One of the proposals for exploiting zero (or negative) electricity prices is storage of the heat at high temperature in refractory solids. This would be used by people who normally burn a fuel for the heat. Instead, they can preheat air and reduce the fuel needed (perhaps reduced to zero).

Firebrick works up to about 1000 C. A spinoff from MIT is using special nickel-doped chromium oxide (chromia) bricks, which can work up to 1800 C, about the temperature of a natural gas-air flame. These bricks are electrically conductive and can act as their own heating elements.

https://www.fastcompany.com/91129126/these-bricks-conduct-el...

https://electrifiedthermal.com/

https://dspace.mit.edu/handle/1721.1/130800