If you put all the earth into a completely insulated box, there is no chance to recharge it randomly from the surrounding earth as we now enjoy - you would have to have a mathematically perfect balance, year after year.
The solar charging pond at Beaufort has an uninsulated bottom precisely so that excess heat or excess chill can be balanced out with the surrounding earth. If it reaches either extreme, the delta-T is such that the earth around will rush in to balance things out.
If you have a closed container, the temperature must not raise higher than the heat source. i.e. if we had a volume of earth equivalent to say, our garage, and that had to rise to 150 degrees C to have enough capacity to heat the house all year, that is not practical, we cannot heat higher than about 40-50 with high performance solar panels.
The way the house works is continuous trickle of heat in and out, but with more taken out in winter, and more put in in during summer. Over the year this must balance. The proportion of heat that the GSHP must find 9800 kWh from the earth annually to meet our requirements (see earlier article) averages at 27 kWh per day, with a peak during January of 55 kWh per day.
We have yet to know what the Sunboxes will put down at their peak, in July, but during May we are getting double figures nearly every day. If the average is 9 kWh per day (3300 kWh per annum) that means we put down 3300 and get up 9800 kWh - the earth has to provide the balance.
This suggests that simply having 3 times the area of Sunbox would make a perfect balance. But it's not only quantity that counts, it is delta-T that also counts. If the Sunboxes only send a max of 17-18 deg C down below, that would be the warmest it would get - it would reach stasis. 18 would not be enough to get through the winter, from a Closed heat source.
If you want a closed box to work all year, you could put glycol down from a pro-quality flat plate panel that gets to higher temperatures (for fewer hours per year). I estimated that if 37degs would be an ideal maximum temperature (same as Blood temperature) and you heated the house all year from that, and permitted it to drop to zero, the volume of clay would be about the same as the house interior - 260 cu.m. The reality is that we are trickle charging, so we never need to rise to 37 or to drop to 0.
Panels accumulating the equivalent of 27 kWh/day would exactly meet the heating demand of the house. But for this to work they would need to have a high peak in July, perhaps double the average, same as the 55 kWh/day that the GSHP must draw in January. How could one put up panels or sunboxes able to absorb and bury 55 kWhr per day when the ones I have now average at 10 ? Five times the area is not possible, there is not enough wall.
So, I am suggesting:
- One could have a caisson type foundation under the footprint of the house, with polystyrene-concrete (Becoform-type polystyrene concrete formwork, such as we have in some of the Uni of Nottingham Ecohouses) as a basement wall,
- The depth could go down 3-4 m, and be backfilled, have a suspended concrete floor above this (to avoid settlement),
- Have an earth bottom (to allow natural earth heat to balance out excess), and
- Add to the backfill a 3D grid of Pipemesh for the solar and GSHP ground loop.
- Have insulation over the earth store so that the house doesnt feel uncomfortably hot when the store builds up.
- Have high performance solar panels injecting heat (not sunboxes)
- Use sunboxes on spare remaining facade, for winter MVHR with their function changed to augmenting the solar charging during the summer months.
Therefore our present arrangement of deep boreholes and sunbox augmentation seems pretty good as a working compromise..... :)