System Level Modeling of Thermal Transients in PEMFC Systems

Fuel cell system models are key tools for automotive fuel cell system engineers to properly size components to meet design parameters without compromising efficiency by over-sizing parasitic components. A transient fuel cell system level model is being developed that includes a simplified transient thermal and parasitics model. Model validation is achieved using a small 1.2 kW fuel cell system, due to its availability. While this is a relatively small stack compared to a full size automotive stack, the power, general thermal behavior, and compressor parasitics portions of the model can be scaled to any number of cells with any size membrane area. With flexibility in membrane size and cell numbers, this model can be easily scaled to match full automotive stacks of any size.

The electrical model employs a generalized polarization curve to approximate system performance and efficiency parameters needed for the other components of the model. General parameters of a stack's individual cells must be known to scale the stack model. These parameters are usually known by the time system level design begins.

The thermal model relies on a lumped capacity approximation of an individual cell system with convective cooling. From the thermal parameters calculated by the model, a designer can effectively size thermal components to remove stack thermal loads. The transient thermal model was found to match experimental data well. The steady state and transient sections of the curve have good agreement during warm up and cool down cycles.

In all, the model provides a useful tool for system level engineers in the early stages of stack system development. The flexibility of this model will be critical for providing engineers with the ability to look at possible solutions for their fuel cell power requirements.