Piwoslaw, that's a great idea about the milk tanker and the links you provided! Unfortunately that is a lot of digging & buying and it would be impossible on my site due to trees, space, underground storm drains, etc. I figured I only need maybe $500 in extra materials plus some boreholes angled out from a small access area cut from the middle of my basement slab. I'll post more on how to accomplish those holes in an existing house shortly. In any case, I want my storage to be within the building envelope so any losses are still useful at least half the year.
Since I am almost at a high point of terrain which slopes down well beyond my house, I think I won't have too much worry about the groundwater carrying my heat away. I figure anyone that can go down around 20' below the slab to be heated before hitting the water table will be able keep enough heat in the soil to be wothwhile, allowing at least 9' depth so the heat returns around 6 months later, another 9' for the heated area, and a bit of buffer room. I'll confirm that my water table is low enough when I dig the boreholes for my GSHP. I hope to lose a lot less heat than usual because of the pattern I am placing them in, shown by the red line:
The boreholes will be spaced 10-12' out from the house and around 10' apart. Since they are in a u-shaped line, they won't be competing with each other for the same heat and it will form a "cage" around any heat trying to escape from under the house, raising the COP of the GSHP. Also check out that new Energystar rated roof (another post) with lots of flat SSE facing area that has only one code-required roof obstacle!
Clay soil weighs around 110lb/cuft and has a specific heat around 0.3btu/lb per degree F. My house footprint is almost exactly 48' square. Assuming at least 20' useful Georgia red clay depth, I think its reasonable that the outer 5' of slab and area under it (9,000cf/990,000lb) could be maintained about 5F above my average 62F soil temp, taking around 1.5 million btu to bring up to temp. The remaining 43' square (37,000cf/4,070,000lb)could easily be maintained 10F warmer at a perfect 72F (warm enough to add winter heat but below air conditioning temps in summer), taking 12.2 million btu to bring up to temp.
Insulating a 3' perimeter around the house with buried 3" thick polyiso then working inward 20' (per Don Stephens) from there to make sure the high temp heat doesn't get away, that leaves a chunk of soil in the center 13' square by 20' deep, or 3,380cuft/372,000lbs to seasonally store heat I am likely to retreive almost all of through the slab or the GSHP field as it cools back down. Taking that soil from 72F to say 150F (very doable with evacuated tube solar and an A/C desuperheater in the summer), that center soil can store 8.7 million btu, plus extra btu as that center heat diffuses towards the edge of the storage area over the course of the "injection season". There will obviously be a temperature gradient that someone better at math than I am might be able to describe more accurately.
The demand will be lowered because the basement is now a net btu adder sitting in that large 72F area instead of in a 62F heat sink (See the details of my calculated demand load in the solar storage tank build thread). I expect I will have a surplus of 27 million btu (4.5 million/month) during the hottest 6 months from the 60 evacuated tubes and the A/C desuperheater, plus maybe half that in the shoulder seasons. Even if I lose 50% of this heat it will be worthwhile.