Wikipedia has a decent article: https://en.wikipedia.org/wiki/Seasonal_thermal_energy_storage https://en.wikipedia.org/wiki/Seasonal_thermal_energy_storag...

Wikipedia has a decent article: https://en.wikipedia.org/wiki/Seasonal_thermal_energy_storage https://en.wikipedia.org/wiki/Seasonal_thermal_energy_storag... At least this one seems to be commercially viable: https://www.solarthermalworld.org/content/denmark-52500-m2-additional-collector-area-vojens https://www.solarthermalworld.org/content/denmark-52500-m2-a... And here seem to be some numbers: https://www.gsenergy.eu/central-solar-heating/ https://www.gsenergy.eu/central-solar-heating/ To me these numbers are a bit confusing, though, it seems like they are production and not storage numbers. Anyway, heat storage capacity goes on third power of the size of the storage size while heat losses go only on second power. So after some point the heat losses are small enough. And as an order of magnitude, storage of tens of GWh seems to be in the commercially viable range. Today, that is.