Case study of heat transfer behavior of helical ground heat exchanger

This paper presents an experimental and numerical case study of the heat transfer around the helical ground heat exchanger. With varying helical pitch, indoor thermal response tests were conducted in a dry sand. The tests were analyzed based on axisymmetric finite element analyses and recently published heat source models for spiral coil heat exchanger including our modifications of them by adding finite-length line source solution to consider vertical return pipe at center of helical heat exchanger. In addition to reasonable agreements in soil temperature rises at exteriors the helical heat exchanger, existence of return pipe was found to be important for estimation of heat transfer inside helical exchanger. Besides, the heat source models were found to be applicable to approximate mean fluid temperature during TRTs. The numerical and analytical models were applied to field thermal response test for an energy pile reported in literature. Compared with numerically estimations, the analytical solutions considerably overestimated temperature rise at the exterior of the energy pile. The results reveal that reasonable considerations in estimation of effective thermal properties may be needed when applying the heat source models for energy pile, even though they would evaluate reliable heat transfer behavior of helical heat exchanger embedded in a homogeneous ground.

► We performed indoor TRTs in a dry sand with varying pitch of helical ground heat exchangers. ► The tests were successfully simulated by axisymmetric FE analyses. ► Existing analytical solutions underestimated temperature rises of soil at the interior of heat exchanger. ► We reliably modified the analytical solutions by considering heat flow from the vertical outlet pipe. ► We analyzed TRT for the actual energy pile and discussed limitation of the analytical solutions.