The parametric optimum analysis of a proton exchange membrane (PEM) fuel cell and its load matching

Based on the irreversible model of a PEM fuel cell working at steady state, expressions for the power output, efficiency and entropy production rate of the PEM fuel cell are analytically derived by using the theory of electrochemistry and non-equilibrium thermodynamics. The effects of multi-irreversibilities resulting from electrochemical reaction, heat transfer and electrical resistance on the key parameters of the PEM fuel cell are analyzed. The curves of the power output, efficiency and entropy production rate of the PEM fuel cell varying with the electric current density are represented through numerical calculation. The general performance characteristics of the PEM fuel cell are revealed and the optimum criteria of the main performance parameters are determined. Moreover, the optimal matching condition of the load resistance is obtained from the relations between the load resistance and the power output and efficiency. The effects of the leakage resistance on the performance of the PEM fuel cell are expounded and the optimally operating states of the PEM fuel cell are further discussed.     

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.     

Performance analysis and parametric study of a solid oxide fuel cell fueled by carbon monoxide

A theoretical model of the solid oxide fuel cell (SOFC) fueled by carbon monoxide is adopted and validated, in which the activation overpotential, concentration overpotential, and ohmic overpotential are regarded as the main sources of voltage losses. Based on the thermodynamic-electrochemical analysis, mathematical expressions of some performance parameters such as the cell potential, power output, efficiency, and entropy production rate are derived. The effects of microstructure parameters such as the electrode porosity, tortuosity, pore size, grain size, etc. on the electrochemical performance characteristics of the SOFC are revealed. Moreover, the effects of some operation conditions such as the current density, anode inlet gas molar fraction, operating temperature, and operating pressure on some important performance parameters of the SOFC are also discussed. It is found that there exist some optimal values of microstructure parameters and operating conditions at which the better performance can be expected. The results obtained in the paper may provide some theoretical guidance for the design and operation of practical SOFCs fueled by coal-derived gases.     

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.     

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.     

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.     

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.     

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.     

A method for estimating the optimum load resistance of a silicon solar cell used in terrestrial power applications

Solar intensity undergoes significant changes from dawn to dusk. Further, the power output of a silicon solar cell is a function of the load resistance. A load resistance (R_m) giving maximum conversi n efficiency at mid-day becomes less efficient at other times of the day under reduced intensity levels. The load resistance must be optimised to derive maximum overall power output for the whole day, taking intensity variations into account. A method for estimating the optimum load resistance (R_opt) is presented here. It is also shown that considerable improvement in the output of a terrestrial power system could be achieved at solar intensities 100 MW cm².     

System analysis of a protonic ceramic fuel cell and gas turbine hybrid system with methanol reformer

The performance of a methanol-fed protonic ceramic fuel cell (PCFC)/gas turbine (GT) hybrid system is investigated in this work. To build the system, Thermolib software is employed with input parameters obtained from references. Effects of air stoichiometry on system performance are analyzed. Results show that, as air stoichiometry is increased, the reformer temperature and CO concentration decrease, while H₂ concentration increases. High air stoichiometry decreases PCFC temperature and performance. GT output power increases with increasing air flow. But, the power consumption by compressor also increases. Overall, to achieve higher system efficiency for this hybrid system, the optimum values of air stoichiometry are from 2.7 to 2.9. An additional heat recovery steam generator can also improve the overall system efficiency from 66.5% to 71.7%. This work helps in understanding the modeling and optimum functioning parameters of high power generation systems.