I've always wondered why we don't build homes with a buried tank of water used as heat storage. In the summer it can be heated with solar thermal to around 90c, and in the winter heat can be drawn out and go through radiators or underfloor heating, with a mixer valve. You just need a few pumps and valves, not even a heat pump is needed.

If you assume a modern house with a heat load of 1800kWh per year (fairly standard for a new build medium sized home where I live, in Northern Europe) that means you'd need a tank roughly 50m3, or 10,000 gallons for Americans. In terms of insulation you'd need around 50cm of XPS foam, and it would be buried a meter below ground.

It's nothing terribly complicated in terms of construction or engineering. Of course you'd pay more upfront, but then your heating bills would be practically zero. In warmer climates it would be much simpler, you could probably get away without burying it.

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.