Thermal storage

Thermal storage is a way to store energy using heat. When electricity is cheap, it is used to heat up a material. Then, when more electricity is needed, the heat is used to generate power through a heat engine. This process can be up to 50-60% efficient. Some materials can stay hot for a long time without losing much energy. The most promising method is to store heat in molten salt, which scientists and companies are working on. Bill Gates's company TerraPower has a goal to use molten salt to store heat generated during the day and convert it to electricity at night when solar power is not available.

Gates, Bill. How to Avoid a Climate Disaster (p. 93). Penguin Books Ltd. Kindle Edition.