So̸ren Knudsen Kær

Dynamic Model of the High Temperature Proton Exchange Membrane Fuel Cell Stack Temperature

The present work involves the development of a model for predicting the dynamic temperature of a high temperature proton exchange membrane (HTPEM) fuel cell stack. The model is developed to test different thermal control strategies before implementing them in the actual system. The test system consists of a prototype cathode air cooled 30 cell HTPEM fuel cell stack developed at the Institute of Energy Technology at Aalborg University. This fuel cell stack uses PEMEAS Celtec P-1000 membranes and runs on pure hydrogen in a dead-end anode configuration with a purge valve. The cooling of the stack is managed by running the stack at a high stoichiometric air flow. This is possible because of the polybenzimidazole (PBI) fuel cell membranes used and the very low pressure drop in the stack. The model consists of a discrete thermal model dividing the stack into three parts: inlet, middle, and end. The temperature is predicted in these three parts, where they also are measured. The heat balance of the system involves a fuel cell model to describe the heat added by the fuel cells when a current is drawn. Furthermore the model also predicts the temperatures when heating the stack with external heating elements for start-up, heat conduction through stack insulation, cathode air convection, and heating of the inlet gases in the manifold. Various measurements are presented to validate the model predictions of the stack temperatures.

A Transient Fuel Cell Model to Simulate HTPEM Fuel Cell Impedance Spectra

This paper presents a spatially resolved transient fuel cell model applied to the simulation of high temperature PEM fuel cell impedance spectra. The model is developed using a 2D finite volume method approach. The model is resolved along the channel and across the membrane. The model considers diffusion of cathode gas species in gas diffusion layers and catalyst layer, transport of protons in the membrane and the catalyst layers, and double layer capacitive effects in the catalyst layers. The model has been fitted simultaneously to a polarization curve and to an impedance spectrum recorded in the laboratory. A simultaneous fit to both curves is not achieved. In order to investigate the effects of the fitting parameters on the simulation results, a parameter variation study is carried out. It is concluded that some of the fitting parameters assume values which are not realistic. In order to remedy this, phenomena neglected in this version of the model must be incorporated in future versions.

Modeling of CO Influence in PBI Electrolyte PEM Fuel Cells

In most PEM fuel cell MEA’s Nafion is used as electrolyte material due to its excellent proton conductivity at low temperatures. However, Nafion needs to be fully hydrated in order to conduct protons. This means that the cell temperature cannot surpass the boiling temperature of water and further this poses great challenges regarding water management in the cells. When operating fuel cell stacks on reformate gas, carbon monoxide (CO) content in the gas is unavoidable. The highest tolerable amount of CO is between 50–100 ppm with CO-tolerant catalysts. To achieve such low CO-concentration, extensive gas purification is necessary; typically shift reactors and preferential oxidation. The surface adsorption and desorption is strongly dependent upon the cell temperature. Higher temperature operation favors the CO-desorption and increases cell performance due to faster kinetics. High temperature polymer electrolyte fuel cells with PBI polymer electrolytes rather than Nafion can be operated at temperatures between 120–200°C. At such conditions, several percent CO in the gas is tolerable depending on the cell temperature. System complexity in the case of reformate operation is greatly reduced increasing the overall system performance since shift reactors and preferential oxidation can be left out. PBI-based MEA’s have proven long durability. The manufacturer PEMEAS have verified lifetimes above 25,000 hours. They are thus serious contenders to Nafion based fuel cell MEA’s. This paper provides a novel experimentally verified model of the CO sorption processes in PEM fuel cells with PBI membranes. The model uses a mechanistic approach to characterize the CO adsorption and desorption kinetics. A simplified model, describing cathode overpotential, was included to model the overall cell potential. Experimental tests were performed with CO-levels ranging from 0.1% to 10% and temperatures from 160–200°C. Both pure hydrogen as well as a reformate gas models were derived and the modeling results are in excellent agreement with the experiments.© 2006 ASME

Flow and Pressure Distribution in Fuel Cell Manifolds

The manifold is an essential part of the fuel cell stack. Evidently evenly distributed reactants are a prerequisite for an efficient fuel cell stack In this study, the cathode manifold ability to distribute air to the cells of a 70 cell stack is investigated experimentally. By means of 20 differential pressure gauges, the flow distribution is mapped for several geometrical and operating conditions. Special attention is given to the inlet conditions of the manifold. Here, a diffuser design was constructed in order to replace the conventional circular inlet design. The diffuser design showed significant improvements to the flow distribution in comparison to the circular design. Moreover, the best flow distribution was found using a U-shaped configuration.

Particle Image Velocimetry and Computational Fluid Dynamics Analysis of Fuel Cell Manifold

The inlet effect on the manifold flow in a fuel cell stack was investigated by means ofnumerical methods (computational fluid dynamics) and experimental methods (particleimage velocimetry). At a simulated high current density situation the flow field wasmapped on a 70 cell simulated cathode manifold. Three different inlet configurationswere tested: plug flow, circular inlet, and a diffuser inlet. A very distinct jet was formedin the manifold, when using the circular inlet configuration, which was confirmed bothexperimentally and numerically. This jet was found to be an asymmetric confined jet,known as the symmetry-breaking bifurcation phenomenon, and it is believed to cause asignificant maldistribution of the stack flow distribution. The investigated diffuser designproved to generate a much smoother transition from the pipe flow to the manifold flowwith a subsequent better flow distribution. A method was found in the literature to probeif there is a risk of jet asymmetry; it is however recommended by the author to implementa diffuser design, as this will generate better stack flow distribution and less head loss.Generally, the numerical and experimental results were found in to be good agreement,however, a detailed investigation revealed some difference in the results.

Fitting a Three Dimensional PEM Fuel Cell Model to Measurements by Tuning the Porosity and Conductivity of the Catalyst Layer

A three-dimensional, computational fluid dynamics (CFD) model of a PEM fuel cell is presented. The model consists of straight channels, porous gas diffusion layers, porous catalyst layers and a membrane. In this computational domain, most of the transport phenomena which govern the performance of the PEM fuel cell are dealt with in detail. The model solves the convective and diffusive transport of the gaseous phase in the fuel cell and allows prediction of the concentration of the species present. A special feature of the model is a method that allows detailed modelling and prediction of electrode kinetics. The transport of electrons in the gas diffusion layer and catalyst layer is accounted for, as well as the transport of protons in the membrane and catalyst layer. This provides the possibility of predicting the three-dimensional distribution of the activation overpotential in the catalyst layer. The current density dependency on the gas concentration and activation overpotential can thereby be addressed. The proposed model makes it possible to predict the effect of geometrical and material properties on the fuel cell’s performance. It is shown how the ionic conductivity and porosity of the catalyst layer affects the distribution of current density and further how this affects the polarization curve. The porosity and conductivity of the catalyst layer are some of the most difficult parameters to measure, estimate and especially control. Yet the proposed model shows how these two parameters can have significant influence on the performance of the fuel cell. The two parameters are shown to be key elements in adjusting the three-dimensional model to fit measured polarization curves. Results from the proposed model are compared to single cell measurements on a test MEA from IRD Fuel Cells.© 2004 ASME

Vapor Delivery Systems for the Study of the Effects of Reformate Gas Impurities in HT-PEM Fuel Cells

The reforming of methanol can be an alternative source of hydrogen for fuel cells, since it has many practical advantages over hydrogen, mainly due to the technological limitations related to the storage, supply and distribution of the latter. However, despite the ease of methanol handling, impurities in the reformate gas, produced from methanol steam reforming can affect the performance and durability of fuel cells. In this paper different vapor delivery systems, intended to assist in the study of the effects of some of the impurities, are described and compared with each other. A system based on a pump and electrically heated evaporator was found to be more suitable for the typical flow rates involved in the anode feed of a H3 PO4 /PBI-based HT-PEMFC unit cell assembly. Test stations composed of vapor delivery system and mass flow controllers for testing the effects of methanol slip, water vapor, CO and CO2 are also illustrated.Copyright