I saw a similar sort of thermal energy storage system that was basically a massive brick of wax in an insulated box. The idea being that as energy is put into the brick, it is stored by melting some wax, and pulling energy causes it to freeze. In physics terms, using the latent heat of a material to store energy rather than its raw temperature.

The box I saw was more designed to be used as a source/sink for a local heat pump than as grid scale storage, but it may be possible to scale it up.