The use of on-line hydrogen sensor for studying inert gas effects and nitrogen crossover in PEMFC system

The design and construction of a polymer electrolyte membrane fuel cell (PEMFC) system test bench suitable for investigating the effects of inert gas build-up and hydrogen quality on the performance of PEMFC systems is reported. Moreover, a new methodology to measure the inert gas crossover rate using an on-line hydrogen concentration sensor is introduced, and preliminary results are presented for an aged 8 kW PEMFC stack. The system test bench was also characterized using the same stack, whereupon its performance was observed to be close to commercial systems. The effect of inert gas accumulation and hence the quality of hydrogen on the performance of the system was studied by diluting hydrogen gas in the anode supply pipeline with nitrogen. During these experiments, uneven performance between cells was observed for the aged stack.     

Uncertainty analysis and robust control of fuel delivery systems considering nitrogen crossover phenomenon

In this paper, the fuel delivery subsystem (FDS) with hydrogen recirculation and anode bleeding is applied in proton exchange membrane fuel cell (PEMFC) system, which is utilized to supply hydrogen to the anode of stack and recirculate fuel back to the supply line. As the diffusion of nitrogen from cathode to anode is inevitable in a real PEMFC during long-term operation. To prevent system performance decline due to nitrogen accumulation. Therefore, this paper firstly develops a control-oriented nonlinear dynamic FDS model involving gas diffusion. Additionally, the FDS is very sensitive to operating environment, uncontrolled operation conditions may cause stack degradation. Specifically, a method based on Monte Carlo simulation is proposed to identify the key parameter boundaries. Then the gas distribution in FDS due to nitrogen crossover is analyzed in detail. After this, a hybrid robust methodology based on sliding mode algorithm is also proposed to maintain adequate hydrogen pressure supply, suitable hydrogen and nitrogen content in the system in presence of nitrogen crossover and renewed uncertainties. Finally, the performance of the presented controller is compared with nonlinear PID (NPID) control and nonlinear multi-input-multi-output (NMIMO) control through a hardware-in-the-loop test bench. Experimental results show that the hybrid controller is accurate and suitable for control purpose, the nitrogen content is restricted to the given range and the variation of output voltage is limited to the desired boundaries, the feasibility and effectiveness are validated.     

Determination of fuel utilisation and recirculated gas composition in dead-ended PEMFC systems

This work presents a fundamental theory and methods for understanding the gas composition dynamics in PEMFC anode fuel supply compartments operated dead-ended with recirculation. The methods are applied to measurement data obtained from a PEMFC system operated with a 1 kW short stack.We show how fuel utilisation and stack efficiency, two key factors determining how well a fuel supply system performs, are coupled through the anode gas composition.The developed methods allow determination of the anode fuel supply molar balance, giving access to the membrane crossover rates and the extent of recirculated gas exchanged to fresh fuel during a purge. A methane tracer gas is also evaluated for estimating fuel impurity enrichment ratios.The above theory and methods may be applied in modelling and experimental research activities related to defining hydrogen fuel quality standards, as well as for developing more efficient and robust PEMFC system operation strategies.     

Operating characteristics of an air-cooling PEMFC for portable applications

Optimal design and proper operation is important to get designed output power of a polymer electrolyte membrane fuel cell (PEMFC) stack. The air-cooling fuel cell stack is widely used in sub kW PEMFC systems. The purpose of this study is to analyze the operating conditions affecting the performance of an air-cooling PEMFC which is designed for portable applications. It is difficult to maintain well balanced operating conditions. These parameters are the relative humidity, the temperature of the stack, the utility ratio of the reactant gas and so on. In this study a 500 W rate air-cooling PEMFC was fabricated and tested to evaluate the design performance and to determine optimal operating conditions. Moreover, basic modeling also is carried out. These results can be used as design criteria and optimal operating conditions for portable PEMFCs.     

Experimental analysis of cathode flow stoichiometry on the electrical performance of a PEMFC stack

The paper describes an experimental analysis on the effect of cathode flow stoichiometry on the electrical performance of a PEMFC stack. The electrical power output of a PEMFC stack is influenced by several independent variables (factors). In order to analyse their reciprocal influence, an experimental design methodology was adopted in a previous experimental session, to determine which factors deserve particular attention. In this work, a further experimental analysis has been carried out on a very significant factor: cathode stoichiometry. Its effects on the electrical power of the PEMFC stack have been investigated. The tests were performed on a 3.5 kW_el ZSW stack using the GreenLight GEN VI FC Test Station. The stack characteristics have been obtained running a predefined loading pattern. Some parameters were kept constant during the tests: anode and cathode inlet temperature, anode and cathode inlet relative humidity, anode stoichiometry and inlet temperature of the cooling water. The experimental analysis has shown that an increase in air stoichiometry causes a significant positive effect (increment) on electric power, especially at high-current density, and up to the value of 2 stoichs. These results have been connected to the cathode water flooding, and a discussion was performed concerning the influence of air stoichiometry on electrode flooding at different levels of current density operation.     

Metamodel based design optimization approach in promoting the performance of proton exchange membrane fuel cells

This research presents a systematization and effectiveness approach in promoting the performance of the power density of a Proton Exchange Membrane Fuel Cell (PEMFC) by Metamodel-Based Design Optimization (MBDO). The proposed methodology of MBDO combines the design of experiment (DoE), metamodeling choice and global optimization. The fractional factorial experimental design method can screen important factors and the interaction effects in DoE, and obtain optimal design of the robust performance parameters by Taguchi method. Metamodeling then adopts the ability to establish a non-linear model of a complex PEMFC system configuration of an artificial neural network (ANN) based on the back-propagation network (BPN). Finally, on the many parameters (factors) of optimization, a genetic algorithm (GA) with a high capability for global optimization is used to search the best combination of the parameters to meet the requirement of the quality characteristics. Experimental results confirmed by the test equipment demonstrate that the MBDO approach is effective and systematic in promoting PEMFC performance of power density.     

Simulation analysis of hydrogen recirculation rates of fuel cells and the efficiency of combined heat and power

The objective of this study was to simulate a proton-electrolyte membrane fuel cell (PEMFC) system, namely a PEMFC stack, an anode gas supply subsystem, an anode gas-recovery subsystem, a cathode gas supply subsystem, and a tail gas exhaustion subsystem. In addition, this paper presents an analysis of the efficiency of combined heat and power (CHP) systems. MATLAB and Simulink were employed for dynamic simulation and statistical analysis. The rates of active and the passive anode hydrogen recirculation were considered to elucidate the mechanism of hydrogen circulation. When recovery involved diverse recovery mechanisms, the recirculation rate was affected by the pressure at the hydrogen outlet of the PEMFC system. The greater the pressure was at that outlet, the higher the recovery rate was. In the hydrogen recovery system, when the temperature of the hydrogen supply end remained the same, increasing the temperature of the gas supply end increased the efficiency of the fuel cells; fixing the flow of the hydrogen supply end and increasing the temperature of the hydrogen supply end increased the efficiency of the PEMFC system. A calculation of the efficiency of the recovery system indicated that the thermal efficiency of the fuel cells exceeded 35%, the power generation efficiency exceeded 45%, and the efficiency of the CHP system exceeded 80%.     

Gravity effect on the performance of PEM fuel cell stack with different gas manifold positions

The importance of gravity effect on the performance of proton exchange membrane fuel cell (PEMFC) has recently been recognized. In this paper, the effect of gravity on the performance of PEMFC has been investigated associating with different gas intake modes. The polarization curves of the stack with different positions of reaction gas inlet and outlet at varied gravitational angles are addressed in detail. The results indicate that the output power of PEMFC stack can be greatly enhanced at the optimized gravitational angle. Gas intake modes that were realized by varying the gas inlet and outlet positions strongly affect the stack performance as well. The optimized performance can be reached at the tilted angle of 90° when both air and hydrogen inlets are placed at the upper side of the stack, whereas the worst performance occurs at the tilted angle of 90° when air and hydrogen flow into the channel from the bottom side of the stack. These results have important implications for PEM fuel cell design and operational strategies. In order to improve the performance, fuel cells should be designed and operated at the optimized gravitational angle and gas inlet/outlet position. Copyright © 2011 John Wiley & Sons, Ltd.     

Experimental analysis of the performance of the air supply system in a 120 kW polymer electrolyte membrane fuel cell system

For analyzing the performance of 120 kW polymer electrolyte membrane fuel cell (PEMFC) system and its air supply system, an air system test bench was built, then applied on a 120 kW PEMFC system test bench composed of air supply subsystem, hydrogen supply subsystem, stack, cooling subsystem and electronic control subsystem. The strategy composed of feedforward table and Piecewise proportional integral (PI) feedback control strategy is employed to regulate the flow rate and pressure of air supply system. Firstly, the air compressor map and the mapping relationship between the speed of air compressor, opening of back-pressure valve and stack current are obtained by carrying out experiments on the PEMFC air system bench. Then, the max output performance, steady-state performance, the startup performance, the dynamic response abilities of PEMFC system are tested, respectively. During the experiments, performances under different test conditions were analyzed by comparing parameters such as voltage inconsistency, average voltage, minimum voltage, voltage range, net power of the PEMFC system, and stack power. The test results show that the air supply system can provide qualified flow rate and pressure for the PEMFC stack. The peak power of the stack is 120 kW and net power of the system is 97 kW when the current is 538 A. The response time from rated net power to idle net power is 12 s and from idle net power to rated net power is 23 s. The overshoot of average voltage and minimum voltage in the process of increasing load is both 0.01 V, which are 0.015 V and 0.02 V lower than that when the load is decreased, respectively. The dynamic response speed and stability of the PEMFC system in the process of decreasing the load are better than those in the process of increasing the load.     

Optimization of PEMFC system operating conditions based on neural network and PSO to achieve the best system performance

PEMFC system is a complex new clean power system. Based on MATLAB/Simulink, this paper develops a system-level dynamic model of PEMFC, including the gas supply system, hydrogen supply system, hydrothermal management system, and electric stack. The neural network fits the electric stack model to the simulation data. The effects of different operating conditions on the PEMFC stack power and system efficiency are analyzed. Combining the power of the reactor and the system efficiency to define the integrated performance index, the particle swarm optimization (PSO) algorithm is introduced to optimize the power density and system efficiency of the PEMFC with multiple objectives. The final optimal operating point increases the power density and system efficiency by 1.33% and 12.8%, respectively, which maximizes the output performance and reduces the parasitic power.     

A comparative study of single and dual ejector concepts for anodic recirculation system in high-performance vehicular proton exchange membrane fuel cells

The fuel supply system is a critical element in Proton Exchange Membrane Fuel Cell (PEMFC). In vehicular applications, an anode recirculation system (ARS) is required to return residual hydrogen to the fuel line. A 1D model of an ejector-based ARS was created and integrated with a vehicular PEMFC in this study. Therefore, two configurations, single-ejector-based and dual-ejector-based ARS, were developed to regulate the fuel supply and, thus, utilized to examine and conduct a comparative study of their advantages and disadvantages to the dynamic behavior of the stack. The use of ARS was found to improve stack performance and fuel utilization by delivering high stoichiometry, recirculation rate, and relative humidity. Contrary to a single-ejector-based ARS, whose use is restricted due to its limited working range, dual-ejectors can cover all fuel cell stack operating ranges. Sensitivity analysis revealed that primary pressure was the most significant parameter affecting ejector performance and flow characteristics.     

High performance PEMFC stack with open-cathode at ambient pressure and temperature conditions

An open-air cathode proton exchange membrane fuel cell (PEMFC) was developed. This paper presents a study of the effect of several critical operating conditions on the performance of an 8-cell stack. The studied operating conditions such as cell temperature, air flow rate and hydrogen pressure and flow rate were varied in order to identify situations that could arise when the PEMFC stack is used in low-power portable PEMFC applications. The stack uses an air fan in the edge of the cathode manifolds, combining high stoichiometric oxidant supply and stack cooling purposes. In comparison with natural convection air-breathing stacks, the air dual-function approach brings higher stack performances, at the expense of having a lower use of the total stack power output. Although improving the electrochemical reactions kinetics and decreasing the polarization effects, the increase of the stack temperature lead to membrane excessive dehydration (loss of sorbed water), increasing the ohmic resistance of the stack (lower performance).     

Development of hybrid photovoltaic-fuel cell system for stand-alone application

In this paper we present firstly the different hybrid systems with fuel cell. Then, the study is given with a hybrid fuel cell–photovoltaic generator. The role of this system is the production of electricity without interruption in remote areas. It consists generally of a photovoltaic generator (PV), an alkaline water electrolyzer, a storage gas tank, a proton exchange membrane fuel cell (PEMFC), and power conditioning units (PCU) to manage the system operation of the hybrid system. Different topologies are competing for an optimal design of the hybrid photovoltaic–electrolyzer–fuel cell system. The studied system is proposed. PV subsystem work as a primary source, converting solar irradiation into electricity that is given to a DC bus. The second working subsystem is the electrolyzer which produces hydrogen and oxygen from water as a result of an electrochemical process. When there is an excess of solar generation available, the electrolyzer is turned on to begin producing hydrogen which is sent to a storage tank. The produced hydrogen is used by the third working subsystem (the fuel cell stack) which produces electrical energy to supply the DC bus. The modelisation of the global system is given and the obtained results are presented and discussed.     

Experimental investigation of the effect of hydrogen recirculation on the performance of a proton exchange membrane fuel cell

The hydrogen recirculation in proton exchange membrane fuel cell (PEMFC) is recommended for the hydrogen supply of PEMFC, and hydrogen ejectors are gradually being used in fuel cell vehicles due to low noise and low energy consumption. However, there is a lack of discussion about the influence of recirculation rate on the stack. Due to passive regulating mechanism of the ejectors, a miniature speed-adjustable peristaltic pump is used to simulate the hydrogen ejector in this study to investigate the effect of hydrogen recirculation on the performance of PEMFC stack. Experiments are conducted under different pump flow rates. The stack with hydrogen recirculation is proven to have better performance, but over high pump flow rate can lead to hydrogen shortage. It is interesting to find that the flow rate fluctuation of hydrogen inlet affects the stability of stack performance, and pressure drop and recovery time during purge process are proposed as effective indicators for performance analysis. Finally, pump flow rates between 60 ml/min and 105 ml/min are defined as “effective area”. Based on the analysis of effective indicators, keeping at “effective area” is further proved to improve the performance of the stack, which is also useful to design hydrogen recirculation.     

Continuous bubble humidification and control of relative humidity of H2 for a PEMFC system

The proton conductivity of perfluorinated ionomer membrane used in a proton exchange membrane fuel cell (PEMFC) depends largely on the extent of hydration state of the membrane. Sufficient membrane hydration is achieved typically through the humidification of gases prior to feeding them into the fuel cell. Further, hydrogen humidification is known to have a larger impact on the performance of a PEMFC than the oxygen humidification. Bubble humidification has been a widely used method to externally humidify hydrogen. However, to-date a continuous bubble humidification system, which is essential to the continuous operation of the PEMFC system, has not been implemented. The main contributions of this work are (i) a design for continuous humidification of hydrogen for the PEMFC system and (ii) a method to maintain the RH of hydrogen between 93 and 95% (at desired temperature) over a wide range of gas flow rates. One of the key advantages of the proposed design is the flexibility of using recirculated stack coolant water to increase the energy efficiency of the PEMFC system. The design is first tested off-line and then online with a 1 kW stack. Results obtained from both the off-line and online tests indicate that the design successfully meets the demands of an online operation. It is observed that with the use of the proposed humidification scheme, the stack efficiency in terms of power output increases by about 6–19% of the power obtained under dry hydrogen conditions.     

Effects of operating conditions on the performances of individual cell and stack of PEM fuel cell

In this study, experiments were carried out to study the effects on the performances of individual cell and stack of PEM fuel cell. In the experiment, there are four key operating conditions that affect the cell performance, and they are gas humidification temperature, cell temperature, assembled torsion, and gas flow rate. A 5-cell stack of PEMFC was used to measure the voltage and current density for individual cell in this experiment. Results reveal that the performances of the center fuel cells are relatively lower than those of the cells on both sides of the stack. It is also shown that stack performance increases with the increase in the anode humidification temperature as well as the center cell of the stack. As for the effect of cell temperature, results indicate that stack performance increases with the increase in cell temperature. It is also disclosed that the performances of individual cell and stack do not change with the increase in the anode gas stoichiometric ratio, but increase with the increase in the cathode gas stoichiometric ratio. In addition, the experiment results also show that the whole stack's performance is enhanced with the increase in the assembling torsion.     

Experimental Investigation of a Novel Design of Proton Exchange Membrane Fuel Cell Stack

A design study of a novel proton exchange membrane fuel cell (PEMFC) is presented in this article. The PEMFC is particularly suited to the automotive and small-scale stationary industries; however, at this stage it fails to be a viable commercial alternative to the internal combustion engine. This is mainly due to large material and manufacturing costs associated with components used in the fuel cell. A new design approach that removes the bipolar plate from the PEMFC stack is investigated. A single PEMFC, which features the design changes that can be integrated in the main stack, has been designed, manufactured, assembled, and tested to obtain performance characteristics for a range of operating conditions. Two different flow configurations for the reactants, that is, dead-end gas flow and through-mode flow, were tested. The new design achieved performance comparable to that with conventional designs reported in literature. The experimental results confirmed that bipolar plate can be removed and it is possible to bring down the costs and weight of the stack drastically. It is envisaged that the new design will allow the PEMFC to potentially inject into the current market.     

Constructal PEM fuel cell stack design

This paper describes a structured procedure to optimize the internal structure (relative sizes, spacings), single cells thickness, and external shape (aspect ratios) of a polymer electrolyte membrane fuel cell (PEMFC) stack so that net power is maximized. The constructal design starts from the smallest (elemental) level of a fuel cell stack (the single PEMFC), which is modeled as a unidirectional flow system, proceeding to the pressure drops experienced in the headers and gas channels of the single cells in the stack. The polarization curve, total and net power, and efficiencies are obtained as functions of temperature, pressure, geometry and operating parameters. The optimization is subjected to fixed stack total volume. There are two levels of optimization: (i) the internal structure, which accounts for the relative thicknesses of two reaction and diffusion layers and the membrane space, together with the single cells thickness, and (ii) the external shape, which accounts for the external aspect ratios of the PEMFC stack. The flow components are distributed optimally through the available volume so that the PEMFC stack net power is maximized. Numerical results show that the optimized single cells internal structure and stack external shape are “robust” with respect to changes in stoichiometric ratios, membrane water content, and total stack volume. The optimized internal structure and single cells thickness, and the stack external shape are results of an optimal balance between electrical power output and pumping power required to supply fuel and oxidant to the fuel cell through the stack headers and single-cell gas channels. It is shown that the twice maximized stack net power increases monotonically with total volume raised to the power 3/4, similarly to metabolic rate and body size in animal design.     

Improvement of thermal management of proton exchange membrane fuel cell stack used for portable devices by integrating the ultrathin vapor chamber

UVC (ultrathin vapor chamber) simultaneously has a high heat-conducting property, excellent temperature uniformity and simple structure. These advantages are very suitable for thermal management of the open-cathode PEMFC (proton exchange membrane fuel cell) stack. In this work, two-type UVCs with different appearances are integrated into a conventional PEMFC stack respectively. The effect of UVC on the output performance, thermal management and operating stability is investigated by the experiment combined with simulation. The results show that UVC can significantly increase the output voltage under high current density. In 35 A, the output voltage of the stack integrated the vertical UVC increases by 20.25% relative to the conventional stack. Thermal management is also improved by UVC. The highest temperature inside the stack decrease by 9 °C in 35 A, and the membrane temperature is decreased obviously. But it still exceeds the optimal operating temperature of open-cathode PEMFC stack due to the poor cooling type in the condensation side of UVC. UVC improves the operation stability of the stack and slows the deteriorative speed of output performance. This work hopes to attract more attention to the application of UVC on the thermal management of portable power sources used open-cathode PEMFC stack.