THE FUEL CELL

Most off-the-grid electric systems include a back-up generator burning fossil fuel. We have a ReliOn 1 kW fuel cell. This is a PEM (proton exchange membrane) unit designed for stationary, telecom applications.

In a PEM fuel cell, hydrogen gas is fed into a catalyst layer which strips off the hydrogen's electrons. The H⁺ ions (protons) travel across an electrolyte membrane to another catalyst layer which is supplied with oxygen (air). Meanwhile the hydrogen electrons take a detour through your electric appliance before returning to the fuel cell. On arrival at the oxygen catalyst, the prodigal electrons, the H⁺ ions and the oxygen combine to form H₂O, which is the sole exhaust (see diagram below). NASA has long used fuel cells to provide both electricity and water for astronauts.

About 40% of the energy of this reaction is electric, 50% is heat (10% is lost to internal loads such as fans). While the heat can be put to good use (cogeneration), we did not do so. Thus the efficiency of our fuel cell is 40% (better than fossil fuel engines). Thus our storage tank holds about 29 kWh of effective electric energy, measured "at the plug".

The fuel cell sends its DC power to the batteries. The battery bank acts as a kind of "slush fund" for electricity- all power passes through it, and it can absorb surplus or fill in deficits. Typically, we turn on the fuel cell when the house loads are drawing the battery voltage down too far.