Seasonal storage of solar energy in borehole heat exchangers

Seasonal storage of solar energy in geothermal boreholes has resurfaced as a means of heating housing communities. Typically, these systems operate at relatively high temperatures leading to high heat losses from the ground storage volume and to low solar collector efficiencies. In this paper, a new seasonal storage strategy is proposed. First, the storage temperature is kept relatively low in order to limit heat losses and improve solar collector efficiencies. Secondly, the seasonal borehole storage is designed in such a way as to enable simultaneous charging and discharging of the ground using four pipe boreholes with two independent counter-current circuits. Finally, the temperature level is raised using heat pumps to supply heat at an acceptable temperature for space heating. The proposed configuration is simulated with TRNSYS using a modified version of the DST model. Results from simulations indicate that it is possible to keep the seasonal storage temperature at an annual average slightly above the annual mean ambient temperature using a relatively small solar collector area (about 11 m2 per house) leading to relatively high solar collector efficiencies. Combined with a heat pump, it is shown that this system can reach a solar fraction of 78 %.