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.     

Parametric analysis of a hybrid power system using organic Rankine cycle to recover waste heat from proton exchange membrane fuel cell

Using fuel cell systems for distributed generation (DG) applications represents a meaningful candidate to conventional plants due to their high power density and the heat recovery potential during the electrochemical reaction. A hybrid power system consisting of a proton exchange membrane (PEM) fuel cell stack and an organic Rankine cycle (ORC) is proposed to utilize the waste heat generated from PEM fuel cell. The system performance is evaluated by the steady-state mathematical models and thermodynamic laws. Meanwhile, a parametric analysis is also carried out to investigate the effects of some key parameters on the system performance, including the fuel flow rate, PEM fuel cell operating pressure, turbine inlet pressure and turbine backpressure. Results show that the electrical efficiency of the hybrid system combined by PEM fuel cell stack and ORC can be improved by about 5% compared to that of the single PEM fuel cell stack without ORC, and it is also indicated that the high fuel flow rate can reduce the PEM fuel cell electrical efficiency and overall electrical efficiency. Moreover, with an increased fuel cell operating pressure, both PEM fuel cell electrical efficiency and overall electrical efficiency firstly increase, and then decrease. Turbine inlet pressure and backpressure also have effects on the performance of the hybrid power system.     

空气抽吸式甲醇燃料电池传热与传质特性

空气抽吸式直接甲醇燃料电池不仅具有被动式燃料电池的优点,同时又便于将其串联成电堆提高输出电压。建立以阴极为管道抽吸式结构的直接甲醇燃料电池的三维、两相、非等温稳态数值模型,研究了质子交换膜性能、供给甲醇浓度以及电堆规模对电池性能及燃料利用率的影响。对于保温较好的大电堆,采用低甲醇穿透的改性质子交换膜能同时提升燃料利用率和比功率;此类电堆若采用穿透率低的改性膜,则2 mol/L的甲醇浓度就能保证电池在较大的电流密度区间内维持较高的功率与效率。作为影响电池运行温度的重要因素,电堆规模的大小将直接影响质子交换膜种类与甲醇浓度等关键参数的设计与选择。     

Multi-objective optimisation analysis and load matching of a phosphoric acid fuel cell system

Based on the current model of a phosphoric acid fuel cell (PAFC) system, the electrolyte concentration is optimised. New analytical expressions for the power output and efficiency of the PAFC system are derived by considering the effects of multi-irreversibilities resulting from the activation overpotential, concentration overpotential, ohmic overpotential, and leakage resistance on the performance of the PAFC system. These parameters are used to evaluate the general performance characteristics of the PAFC system. Accordingly, the upper and lower limits of the optimised values for some main parameters, such as the current density, power output, and efficiency, are determined. Moreover, a multi-objective function, including both the power output and efficiency, is introduced and used to further subdivide the parametric optimum regions according to different requirements. In addition, a general formula for the load of the system is derived. The relations between the power output and efficiency of the system and the load are discussed in detail, and the optimum matching conditions of the load are obtained.     

Experimental investigation on the mechanism of variable fan speed control in Open cathode PEM fuel cell

This study aims to reveal the mechanism of variable fan speed control in Open cathode PEM fuel cell (OC-PEMFC) by the experiments, which analyze the effects of variable fan speed on the operating parameters under different fuel cell loads. The operating parameters are cell temperature, stack voltage, voltage uniformity, and parasitic power. The results reveal that the fan speed has strong effects on the operating parameters: (1) it affects the temperature distribution and overheats the middle cells of fuel cell stack, especially, when it operates at high power; (2) it affects the variation trend of the voltage under different loads; (3) it affects the voltage uniformity, which can be improved via forced convection; (4) it affects the system efficiency due to the parasitic load consumed by the fan. Considering the varying effects of these operating parameters on performance, durability and system efficiency under different fuel cell loads, the variable fan speed control should adapt the variation of fuel cell loads and weigh multiple operating parameters.     

A new analytical approach to model and evaluate the performance of a class of irreversible fuel cells

An irreversible model of a class of hydrogen–oxygen fuel cells working at steady-state is established, in which the irreversibilities resulting from electrochemical reaction, electrical resistance, and heat transfer to the environment are taken into account. The entropy production analysis is introduced and applied to investigate the physical and chemical performances of the fuel cell by using the theory of electrochemistry and non-equilibrium thermodynamics. Expressions for the power output and efficiency of the fuel cell are derived by introducing the equivalent internal and leakage resistances. With the help of the model being applied to high temperature solid oxide fuel cells, the performance characteristic curves of the fuel cell are presented and the influence of some design and operating parameters on the performance of the fuel cell are discussed in detail. Moreover, the optimum criteria of some important parameters such as the power output, efficiency, and current density are given. The results obtained may provide a theoretical basis for both the optimal design and operation of real fuel cells. This new method can also be used in the investigation and optimization of similar energy conversion settings and electrochemistry systems.     

Experimental investigation on water and heat management in a PEM fuel cell using response surface methodology

Water and heat management are the most critical issues for the performance of proton exchange membrane (PEM) fuel cells. They can be provided by keeping hydrogen flow rate, oxygen flow rate, cell temperature and humidification temperature under control. In this study, the effects of these parameters on the power density of proton exchange membrane (PEM) fuel cell which has 25 cm² active area have been examined experimentally using hydrogen on the anode side and oxygen on the cathode side. Response Surface Methodology (RSM) has been applied to optimize these operation parameters of proton exchange membrane (PEM) fuel cell. The test responses are the maximum output power density. ANOVA (analysis of variance) analyses are used to compute the effects and the contributions of the various factors to the fuel cell maximal power density. The use of this design shows also how it is possible to reduce the number of experiments. Hydrogen flow rate, oxygen flow rate, humidification temperature and cell temperature were the main parameters to have been varied between 2.5–5 L/min, 3–5 L/min, 40–70 °C and 40–80 °C in the analyses. The maximum power density was found as 241.977 mW/cm².     

Three‐dimensional optimisation of a fuel gas channel of a proton exchange membrane fuel cell for maximum current density

Proton exchange membrane (PEM) fuel cells operated with hydrogen and air offer promising alternative to conventional fossil fuel sources for transport and stationary applications because of its high efficiency, low‐temperature operation, high power density, fast start‐up and potable power for mobile application. Power levels derivable from this class of fuel cell depend on the operating parameters. In this study, a three‐dimensional numerical optimisation of the effect of operating and design parameters of PEM fuel cell performance was developed. The model computational domain includes an anode flow channel, membrane electrode assembly and a cathode flow channel. The continuity, momentum, energy and species conservation equations describing the flow and species transport of the gas mixture in the coupled gas channels and the electrodes were numerically solved using a computational fluid dynamics code. The effects of several key parameters, including channel geometries (width and depth), flow orientation and gas diffusion layer (GDL) porosity on performance and species distribution in a typical fuel cell system have been studied. Numerical results of the effect of flow rate and GDL porosity on the flow channel optimal configurations for PEM fuel cell are reported. Simulations were carried out ranging from 0.6 to 1.6 mm for channel width, 0.5 to 3.0 mm for channel depth and 0.1 to 0.7 for the GDL porosity. Results were evaluated at 0.3 V operating cell voltage of the PEM fuel cell. The optimisation results show that the optimum dimension values for channel depth and channel width are 2.0 and 1.2 mm, respectively. In addition, the results indicate that effective design of fuel gas channel in combination with the reactant species flow rate and GDL porosity enhances the performance of the fuel cell. The numerical results computed agree well with experimental data in the literature. Consequently, the results obtained provide useful information for improving the design of fuel cells. Copyright © 2011 John Wiley & Sons, Ltd.     

The power performance curve for engineering analysis of fuel cells

The power delivered by a fuel cell to an external load is controlled by the impedance of the external load. The power performance curve is a new metric that relates the power delivered to the external load to its impedance. The power delivered is 0 for both an open circuit and a short circuit (infinite and zero external impedance) and is a maximum when the external load impedance matches the internal resistance of the fuel cell. Fuel efficiency is 50% at maximum power output. Higher fuel efficiency is achieved when the load impedance is much greater than the internal resistance of the electrolyte. A simple equivalent circuit for the fuel cell consisting of a battery, diode and resistor captures the essential characteristics of a fuel cell as part of an electrical circuit and can be used to analyze of the response to changes in load. Simple circuit analysis can be employed to elucidate the power output and efficiency of large area fuel cells and fuel cell stacks. Non-uniformities in large area fuel cells create internal potential differences that drive internal currents dissipating energy. Non-uniformities in fuel cell stacks can drive low power cells into an electrolytic state, eventually leading to failure. The power performance curve simplifies analysis of control and operation of fuel cell systems.     

Maximum power point tracking of a proton exchange membrane fuel cell system using PSO-PID controller

Fuel cells output power depends on the operating conditions, including cell temperature, oxygen partial pressure, hydrogen partial pressure, and membrane water content. In each particular condition, there is only one unique operating point for a fuel cell system with the maximum output. Thus, a maximum power point tracking (MPPT) controller is needed to increase the efficiency of the fuel cell systems. In this paper an efficient method based on the particle swarm optimization (PSO) and PID controller (PSO-PID) is proposed for MPPT of the proton exchange membrane (PEM) fuel cells. The closed loop system includes the PEM fuel cell, boost converter, battery and PSO-PID controller. PSO-PID controller adjusts the operating point of the PEM fuel cell to the maximum power by tuning of the boost converter duty cycle. To demonstrate the performance of the proposed algorithm, simulation results are compared with perturb and observe (P&O) and sliding mode (SM) algorithms under different operating conditions. PSO algorithm with fast convergence, high accuracy and very low power fluctuations tracks the maximum power point of the fuel cell system.     

A reforming system for co-generation of hydrogen and mechanical work from methanol

The paper describes a reforming system for converting methanol into pure hydrogen. The system is based on an autothermal reforming reactor operating at elevated pressures followed by membrane-based hydrogen separation. The high-pressure membrane retentate stream is combusted and expanded through a turbine generating additional power. Process simulation illustrates the effects of the system operating parameters on performance and demonstrates system reforming efficiency up to ∼90%. When coupled with a PEM fuel cell and an electrical generator, overall fuel to electricity efficiency can be >48% depending upon the efficiency of a PEM fuel cell stack.     

Convenient two-dimensional model for design of fuel channels for proton exchange membrane fuel cells

A theoretical, two-dimensional, along-the-channel model has been developed to design fuel channels for proton exchange membrane (PEM) fuel cells. This has been implemented by solving the resultant ordinary differential equation with a straightforward shooting computational scheme. With such a design tool, an analysis can be made of the effects due to some operation and design parameters, such as inlet velocity, inlet pressure, catalyst activity, height of channel, and porosity of gas-diffusion layer to obtain a fuel cell with high performance. Present results indicate that there is always a trade-off between higher power density and higher efficiency of the fuel cell. Namely, a design for higher power density (a better performance) is always accompanied with a higher fuel efficiency (or a larger fuel consumption rate and a higher fuel cost), and vice versa. When some relevant physical parameters are determined experimentally and applied in the present model, a quantitative design for a fuel cell of either high efficiency or high performance is feasible.     

Performance analysis of a new designed PEM fuel cell

In this paper, a new design for the flow channels is presented, and a parametric study of the proton exchange membrane (PEM) fuel cell is conducted in order to investigate the effect of the new flow channels, as well as different operating parameters, on the efficiency and energy output of the cell. Design parameters are selected based on studies presented in the literature to build a physical and practical model. With the new design of the flow channels, it is noticed that the cell efficiency increases from 33.8% to 47.7% if the temperature of the cell is increased. The power output of the cell increases from 2.6 to 282.5 W when the cell temperature and the current density are increased. Moreover, decrease in the efficiency of the cell ranges from 45.5% to 28.4% with the increase in the current density and membrane thickness. Based on the analytical model, design parameters were selected to manufacture a fuel cell that has a power output of 175 W and an efficiency of 35% running at 353 K and 3 bar, with an effective membrane area of 450 cm². Experiments are conducted to investigate the effect of newly designed flow channels on pressure distribution. It is found that when hydrogen is supplied from both inlets, pressure across the channels become symmetric and, therefore increasing the power output. This study reveals that, with the proper choice of design parameters, a PEM fuel cell is an attractive economical, efficient, and environmental solution when compared with conventional systems of power generation such as gas turbines. Copyright © 2011 John Wiley & Sons, Ltd.     

Maintaining equal operating conditions for all cells in a fuel cell stack using an external flow distributor

This paper presents a novel fuel cell stack architecture that allows each fuel cell to work at the same condition, maintaining the same performance from each individual cell and creating a maximum power output from the cell stack. A fuel cell stack having four PEM fuel cells was fabricated to experimentally compare its performance when fuel and air supplying/distribution schemes are different. The performance of the fuel cell stack and individual cells in the stack is measured to achieve a detailed evaluation of the effect of the different fuel and air supplying schemes. Experimental data shows that non-uniform flow distribution to individual cells has a considerable influence on individual cell performance, which affects the power output of the fuel cell stack. The fuel cell stack with the novel approach of fuel and air feeding shows a better power output performance compared to a different fuel and air feeding approach to the fuel cell stack.     

The dynamics analysis and controller design for the PEM fuel cell under gas flowrate constraints

This paper is on the dynamics analysis and controller design for the PEM fuel cell under the flowrate constraints of the supplied hydrogen and oxygen. By linearization around the equilibrium trajectories defined by the quantities of hydrogen and oxygen input flowrate, the nonlinear dynamics of the PEM fuel cell can be expressed as a linear parameter varying system with the output current and temperature as the system parameters. The state-feedback controller design is performed based on the linear time-invariant model obtained from the derived linear parameter varying system evaluated at the half load operation condition. The control objective is to achieve a maximized relative stability or equivalently the maximum decay rate under the specified magnitude constraints on the input flowrate of hydrogen and oxygen. The convex linear matrix inequality algorithm is utilized for numerical construction of the state-feedback control law. Under the fixed load resistance corresponding to the half load condition, the time response simulations are conducted for both the cases of initial condition regulation and external command tracking. For the simulation of regulation, the initial deviation of state variables diminishes quickly that agrees with the obtained large delay rate during controller design. In the case of command tracking for the same amount of state variables, the controlled system can follow the issued command in the right direction but leave large tracking error, which is due to the weak controllability of the gas flowrates on the activation overvoltage for the PEM fuel cell system dynamics.     

Integrated modeling and control of a PEM fuel cell power system with a PWM DC/DC converter

A fuel cell powered system is regarded as a high current and low voltage source. To boost the output voltage of a fuel cell, a DC/DC converter is employed. Since these two systems show different dynamics, they need to be coordinated to meet the demand of a load. This paper proposes models for the two systems with associated controls, which take into account a PEM fuel cell stack with air supply and thermal systems, and a PWM DC/DC converter. The integrated simulation facilitates optimization of the power control strategy, and analyses of interrelated effects between the electric load and the temperature of cell components. In addition, the results show that the proposed power control can coordinate the two sources with improved dynamics and efficiency at a given dynamic load.     

Development of cylindrical PEM fuel cells with semi-cylindrical cathode current collectors

Proton exchange membrane (PEM) fuel cells are attractive because of advantages such as low-temperature operation, no emission of harmful gases and high efficiency. However, the bipolar plates used in the state-of-the-art planar architecture are costly and increase the dead weight of the cell. In addition, the flow channels in the planar fuel cell increase the difficulty in removing the water produced in the cathode during cell operation. Cylindrical PEM fuel cells, on the other hand, do not require bipolar plates and there is no need for precisely machined flow channels. Thus, cylindrical PEM fuel cells are cheap, efficient in water management, and possess higher volumetric and gravimetric power density compared to planar PEM fuel cells. The design of a cylindrical fuel cell is very simple, but the fabrication of the same is fairly complex. In this work, a novel cathode current collector design for cylindrical PEM fuel cell has been developed. The cell performance was limited by low open circuit voltage and high ohmic resistance. The open circuit voltage of the cell is increased from 0.85 V to 0.95 V using an acrylic based adhesive to seal the membrane edges. The contact resistance of the cell is reduced from 75 mOhm to 50 mOhm by increasing the contact pressure on the membrane electrode assembly and it is further reduced to 30 mOhm by gold coating the current collectors. Furthermore, a cumulative 40% increase in peak power has been achieved from the optimization of cathode rib width and hydrogen flow rate. The optimized cell delivered a current density of 400 mA/cm² at 0.6 V and peak power of 2 W, which is appreciable considering the fact that the cell is air-breathing and operated with very minimal subsystems.     

Thermodynamic analysis of polymer-electrolyte-membrane fuel-cell performance under varying cooling conditions

The cooling capacity and cooling load of a fuel-cell cooling loop govern the operating temperature of the fuel-cell module and its electrical output, efficiency and other thermodynamic aspects. The aim of this work was to analyze the performance of a polymer-electrolyte-membrane fuel-cell (PEMFC) under changing cooling conditions. A back-iteration algorithm was employed to determine the operating temperature of a PEMFC for which thermodynamic performance models were developed for the entropy generation, exergy-destruction and second-law efficiency using an entropy-analysis method. Electrochemical equations for the calculation of the voltage, power and first-law efficiency of the cell were also formulated. A parametric study was performed to evaluate the effects of varying cooling conditions on the energy and exergy efficiency of the PEMFC. The parameters considered include the electric-current density governing the cooling load, the mass flow rate of the coolant and the external thermal resistance of the cooler, which together determine the cooling ability of the fuel-cell cooling loop. Their influences on operating temperature, voltage, power, energy and exergy efficiencies were numerically investigated. The results indicate that although the power output and exhaust heat of PEMFC is mainly dominated by the electric-current density, the impacts of the coolant's mass flow rate and the cooler's external thermal resistance on the voltage, energy and exergy efficiencies of PEMFC module can't be neglected. In the investigated ranges, the gross energy and exergy efficiencies increase with the cooler's external thermal resistance by 3.2% and 2.45%, and decrease with the increase in coolant's mass flow rate by 1.2% and 0.92%, respectively.     

Dynamic characteristics of a PEM fuel cell system for individual houses

The method of determination of the control variables for a system controller, which controls the electric power output of a solid‐polymer‐membrane (PEM) fuel cell system during electric power load fluctuations, was considered. The operation was clarified for the response characteristics of electric power generation for setting the control variables of proportional action and integral action considered to be the optimal for the system controller. The power load pattern of an individual house consists of loads usually moved up and down rapidly for a short time. Until now, there have been no examples showing the characteristics of the power generation efficiency of a system that follows a load pattern that moves up and down rapidly. Therefore, this paper investigates the relation of the control variables and power generation efficiency when adding change that simulates the load of a house to PEM fuel cell cogeneration. As a result, it was shown that an operation, minimally influenced by load fluctuations, can be performed by changing the control variables using the value of the electric power load of a system. Copyright © 2006 John Wiley & Sons, Ltd.     

Exergoeconomic analysis of a vehicular PEM fuel cell system

In this study, we deal with the exergoeconomic analysis of a proton exchange membrane (PEM) fuel cell power system for transportation applications. The PEM fuel cell performance model, that is the polarization curve, is previously developed by one of the authors by using the some derived and developed equations in literature. The exergoeconomic analysis includes the PEM fuel cell stack and system components as compressor, humidifiers, pressure regulator and the cooling system. A parametric study is also conducted to investigate the system performance and cost behaviour of the components, depending on the operating temperature, operating pressure, membrane thickness, anode stoichiometry and cathode stoichiometry. For the system performance, energy and exergy efficiencies and power output are investigated in detail. It is found that with an increase of temperature and pressure and a decrease of membrane thickness the system efficiency increases which leads to a decrease in the overall production cost. The minimization of the production costs is very crucial in commercialization of the fuel cells in transportation sector.