Sensitive analysis on the effective soil thermal conductivity of the Thermal Response Test considering various testing times, field conditions and U-pipe lengths

Abstract

Thermal Response Test (TRT) is commonly conducted in geothermal projects to determine the effective ground thermal conductivity. However, the tests are hardly conducted in small or medium projects due to its considerable costs. Moreover, these projects are more likely to be influenced by the environmental conditions (climate and groundwater level) due to the shallow installation depth of the Borehole Heat Exchangers (BHEs). The objective of this investigation is to find an alternative approach for the estimation of the field effective soil thermal conductivity by considering various testing times, field conditions and U-pipe lengths. To reach this objective, a numerical framework is initially validated by the in-situ measurements. Then, it is used to simulate the TRTs conducted in clay, sandy loam and sand during the hottest day(s) of a year in summer and the coldest day(s) of a year in winter with various U-pipe lengths and groundwater levels. The results show that there is a higher effective soil thermal conductivity in winter than in summer while this difference becomes less significant for the U-pipes longer than 30 m. Among the studied soils, clay has the least effective thermal conductivity. Sandy loam shows a higher effective thermal conductivity than sand especially in the short U-pipes. The effective soil thermal conductivity varies linearly with a newly defined λ ratio (the water level depth divided by the U-pipe length). It also varies with the U-pipe length in a polynomial form (2nd degree). Finally, an analytical approach is proposed to estimate the effective soil thermal conductivity. The approach is validated by 6 in-situ measurements. The capacity of the approach is also evaluated by a regional multi-layered field.