Development of TPRT (Thermal Performance-Response Test) for borehole heat exchanger design

Abstract

To obtain the effective thermal conductivity and borehole thermal resistance required to design borehole heat exchangers (BHEs), thermal response tests (TRTs) are usually conducted. Although many advanced TRT methods have been proposed, most TRTs cannot directly provide the actual thermal performance of an installed BHE. Because many uncertainties exist in constructing even conventional BHEs, examining the transient heat exchange rate allows inspection of the construction quality; the rate can also be used as a reference value for design. To determine the actual heat exchange rates of BHEs, it is necessary to conduct thermal performance tests (TPTs) under a constant inlet fluid temperature. However, TPT requires expensive equipment, including a water tank and a complex control system; thus, generally only TRT is conducted. To overcome the existing problems of TPT, in this study, we proposed a thermal performance-response test (TPRT) that combines TRT and TPT. This method involved the construction of a cost-effective TPRT apparatus by adding only a general PID controller and a solid-state relay to an existing TRT apparatus. Using the apparatus constructed by the proposed method, two TPRTs were conducted to confirm the performance of the apparatus and the validity of the TPRT method. Additionally, by defining the new parameter of the unit heat exchange rate, one potential simple and reliable design method for BHEs was explored.