Why not just use vertical boreholes to extract heat from the ground?
It is more expensive to drill vertical boreholes. Drilling rigs are expensive. Drilling a set of boreholes takes time. There are some uncertainties attached to drilling boreholes: the ground may be difficult to drill, the thermal properties of the ground may alter with depth, the level of the water table may introduce complications and a moving water course might allow your saved summer heat to escape before the winter.
