A Comparison of High-Pressure and Low-Pressure Operation of PEM Fuel Cell Systems 2001-01-0538
This paper compares the merits of operating a direct-hydrogen fuel cell (DHFC) system using a high-pressure air supply (compressor) versus one using a low-pressure air supply (blower). Overall, for the system modeled, it is shown that there is no inherent performance advantage for either mode of operation at the DHFC stack level. However, in practical applications, as will be shown in this paper, a systems analysis (stack and air supply) of power and efficiency needs to be performed.
Equivalent PEM DHFC stack peak power values can be obtained using both high-pressure and low-pressure air supply systems. For each air supply configuration, air mass flow and pressure operating conditions can be found that result in an equal value of the oxygen partial pressure at the cathode catalyst layer surface.
However, at the system level, the required air supply power needed to achieve the same DHFC stack performance values can be drastically different for high and low pressure operation. In order to compare the two systems, an optimal air supply control strategy is first developed to obtain the desired stack operating conditions with minimal parasitic loads based on each air supply configuration. Second, the resulting air supply parasitic loads are compared directly between the two configurations - both comparisons are set in the context of the system performance. In other words, the systems are sized such that the peak net power values are equal while the stack gross power may be different.
The results of the study demonstrate the well-known fact that equivalent DHFC peak net system power values (86kW) can be obtained with both types of air supply configurations but require different stack sizes. For the blower application, the stack size had to be increased by 16.3% (500 vs. 430 cells in this example) for a peak net power of 86kW. Differences are also apparent with the WTM sub-system. Quantitative results will be presented for both the high pressure and the low pressure applications.