Was randomly trawling t'internet and came across https://tepeo.com/ - linked via Octopus energy so seems to have some credibility.   The website is a little devoid on detail but it seems to be using storage heater 'type' technology for energy storage, resistive heating for charging. Interesting claims around storing >40kWh of heat energy and with a higher energy density than PCM storage.   I've dropped them an e-mail to find out more but would be interested to hear if anyo