool. Power output, cooling-heating and produced hydrogen rates are determined using energetic and exergetic analysis methods. Results are obtained numerically and plotted. The maximum power output from the system is 16.14 kW while maximum energy efficiency and exergy efficiency are 86.8% and 80.4%,. The largest exergy destruction is due to the MCFC.     

Exergetic performance coefficient analysis of a simple fuel cell system

In this paper, a fuel cell power generation system fed by hydrogen is analyzed by different performance criteria over the entire range of potential operating conditions. First law efficiency and net power output are considered for conventional energetic indices of performance, and exergy destruction rate is taken into consideration as an exergetic performance criteria. A new exergetic criterion called the exergetic performance coefficient (EPC) is introduced and is applied to the system model based on zero-dimensional approach. The system model consists of the following components: fuel cell stack, afterburner, fuel and air compressors, and heat exchangers. The effects of the operating conditions on the system performance are studied parametrically. The obtained results based on the exergetic performance coefficient criterion are compared with first law efficiency, power output and exergy destruction rate. Results show that design insights of fuel cell systems can be considerably improved when conventional energetic analyses are supplemented with EPC criterion.     

Multi-criteria design optimization and thermodynamic analysis of a novel multi-generation energy system for hydrogen,cooling, heating,power, and freshwater

The main focus of this paper is to present thermodynamic and economic analyses and multi-objective optimization of a novel geothermal-solar multigeneration system. The system aims to produce hydrogen, freshwater, electricity, cooling load, and hot water and designed based on geothermal and solar energy. After modeling and thermodynamic and economic analysis, exergy destruction rate, exergy efficiency and, cost rate were calculated for each component of the system. The results showed that the highest amount of exergy destruction was related to parabolic trough collectors (PTCs) and absorption chillers. To select the geothermal fluid of the organic Rankine cycle (ORC), several different fluids were investigated, among which isobutene was selected. By using the Group method of data handling (GMDH) neural network, a mathematical relationship was obtained between the inputs and outputs of the problem and were given as inputs to the non-dominated sorting genetic algorithm II (NSGAII)alg. The final optimal point was obtained applying the technique for order of preference by similarity to ideal solution (TOPSIS) decision criterion at which the exergy efficiency and cost rate were calculated to be 21.63% and 63.89 $/h, respectively. The meteorological data of the Zanjan, Isfahan, and Bandar Abbas cities were used to calculate the performance accurately at the TOPSIS selection point. To provide a comparison between different cities, the performance of the system was evaluated on September 17 as a sample day. On this day, the proposed system produces 26.38 kg of hydrogen and 373.8 m³ of freshwater in Isfahan.     

Exergy analysis of biomass-to-synthetic natural gas (SNG) process via indirect gasification of various biomass feedstock

This paper presents an exergy analysis of SNG production via indirect gasification of various biomass feedstock, including virgin (woody) biomass as well as waste biomass (municipal solid waste and sludge). In indirect gasification heat needed for endothermic gasification reactions is produced by burning char in a separate combustion section of the gasifier and subsequently the heat is transferred to the gasification section. The advantages of indirect gasification are no syngas dilution with nitrogen and no external heat source required. The production process involves several process units, including biomass gasification, syngas cooler, cleaning and compression, methanation reactors and SNG conditioning. The process is simulated with a computer model using the flow-sheeting program Aspen Plus. The exergy analysis is performed for various operating conditions such as gasifier pressure, methanation pressure and temperature. The largest internal exergy losses occur in the gasifier followed by methanation and SNG conditioning. It is shown that exergetic efficiency of biomass-to-SNG process for woody biomass is higher than that for waste biomass. The exergetic efficiency for all biomass feedstock increases with gasification pressure, whereas the effects of methanation pressure and temperature are opposite for treated wood and waste biomass.     

Exergy analysis of Joule–Thomson cryogenic refrigeration cycle with an ejector

In this paper, exergy method is applied to analyze the ejector expansion Joule–Thomson (EJT) cryogenic refrigeration cycle. The exergy destruction rate in each component of the EJT cycle is evaluated in detail. The effect of some main parameters on the exergy destruction and exergetic efficiency of the cycle is also investigated. The most significant exergy destruction rates in the cycle are in the compressor and ejector. The ejector pressure ratio and compressor isothermal efficiency have a significant effect on the exergetic efficiency of the EJT cycle. The exergy analysis results show the EJT cycle has an obvious increase in the exergetic efficiency compared to the basic Joule–Thomson refrigeration cycle. A significant advantage from the use of the ejector is that the total exergy destruction of the EJT cycle can be reduced due to much more decreasing of the exergy destruction rates in the compressor and expansion valve. The exergy analysis also reconfirms that applying an ejector is a very important approach to improve the performance of the Joule–Thomson cryogenic refrigeration cycle.     

Analysis and performance assessment of a combined geothermal power-based hydrogen production and liquefaction system

In this paper, the thermodynamic study of a combined geothermal power-based hydrogen generation and liquefaction system is investigated for performance assessment. Because hydrogen is the energy of future, the purpose of this study is to produce hydrogen in a clear way. The results of study can be helpful for decision makers in terms of the integrated system efficiency. The presented integrated hydrogen production and liquefaction system consists of a combined geothermal power system, a PEM electrolyzer, and a hydrogen liquefaction and storage system. The exergy destruction rates, exergy destruction ratios and exergetic performance values of presented integrated system and its subsystems are determined by using the balance equations for mass, energy, entropy, energy and exergy and evaluated their performances by means of energetic and exergetic efficiencies. In this regard, the impact of some design parameters and operating conditions on the hydrogen production and liquefaction and its exergy destruction rates and exergetic performances are investigated parametrically. According to these parametric analysis results, the most influential parameter affecting system exergy efficiency is found to be geothermal source temperature in such a way that as geothermal fluid temperature increases from 130 °C to 200 °C which results in an increase of exergy efficiency from 38% to 64%. Results also show that, PEM electrolyzer temperature is more effective than reference temperature. As PEM electrolyzer temperature increases from 60 °C to 85 °C, the hydrogen production efficiency increases from nearly 39% to 44%.     

Modeling and analysis of energetic and exergetic efficiencies of a LiBr/H<Subscript>2</Subscript>0 absorption heat storage system for solar space heating in buildings

The development of efficient long-term heat storage systems could significantly increase the use of solar thermal energy for building heating. Among the different heat storage technologies, the absorption heat storage system seems promising for this application. To analyze the potential of this technology, a numerical model based on mass, species, energy, and exergy balances has been developed. The evolution over time of the storage imposes a transient approach. Simulations were performed considering temperature conditions close to those of a storage system used for space heating coupled to solar thermal collectors (as the heat source), with ground source heat exchangers (as the cold source). The transient behavior of the system was analyzed in both the charging and discharging phases. This analysis highlights the lowering of energetic and exergetic performance during both phases, and these phenomena are discussed. The thermal efficiency and the energy storage density of the system were determined, equal to 48.4 % and 263 MJ/m³, respectively. The exergetic efficiency is equal to 15.0 %, and the exergy destruction rate is 85.8 %. The key elements in terms of exergy destruction are the solution storage tank, the generator, and the absorber. The impact of using a solution heat exchanger (SHX) was studied. The risk of the solution crystallizing in the SHX was taken into account. With a SHX, the thermal efficiency of the system can reach 75 %, its storage density was 331 MJ/m³, and its exergetic efficiency and exergy destruction rate was 23.2 and 77.3 %, respectively.     

Multi-objective approach in thermoenvironomic optimization of a benchmark cogeneration system

Multi-objective optimization for designing of a benchmark cogeneration system known as CGAM cogeneration system has been performed. In optimization approach, the exergetic, economic and environmental aspects have been considered, simultaneously. The thermodynamic modeling has been implemented comprehensively while economic analysis conducted in accordance with the total revenue requirement (TRR) method. The results for the single objective thermoeconomic optimization have been compared with the previous studies in optimization of CGAM problem. In multi-objective optimization of the CGAM problem, the three objective functions including the exergetic efficiency, total levelized cost rate of the system product and the cost rate of environmental impact have been considered. The environmental impact objective function has been defined and expressed in cost terms. This objective has been integrated with the thermoeconomic objective to form a new unique objective function known as a thermoenvironomic objective function. The thermoenvironomic objective has been minimized while the exergetic objective has been maximized. One of the most suitable optimization techniques developed using a particular class of search algorithms known as multi-objective evolutionary algorithms (MOEAs) has been considered here. This approach which is developed based on the genetic algorithm has been applied to find the set of Pareto optimal solutions with respect to the aforementioned objective functions. An example of decision-making has been presented and a final optimal solution has been introduced. The sensitivity of the solutions to the interest rate and the fuel cost has been studied.     

Design,modeling and optimization of a renewable-based system for power generation and hydrogen production

Due to the environmental concerns caused by fossil fuels, renewable energy systems came into consideration. In this study, a renewable hybrid system based on ocean thermal, solar and wind energy sources were designed for power generation and hydrogen production. To analyze the system, a techno-economic model was exerted in order to calculate the exergy efficiency as well as the cost rate and the hydrogen production. The main parameters that affect the system performance were identified, and the impact of each parameter on the main outputs of the system was analyzed as well. The thermo-economic analysis showed that the most effective parameters on the exergy efficiency and total cost rate are the wind speed and solar collector area, respectively. To reach the optimum performance of the system, multi-objective optimization, by using genetic algorithm, was applied. The optimization was divided into two separate case studies; in case A, the cost rate and the exergy efficiency were considered as two objective functions; and in case B, the cost rate and the hydrogen production were assigned as two other objective functions. The optimization results of the case A showed that for the total cost rate of 30.5 $/h, the exergy efficiency could achieve 35.57%. While, the optimization of the case B showed that for the total cost rate of 28.06 $/h, the hydrogen production rate could reach 5.104 kg/h. Furthermore, after optimizing, an improvement in exergy efficiency was obtained, approximately 19%.     

The effects of hydrogen fuel usage on the exergetic performance of a turbojet engine

In this study, the hydrogen fuel effect on the exergetic performance of a turbojet engine used in a military trainer aircraft is investigated. For the first step, the performance assessments of the exergetic performance are conducted according to jet fuel usage and the actual test cell data of the engine. For the second step, an exergetic evaluation is parametrically estimated to use the hydrogen fuel in the engine. Finally, the performance results of the engine run by jet fuel are compared with the performance results of the engine run by hydrogen fuel. Regarding the results of this study, by using hydrogen fuel in the engine, the exergy efficiency of the engine decreases from 15.40% to 14.33%, while the waste exergy rate increases from 6196.51 kW to 6669.4 kW. At the same time, the exergy rate of the fuel rises from 7324.87 kW to 7785.25 kW, hence the specific fuel exergy of the hydrogen fuel is higher than that of the jet fuel. The waste exergy flow cost of the engine rises from 16.52 × 10^?3 US$/kW to 17.79 × 10^?3 US$/kW. The environmental effect factor of the engine escalates from 5.49 to 5.98 and the ecological effect factor increases from 6.49 to 6.98. On the other hand, the exergetic sustainability index of the engine reduces from 0.182 to 0.167 when the sustainable efficiency factor of the engine goes down from 1.182 to 1.167. Between the components, for both jet fuel and hydrogen fuel, the CC has the highest values of the fuel exergy waste ratio, the relative waste exergy ratio, the product exergy waste ratio, the fuel ratio indicator, the product ratio indicator, the waste exergy cost flow, the environmental effect factor, the ecological effect factor, and the exergetic improvement potential when the CC has the lowest values of the exergy efficiency, exergetic sustainability index, and sustainable efficiency factor, respectively. The reason for this result is that the combustion process contains high irreversibities. The obtained results indicate that the hydrogen fuel usage in the turbojet engine badly affects the exergetic performance of the engine and its components (especially the combustion chamber) hence the specific exergy of the hydrogen fuel is higher than the jet fuel's. On the other hand, the exhaust emissions emitted to the environment decrease from 0.509 kg/s to 0.0045 kg/s with the hydrogen fuel usage.     

Energy,exergy, and exergoeconomics (3E) analysis and multi-objective optimization of a multi-generation energy system for day and night time power generation - Case study: Dezful city

The present study aimed to investigate a multi-generation energy system for the production of hydrogen, freshwater, electricity, cooling, heating, and hot water. Steam Rankine cycle (SRC), organic Rankine cycle (ORC), absorption chiller, Parabolic trough collectors (PTCs), geothermal well, proton exchange membrane (PEM) electrolyzer, and reverse osmosis (RO) desalination are the main subsystems of the cycle. The amount of exergy destruction is calculated for each component after modeling and thermodynamic analysis. The PTCs, absorption chiller, and PEM electrolyzer had the highest exergy destruction, respectively. According to meteorological data, the system was annually and hourly tested for Dezful City. For instance, it had a production capacity of 13.25 kg/day of hydrogen and 147.42 m³/day of freshwater on 17th September. Five design parameters are considered for multi-objective optimization after investigating objective functions, including cost rate and exergy efficiency. Using a Group method of data handling (GMDH), a mathematical relation is obtained between the input and output of the system. Next, a multi-objective optimization algorithm, a non-dominated sorting genetic algorithm (NSGA-II), was used to optimize the relations. A Pareto frontier with a set of optimal points is obtained after the optimization. In the Pareto frontier, the best point is selected by the decision criterion of TOPSIS. At the TOPSIS point, the exergy efficiency is 31.66%, and the total unit cost rate is 21.9 $/GJ.     

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.     

Exergy analysis of glycerol steam reforming in a heat recovery reactor

A detailed exergy analysis was performed for the steam reforming process of glycerol by means of a series of experiments in a bench scale apparatus. The reforming was conducted in a fixed bed reactor, which operated in heat recovery mode by extracting the demanded energy from hot exhaust gases provided by a diesel engine. In order to determine the role of the main operational parameters into the exergy efficiency of the studied process, the experiments were carried out with glycerol feed concentrations in water ranging from 10% up to 90% weight, whereas the outlet reactor temperature was varied from 600 °C up to 800 °C. Detailed exergy balances revealed a compromise between the exergy destruction within the reforming reactor and liquid separator versus the exergy losses associated to the tar and char outputs. This trade-off was favourable to the 50% and 70% glycerol feed concentration regimes and plateaus of about 74% exergy efficiency and 24 MJ/kg dry syngas exergy content were verified from 650 to 800 °C reactor temperatures.     

Thermodynamic and electrochemical analyses of a solid oxide electrolyzer for hydrogen production

In this paper, a modeling of the Solid Oxide Electrolysis Cell (SOEC), through energetic, exergetic and electrochemical modeling approaches, is conducted, and its performance, particularly through exergy efficiency, is analyzed under various operating conditions and state properties for optimum hydrogen production. In a comprehensively performed parametric study, at a single electrolysis cell scale, the effects of varying some operating conditions, such as temperature, pressure, steam molar fraction and the current density on the cell potential and hence the performance are investigated. In addition, at the electrolyzer system scale, the overall electrolyzer performance is investigated through energy and exergy efficiencies, in addition to the system's power density consumption, hydrogen production rate, heat exchange rates and exergy destruction parameters. The present results show that the overall solid oxide electrolyzer energy efficiency is 53%, while the exergy efficiency is 60%. The exergy destruction at a reduced operating temperature increases significantly. This may be overcome by the integration of this system with a source of steam production.     

有机工质低温余热发电系统多目标优化设计

针对现有有机朗肯循环单目标优化设计的局限性,从热力性、经济性等多方面对有机工质低温余热发电系统进行多目标优化设计.以系统效率最大和总投资费用最小为目标函数,选取透平进口温度、透平进口压力、余热锅炉节点温差、接近点温差和冷凝器端差等5个关键热力参数作为决策变量,利用非支配解排序遗传算法(NSGA-II)分别对采用R123、R245fa和异丁烷的有机工质余热发电系统进行多目标优化,获得不同工质的多目标优化的最优解集(Pareto最优前沿),并采用理想点辅助法从最优解集中选择出最优解及相应的系统最佳热力参数组合.结果表明:在给定余热条件下,从热力性能和经济性两方面考虑,R245fa是最优的有机工质,从多目标优化的最优解集中选择出的最佳效率为10.37%,最小总投资费用为455.84万元.     

Performance assessment and optimization of a biomass-based solid oxide fuel cell and micro gas turbine system integrated with an organic Rankine cycle

Rice straw is a potential energy source for power generation. Here, a biomass-based combined heat and power plant integrating a downdraft gasifier, a solid oxide fuel cell, a micro gas turbine and an organic Rankine cycle is investigated. Energy, exergy, and economic analyses and multi-objective optimization of the proposed system are performed. A parametric analysis is carried out to understand the effects on system performance and cost of varying key parameters: current density, fuel utilization factor, operating pressure, pinch point temperature, recuperator effectiveness and compressors isentropic efficiency. The results show that current density plays the most important role in achieving a tradeoff between system exergy efficiency and cost rate. Also, it is observed that the highest exergy destruction occurs in the gasifier, so improving the performance of this component can considerably reduce the system irreversibility. At the optimum point, the system generates 329 kW of electricity and 56 kW of heating with an exergy efficiency of 35.1% and a cost rate of 10.2 $/h. The capability of this system for using Iran rice straw produced in one year is evaluated as a case study, and it is shown that the proposed system can generate 6660 GWh electrical energy and 1140 GWh thermal energy.     

Exergetic optimization of a solar photovoltaic thermal (PV/T) air collector

In this paper, an exergetic optimization has been developed to determine the optimal performance and design parameters of a solar photovoltaic thermal (PV/T) air collector. A detailed energy and exergy analysis has been carried out to calculate the thermal and electrical parameters, exergy components, and exergy efficiency of a typical PV/T air collector. The thermal and electrical parameters of a PV/T air collector include solar cell temperature, back surface temperature, outlet air temperature, open‐circuit voltage, short‐circuit current, maximum power point voltage, maximum power point current, etc. An improved electrical model has been used to estimate the electrical parameters of a PV/T air collector. Furthermore, a new equation for the exergy efficiency of a PV/T air collector has been derived in terms of design and climatic parameters. A computer simulation program has been also developed to calculate the thermal and electrical parameters of a PV/T air collector. The results of numerical simulation are in good agreement with the experimental measurements noted in the previous literature. Moreover, the simulation results obtained in this paper are more precise than the one given by the previous literature, and the new exergy efficiency obtained in this paper is in good agreement with the one given by the previous literature. Finally, exergetic optimization has been carried out under given climatic, operating, and design parameters. The optimized values of inlet air velocity, duct length, and the maximum exergy efficiency have been found. Parametric studies have been also carried out. Copyright © 2010 John Wiley & Sons, Ltd.