Improving dynamic performance of proton-exchange membrane fuel cell system using time delay control

Transient behaviour is a key parameter for the vehicular application of proton-exchange membrane (PEM) fuel cell. The goal of this presentation is to construct better control technology to increase the dynamic performance of a PEM fuel cell. The PEM fuel cell model comprises a compressor, an injection pump, a humidifier, a cooler, inlet and outlet manifolds, and a membrane-electrode assembly. The model includes the dynamic states of current, voltage, relative humidity, stoichiometry of air and hydrogen, cathode and anode pressures, cathode and anode mass flow rates, and power. Anode recirculation is also included with the injection pump, as well as anode purging, for preventing anode flooding. A steady-state, isothermal analytical fuel cell model is constructed to analyze the mass transfer and water transportation in the membrane. In order to prevent the starvation of air and flooding in a PEM fuel cell, time delay control is suggested to regulate the optimum stoichiometry of oxygen and hydrogen, even when there are dynamical fluctuations of the required PEM fuel cell power. To prove the dynamical performance improvement of the present method, feed-forward control and Linear Quadratic Gaussian (LQG) control with a state estimator are compared. Matlab/Simulink simulation is performed to validate the proposed methodology to increase the dynamic performance of a PEM fuel cell system.     

Feedback-Linearization-Based Nonlinear Control for PEM Fuel Cells

This paper presents a dynamic nonlinear model for polymer electrolyte membrane fuel cells (PEMFCs). A nonlinear controller is designed based on the proposed model to prolong the stack life of the PEM fuel cells. Since it is known that large deviations between hydrogen and oxygen partial pressures can cause severe membrane damage in the fuel cell, feedback linearization is applied to the PEM fuel cell system so that the deviation can be kept as small as possible during disturbances or load variations. A dynamic PEM fuel cell model is proposed as a nonlinear, multiple-input multiple-output system so that feedback linearization can be directly utilized. During the control design, hydrogen and oxygen inlet flow rates are defined as the control variables, and the pressures of hydrogen and oxygen are appropriately defined as the control objectives. The details of the design of the control scheme are provided in the paper. The proposed dynamic model was tested by comparing the simulation results with the experimental data previously published. The simulation results show that PEMFCs equipped with the proposed nonlinear controls have better transient performances than those with linear controls.     

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.     

Time delay control for fuel cells with bidirectional DC/DC converter and battery

Transient behavior is a key property in the vehicular application of proton exchange membrane (PEM) fuel cells. A better control technology is constructed to increase the transient performance of PEM fuel cells. A steady-state isothermal analytical fuel cell model is constructed to analyze mass transfer and water transport in the membrane. To prevent the starvation of air in the PEM fuel cell, time delay control is used to regulate the optimum stoichiometric amount of oxygen, although dynamic fluctuations exist in the PEM fuel cell power. A bidirectional DC/DC converter connects the battery to the DC link to manage the power distribution between the fuel cell and the battery. Dynamic evolution control (DEC) allows for adequate pulse-width modulation (PWM) control of the bidirectional DC/DC converter with fast response. Matlab/Simulink/Simpower simulation is performed to validate the proposed methodology, increase the transient performance of the PEM fuel cell system and satisfy the requirement of energy management.     

On-line implementation of model free controller for oxygen stoichiometry and pressure difference control of polymer electrolyte fuel cell

This paper proposes and validates a model free controller to improve the real time operating conditions of Proton Exchange Membrane Fuel Cells (PEMFC). This approach is based on an ultra-local model that does not depend on a precise knowledge of the system. It is perfectly adapted to a complex system such as the fuel cell, while benefiting from the ease of online implementation and low computational cost. The designed controller is used to regulate both the oxygen stoichiometry and the membrane inlet pressure, which are crucial operating conditions for the fuel cell's lifetime. The objectives of the proposed control strategy are twofold: preventing the starvation failure, and limiting the potential for mechanical degradation of the membrane during a large pressure difference. The performance of the proposed control strategy is initially evaluated by a simulation environment for both oxygen stoichiometry and inlet pressure difference control of fuel cell stack. An online validation on 1.2 KW fuel cell stack is conducted to control the membrane pressure drop. Two case studies are comprehensively investigated in relation to stoichiometry control: set point tracking and rejection of unmeasured disturbances caused by current variations. Simulations and experimental results reveal that the proposed controller provides significantly better performance in terms of fast trajectory tracking, and ensures less overshoot compared to the Fuzzy PID and PID controller. This efficiency is proven using the Integral Absolute Error (IAE), Integral Squared Error (ISE) and Integral of the Square input (ISU) performance indexes.     

Improved dynamic performance of hybrid PEM fuel cells and ultracapacitors for portable applications

Transient power demand fluctuations and maintaining high energy density are important for many portable devices. Small fuel cells are potentially good candidates as alternative energy sources for portable applications. Hybrid power sources have some inherent properties which may be effectively utilized to improve the efficiency and dynamic response of the system. In this paper, an improved dynamic model considering the characteristics of the temperature and equivalent internal resistance is presented for proton exchange membrane (PEM) fuel cells. The dynamic behavior of a system with hybrid PEM fuel cells and an ultracapacitor bank is simulated. The hybrid PEM fuel cell/ultracapacitor bank system is used for powering a portable device (such as a laptop computer). The power requirement of a laptop computer varies significantly under different operation conditions. The analytical models of the hybrid system with PEM fuel cells and an ultracapacitor bank are designed and simulated by developing a detailed simulation software using Matlab, Simulink and SimPowerSystems Blockset for portable applications.     

Modeling and control of a portable proton exchange membrane fuel cell-battery power system

A portable proton exchange membrane (PEM) fuel cell-battery power system that uses hydrogen as fuel has a higher power density than conventional batteries, and it is one of the most promising environmentally friendly small-scale alternative energy sources. A general methodology of modeling, control and building of a proton exchange membrane fuel cell-battery system is introduced in this study. A set of fuel cell-battery power system models have been developed and implemented in the Simulink environment. This model is able to address the dynamic behaviors of a PEM fuel cell stack, a boost DC/DC converter and a lithium-ion battery. To control the power system and thus achieve proper performance, a set of system controllers, including a PEM fuel cell reactant supply controller and a power management controller, were developed based on the system model. A physical 100 W PEM fuel cell-battery power system with an embedded micro controller was built to validate the simulation results and to demonstrate this new environmentally friendly power source. Experimental results demonstrated that the 100 W PEM fuel cell-battery power system operated automatically with the varying load conditions as a stable power supply. The experimental results followed the basic trend of the simulation results.     

Solar-powered regenerative PEM electrolyzer/fuel cell system

An electrolyzer/fuel cell energy storage system is a promising alternative to batteries for storing energy from solar electric power systems. Such a system was designed, including a proton-exchange membrane (PEM) electrolyzer, high-pressure hydrogen and oxygen storage, and a PEM fuel cell. The system operates in a closed water loop. A prototype system was constructed, including an experimental PEM electrolyzer and combined gas/water storage tanks. Testing goals included general system feasibility, characterization of the electrolyzer performance (target was sustainable 1.0 A/cm² at 2.0 V per cell), performance of the electrolyzer as a compressor, and evaluation of the system for direct-coupled use with a PV array. When integrated with a photovoltaic array, this type of system is expected to provide reliable, environmentally benign power to remote installations. If grid-coupled, this system (without PV array) would provide high-quality backup power to critical systems such as telecommunications and medical facilities.     

Nonlinear model predictive control for efficient and robust airpath management in fuel cell vehicles

The fuel cell airpath multivariable control problem of optimally coordinating the electric compressor motor and the back-pressure valve to achieve efficient and safe conditions, for both steady state and transient operation, has not been completely addressed in the literature. This paper proposes a nonlinear model predictive control strategy, implemented via the Garrett Motion proprietary NMPC toolbox, to regulate the oxygen stoichiometry and the cathode pressure of an automotive fuel cell airpath system, while avoiding compressor surge and air starvation. The controller set-points are optimized, using the nonlinear model, to achieve the maximum system power as a function of the operating stack condition. The effectiveness and robustness of the proposed control strategy have been validated by means of a simulated World harmonized Light-duty vehicles Test Cycle (WLTC), under both state feedback and model parameters uncertainties.     

Model-based control of cathode pressure and oxygen excess ratio of a PEM fuel cell system

For PEM fuel cells supplied with air, pressure and flow control is a key requirement for an efficient and dynamic operation because fuel cells are in risk of starvation when the partial pressure of oxygen at the cathode falls below a critical level. To avoid oxygen starvation and, at the same time, to allow for a dynamic operation of the fuel cell system, both excess ratio of oxygen and cathode pressure need to be adjusted rapidly.     

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.     

Exergy analysis of PEM fuel cells for marine applications

Fuel cells have a promising potential use in stationary and mobile power generation systems, as well as in automotive, aerospace or marine industries. At present, the main field of marine applications of fuel cells is submarines. Hydrogen/oxygen polymer electrolyte membrane (PEM) fuel cells are commonly used in this field. Storage of oxygen in liquid form is the optimal solution. Hydrogen can be stored in carbon-nanofibres or metallic hydrides, for example, or in liquid fuels, as alcohols, with further generation of the hydrogen required on-board. The objective of this study is to perform an exergetic analysis of two possibilities of using PEM fuel cells on surface ships and submarines: hydrogen/oxygen PEM fuel cells fed with hydrogen generated by reforming of methanol, and Direct Methanol Fuel Cells directly fed with liquid methanol. To do this, exergy losses and exergetic efficiencies are calculated for both configurations at selected optimal operation points.     

Optimal vehicle control strategy of a fuel cell/battery hybrid city bus

In this article, an optimal vehicle control strategy based on a time-triggered controller area network (TTCAN) system for a polymer electrolyte membrane (PEM) fuel cell/nickel-metal hydride (Ni-MH) battery powered city bus is presented. Aiming at improving the fuel economy of the city bus, the control strategy comprises an equivalent consumption minimization strategy (ECMS) and a braking energy regeneration strategy (BERS). On the basis of the introduction of a battery equivalent hydrogen consumption model incorporating a charge-sustaining coefficient, an analytical solution to the equivalent consumption minimization problem is given. The proposed strategy has been applied in several city buses for the Beijing Olympic Games of 2008. Results of the “China city bus typical cycle” testing show that, the ECMS and the BERS lowered hydrogen consumption by 2.5% and 15.3% respectively, compared with a rule-based strategy. The BERS contributes much more than the ECMS to the fuel economy, because the fuel cell system does not leave much room for the optimal algorithm in improving the efficiency.     

A simple electric circuit model for proton exchange membrane fuel cells

A simple and novel dynamic circuit model for a proton exchange membrane (PEM) fuel cell suitable for the analysis and design of power systems is presented. The model takes into account phenomena like activation polarization, ohmic polarization, and mass transport effect present in a PEM fuel cell. The proposed circuit model includes three resistors to approach adequately these phenomena; however, since for the PEM dynamic performance connection or disconnection of an additional load is of crucial importance, the proposed model uses two saturable inductors accompanied by an ideal transformer to simulate the double layer charging effect during load step changes. To evaluate the effectiveness of the proposed model its dynamic performance under load step changes is simulated. Experimental results coming from a commercial PEM fuel cell module that uses hydrogen from a pressurized cylinder at the anode and atmospheric oxygen at the cathode, clearly verify the simulation results.     

Multi-input and multi-output neural model of the mechanical nonlinear behaviour of a PEM fuel cell system

A fuel cell system model is necessary to prepare and analyse vibration tests. However, in the literature, the mechanical aspect of the fuel cell systems is neglected. In this paper, a neural network modelling approach for the mechanical nonlinear behaviour of a proton exchange membrane (PEM) fuel cell system is proposed. An experimental set is designed for this purpose: a fuel cell system in operation is subjected to random and swept-sine excitations on a vibrating platform in three axes directions. Its mechanical response is measured with three-dimensional accelerometers. The raw experimental data are exploited to create a multi-input and multi-output (MIMO) model using a multi-layer perceptron neural network combined with a time regression input vector. The model is trained and tested. Results from the analysis show good prediction accuracy. This approach is promising because it can be extended to further complex applications. In the future, the mechanical fuel cell system controller will be implemented on a real-time system that provides an environment to analyse the performance and optimize mechanical parameters design of the PEM fuel system and its auxiliaries.     

Output-feedback voltage tracking control for input-constrained PEM fuel cell systems

An output-feedback voltage control system for nonlinear PEM fuel cells is presented. For voltage tracking around equilibrium operating points, the controller design minimizes the energy ratio between tracking error and normalized command while hydrogen and oxygen flowrates satisfy specified magnitude constraints and closed-loop poles meet desired placement constraints. Time response simulations based on Ballard 5 kW PEM fuel cell system parameters verify the design. Simulated controllers constructed numerically via the linear matrix inequality algorithm elaborate relationships between designed input flowrate and voltage tracking error. With controller design based on the same nominal input flowrate constraints, the achieved voltage tracking capability is comparable to our published state-feedback design study. To reduce voltage tracking error under fixed external resistance, gas flowrate magnitude constraints must be relaxed, requiring more fuel energy to manipulate the system variables for operation away from equilibrium conditions. Whereas state-feedback designs depend on internal state variables which are not always measurable, output-feedback control using only voltage tracking error as measurement simplifies practical implementation.     

Modelling CO poisoning and O2 bleeding in a PEM fuel cell anode

Fuel gas containing carbon monoxide severely degrades the performance of a polymer electrolyte membrane (PEM) fuel cell. However, CO poisoning can be mitigated by introducing oxygen into the fuel (oxygen bleeding). A mathematical PEM fuel cell model is developed that simulates both CO poisoning and oxygen bleeding, and obtains excellent agreement with published, experimental data. Modelling efforts indicate that CO adsorption and desorption follow a Temkin model. Increasing operating pressure or temperature mitigates CO poisoning, while use of reformate fuel increases the severity of the poisoning effect. Although oxygen bleeding mitigates CO poisoning, an unrecoverable performance loss exists at high current densities due to competition for reaction sites between hydrogen adsorption and the heterogeneous catalysis of CO. Copyright © 2003 John Wiley & Sons, Ltd.     

Methanol fuel processor and PEM fuel cell modeling for mobile application

The use of hydrocarbon fed fuel cell systems including a fuel processor can be an entry market for this emerging technology avoiding the problem of hydrogen infrastructure. This article presents a 1 kW low temperature PEM fuel cell system with fuel processor, the system is fueled by a mixture of methanol and water that is converted into hydrogen rich gas using a steam reformer. A complete system model including a fluidic fuel processor model containing evaporation, steam reformer, hydrogen filter, combustion, as well as a multi-domain fuel cell model is introduced. Experiments are performed with an IDATECH FCS1200™ fuel cell system. The results of modeling and experimentation show good results, namely with regard to fuel cell current and voltage as well as hydrogen production and pressure. The system is auto sufficient and shows an efficiency of 25.12%. The presented work is a step towards a complete system model, needed to develop a well adapted system control assuring optimized system efficiency.     

Computational modeling and experimental verification of cathode catalyst layer on PEM fuel cells

Fuel cell systems are environmentally friendly energy converters that directly transform the chemical energy of the fuel to electricity. The proton exchange membrane (PEM) fuel cells are one of the most common type of fuel cells since they deliver high power density and are lighter and smaller when compared to the other cells. However, commercialization of the PEM fuel cells is challenging due to the high cost of its components. In addition to high catalyst costs, the problem of poor water management is also a vital issue that needs to be overcome. While the gas diffusion layer of a fuel cell is essential for removing the by-product water, the Nafion solution contained in the catalyst layer has hydrophobic properties and is crucial for both preventing the water accumulation and increasing the effectiveness of the fuel cell. In this study, the effects of Carbon:Nafion ratio on the reduction potential was investigated. The cyclic voltammograms (CV) was produced for each ratio, and it was shown that the CVs exhibit characteristics of hydrogen adsorption/desorption peaks. All the linear sweep voltammogram (LSV) curves revealed well distinguished regions of kinetic, mixed and diffusion limited reaction rate. As a result, it was observed that the ratio of 1:5 resulted higher reduction potential compared to 1:3 and 1:7. Finally, a mathematical model was purposed, in which related the rotation rate and platinum coating with the current density, in order to gain insight about the responses of the fuel cell system. The constructed model is tested and validated experimentally for various parameters that are present in the system, and it may be utilized to determine oxygen reaction activities of the catalysts without performing any unnecessary electrochemical tests.     

The potential of PEM fuel cell for a new drinking water source

The worldwide water scarcity, especially in the developing countries and arid regions, forces people to rely on unsafe sources of drinking water. There is a pressing need for these regions to develop decentralized, small-scale water utilities. However, more than 50% of the total operating costs associated with such small-scale, water-utility operations are the cost of providing electricity to run water pumps. We think that advances in a variety of renewable and sustainable energy technologies offer considerable promise for reducing the energy required for the production and distribution of water by small-scale water utilities. This paper provides a comprehensive review of the potential for using proton exchange membrane (PEM) fuel cells to provide an alternative supply of drinking water. This system can eliminate the excessive energy requirements that are currently associated with water production. Such alternative water production processes are designed to increase the production rate of drinking water by reducing the amount of water required to humidify the reactant gases during stable cell performance. The principal operational components of PEM fuel cells are reviewed and evaluated, including air stoichiometry, pressure, and cell temperature. Hydrogen-fed fuel cell systems provide sufficient water to meet the potable water needs of a typical household. Furthermore, it is concluded that PEM fuel cells have great promise for decentralized, small-scale, water-production applications, because they are capable of generating sufficient quantities of potable water by operating at maximum power and by increasing the number of polymer membranes.