Since fuel cells can generate useful power when all components of the system are at the same temperature ( T = T H = T C ), they are clearly not limited by Carnot's theorem, which states that no power can be generated when T H = T C . This is because Carnot's theorem applies to engines converting thermal energy to work, whereas fuel cells instead convert chemical energy to work. Nevertheless, the second law of thermodynamics still provides restrictions on fuel cell energy conversion.