Maximum equivalent efficiency and power output of a PEM fuel cell/refrigeration cycle hybrid system

With the help of the current models of proton exchange membrane (PEM) fuel cells and three-heat-source refrigeration cycles, the general model of a PEM fuel cell/refrigeration cycle hybrid system is originally established, so that the waste heat produced in the PEM fuel cell may be availably utilized. Based on the theory of electrochemistry and non-equilibrium thermodynamics, expressions for the efficiency and power output of the PEM fuel cell, the coefficient of performance and cooling rate of the refrigeration cycle, and the equivalent efficiency and power output of the hybrid system are derived. The curves of the equivalent efficiency and power output of the hybrid system varying with the electric current density and the equivalent power output versus efficiency curves are represented through numerical calculation. The general performance characteristics of the hybrid system are discussed. The optimal operation regions of some parameters in the hybrid system are determined. The advantages of the hybrid system are revealed.     

Performance evaluation of an alkaline fuel cell/thermoelectric generator hybrid system

A hybrid system consisting of an AFC (Alkaline Fuel Cell), a TEG (Thermoelectric Generator) and a regenerator is put forward, where the AFC converts the chemical energy in the hydrogen into electrical energy and thermal energy, and the released thermal energy is subsequently converted into electrical energy through the bottoming TEG. The main irreversible losses in each element of the hybrid system are characterized, and numerical expressions for the efficiency and power output of the AFC, TEG and hybrid system are respectively derived. The fundamental relation between the operating current density of the AFC and the dimensionless current of the TEG is obtained, from which the region of the operating current density of the AFC that the TEG exerts its function is determined. By employing such a hybrid system, the equivalent maximum power density of the AFC can be increased by up to 23%. The effects of the operating current density, operating temperature, heat conductivity, and integrated parameter on the performance of the hybrid system are revealed. The results obtained in the present paper will provide some theoretical guidance for the performance improvement of the AFC.     

Recycling alkaline fuel cell waste heat for cooling production via temperature-matching elastocaloric cooler

To recycle byproduct waste heat, a new hybrid system model that integrates alkaline fuel cell (AFC) with temperature-matching elastocaloric cooler (ECC) is proposed. Considering diverse irreversible effects in thermodynamic and electrochemical processes, the mathematical model of hybrid system is formulated, and the performance metrics of the hybrid system are obtained. The optimally operating current density range of AFC enabling ECC to work is determined. Numerical calculation shows that the equivalent peak output power density and its according efficiency can be 51.64% and 20.88% higher than that of the single AFC, respectively. ECC is demonstrated to be an effective and competitive technology for AFC waste heat harvesting. In addition, considerable sensitivity analyses are performed to check the dependence of the proposed system performance on some key variables, including operating pressure, operating temperature, thermodynamic loss composite parameter, elastocaloric material types and elastocaloric properties, cross-sectional area ratio and length ratio for ECC. The results derived may offer some insights into the design or operation of practical AFC-ECC hybrid systems.     

Influence of multiple irreversible losses on the performance of a molten carbonate fuel cell-gas turbine hybrid system

In this study, a new molten carbonate fuel cell-gas turbine hybrid system, which consists of a fuel cell, three heat exchangers, a compressor, and a turbine, is established. The multiple irreversible losses existing in real hybrid systems are taken into account by the models of a molten carbonate fuel cell and an open Brayton cycle with a regenerative process. Expressions for the power outputs and efficiencies of the subsystems and hybrid system are derived. The maximum power output and efficiency of the hybrid system are numerically calculated. It is found that compared with a single molten carbonate fuel cell, both the power output and efficiency of the hybrid system are greatly enhanced. The general performance characteristics of the hybrid system are evaluated and the optimal criteria of the main performance parameters are determined. The effects of key irreversibilities on the performance of the hybrid system are investigated in detail. It is found that the use of a regenerator in the gas turbine can availably improve the power output and efficiency of the system. The results obtained here are significant and may be directly used to discuss the optimal performance of the hybrid system in special cases.     

一种新型水泥工业余热与生物质能互补发电系统

本文提出一种新型水泥工业余热与生物质能互补发电系统,该系统采用水泥窑低温余热和生物质补燃有机结合的方式大幅提高水泥窑余热发电蒸汽参数与系统效率。来自水泥生产线窑头和窑尾的低温余热烟气全部用来加热工质水产生饱和蒸汽,饱和蒸汽进入生物质补燃系统中进行过热后送入汽轮发电机组中做功发电,补燃燃料为生物质气化燃气。本研究建立了单压和双压2种互补发电系统,分析了其热力学性能,结果表明:单压互补发电系统与传统单压纯低温发电系统相比,系统循环热效率和系统发电效率分别提高了1.63和1.92个百分点。双压互补发电系统与传统双压纯低温发电系统相比,系统循环热效率和系统发电效率分别提高了1.05和1.53个百分点。     

MCFC-并联TTEG混合系统的性能优化

为有效回收熔融碳酸盐燃料电池产生的余热,提出一种由熔融碳酸盐燃料电池(MCFC)、两级并联温差发电器(TTEG)和回热器组合而成的混合系统模型.考虑MCFC电化学反应中的过电势损失和混合系统中的不可逆损失,通过数值分析得出混合系统的输出功率和效率的数学表达式,获得混合系统的一般性能特征,讨论MCFC电流密度与温差发电器...     

The performance analysis and multi-objective optimization of a typical alkaline fuel cell

Based on the model of a typical alkaline fuel cell (AFC) with circulating potassium hydroxide (KOH) solution as electrolyte and oxygen as oxidant and the experimental data available in the current literature, thermodynamic-electrochemical analyses on the performance of the AFC are carried out, in which multi-irreversibilities such as charger-transfer, concentration and ohmic overpotentials are taken into account. Expressions for the power output and efficiency of the AFC are derived, from which the general performance characteristics of the AFC are discussed in detail. It is found that the power output and efficiency of the AFC first increase and then decrease as the electrolyte concentration is increased, and consequently, there exist the optimal electrolyte concentrations for different temperatures. It is also found that the power output is not a monotonic function of the electric current density while the efficiency is a monotonically decreasing function of the electric current density. According to the performance characteristic curves of the AFC, the optimal operation regions of some main parameters are determined. Moreover, a new multi-objective function is used to further optimize the characteristics of the AFC. Some significant results for the optimal design and operation of practical AFCs are obtained.     

Performance analysis and parametric optimum criteria of a class of irreversible fuel cell/heat engine hybrid systems

Based on the current models of solid oxide fuel cells and two-heat-source heat engines consisting of two isothermal and two polytropic processes, a general model of a class of fuel cell/heat engine hybrid systems is established, in which multi-irreversibilities existing in real hybrid systems are taken into account. Expressions for the efficiency and power output of the hybrid systems are analytically derived from the model. The curves of the efficiency and power output of the hybrid systems varying with the current density and the efficiency versus power output curves are represented through numerical calculation. The general performance characteristics of the hybrid systems are revealed and the optimum criteria of the main performance parameters are determined. The effects of some key irreversibilities existing in the fuel cell, regenerator and two-heat-source heat engine on the performance of the hybrid systems are discussed in detail. The results obtained here are very general and may be directly used to derive the various interesting conclusions of the hybrid systems which are operated under different special cases.     

Theoretical analysis of a thermodynamic cycle for power and heat production using supercritical carbon dioxide

A numerical study of a thermodynamic cycle is described: solar energy powered Rankine cycle using supercritical carbon dioxide as the working fluid for combined power and heat production. A model is developed to predict the cycle performance. Experimental data is used to verify the numerical formulation. Of interest in the present study is the thermodynamic cycle of 0.3–1.0 kW power generation and 1.0–3.0 kW heat output. The effects of the governing parameters on the performance are investigated numerically. The results show that the cycle has a power generation efficiency of somewhat above 20.0% and heat recovery efficiency of 68.0%, respectively. It is seen that the cycle performance is strongly dependent on the governing parameters and they can be optimized to provide maximum power, maximum heat recovery or a combination of both. The power generation and heat recovery are found to be increased with solar collector efficient area. The power generation is also increased with water temperature of the heat recovery system, but decreased with heat exchanging area. It is also seen that the effect of the water flow rate in the heat recovery system on the cycle performance is negligible.     

Performance analysis and parametric optimum criteria of an irreversible Atkinson heat-engine

Multi-irreversibilities, mainly resulting from the adiabatic processes, finite-time processes and heat loss through the cylinder wall, are considered in the cycle model of an Atkinson heat engine. The power output and efficiency of the cycle are derived by introducing the pressure ratio and the compression and expansion efficiencies. The performance characteristic curves of the cycle are presented. The bounds of the power output and efficiency are determined. The optimum criteria of some important parameters, such as the power output, efficiency and pressure ratio are given. The influences of the various design parameters on the performance of the cycle are analyzed in detail. The results obtained may provide a theoretical basis for both the optimal design and operation of real Atkinson heat engines.     

Ecological performance analysis of an endoreversible modified Brayton cycle

An endoreversible closed modified simple Brayton cycle model with isothermal heat addition coupled to variable-temperature heat reservoirs is established using finite-time thermodynamics. Analytical expressions of dimensionless power output, thermal efficiency, dimensionless entropy generation rate and dimensionless ecological function are derived. Influences of cycle thermodynamic parameters on ecological performance and optimal compressor pressure ratio, optimal power output, optimal cycle thermal efficiency and optimal entropy generation rate corresponding to maximum ecological function are obtained and compared with those corresponding to maximum power output. The results show that cycle thermal efficiency improvement and entropy generation rate reduction are obtained at the expense of higher compressor pressure ratio and a little sacrifice of power output at maximum ecological function. The compromises between power output and entropy generation rate and between power output and cycle thermal efficiency, respectively, are achieved.     

Thermodynamic analysis of a transcritical CO2 power cycle driven by solar energy with liquified natural gas as its heat sink

This paper proposes a transcritical CO₂ power cycle driven by solar energy while utilizing the cold heat rejection to an liquified natural gas (LNG) evaporation system. In order to ensure a continuous and stable operation for the system, a thermal storage system is introduced to store the collected solar energy and to provide stable power output when solar radiation is insufficient. A mathematical model is developed to simulate the solar-driven transcritical CO₂ power cycle under steady-state conditions, and a modified system efficiency is defined to better evaluate the cycle performance over a period of time. The thermodynamic analysis focuses on the effects of some key parameters, including the turbine inlet pressure, the turbine inlet temperature and the condensation temperature, on the system performance. Results indicate that the net power output mainly depends on the solar radiation over a day, yet the system is still capable of generating electricity long after sunset by virtue of the thermal storage tank. An optimum turbine inlet pressure exists under given conditions where the net power output and the system efficiency both reach maximum values. The net power output and the system efficiency are less sensitive to the change in the turbine inlet temperature, but the condensation temperature exerts a significant influence on the system performance. The surface area of heat exchangers increases with the rise in the turbine inlet temperature, while changes in the turbine inlet pressure have no significant impact on the heat exchanging area under the given conditions.     

Modeling of natural gas fueled quadruple cycle for power applications

Performance improvement being a major need of the power sector aims at increasing efficiency, lowering air pollutants and ultimately cost. This paper explores a quadruple cycle, a hybrid of solid oxide fuel cell integrated with gas turbine, steam turbine and organic Rankine cycle totaling four cycles (SOFC-GT-ST-ORC), fueled primarily by natural gas for stationary power generation. A mathematical model of the configuration of the quadruple cycle is developed and the performance investigated through a parametric study of the thermodynamic components. The power output, efficiency and other results were validated with those found in literature. The quadruple cycle produced an efficiency of 66.1% with 1,1,1,2-tetrafluoroethane, R134a as the organic working fluid. This efficiency exceeded the performance of traditional thermodynamic cycles like single steam cycle, combined and triple cycle at similar operating conditions. Lastly, the quadruple cycle presents a potential for optimization with waste heat recovery.     

An irreversible heat engine model including three typical thermodynamic cycles and their optimum performance analysis

An irreversible Dual heat engine model, which can include the Otto and Diesel cycles, is established and used to investigate the influence of the multi-irreversibilities mainly resulting from the adiabatic processes, finite time processes and heat leak loss through the cylinder wall on the performance of the cycle. The power output and efficiency of the cycle are derived and optimized with respect to the pressure ratio of the working substance. The maximum power output and efficiency are calculated. The influence of the various design parameters on the performance of the cycle is analyzed. The optimum criteria of some important parameters such as the power output, efficiency and pressure ratio are given. Several special interesting cases are discussed. The results obtained are general, so that the optimal performance of irreversible Otto and Diesel cycles are included in two special cases of the Dual cycle and may be directly derived from that of the Dual heat engine. Moreover, the performance characteristic curves of the three heat engines are presented by using numerical examples.     

A unified model of combined energy systems with different cycle modes and its optimum performance characteristics

A unified model is presented for a class of combined energy systems, in which the systems mainly consist of a heat engine, a combustor and a counter-flow heat exchanger and the heat engine in the systems may have different thermodynamic cycle modes such as the Brayton cycle, Carnot cycle, Stirling cycle, Ericsson cycle, and so on. Not only the irreversibilities of the heat leak and finite-rate heat transfer but also the different cycle modes of the heat engine are considered in the model. On the basis of Newton’s law, expressions for the overall efficiency and power output of the combined energy system with an irreversible Brayton cycle are derived. The maximum overall efficiency and power output and other relevant parameters are calculated. The general characteristic curves of the system are presented for some given parameters. Several interesting cases are discussed in detail. The results obtained here are very general and significant and can be used to discuss the optimal performance characteristics of a class of combined energy systems with different cycle modes. Moreover, it is significant to point out that not only the important conclusions obtained in Bejan’s first combustor model and Peterson’s general combustion driven model but also the optimal performance of a class of solar-driven heat engine systems can be directly derived from the present paper under some limit conditions.     

Influence of heat loss on the performance of an air-standard Atkinson cycle

This study is aimed at investigating the effects of heat loss, as characterized by a percentage of fuel’s energy, friction and variable specific heats of the working fluid, on the performance of an air-standard Atkinson cycle under the restriction of the maximum cycle-temperature. A more realistic and precise relationship between the fuel’s chemical-energy and the heat leakage is derived through the resulting temperature. The variations in power output and thermal efficiency with compression ratio, and the relations between the power output and the thermal efficiency of the cycle are presented. The results show that the power output as well as the efficiency, for which the maximum power-output occurs, will rise with the increase of maximum cycle-temperature. The temperature-dependent specific heats of the working fluid have a significant influence on the performance. The power output and the working range of the cycle increase while the efficiency decreases with the rise of specific heats of working fluid. The friction loss has a negative effect on the performance. Therefore, the power output and efficiency of the Atkinson cycle decrease with increasing friction loss. It is noteworthy that the results obtained in the present study are of significance for providing guidance with respect to the performance evaluation and improvement of practical Atkinson-cycle engines.     

Thermal‐economic analysis of a transcritical Rankine power cycle with reheat enhancement for a low‐grade heat source

A thermal‐economic analysis of a transcritical Rankine power cycle with reheat enhancement using a low‐grade industrial waste heat is presented. Under the identical operating conditions, the reheat cycle is compared to the non‐reheat baseline cycle with respect to the specific net power output, the thermal efficiency, the heat exchanger area, and the total capital costs of the systems. Detailed parametric effects are investigated in order to maximize the cycle performance and minimize the system unit cost per net work output. The main results show that the value of the optimum reheat pressure maximizing the specific net work output is approximately equal to the one that causes the same expansion ratio across each stage turbine. Relative performance improvement by reheat process over the baseline is augmented with an increase of the high pressure but a decrease of the turbine inlet temperature. Enhancement for the specific net work output is more significant than that for the thermal efficiency under each condition, because total heat input is increased in the reheat cycle for the reheat process. The economic analysis reveals that the respective optimal high pressures minimizing the unit heat exchanger area and system cost are much lower than that maximizing the energy performance. The comparative analysis identifies the range of operating conditions when the proposed reheat cycle is more cost effective than the baseline. Copyright © 2012 John Wiley & Sons, Ltd.     

具有热阻、热漏和补燃的卡诺和朗肯联合热机循环性能分析

在原有的联合循环模型的基础上,建立了一个存在热阻、热漏和补燃的卡诺和朗肯联合循环热机模型。研究其在补燃作用下的功率、效率特性并对其进行优化,导出功率、效率的基本优化关系,分析补燃对最优性能的影响。     

Power,power density and efficiency optimization of an endoreversible Braysson cycle

The performance optimization of an endoreversible Braysson cycle with heat resistance losses in the hot- and cold-side heat exchangers is performed by using finite-time thermodynamics. The relations between the power output and the working fluid temperature ratio, between the power density and the working fluid temperature ratio, as well as between the efficiency and the working fluid temperature ratio of the cycle coupled to constant-temperature heat reservoirs are derived. Moreover, the optimum heat conductance distributions corresponding to the optimum dimensionless power output, the optimum dimensionless power density and the optimum thermal efficiency of the cycle, and the optimum working fluid temperature ratios corresponding to the optimum dimensionless power output and the optimum dimensionless power density are provided. The effects of various design parameters on those optimum values are studied by detailed numerical examples.     

Parametric study and optimization of a transcritical power cycle using a low temperature source

A parametric study and optimization is performed on a transcritical power cycle using six performance indicators: thermal efficiency, specific net output, exergetic efficiency, total UA and surface of the heat exchangers as well as the relative cost of the system. The independent parameters are the maximum temperature and pressure of the cycle as well as the net power output. Results show that it is impossible to simultaneously optimise all six performance indicators, i.e. that the values of the independent parameters are not the same for all the optimizations. The design value for these parameters is therefore a matter of choice, or compromise, among the combinations optimising the performance indicators. For a limited low temperature heat source the parametric studies reveal the existence of a maximum value for the net power output of the system and of another net power output minimising the cost per kW. A comparison of optimised results for three working fluids (CO₂, ethane, R125) shows that the better fluid depends on the optimised indicator and clearly indicates that a simple first law analysis is not sufficient for the selection of a working fluid. In summary, this paper demonstrates the need to achieve a multi-point optimization and comparison in order to study adequately a transcritical power system.