Engineering Considerations in the Design of Small PEM Fuel Cell Systems

Michael M.D. Ross and Matti Noponen
Helsinki University of Technology

Note on Authorship:

This article was co-authored by Michael M.D. Ross, principal of RER Renewable Energy Research, while he was an employee of the Advanced Energy Systems Group, Department of Engineering Physics and Mathematics, Helsinki University of Technology.

Citation:

Ross, Michael and Matti Noponen. Engineering Considerations in the Design of Small PEM Fuel Cell Systems. Unpublished internal report, Helsinki University of Technology, Otaniemi, Finland, May 2000.

Summary:

While most fuel cell work has focused on large fuel cells, there are niche markets for simple, small fuel cell systems. We compare the constraints and the criteria for the power supply for three niche applications: a small remote stand-alone system consisting of stack, photovoltaic generator, electrolyzer, and seasonal storage; a portable computer; and a mobile phone. For small portable applications, the combination of fuel cell and metal hydride faces stiff competition from advanced lithium-ion batteries. Compared with the metal-hydride, these have the same or lower costs, have similar gravimetric energy densities, and can achieve volumetric energy densities half as high. When the volume and weight of the stack are added to that of the metal hydride, the battery's performance seems quite respectable. It will be hardest for the metal-hydride/fuel cell combination to compete in very small applications, such as the mobile phone; larger applications, such as the portable computer, are more promising.

A commercial 22 cell stack was operated under a wide range of conditions to experimentally investigate its capabilities and how they can be improved. We modified the stack testbench so that it measures the voltage of every cell and the temperature of every third cell; this provided sufficient information to determine when flooding, dryout, crossover, hydrogen starvation, and oxygen starvation were occurring in individual cells. We removed five weak cells from the stack. Free convection currents can be strengthened by placing a cardboard chimney over the stack, and this improves the stack performance. The net power output of the stack can be raised significantly by forcing air through the cathode air channels with a fan, especially at low temperatures, e.g., at start-up, or at high currents. The fan tends to dry out the cell membranes, however, especially at temperatures above 40ºC. At temperatures below this, flooding of the cathode tends to occur, especially with free convection.

A model of the thermal processes in a free convection stack has been developed. Modeling of heat losses and air flows in the cathode air channels with free convection is the most challenging aspect of this work. The density gradient that engenders the air flow is caused not simply by heat transfer from the stack, but also by water evaporation from the cathode and the consumption of oxygen by the cathode. We found a relation that accounted for the heat transfer from the stack and the water vapour, but it is only applicable when flows are fully developed. Nevertheless, we use this to investigate the relationship of stack power and temperature, and the conditions under which membrane dryout will occur. Various ways to improve cooling through free convection and forced convection are investigated with the model. Ultimately, however, a numerical approach would be required to do this work accurately.

Manufacturers of small stacks, miniature valves, miniature pressure regulators, miniature compressors, and very low voltage DC-DC converters are surveyed, and promising components identified. The requirements for small fuel cell systems, such as explosion hazard compliance for valves, are discussed.

Created 2007/03/08   Updated 2007/03/08     ©2007 RER Renewable Energy Research