Am I wrong or are there extremely few actual implementations of seasonal heat storage systems out there, in private houses?
I'm guessing this is because different houses have different requirements and different conditions (climate, size of house, energy leaks, ground quality, heat distribution system, etc). IRL this means that you must do a LOT of research for your specific conditions... meaning you must know how to do this research and also how to match those conditions with a solution. Bascially, you'll have to be both a theoretical physicist and a practical one. When I realized this, I also realized I should give up on finding that perfect solution. And here's the killer; what if you got it wrong? You really over sized it, or it needs more insulation after all, or...
This is very problematic and it is not surprising see very little actually accomplished in this area.
So, I've realized maybe an alternative approach is more fruitful: Instead of aiming for optimal go for adjustable.
If you're doing the "big water tank", then don't dig it down. Instead build an insulated shed around it. It is probably cheaper but, more importantly, you can manipulate it all much more easily. You could even build a solar house around it. And when you realize you're not feeding it enough sun, then you can add another solar panel next to it. And one more. You can add or remove insulation. You can change the whole tank much more easily. Or even daisy chain a second one. Again, it'll not be optimal from an energy point but it might just become something that works.
Thinkin' out loud.
Sweden. Small farm/homestead since 2012.
Climate: Köppen Dfb (like south Canada). Harsh/dark winters, perfect summers, fair humidity, no draughts.
Soil: Mostly clay/stone
Lat: ~60 deg