Borehole temperature evolution during thermal response tests

The measurement of temperature inside a borehole at specified depths during a thermal response test, used to infer the subsurface and the borehole thermal properties for the design of a ground-coupled heat pump system, allows the correlation of the subsurface thermal conductivity with stratigraphy. The temperature signal measured in the borehole during heat injection in a ground heat exchanger made with a single U-pipe, however, depends on the location of the temperature sensor in the borehole, which is difficult to determine in practice. Two-dimensional numerical simulations of the borehole temperature evolution during thermal response tests show that the temperature inside the borehole homogenizes rapidly after heat injection is stopped. Monitoring temperature recovery consequently helps to analyze measurements conducted at depth inside the borehole, since recovery measurements are not significantly influenced by the position of the sensor in the borehole. Numerical simulations also indicate that the borehole thermal resistance is best determined using a combination of recovery and heat injection data.