Optimal control strategy for the operation of a PEMFC system under low-pressure conditions

Abstract

The effect of low ambient pressure on a PEMFC system is investigated by operating the entire non-pressurized fuel cell system inside a low-pressure chamber. Polarization curve measurements are conducted at various pressure levels between 980 and 491 hPa by using the manufacturer’s default parameters for stack temperature and cathode stoichiometry control. Significant net power losses of up to 59 % at 491 hPa are observed with respect to 980 hPa, which result from increasing stack drying and growing parasitic power losses with decreasing pressure. The latter is due to the decreasing fluid density combined with a constant mass flow requirement. Using statistical design of experiment methods, a comprehensive parameter study is performed to derive model equations for the response of the cell voltage, the net power and the relative humidity at low-pressure operation. The model predictions are in very good agreement with validation measurements. Model evaluation yields the pressure and current density-dependent optimal parameters for stack temperature and cathode stoichiometry control in terms of the net power output. Applying the optimal control parameters to the experiment leads to significantly higher cell efficiency and net power compared to the use of the default parameters. At 497 hPa, a net power increase of 29 % is observed experimentally. The model analysis shows that, in good approximation, the optimal relative humidity is independent of the pressure and decreases linearly with rising current density. Moreover, the optimal stack temperature is, in a first approximation, independent of the current density and decreases linearly with decreasing pressure. Based on these two linear relations, a simplified optimal operation strategy is proposed to obtain high net power output at low ambient pressure.

Topics & Concepts