Accounting for Borehole Thermal Capacity When Designing Vertical Geothermal Heat Exchangers (ST-16-C027)

Steady-state heat transfer inside boreholes is usually assumed when sizing geothermal boreholes and a constant borehole thermal resistance is used to calculate the temperature difference from the fluid to the borehole wall. Thus, heat rejected into the fluid is assumed to be transferred immediately at the borehole wall. In reality, steady-state borehole heat transfer is rarely present. Rejected heat will heat the fluid and the grout before reaching the borehole wall and be transferred to the ground. These transient effects, caused by the fluid and grout thermal capacities, are beneficial as they reduce the peak ground loads and, consequently, the required borehole length. This paper proposes improvements to the ASHRAE vertical borehole sizing equation to account for borehole thermal capacity. In the first part of this study, annual TRNSYS energy simulations are performed on a residential ground-source heat pump system. Borehole models that account for thermal capacity are used to quantify the borehole transient effects for a range of operating conditions. In the second part of the paper, modifications to the current ASHRAE sizing equation are proposed to consider borehole thermal capacity. Results show that neglecting borehole transient effects leads to oversized boreholes and overestimated heat pump energy consumption. By considering the fluid and grout thermal capacity, it appears that borehole length can be reduced by about 10% and heat pump energy consumption by 5%. The largest reductions occur when heat pumps operate intermittently.