The distance that heat travels depends on the delta T. We now know that we can't raise the temperature massively, we are there to reduce chilling. Therefore the delta T between our pipes and the broad earth mass around will be smaller. I have reconfigured the theoretical radius of the boreholes from 5 metres to 3.6. It might be less than that, even.
Both David Atkins agrees that the amount of soil that we are affecting is a lot smaller, diurnally, perhaps only a metre around the pipes (before it smooths out), but the Annual thermal store for the GSHP serving the entire house (in the absense of sunboxes) is likely to be more like 3.6 around. This accords with the figures I derived from reverse engineering the borehole calculation give in the VPW2100 website.
It is all theoretical, but makes more sense. I am debating myself whether to go back and alter figures in the blog, or just make new entries, leaving the previous ones unchanged. handed in yet? I think I will change the Introduction and permanent documents, and leave the blog entries as they were.
Changing the RANGE of the borehole bottleshape from 5 to 3.6 metre makes a change in the other numbers, it makes the volume to a rather neat 3,600 cubic metres (the numerical coincidence is just that!), and that makes the tonnage to 8000 metric tons. Thats a lot easier for thermal calcs, and a lot less discouraging.
On this basis, the thermal capacity of that mass is:
Q (heat required in kJ) = 0.92 x 8,000 x 1000 x 1 kJ for one degree
Q=7,352,000 kJ/K, which is equivalent of 2,042 kWh per degree change.
That is a lot more realistic. Only 200 days of continuous Sunbox pumping (without pulling heat out) to raise it by one degree.