Effect of heat and mass transfer related parameters on the performance of deep borehole heat exchangers

Deep borehole heat exchangers (DBHE) are a highly potential clean heat source. Using 3-D numerical simulations, a comprehensive analysis on the effect of design parameters and geological conditions for 1-3 km deep DBHEs were undertaken. Key parameters included the inner and outer pipe diameter of the well, insulation between them, borehole depth, mass flow rate, and thermal conductivity and geothermal temperature gradient of ground. The whole heating system performance with the DBHE, heat pump, and circulation-pump-unit of the well was also analyzed. The results show that the performance of the well is strongly influenced by its design and mass flow rate with increasing well depth. For a 1 km to 3 km DBHE, the thermal output ranges from 46 kW to 240 kW at steady state in Scandinavian conditions. A smaller inner pipe in the well could increase the output by 7-11 % compared to a larger pipe, but a higher mass flow rate could produce even 75 % more heat than a lower one. However, the effect of the inner pipe and mass flow rate would be the opposite on the whole energy system efficiency when considering the COP of a heat pump connected to well and the well pressure losses. The geological conditions of the rock have a major effect, too.