Experimental evaluation of thermal response tests performed on borehole strings

Abstract

This thesis investigates the validity of the standard thermal response test (TRT) results when performed on a series of boreholes (string). The typical TRT consists of subjecting a single borehole to a constant heat injection rate to obtain the temperature response in the ground which can then be used to determine the ground thermal conductivity. When completed on a single borehole, the results may be analyzed with the line source theory, since the assumption of a single line heat source is valid. For multiple boreholes, the assumption of a single line source becomes invalid if the spacing between the boreholes is small enough for borehole thermal interaction to occur. Moreover, for boreholes that are charged in series, heat transfer from the horizontal pipes that connect the vertical boreholes may also influence the ground thermal response. This thesis takes an in-depth look at the different factors that affect the results of TRTs performed on borehole strings. Different analysis methods are implemented to determine areas of improvement for determining the thermal conductivity of the soil surrounding the borehole string. For the analysis, the infinite line source (ILS) model and a model developed using TRNSYS 18 were used to determine the effective thermal conductivity. The results show that TRNSYS is unable to accurately model a TRT performed on a borehole string. The horizontal pipe model within TRNSYS proved to have significant fundamental issues, as the effective thermal conductivity is greatly underestimated with values of 1.2±0.1W/mK and the results of increasing the horizontal length both increased and decreased the effective thermal conductivities. The results from the ILS demonstrate that an effective thermal conductivity of 1.7±0.2W/mK is an appropriate estimate of the soil at the BTES field tested, as the borehole string with the furthest spacing between boreholes gave an effective thermal conductivity of 1.7W/mK. Performing multiple thermal response tests within the same BTES field also provided evidence of the need to implement multiple TRTs as common practise. The testing presented shows that the effective thermal conductivity can vary within ±0.2W/mK within the same relative location. With better knowledge of the thermal properties within the BTES field location comes the opportunity for improved planning of operation and control of thermal distribution within the field. This would be especially beneficial when dealing with seasonal BTES fields