Question

Could a fuel cell which combines H2 and O2 to form H2O and produce electricity directly be more efficient than a Carnot heat engine attached to an electric generator? Explain.

Answer 1

First thing ideal Carnot heat engine is a ideal case where efficiency can be 100% as the temperature of sink (T_low) can be reduced to very low temperature as efficiency of Carnot heat engine is = 1- T_low/T_high where T_low temperature of sink and T_high is temperature of source and in ideal case temperature of T_low can be equal to or nearly to 0 Kelvin. Now according to second law of thermodynamics it is tough to make an engine that converts all given power to mechanical work or there are no other output than mechanical work. Again in actual engine this is not possible and we have other losses in engine as friction , so 100 % efficiency is not possible, while a actual internal combustion engine gives 25 to 35 %. In case of fuel cell there energy is generated by chemical reaction which gives it the thermodynamic efficiency of approx 80 % to produce electricity . While in case of vehicle, after converting electrical energy to mechanical energy efficiency is 40 to 60%. So ideal Carnot heat engine has higher efficiency while actual gas engine has lower efficiency than fuel cell in case of vehicle.