how to install a geothermal heat pump?
How many boreholes will I need and how deep do they need to be?
Vertical borehole collectors can provide between 25 watts (in loose gravels) and 60-70 watts (in saturated stone) per metre of active collector. (active collector denotes the actual probe that is in contact with geothermal grout)
For example calculation, we will assume a fairly modest figure of 35 watts per metre, a ground source heat pump CWR-12XB size of 12kw.
Assumed COP of 4 (Coefficient Of Performance of 4) – 1kw in = 4kw out = net gain of 3kw.
(I.E. for every 1kw input of electricity to drive the heat pump, 4Kw of heat is produced from the ground, so a net gain of 3)
So, using our average figure of 35 watts/metre, a heat pump size of 12kw and a COP of 4 we can apply the following calculation:12 (heat pump size) / by 4 x 3 (COP) = 9 kw or 9000 watts / 35 (watts per metre) = 258m.
This would be 3 x86m boreholes.
(Please note however that this could be as much as 360m or as little as 150m – depending on the watts per metre produced in the given geology)
What size loop do I need?
Borehole depths below 110m operate better and are more efficient with a32mm diameter loop. if above 110m operate better and are more efficient with a40mm diameter loop.
if 100-120m operate equally as well with either diameter.
How far apart should they be?
They should be spaced at least 6m apart to prevent thermal linkage (and efficiency loss) between the boreholes.
They should be at least 5m from any buildings.