News & HighlightsResearch in Flow Control in Fuel Cells at the University of PittsburghContact: Jeffery S. Vipperman, jsv@pitt.edu
A fuel cell is a chemical reaction device in which hydrogen (sometimes from a hydrocarbon source) and oxygen combine with the help of a catalyst, an electrolyte, and a separating membrane to form water and an excess of electrons that are used as a power source. The appeal of fuel cells is their lack of moving parts, their negligible emissions, and their wide scalability from the micro scale (for portable electronics) to the macro scale (for automotive or land-based power plants). A fuel “cell” is made up of channels for fuel flow, channels for air flow, and an electrolyte (along with catalysts and membranes) that separates the two. Fuel cells are typically classified according to their electrolyte. Individual cells can be combined to form a fuel cell “stack” that has increased electrical power output. Existing fuel cell systems such as proton exchange membrane (PEM) and solid oxide fuel cells use inlet fuel flow manifold designs. The cell geometry within a fuel cell stack provides many flow paths for reactants to flow, which inherently create flow distribution problems that can potentially cause inefficiencies and damage to the fuel cell membrane. For example, the PEM fuel cell is considered for military portable power applications, which require the fuel cell to perform consistently during variable load operations, and most importantly provide the power needed for an acceptable length of time without refueling. Unfortunately, the existing PEM fuel cell stack systems experience problems associated with poor distribution of reactant fuel across the membrane surface, which inherently causes a decrease in the membrane life cycle. In addition, excess reactant flow is commonly used to correct for insufficient flow distribution across the membrane surface, and this causes a decrease in fuel utilization efficiency. To further illustrate the problems that arise due to mal-distribution of reactants, consider a PEM stack employing cells with parallel flow channels. Problems arise when the generated water vapor condenses onto the channel surfaces thereby blocking or restricting the flow through the channel, and the amount of hydrogen reacting with the catalyst. When this happens, the associated cell will perform poorly (generate low voltage and current) and cause the cell to overheat locally at the misbehaved channel. Damage can result if the stack is operated for too long at these conditions. This makes the design of the channel and manifold geometry difficult, especially if the system is to be used under a wide range of operating conditions. To address this problem the Department of Energy’s National Energy Technology Lab (NETL) is investigating a novel technique of integrated flow distribution for individual cells within the stack. However, using conventional valves to perform flow distribution control can be very expensive and significantly increase the overall cost and size of the system. It is anticipated that microelectromechanical systems (MEMS) technology can provide micro-valves for internal flow control capability, at a fraction of the cost of conventional technology and with no size penalty. Therefore, we are working closely with NETL in developing microvalves for controlling flow within the stack to increase performance and decrease life cycle cost of fuel cell systems. |
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