Since you can store a lot heat in many materials they can act as a battery if you only know how to turn heat to electricity efficiently. The reason to develop heat to electricity capacity would be the very low cost of the energy storage.
My idea is to use a magnetic fluid to turn a temperature difference to electricity. The temperature difference I imagine could be created using heat pumps to create the very hot side and the very cold side. For the electricity generating invention I took inspiration from the lava lamp. Hot fluid rises to the top and gets replaced by colder fluid. I therefore wonder if you can have a magnetic fluid that moves between a hot and cold places. Since its magnetic it can then induce a current in the wire coil.