[asfd]

You should have a look at paraffin as a heat storage medium. It can store heat much more effectively than water in the form of latent heat when heated (melted). I quick read a document on that topic whre it says that
"The paraffin shows the best performance (of their test), with a useful thermal storage density of more than 2 times that of water.".
Paraffin is also cheap, non-toxic and readily available.

Regards,