Critical comparison between thermal performance test (TPT) and thermal response test (TRT): Differences in heat transfer process and extractable information

Abstract

In the field of ground-source heat pumps (GSHPs), thermal response tests (TRTs) with constant heat rate have been conducted to estimate the design parameters of ground heat exchangers (GHEs), and thermal performance tests (TPTs) with constant inlet temperature have been conducted to examine the heat exchange performance of GHEs. These two experimental methods have been discussed insufficiently, and therefore, they are sometimes perceived as completely different experiments with mutually exclusive extractable information. However, both methods provide similar information, although there are differences in the heat transfer process. For better understanding these two methods and extending the utilization potential of TPT and TRT data, this study analyzes and discusses the following two issues using numerically generated datasets: (1) differences in heat transfer process between TRTs and TPTs and (2) effect of different physical processes on the extractable information and its accuracy, such as the ground thermal conductivity, borehole thermal resistance, unit heat exchange rate, and transient ground thermal resistance. From the viewpoint of the heat transfer process, we found that the GHE’s heat storage effect disappears faster in TPT than in TRT. When estimating ground thermal properties, this difference allows the TPT to be more accurate with shorter experiment time than the TRT. Additionally, we found that the TRT overestimated the GHE thermal performance compared to the TPT. However, when the heat storage effect was removed, both methods provided almost identical results. A comparison of experimentally obtained ground thermal resistances, which are required in the ASHRAE design method, with those obtained from the infinite cylindrical source model indicated that the TPT showed discrepancy rates less than ±8% whereas the TRT showed larger discrepancy rates. In particular, when the heat storage effect is not removed, the discrepancy rate for the short-term ground thermal resistance increased by ~27%.