System Efficiency
The efficiency of the fuel cell stack (EffFC), the utilization of the hydrogen (UtilH2) and power consumed by the balance of plant components (PowerBOC) determine the total system efficiency:
Effel, sys = EffFC * UtilH2 * (1-(Power BOC / Power Fuel cell system ))
Efficiency gains can be obtained by:
- operating at a high cell voltage; MEAs that offer high power densities at high cell voltages are clearly to be preferred: The fuel cell efficiency for hydrogen/oxygen fuel cells, can be obtained by dividing the cell voltage at operation by 1.23 V. Hydrogen/oxygen fuel cells operated at 0.7 V thus have an electrical efficiency of 0.57. This energy efficiency number is based on the lower heating value of hydrogen.
- maximizing the utilization of hydrogen. Open systems with low hydrogen stoichiometries, or closed systems with low purging frequencies, lead to less waste of hydrogen and thus to an increased efficiency.
- minimizing the flow of air. As the flow of air is generally a factor of 4 higher than the flow of hydrogen, the energy needed for supplying this air can pose a significant parasitic loss.
- minimizing the pressure drop at anode and cathode. The pressure drop of the flow field and manifolds requires an increase of the reactant pressure at the inlet that directly leads to an increase in parasitic energy consumption.