A heat engine also converts chemical energy into electric energy, but through intermediate steps. The chemical energy is first converted into thermal energy through combustions, then thermal energy is converted into mechanical energy by the heat engine, and finally the mechanical energy is converted into electric energy by an electric generator. This multistep energy process requires several devices in order to obtain electricity. The maximum efficiency is limited by Carnot’s law because the conversion process is based upon a heat engine, which operates between a low and high temperature. The process also involves moving parts, which implies that they wear over time. Regular maintenance of moving components is required for proper operation of the mechanical components. Figure shows a comparison between a fuel cell and a heat engine/electrical generator.