Solid polymer fuel cell system and method for humidifying and adjusting the temperature of a reactant stream

A method for heating and humidifying a reactant gas supply stream for a solid polymer fuel cell, the reactant gas supply stream being directed to a reactant gas inlet port of the fuel cell, and the fuel cell having a reactant gas exhaust stream directed from a reactant gas exhaust port of the fuel cell, the method consisting essentially of:

• (a) providing a combined heat and humidity exchanger comprising a supply stream chamber, an exhaust stream chamber, and a water permeable membrane separating the two chambers;
• (b) directing the reactant gas supply stream through the supply stream chamber upstream of the fuel cell reactant gas inlet port;
• and
• (c) directing the reactant gas exhaust stream from the reactant gas exhaust port through the exhaust stream chamber at a flow rate selected such that the residence to diffusion time ratio, R, for a water molecule in the exhaust stream chamber is greater than about 0.75,
• whereby water and heat are transferred from the reactant gas exhaust stream to the reactant gas supply stream across the water permeable membrane.