This is essentially what a ground source heat pump system is. Except instead of a sealed water tank you just make a tall hole that fills with water and the sun will warm it for you during the summer automatically.

1800 kWh is very little. We use around 12000 kWh and our neighbours' new house uses around 8000 kWh annually and most of that is heating. I'm not sure how many houses can hit 1800.

I can't find the link now, but there was an episode of Grand Designs here in the UK (a show detailing private individuals developing interesting or unusual homes) where the owner was building a passively heated house based on an idea by his architect father.

The ground beneath the footprint of the house was insulated around the sides to a depth of about 2m, effectively extending the thermal mass of the house into the ground. After construction, it took about 2 years (IIRC) to warm to a stable level, but thereafter required little to no energy to stay at a comfortable temperature year round.

50m³ is huge. IMO that would be an engineering challenge that would probably impact the sability of the foundation if not done right.

Ground source heat pumps are expensive because of the buried piping, I imagine this would be even more costly.

Something like that was attempted south of Calgary, in Canada: https://en.wikipedia.org/wiki/Drake_Landing_Solar_Community

Its kind of done. Active heating systems often have the intake air go through the foundation so it heats up in summer and cools down in winter reducing both heating and cooling costs.

Larger storage structures are easier to (thermally) insulate. Because geometry.

But going with larger structures probably means aggregation (fewer of them are built, and further apart). Assuming homes to be heated are staying where they are, that requires longer pipes. Which are harder to insulate. Because geometry.

Pipes are competing with wires, which are much less technically complicated than pipes.

The issue we have in Finland is the assymetric electricity usage between winter and summer. This is driven by the need for heating.

In the past, district heating systems burned coal. Now that's out the window we haven't got enough to burn. We do burn waste products from forestry, trash and the like but there's not enough to go around before you start felling trees en-mass just to heat a city.

A lot of municipalities in Finland are now starting to play with thermal storage. There's this sand battery, but there's even more hot water storage being built and has been built.

In the medium term, winter electricity production and consumption is starting to become a bit of a risk for us.

> near the North Pole.

Finland is not near the North Pole. Lahti is at 61°, right in the middle between Greece and the North Pole.

But yes, heating needs are higher than in most European or North American populated areas.