Engineering chart for thermal performance of cast-in-place energy pile considering thermal resistance

Abstract

An energy pile is one of the novel types of ground heat exchanger (GHEX), which has been introduced as an economical alternative to conventional closed-loop vertical GHEXs. The energy pile contains heat exchange pipes inside a pile foundation and circulates a working fluid through the pipe inducing heat exchange with the surrounding ground formation. Using the existing foundation structure, the energy pile can reduce the initial construction cost such as drilling and grouting cost, and eliminate the need for additional construction space. However, most commercial design programs and analytical models for estimating the thermal performance of GHEXs are mainly applicable to the closed-loop vertical GHEXs. In this paper, an engineering chart is proposed for evaluating the thermal performance of cast-in-place energy piles considering various layouts of heat exchange pipe. First, parametric studies on different ground and operation conditions were performed with the aid of a series of computational fluid dynamic (CFD) analyses to determine design factors of the cast-in-place energy pile. Based on the design factors, the engineering chart for the thermal performance of cast-in-place energy piles was developed. Since the CFD simulations were exclusively carried out for the typical 5-pair-parallel U-type energy pile, the concept of thermal resistance was adopted for estimating the thermal performance of other types of cast-in-place energy piles. The accuracy of the developed engineering chart was verified by comparing with the results of CFD simulations, and the maximum error was measured to be about 9%.