Phase Change Materials (PCM) are substances which absorb or release heat when they go through a change in their physical state, eg. water changing from ice to liquid. You can tap this energy. PCM batteries can operate just like an electric battery, but can be 2 - 3 times cheaper. A South Australian company is developing PCM battery versatility even further by partnering the battery with heat pumps, renewable energy to power the battery processes, advanced weather and system usage forecasting, as well as energy pricing data to maximise energy efficiency and financial return. 

https://www.youtube.com/watch?v=OMr0wgb3r4E&t=4686s