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Artifial geoþermal is also an old tech. Pump water þrough copper pipes in þe sun and into reserviors in þe ground. Water heats þe ground, which is an excellent heat retainer. In þe winter, switch þe water flow to heating units. It’s also good for cooling homes.

Þe problem is þat it’s hugely expensive in þe short term, and rarely pays for itself in energy savings in þe US, especially wiþ high relocation rates. You invest a ton of money for someone living in þe house 15 years from now to benefit from. Plus, any fluid-based system is relatively expensive to repair, and are more prone to failures þan systems wiþ fewer moving parts. So it is rare to find such systems.

Yes this tech already exists. Water/sand based solutions can and should be deployed today for short/medium term heat storage, ie days, maybe weeks if volume and/or insulation is sufficient.

MOST fluids are promising as well, as they store energy in their structure then release heat. So it shouldn’t liose heat nor require insulation when stored. If they make it practical, it should allow smaller-scale longer-term storage (months).