Ground source heat pumps with shallow heat collection coils extract heat stored by the sun during the previous warm season. With deeper boreholes you're extrac

Ground source heat pumps with shallow heat collection coils extract heat stored by the sun during the previous warm season. With deeper boreholes you're extracting heat from radioactivity decay. In both cases the rate at which you can extract that heat depends in the heat transfer rate in the ground around the coils/boreholes.