The long-term performance of double U-tube Borehole Heat Exchangers (BHEs) is studied numerically by considering three different time-dependent heat fluxes exchanged between each BHE and the ground. Reference is made to the following geometrical configurations: a single BHE surrounded by infinite ground; a field of 4 BHEs surrounded by infinite ground; the limiting case of a field of infinite BHEs. Since the temperature distribution along the vertical direction has a negligible influence on long-term BHE performance, the problem is studied by means of a 2D conduction model, where the energy transfer due to water flow is replaced by a uniform, time dependent, heat generation. The fluid mean temperature in the BHE is determined as a function of time. Results point out that, in the absence of groundwater movement, when a BHE field is considered, at least a partial compensation of the thermal load is necessary to avoid long-term system breakdown.
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