Israel’s Brenmiller Energy (reported here previously) is piloting a new water heating system to save 36,000 liters of diesel and reduce pollutant emissions at Israeli army outposts. Residual heat from the base’s power generators is stored in crushed rocks and discharged when hot water is required.

https://www.calcalistech.com/ctech/articles/0,7340,L-3829261,00.html