Effect of groundwater movement on the long term efficiency of borehole heat exchangers

The long-term performance of double U-tube borehole heat exchanger (BHE) fields is investigated by finite element simulations, performed through the software package COMSOL Multiphysics (COMSOL, Inc.), taking into account also the effects of groundwater movement. Two time periodic heat loads with period of one year are examined, with 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, four staggered lines of infinite BHEs. The distance of 6 m between adjacent BHEs and three values of the groundwater velocity 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 configuration analyzed, plots of the minimum and maximum annual value of the fluid temperature for a period of 50 years are reported