Development of thermal response test equipment applicable to water wells of large diameters

Abstract

In this work, a novel TRT equipment applicable to water wells of large diameters was developed and tested in an actual GSHP site. The equipment consists of an electric heater and an optical fiber sensor and is adhered to the well casing by electromagnets. The temperature change at the well wall is measured during and after the heating period for the estimation of the thermal conductivity of the ground (λs). First, TRTs using a heating cable were conducted in a grouted GHE close to a water well of 50 m deep located in Saga City, Kyushu, Japan, to obtain a vertical profile of λs as a reference. Then, four TRTs were conducted in the water well completed with a steel casing using the new TRT equipment. In the TRTs, the length of the heating period was changed from 0.5 days to 4 days to investigate the necessary length of the heating period, while the heat load was set around 43 W/m. λs values of each depth were estimated by graphical method based on Kelvin’s line source theory in the heating and the recovery periods in each TRT. The comparison between the λs profiles estimated from the heating periods and the reference profile showed significant differences due to the generation of natural convection of groundwater and the difficulty in controlling the position of the heater and the temperature sensor. The profiles obtained from the recovery periods, on the other hand, showed a good agreement with the reference profile when the heating period was set as 2 days or more, indicating the applicability of the new equipment in water wells of diameters up to 200 mm.