Design Methods for PEM Fuel Cells Influence of operating pressure on cell design

摘要

The work presented in this paper examines the relationship between cathode air pressure and proton exchange membrane (PEM) fuel cell design. As with most engineering problems, compromise has to be made between conflicting criteria in order to achieve a satisfactory result. For a PEM fuel cell system with a high temperature fuel processor, greater electrical efficiency can be achieved with a high air pressure than with a low air pressure. This is because energy in the flue gases can be utilised in the expander to provide power for the compressor. A typical optimised system should be able to achieve an efficiency of > 40%. For systems without an expander, the parasitic losses associated with the compressor can become unacceptable as the pressure increases, therefore the system efficiency can be greater at low air pressure. For a given cell voltage, the operating pressure influences the distribution of the waste heat between the stack cooling system and the thermal energy emitted in the cathode exhaust. At high-pressure, more heat is generated in the stack than at low pressure, where more heat is transmitted out through the cathode exhaust. Therefore, although the cell performance is higher at high pressures the cooling system needs to be larger. For operation below 1.5 bara it is possible to provide all of the cooling requirements through evaporative cooling by injecting water into the cathode air stream (and/or anode fuel stream). By controlling the pressure drop down the flow channel it is possible to control the water evaporation rate by keeping the relative humidity at 100%. Thus, as the pressure drops, the mass of water in the vapour phase increases and the cooling effect is distributed over the entire cell. Obviously this needs to be achieved without flooding the electrodes.