Probabilistic uncertainty quantification of borehole thermal resistance in real-world scenarios

Abstract

The thermal resistance of a borehole heat exchanger (BHE) varies considerably depending on the position of the pipe legs. However, the pipe position cannot be fully controlled during construction because of the extreme slenderness of the BHEs. Therefore, the borehole's thermal resistance (Rb) estimated from one BHE is not representative of the entire borefield. This study probabilistically quantified the uncertainty of Rb caused by the inherent uncertain characteristics based on Monte Carlo simulations. The following four scenarios were considered: 1) pipe legs that moved along the central axis of the borehole, 2) insertion of a U-tube using spacers to maintain the shank spacing, 3) pipe legs that were freely located within the borehole, and 4) consideration of the borehole radius and thermal conductivity of the grout and ground in addition to an uncertain pipe location. The results demonstrated that the use of spacers considerably improved Rb and reduced its uncertainty range. The results of the final scenario revealed that the maximum Rb value could be more than twice the minimum value. Moreover, the spatial variation of Rb was analyzed systematically as a function of the pipe position to clarify the dynamics of the change in Rb. The results suggest that Rb obtained based on a thermal response test cannot represent the entire borefield adequately.