Long-term performance of BHE (borehole heat exchanger) fields with negligible groundwater movement

The long-term performance of double U-tube BHE (borehole heat exchanger) fields is investigated by finite element simulations, performed through the software package COMSOL Multiphysics (©COMSOL, Inc.), for grounds in which the effects of groundwater movement are negligible. Six time periodic heat loads with period of 1 year are examined, with either full compensation, or partial compensation or no compensation of winter heating with summer cooling. A single BHE surrounded by infinite ground and the following BHE field configurations are analyzed: a single line of infinite BHEs, two staggered lines of infinite BHEs, a square field of infinite BHEs. For each BHE field configuration, four different distances between adjacent BHEs and two values of the ground thermal conductivity are considered. The undisturbed ground temperature is assumed equal to 14 °C, and −5 °C is prescribed as the lowest allowed temperature for the working fluid. For each BHE field geometry, heat load and ground thermal conductivity, plots of the minimum annual value of the fluid temperature for a period of 50 years are reported, and the pairs “distance – heat load” which keep the fluid temperature above the prescribed limit are evidenced.