It seems you can store warmth from season to season:
https://www.icax.co.uk/thermalbank.h...sulated%20tank.

Apologies for doubting you, Mossdog.

So this is a ground source heat pump, but you're also putting some heat into it in summer. They claim it's a good way to store heat. I suspect the efficiency isn't great, but if it works even a bit, it makes sense.

Of course, it'll still use electricity to get the heat out of the ground, just as a normal ground source heat pump does.