[Mobile Master Tech]

BBP,

I agree with others on crushing problems. I've studied underground bunkers for prepping. Nearly all instances of using a square box (Intermodal transport containers), the container has crushed from the backfill. Anything underground needs to have purposely strong walls (like a concrete septic tank) or be spherical, round, or arched (like corrugated drain culvert).

If the purpose is seasonal energy storage, could you instead use the dirt under the house? As long as groundwater is low enough to not steal your heat away, you can store an awful lot of BTU's in the tons of soil beneath your home just by bringing it closer to your house's ambient temperature through a perimeter of shallow boreholes, then retrieve it as needed with a heatpump working extremely efficiently due to the low temperature differential. I touched on this in my "don't waste that heat" thread, and a number of others have worked on annual heat storage systems.

Most other methods of storage for more than a few days involve massive amounts of water in expensive vessels with a high enough temperature differential that much heat is lost to the soil anyway.