Apologies; I just find it rather absurd that thermal energy could be stored in what apparently are water tanks, for months at a time, without losing most of the energy to conduction/convection with whatever surrounds the tanks.
After all, if we could do that, then certainly the same materials could allow smaller houses in cold climates to maintain a comfortable temperature throughout an entire winter without needing any active heating.
For underground storage, it's basically just brute force. Or rather, a riff on geothermal.
For building-scale storage, there are two factors. One is obviously very good insulation. Both for the building, and for the heat storage. The other is storing latent heat in a medium that changes phase, usually liquid to solid.
And yes, that's the idea. In summer, store heat from solar collectors, and from air conditioning. In winter, use that heat, plus PV electricity.
the same materials could allow smaller houses in cold climates to maintain a comfortable temperature throughout an entire winter without needing any active heating.
That's the general notion behind PassivHaus.
Integral design, internal thermal storage, situational orientation appropriate to site, thermal mass, extensive insulation, minimised building penetrations, and mitigations to address resulting consequences, particularly air exchange, moisture, and condensation.
The engineering is impressive. I recommend the videos, which are long, but information-dense and detailed.
0) https://www.delta.tudelft.nl/article/can-underground-heat-st...
1) https://en.wikipedia.org/wiki/Seasonal_thermal_energy_storag...