Storage heaters came with the flat I own back in the UK, which dates back to the late 80s or early 90s. Not even vaguely new. A quick search says the tech dates to 1940s.
Here is a Swedish company making a heat storage system that outputs electricity using a Stirling Engine. They claim 165kWh electrical storage capacity.
> Every non-renewable power source involves a heat engine.
Even some renewable power sources like wind involve a "heat engine" in the sense that they exploit a temperature differential driving a working fluid (AKA wind) to spin a turbine.
