Analysis of short helical and double U-tube borehole heat exchangers: A simulation-based comparison

This paper makes a comparative analysis between a helical-shaped pipe and a double U-tube ground heat exchanger. The two configurations were installed at a shallow depth and studied, since a depth limit is mandatory in some areas. When installed this way, the interaction between the ground surface and the ambient environment needs to be taken into account when the thermal behaviour of the ground heat exchangers is investigated. A detailed numerical simulation tool was used to carry out this research. The problem of heat transfer was solved with an equivalent electrical circuit of thermal resistances and capacitances. This simulation tool was also used on a double U-tube heat exchanger in order to include axial heat conduction in the ground and the borehole. The two borehole heat exchangers were analyzed over both long- and short-term periods; the thermal properties of the ground, energy loads and the axial effects of weather conditions were also taken into account. Finally, a reduction in the borehole length for the helical-shaped pipe was also assessed and compared with the performance of the double U-tube.