Long-Term Performance of Borehole Heat Exchanger Fields with Groundwater Movement

A numerical investigation of the longterm performance of double U-tube borehole heat exchanger (BHE) fields is performed by means of COMSOL Multiphysics, in the case of non-negligible effects of groundwater movement. Two time periodic heat loads, with a period of one year, are studied: Q1, with a partial compensation between winter heating (main load) and summer cooling; Q2, with no summer cooling. By assuming a distance of 6 m between adjacent BHEs, the following BHE field configurations are analyzed for a 50-year period: a single line of infinite BHEs; two staggered lines of infinite BHEs; four staggered lines of infinite BHEs. The BHE working fluid is a water-ethylene glycol 20% solution with a minimum allowed temperature equal to ‒ 5°C. The ground is modelled as a Darcian porous medium with an undisturbed temperature of 14°C. Three values of the groundwater velocity are investigated: W = 0, 10-7 and 10-6 m/s.