Removal of excess product water in a PEM fuel cell stack by vibrational and acoustical methods

The research presented here investigates the use of vibro-acoustic methods to improve the performance of a PEM fuel cell by enhancing water removal from the active reaction sites within the fuel cell. Removing the water increases the available reaction sites and thus increases the available power for a given operating condition. To examine the new water removal methods, first, the production of water in fuel cells and current water removal methods are reviewed. Then, the new methods are proposed that are based on structural and acoustical excitation of the stack. Specifically, the use of flexural waves, acoustic waves and surface waves to remove water from a fuel cell stack are examined. Analytical formulations are given in order to calculate the excitation frequency and amplitude required to move a droplet resting on a vibrating bipolar plate. Depending on the droplet radius and other parameters, it is estimated that a water droplet resting on a bipolar plate can be moved by structural displacement levels as low as 1μm. The different approaches to droplet removal are compared in terms of the minimum vibration energy required per droplet. Water production in a commercial fuel cell stack is then estimated and used as a test case to compare the power required to effect removal of a certain number of droplets with the amount of power produced by the stack. It is shown that a water droplet clogging a plate channel may be moved with parasitic power requirements as low as 21mW. For each method, the energy required to effect droplet removal is quite small, although among the three, the use of surface acoustic waves may be the best option in terms of minimal vibration energy and implementation feasibility.