Coupling heat and buoyant fluid flow for thermal performance assessment of geothermal piles

Abstract

Traditional analyses of ground heat exchangers consider conduction as the sole mode of heat transport within ground. However, research provides evidences of thermally induced pore fluid flow in soil. This paper demonstrates that temperature induced fluid flow should be considered to accurately analyze the performance of geothermal piles. Heat transfer, momentum, and flow continuity equations are coupled to quantify performance of heat exchanger piles. A hybrid explicit-implicit solution scheme is utilized for simultaneous and efficient solution of the system of equations. Results from pile-soil heat exchange analysis of a model geothermal pile installed in saturated sand indicate that temperature increments in saturated ground induces pore fluid flow due to alteration in fluid density with temperature. Direction of pore fluid flow is different at inlet and outlet sides of a pile with U-shaped circulation tube. For the conditions explored within the scope of this paper, power output of a model heat exchanger pile increases by around 12% when contribution of convective heat flow is included in addition to that from conductive heat transfer in saturated sand. Therefore, convective heat transfer due to buoyant pore fluid flow may have considerable contribution towards thermal performance of ground heat exchangers in saturated sand.