Fuel cell stack power systems which use a water coolant will typically have the coolant in a two-phase water/steam mode throughout substantially the entire passage of the coolant through the stack. The temperature of the water in the two-phase coolant will be substantially constant as the coolant traverses th stack, with the percentage of steam increasing in the two-phase coolant from coolant inlet, to coolant outlet. The coolant thus remains in a substantially isothermal condition as it passes through the stack cooling passages. In order to preserve the isothermal nature of the coolant, systems cooled in this manner will typically utilize steam condensers and steam separators in concert so that the water will not significantly cool before it is returned to the stack. Thus, the coolant will be exhausted from the cooling passages of the stack as a two-phase water/steam mixture, will be passed to a steam condenser to condense more water out of the steam phase, and also passed to a steam separator where the steam and water phases will be separated, the steam going to a fuel reformer and the water going back to the coolant passages in the stack. The condenser and separator may be used in either order. Before returning to the stack, typically, makeup water will be mixed with the returning water, but the temperature of the returning water will not be lowered during this recycling more than a very few degrees. Thus, where the coolant water is returned to the stack, almost immediately it begins to boil creating more steam.
There are several problems which exist in the aforesaid fuel cell stack cooling system scheme. One of the problems relates to the handling of the steam phase and the fact that the steam condensers must be physically elevated above the steam separator, and the steam separators must be physically elevated above the cell stack. This creates packaging or housing problems for the fuel cell system relating to the necessary height of the housing in which the system is contained. This problem is particularly apparent with small to mid-range power systems. Another problem created by isothermal cooling of the fuel cell stacks occurs in cells which use aqueous electrolyte solutions, such as acid or alkaline cells. This second problem concerns the evaporation of electrolyte into the oxidation gas. This evaporated electrolyte leaves the cell area with the exhausted oxidant gas. Without special modifications to the cell structure, this will be a serious problem which is continuous at high load outputs. This requires electrolyte replacement, and special stack construction to combat corrosion in stack manifolds and the like.