Thermal response tests using optical fiber thermometers

Thermal response tests (TRTs) were conducted using optical fiber thermometers in two types of ground heat exchangers (GHEs), coaxial type and U-tube type, drilled in a heterogeneous deposit in Fukuoka City, Japan. For the interpretation of the TRTs, the ground was sliced into sub-layers and the cylindrical source function was applied to each sub-layer to calculate the heat exchange performance between the ground and the GHEs. A nonlinear regression method was used to estimate the vertical distribution of thermal conductivities of the ground by minimizing the sum of squared errors between measured and calculated temperatures of ground and heat medium. The estimated thermal conductivity distribution showed good agreements with the rock type distributions and the groundwater flow information at the well site, indicating the reliability of the testing and interpretation method.