[xyzzyz]

Water has really good thermal mass already. I think that adding huge water tanks in basements would be pretty beneficial: if the water in those tanks is operated in closed cycle, minerals from water wouldn't accumulate, and if we used rust proof materials, they tanks could last for the lifetime of the house. Then, we could use them to store heat in winter and cold in summer, when electricity is cheap.

[epistasis]

District heating already deploys rather large thermal storage in surprisingly compact volumes. The picture here is 2GWh:

https://en.wikipedia.org/wiki/District_heating#Heat_accumula...

When powered by heat pumps, I imagine this could be used for cooling in the summer as well.

[_ph_]

There were concept houses in Germany, where they had a water tank of 6 cubic meters which was heated through the summer via thermal solar collectors and stored enough energy for the house heating in the winter (and even in the winter the collectors would collect heat on sunny days).