Deep Borehole Heat Exchangers, Application to Ground Source Heat Pump Systems

Abstract

Deep borehole heat exchangers (BHE) can be used as a complementary heat source in ground source heat pump (GSHP) installations with a negatively balanced thermal load, i.e. when more thermal energy is extracted than recharged. GSHP systems can be made space effective and with a small or negligible visual footprint. Larger installations may, however, require certain amount of available drilling area for the boreholes. This area can be reduced by placing the boreholes closely together. This creates a system susceptible to the load balance. If the possibilities for thermal recharge are limited, the temperature in the boreholes will decline, which also degrades the performance of the system. To balance the thermal load, deep BHEs have to sustain the required temperature level without the need for thermal recharge. A numerical model is applied to simulate the performance of deep BHEs over time, and to determine the average specific thermal load and amount of energy extraction that can be sustained. The results are used to propose an alternative solution for a GSHP installation having a limited construction area and a negatively balanced thermal load. It is seen that the deep BHEs can support a high (increasingly with depth) thermal load, and that the required temperature level can be sustained over the life time of the system. Deep BHEs reduce the required borehole length, and are a viable option for GSHP installations in areas with scarcity of space and/or with negatively balanced loads.