Consider the possibility of a "thermal battery," i.e., a device for storing mechanical or electrical energy. Use two large containers as heat reservoirs in which one container contains ice and water in equilibrium while the other contains steam and water in equilibrium, and both are held at a pressure of 1 atm. Energy is stored by using a Carnot cycle to transfer heat from the colder reservoir to the hotter reservoir.
How much ice is created and how much water is vaporized when 1.0 kilowatt hour of energy is stored?