Initially demonstrated in 2013 as a UC-Irvine student project with financial support from the Bill and Melinda Gates Foundation, the stored energy cook stove was showcased as a labor-saving device for women in India. It is simply a permanently sealed, partially insulated box containing two types of salt, combined in a ratio to provide a melt temperature around 420F. The salt stores energy, via phase change, provided via a common, commercially available solar reflector. The box of melted salt can used as a thermal source for cooking after the sun is no longer high in the sky, or even after dark, and provides enough energy to boil a pot of water, make bread, or grill a serving of meat or vegetables.

https://engineering.uci.edu/news/2013/11/uci-engineering-school-gets-grant-bill-melinda-gates-foundation
https://www.ocregister.com/2013/07/22/cooking-with-the-sun/