Expert diagnosis of polymer electrolyte fuel cells

Diagnosing faulty conditions of engineering systems is a highly desirable process within control structures, such that control systems may operate effectively and degrading operational states may be mitigated. The goal herein is to enhance lifetime performance and extend system availability. Difficulty arises in developing a mathematical model which can describe all working and failure modes of complex systems. However the expert's knowledge of correct and faulty operation is powerful for detecting degradation, and such knowledge can be represented through fuzzy logic. This paper presents a diagnostic system based on fuzzy logic and expert knowledge, attained from experts and experimental findings. The diagnosis is applied specifically to degradation modes in a polymer electrolyte fuel cell. The defined rules produced for the fuzzy logic model connect observed operational modes and symptoms to component degradation. The diagnosis is then tested against common automotive stress conditions to assess functionality.     

Fast start-up of a diesel fuel processor for PEM fuel cells

Fuel cell systems based on liquid fuels are particularly suitable for auxiliary power generation due to the high energy density of the fuel and its easy storage. Together with industrial partners, Oel-Waerme-Institut is developing a 3 kW_el PEM fuel cell system based on diesel steam reforming to be applied as an APU for caravans and yachts. The start-up time of a fuel cell APU is of crucial importance since a buffer battery has to supply electric power until the system is ready to take over. Therefore, the start-up time directly affects the battery capacity and consequently the system size, weight, and cost.     

Flooding of the diffusion layer in a polymer electrolyte fuel cell: Experimental and modelling analysis

Water management is widely investigated because it affects both the performance and the lifetime of polymer electrolyte fuel cells. Membrane hydration is necessary to ensure the high proton conductivity, but too much water can cause flooding and pore obstruction within the cathode gas diffusion layer and the electrode. Experimental studies prove that the characteristics of the diffusion layer have great influence on water transport; the introduction of a micro-porous layer between the gas diffusion layer and the electrode reduces flooding and stabilizes the performance of the fuel cell, although the reason is not fully explained. A quantitative method to characterize water transport through the diffusion layers was proposed in our previous work, and the present work aims to further understand the flooding phenomenon and the role of the micro-porous layer. The improved experimental setup and methodology allow an accurate and reliable evaluation of water transport through the diffusion layer in a wide range of operating conditions. The proposed 1D + 1D model faithfully reproduces the experimental data adopting effective diffusivity values in agreement with literature. The presented experimental and modelling analysis allows us to evaluate the influence of pore obstruction on the effective diffusivity, the overall transport coefficient and water flow through the diffusion layer, elucidating the effect of the micro-porous layer on fuel cell performance and operation stability.     

35-We polymer electrolyte membrane fuel cell system for notebook computer using a compact fuel processor

A polymer electrolyte membrane fuel cell (PEMFC) system is developed to power a notebook computer. The system consists of a compact methanol-reforming system with a CO preferential oxidation unit, a 16-cell PEMFC stack, and a control unit for the management of the system with a d.c.–d.c. converter. The compact fuel-processor system (260 cm³) generates about 1.2 L min⁻¹ of reformate, which corresponds to 35 We, with a low CO concentration (<30 ppm, typically 0 ppm), and is thus proven to be capable of being targetted at notebook computers.     

A complete miniaturized microstructured methanol fuel processor/fuel cell system for low power applications

A complete miniaturized methanol fuel processor/fuel cell system was developed and put into operation as compact hydrogen supplier for low power application. The whole system consisting of a micro-structured evaporator, a micro-structured reformer and two stages of preferential oxidation of CO (PROX) reactor, micro-structured catalytic burner, and fuel cell was operated to evaluate the performance of the whole production line from methanol to electricity. The performance of micro methanol steam reformer and PROX reactor was systematically investigated. The effect of reaction temperature, steam to carbon ratio, and contact time on the methanol steam reformer performance is presented in terms of catalytic activity, selectivity, and reformate yield. The performance of PROX reactor fed with the reformate produced by the reformer reactor was evaluated by the variation of reaction temperature and oxygen to CO ratio. The results demonstrate that micro-structured device may be an attractive power source candidate for low power application.     

Numerical analysis of gas crossover effects in polymer electrolyte fuel cells (PEFCs)

The gas crossover phenomenon in polymer electrolyte fuel cells (PEFCs) is an indicator of membrane degradation. The objective of this paper is to numerically investigate the effects of hydrogen and oxygen crossover through the membrane in PEFCs. A gas crossover model is newly developed and implemented in a comprehensive multi-dimensional, multi-phase PEFC model developed earlier. A parametric study is carried out to investigate the effects of the crossover diffusion coefficients for hydrogen and oxygen as well as the membrane thickness. The simulation results demonstrate that the hydrogen crossover induces an additional oxygen reduction reaction (ORR) and consequently causes an additional voltage drop, while the influence of oxygen crossover on PEFC performance is relatively insignificant because it leads to the hydrogen/oxygen chemical reaction at the anode side. Finally, using the time-dependent gas crossover data that are available in the literature (measured in days), we conduct gas crossover simulations to examine the effects of increased gas crossover due to membrane degradation on PEFC performance and successfully demonstrate decaying polarization curves with respect to time. This study clearly elucidates the detailed mechanisms of the hydrogen and oxygen crossover phenomena and their effect on PEFC performance and durability.     

Static and dynamic modeling of a diesel fuel processing unit for polymer electrolyte fuel cell supply

This article introduces the energetic macroscopic representation (EMR) as approach for the dynamic modeling of a diesel fuel processing unit. The EMR is the first step toward model-based control structure development. The autothermal fuel processing system containing: heat exchanger, reformer, desulfurization, water gas shift, preferential oxidation and condensation is divided into a multitude simple subblocks. Each subblock describes an elementary step of the fuel conversion, several of these blocks may occur in a single module. Calculations are carried out using two basic principles: mass and energy balances. For model-based control development, it is imperative that the model represents dynamic behavior, therefore temperature and pressure dynamics are taken into account in the model. It is shown that the model is capable to predict the stationary behavior of the entire fuel processing unit correctly by comparison with given data. Predictions regarding reformer heat up, temperature and pressure dynamics are also provided.     

A 5 kWt catalytic burner for PEM fuel cells: Effect of fuel type,fuel content and fuel loads on the capacity of the catalytic burner

For proton exchange membrane fuel cell systems (PEMFC) integrated with fuel processors, the calorific value of reformate gases produced during the start-up phase must be recovered. An appropriate exhaust after treatment system has crucial importance for PEMFC systems. Catalytic combustion is a promising alternative regarding its total oxidation capability of low calorific value gases at low temperatures, thereby reducing environmentally hazardous emissions. The aim of the study is to develop an after treatment system using a catalytic burner with a nominal capacity of 5 kW_t, which is also adaptive to partial loads of PEM fuel cell capacity. Fuel type, fuel composition and fuel loads are important parameters determining the operating window of the catalytic burner. Precious metal based catalysts, as proved to be the most active catalysts for the oxidation of hydrocarbons, can withstand temperatures of about 1073 K without exhibiting a rapid deactivation. This is the main barrier dictating the operating window and thereby determining the capacity of the burner. In this work, 1.5% natural gas (NG) alone was found to be the upper limit to control the catalyst bed temperature below 1073 K. In the case of catalytic combustion of hydrogen–NG mixture, 7% of hydrogen with NG up to 0.6% could be totally oxidized below 1073 K. Within the experimented ranges of fuel loads, between 2.5 kW_t and 5.5 kW_t, the temperature of the catalyst bed was seen to increase with increasing the fuel load at constant fuel percentages. It has been observed that fuel type was another parameter affecting the exhaust gas temperature.     

Experimental and theoretical analysis of a natural gas fuel processor

In this study, a natural gas fuel processor was experimentally and theoretically investigated. The constructed 2.0 kWth fuel processor is suitable for a residential-scale high temperature proton exchange membrane fuel cell. The system consists of an autothermal reformer; gas clean-up units, namely high and low-temperature water-gas shift reactors; and utilities including feeding unit, burner, evaporator and heat exchangers. Commercial monolith catalysts were used in the reactors. The simulation was carried out by using ASPEN HYSYS program. A validated kinetic model and adiabatic equilibrium model were both presented and compared with experimental data. The nominal operating conditions which were determined by the kinetic model were the steam-to-carbon ratio of 3.0, the oxygen-to-carbon ratio of 0.5 and the inlet temperatures of 450 °C for autothermal reformer, 400 °C for high-temperature water-gas shift reactor and 310 °C for low-temperature water-gas shift reactor. Experimental results at the nominal condition showed that the performance criteria of the hydrogen yield, the fuel conversion and the efficiency were 2.53, 93.5% and 82.3% (higher heating value-HHV), respectively. The validated kinetic model was further used for the determination of 2–10kWthermal fuel processor efficiency which was increasing linearly up-to 86.3% (HHV).     

Self-sustained operation and durability testing of a 300 W-class micro-structured LPG fuel processor

A miniaturized fuel processor for LPG has been developed and put into operation as compact hydrogen supply system for low power applications. The fuel processor consists of an integrated micro-structured evaporator and a micro-structured reformer both integrated with micro-structured catalytic burners, heat exchangers, and a micro-structured water-gas shift (WGS) stage. In the current paper, performance data of a coupled LPG steam reformer/catalytic burner are presented, which has been running stably over 1060 h with repeated start-up and shut-down cycles. On top of that, some performance data of complete LPG fuel processors will be shown, which have been operated up to 3500 h in combination with high temperature PEM fuel cell stacks. These fuel processing systems are capable to convert LPG with a nominal hydrogen production rate of 0.263 Nm³ h⁻¹. It could be demonstrated, that the micro-structured devices are not only compact but show also high reliability and durability.     

Measuring method for flow rate distribution between cells in a polymer electrolyte fuel cell stack

The fuel cell transmogrified from a single cell that was the research object to stack is used in various fields such as cars, portable power sources, and fuel-cell cogeneration systems. It is preferable in the stack for the flow rate distribution between cells to be uniform because of the performance gain in power generation efficiency and longevity. As for the flow rate distribution between cells, the method for measuring using smoke in the measuring method and making visible the heat distribution in the stack is reported. However, a research stack was used with these measures, not a stack for practical use. In this report, a method for measuring the flow rate distribution between cells which can also be used for a cell that uses hydrogen limiting current in practical use was examined.     

Simulated start-stop as a rapid aging tool for polymer electrolyte fuel cell electrodes

The corrosion stability of supported catalysts as employed in state of the art intermediate temperature polymer electrolyte fuel cells has been studied by means of simulated start-stop cycling (150 cycles). The carbon dioxide formation from the air electrode has been monitored during repeated cycling runs and the loss of catalyst support has been correlated with performance drops. Degradation effects have been studied at different current densities in order to differentiate between kinetic and mass transport effects. Finally, correlations of this accelerated aging tool with a more realistic durability test over 4000 h and 157 start-stop cycles have been made and the good agreement between simulated and realistic approaches has been confirmed, demonstrating the high value of the experimental approach and analysis.     

One-dimensional thermal model of cold-start in a polymer electrolyte fuel cell stack

A transient, one-dimensional thermal model for a generic polymer electrolyte fuel cell (PEFC) stack is developed to investigate the cold-start ability and the corresponding energy requirement over different operating and ambient conditions. The model is constructed by applying the conservation of energy on each stack component and connecting the component's relevant boundaries to form a continuous thermal model. The phase change of ice and re-circulation of coolant flow are included in the analytical framework and their contribution to the stack thermal mass and temperature distribution of the components is also explored. A parametric study was conducted to determine the governing parameters, relative impact of the thermal mass of each stack component and ice, and anticipated temperature distribution in the stack at start-up for various operating conditions. Results indicate that 20 cells were sufficient to accurately experimentally and computationally simulate the full size stack behavior. It was observed that an optimum range of operating current density exists for a chosen stack design, in which rapid start-up of the stack from sub-zero condition can be achieved. Thermal isolation of the stack at the end plates is recommended to reduce the start-up time. Additionally, an end plate thickness exceeding a threshold value has no added effect on the stack cold-start ability. Effect of various internal and external heating mechanisms on the stack start-up were also investigated, and flow of heated coolant above 0 °C was found to be the most effective way to achieve the rapid start-up.     

Preferred test conditions for measuring flow rate distribution between cells in a polymer electrolyte fuel cell stack

A new and practical testing technique was developed for measuring the flow rate distribution between cells in a stack that did not contain any internal sensors. The flow rate distribution is obtained by measuring the hydrogen limiting current of each cell in the stack while a mixed gas of hydrogen and dimethyl ether is supplied to the anode and hydrogen to the cathode. In order to measure large flow rate deviations between cells, it is necessary to decrease the flow rate of the anode hydrogen and to sufficiently humidify the cells. The faster the increasing rate of the current, the more the apparent hydrogen limiting current increases than the theoretical electrochemical equivalent current. However, the relative flow rate deviations between cells can be obtained by a practical accuracy using the ratio of the apparent hydrogen limiting current. Humidification of the cell is indispensable for the measurement and a method using dry anode gas and humidified cathode gas is recommended. The preferred test conditions for measuring the flow rate distribution between cells in a PEFC stack are proposed.     

Deuterium isotope separation by polymer electrolyte fuel cells connected in series

The separation of hydrogen isotopes has important applications for fundamental science and nuclear engineering. This study investigates isotope separation by stacked polymer electrolyte fuel cells that form part of a combined electrolysis and fuel cell (CEFC) system. Fuel gas containing deuterium (D) was generated by water electrolysis and passed through three fuel cells (FCs) connected in series. Increasing the number of operating FCs in the series greatly improved D separation, but had only a modest impact on power consumption. When all three FCs were individually controlled, the separation efficiency depended on the power condition in each FC. At high current the separation factor of the CEFC system reached over 100 owing to the relationship between fuel gas utilization and separation efficiency.     

Non intrusive diagnosis of polymer electrolyte fuel cells by wavelet packet transform

Fuel cell is a promising technology for both automotive and stationary applications. However, its reliability and its lifetime remain major hurdles to its wide access to these markets.It is therefore necessary to develop reliable diagnosis tools adapted to these two applications’ requirements. More particularly, online and real time tools for diagnosis will permit an early faults diagnosis and therefore an increase of the system reliability and performance.Most of the existing fault diagnosis methodologies in fuel cells require the knowledge of numerous parameters that may lead to a special inner parameter monitoring setup, which is difficult, even impossible to obtain, considering constraints like fuel cell stacks’ geometry. Moreover, considering the final fuel cell stack end-uses, for instance in transportation applications in which the “on-board” instrumentation has to be minimized, a model using a minimal number of parameter is highly desirable.In this paper, a simple and low-cost flooding diagnosis method applied to a PEFC (Polymer Electrolyte Fuel Cell) is described. This method only uses the stack voltage and can be adapted to a large set of fuel cell configurations and applications.Coming from the signal-processing domain, the diagnosis consists in a signal feature extraction by multiscale decomposition using discrete wavelet transform, followed by fault identification and classification. Results obtained in this work showed that the wavelet analysis method allows the identification of the flooding based on the patterns obtained from the wavelet packet coefficients.The application of wavelet theory to fuel cell diagnosis is innovative and very promising and the experimental results obtained in this study proved its feasibility and reliability to classify correctly PEFC experimental states into flooded and non-flooded state of health.     

Dynamic modeling of a high-temperature proton exchange membrane fuel cell with a fuel processor

A dynamic model of a high-temperature proton exchange membrane fuel cell with a fuel processor is developed in this study. In the model, a fuel processing system, a fuel cell stack, and an exhaust gas burner are modeled and integrated. The model can predict the characteristics of the overall system and each component at the steady and transient states. Specifically, a unit fuel cell model is discretized in a simplified quasi-three-dimensional geometry; therefore, the model can rapidly predict the distribution of fuel cell characteristics. Various operating conditions such as the steam-to-carbon ratio, oxygen-to-carbon ratio, and autothermal reforming inlet temperature are varied and investigated in this study. In addition, the dynamic characteristics exhibited during the transient state are investigated, and an efficiency controller is developed and implemented in the model to maintain the electrical efficiency. The simulation results demonstrate that the steam-to-carbon ratio and the oxygen-to-carbon ratio affect the electrical and system efficiency and that controlling the fuel flow rate maintains the electrical efficiency in the transient state. The model may be a useful tool for investigating the characteristics of the overall system as well as for developing optimal control strategies for enhancing the system performance.     

A techno-economic evaluation of the hydrogen production for energy generation using an ethanol fuel processor

The techno-economic analysis of a process to convert ethanol into H₂ to be used as a fuel for PEM fuel cells of H₂-powered cars was done. A plant for H₂ production was simulated using experimental results obtained on monolith reactors for ethanol steam reforming and WGS steps. The steam reforming (Rh/CeSiO₂) and WGS (Pt/ZrO₂) monolith catalysts remained quite stable during long-term startup/shut down cycles, with no carbon deposition. The H₂ production cost was significantly affected by the ethanol price. The monolith catalyst costs contribution was lower than that of conventional reactors. The H₂ production cost obtained using the expensive Brazilian ethanol price (0.81 US$/L ethanol) was US$ 8.87/kg H₂, which is lower than the current market prices (US$ 13.44/kg H₂) practiced at H₂ refueling stations in California. This result showed that this process is economically feasible to provide H₂ as a fuel for H₂-powered cars at competitive costs in refueling stations.     

Potential of hydrolyzed oxymethylene dimethyl ether for the suppression of fuel crossover in polymer electrolyte fuel cells

Fuel crossover is a crucial issue for polymer electrolyte fuel cells (PEFCs) supplied directly with liquid fuels. Therefore, finding a type of fuel with a low fuel crossover rate is required. In this study, we investigated fuel crossover characteristics for oxymethylene dimethyl ether (OME), which has attracted attention for use in engines, and compared it with methanol. Cell performance tests and exhaust gas analyses showed that fully hydrolyzed OME (h-OME), which consisted of methanol and formaldehyde, yields high cell performance at high fuel concentrations, due to the low fuel crossover rate, compared with a direct methanol fuel cell (DMFC). To clarify a factor suppressing fuel crossover, h-OME's effective diffusion coefficient in the membrane was measured. Although an effective diffusion coefficient for fuels like methanol commonly increases with increased fuel's concentration, h-OME's effective diffusion coefficient decreased with increased fuel concentration, leading to a low fuel crossover rate at high h-OME concentrations.     

Two-phase flow measurement system for polymer electrolyte fuel cells

In this paper, a sensory system capable of measuring two-phase flow of water at the PEFC output is introduced. It works based on collecting and evaporating the liquid water that exits the PEFC in a vessel that is heated to a temperature above that of the fuel cell temperature. By measuring the vessel dew point temperature and flow rate, the mass of water in liquid and vapor phases are calculated. To demonstrate the capabilities of this measurement system, it is placed at the output of a PEFC cathode during membrane conditioning. The effect of two-phase flow on cell voltage reveals two distinct modes of liquid water transport in the PEFC cathode during membrane conditioning.