Numerical analysis of a distributed thermal response test on a U-pipe borehole heat exchanger

Abstract: Ground Source Heat Pumps (GSHPs) are relevant systems for space heating and cooling, very common in Sweden since 1970s. More than one million heat pumps have been sold by Swedish companies. The Energy Technology Department of KTH is working on the optimization of such application in the context of EFFSYS+ which is a national swedish project to keep the industry in a front position for the refrigeration systems and heat pump technology by a strong cooperation with the research institutes. Thermal Response Tests (TRT) were in earlier years developed in order to give an estimation of the average thermal properties of the ground over the length of the borehole heat exchanger (BHE). Further studies have been recently carried out using Distributed Thermal Response Tests (DTRT) based on temperature measurements along the borehole depth. This test permits the determination of the local ground thermal properties and local thermal resistances inside theborehole, which may be essential in the design of large Ground Coupled Heat Pump system (GCHP). A DTRT may provide information about different rock types, presence of groundwater flow and rock fissures, influence of surrounding buildings on ground temperature, among others. The analysis of the measured temperatures has been done previously using the so-called Line-Source model considering a stratified subsurface. In order to compare with the analytical approach, this numerical work simulates the ground with the commercial software COMSOL taking into account the temporal variations of the heat injection rate. An optimization module using a global least square approach is implemented to get a model which fits best the experimental data.