THERMAL RESISTANCE EVALUATION AND PERFORMANCE ASSESSMENT OF HEAT EXCHANGER PILES

Traditionally, boreholes have been utilized in ground-source heat pump (GSHP) systems for heating and cooling of buildings. Recently the concept has been expanded to heat exchanger piles, which are structural foundation elements retrofitted with circulation pipes. These dual-purpose elements provide structural support and can be used as heat exchanger elements with the use of the circulation pipes integrated into the piles. Similar to traditional borehole systems, the overall thermal resistance of the heat exchanger pile is the prominent factor on the thermal performance. Higher thermal resistance of the heat exchanger piles due to their larger diameter lowers the heat injection/extraction capacity, reducing the efficiency of the system. Many factors can impact the pile thermal resistance, including the diameter of the pile, thermal conductivities of the pile grout and the ground, thermal conductivity and the dimensions of the U-tubes, their shank spacing and loop spacing, flow rate of the fluid and its thermal properties, as well as the flow circuit arrangement. In this study, a two-dimensional numerical model is developed along with a methodology to accurately evaluate the thermal performance of various heat exchanger pile configurations in different ground conditions. The model is used in a parametric study to identify the most significant design parameters and their effect on the thermal performance.