building a thermal mass with soil
 

Hello friends!
We are in the permitting process for our new home and it originally had two Ground Source Heat Pumps, one for hydronics and the other for HVAC for the brief air conditioning season.
Now I am exploring the idea of collecting heat in vacuum tube collectors and storing it in an insulated mass for a seasonal storage system.
I have a contact for free insulation from a manufacturing process, which changes the economics considerably. I can get an R value of R80 around the mass.
I see that traffic is way down here. I will wait and see if there is any interest from the old guard for discussing it further.

Yes, there is interest!

Just a few months ago I was reading about long-term heat storage in the soil under buildings.

One fellow stated that before pouring the foundations he buried some plumbing in the ground, which he later connected to solar collectors. After 2 years his feedback is that it hardly works: Yes, the soil (in his case mostly sand) warms up, and indeed the ground floor is warmer at the end of summer. But once it gets cold outside the soil loses heat very fast. He estimates the only difference it makes is that he turns on the house's heating maybe 2-3 days later in the autumn than he would without the system.

He suspected that ground water is flowing through the sand and robbing it of the heat, so he later dug around the house, only a bit deeper than the foundations, and added a barrier of insulation. That did help, but only slightly. Again, the return was not worth the investment.

In your case I assume you will have better shielding and more insulation (you can never have enough!!), so hopefully you will have much better results.