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Fuel Cell Efficiency

Efficiency figures can be defined in many ways, conventionally they are based upon the maximum energy you could obtain from a fuel by burning it, called the heating or calorific value (HV or CV). In a fuel cell, the energy available is called the Gibbs energy, and represents the maximum amount of electricity that can be gained from the cell. The Gibbs energy is smaller than the calorific value. Fuel cell efficiencies related to Gibbs energies are nearly always 100%, so efficiency is normally defined as the electrical energy extracted divided by the calorific value of the fuel. This enables fuel cells to be compared directly to combustion-based processes, but places an upper limit on fuel cell efficiencies due to the chemical properties of the fuel. A hydrogen fuel cell operating at 25°C has a maximum theoretical efficiency of 83%, even though the fuel cell is extracting all the electrical energy possible. This compares to a maximum theoretical efficiency in a combustion engine at 500°C of 58% (assuming heat rejection at 50°C).

In a perfect system, the efficiency of a fuel cell would decrease with increasing temperature, as the Gibbs energy decreases with higher temperatures. However in practice there are efficiency drops at low temperature, and so the change in Gibbs energy is not significant. High temperature (800-1000°C) fuel cells can be used with a turbine to produce electricity from the waste heat. It is in this application where the really high efficiency figures (near 80%) could be realised, and heat could still be provided for district heating. However low temperature fuel cells, or those not operated with a turbine to recover waste heat, are still more efficient than existing technologies.