Exploiting heat gains along horizontal connection pipes in existing borehole heat exchanger fields

This study investigates three options for utilising the additional capacity of borehole heat exchanger (BHE) fields through gains along horizontal connection pipes, whose contribution is routinely ignored. The analysis considers thermal load profiles with different heating to cooling ratios and finds that the effect of horizontal pipes becomes more significant with unbalanced loads. The study explores the potential of extended operation, increased loads and an optimised operating strategy to exploit the idle capacity gain from the horizontal connection pipes. It is shown that for the scenarios investigated, the BHE field operational time can be extended by more than 25 years without violating the critical fluid temperatures in the design phase. Alternatively, the thermal load can be increased by up to 26%. In addition, the study highlights the potential of an optimised operating strategy involving adjustments to the number of BHEs operated to reduce power consumption and therefore reducing operating costs in existing systems by utilising heat gains from the horizontal connection pipes.