SE-14-012 -- Evaluation of the Thermal Performance of Two Nonstandard Borehole Configurations PDF

Atheoretical study on the alternatives to single and double U-tube boreholes is presented in this work. These nonstandard configurations consist of channels arranged in a semicircle or in quadrants. Channels are thermally insulated to reduce the thermal short circuit between the downward and the upward legs.

The steady-state thermal performance of the proposed configurations is evaluated using an analytical solution for a set of coupled linear differential equations of energy balances with and without thermal interaction. The borehole thermal resistance and the circulating-fluid temperature during heat extraction are evaluated in the turbulent and laminar regimes and are used for comparison purposes. Results show that the new configurations offer up to a 20% reduction in borehole thermal resistance leading to shorter boreholes. For low mass flow rates (laminar flow), where thermal interaction between the upward and downward flowing fluid becomes more important, thermal insulation between channels can reduce the borehole thermal resistance of the proposed configurations by up to15%over a standard double U-tube borehole configuration.