Long-term storage of electricity generated from variable renewable energy resources sources can be achieved by converting the electricity into the chemical energy of a combustible fuel. The conversion process usually involves electrolysis of water to produce hydrogen, which may either be stored directly as a high pressure gas or converted to a more energy-dense fuel such as ammonia. In either case, the fuel can be stored for days, weeks, or even months and then, when its energy is needed, it can be burned in a heat engine. The heat engine can be used to drive an electrical generator, thereby reclaiming…