2010 Volume 32 Issue 1 Pages 31-40
In Japan, the annular space between heat exchange pipes and well walls are commonly grouted in ground heat exchangers (GHEs) to avoid the formation collapse and to secure the contact between the heat exchange pipes and the formation. As is frequently applied in northern European countries, however, GHEs can be completed without grouting in case the formation rock is rigid and groundwater level is located near the ground surface. To investigate the applicability of GHEs without grouting in Japan, the evaluation of heat exchange capacity of such GHEs is quite important for the design of optimum GHE length and hence to minimize the initial cost of geothermal heat pump systems.
In this research, an ungrouted GHE was drilled in a hard formation of granite in Fukuoka City, Japan and series of thermal response tests (TRTs) were carried out under various heat loads. On the basis of the chronological changes of heat medium temperatures and vertical temperature profiles measured using an optical fiber thermometer, the thermal resistivity of the GHE and the vertical distribution of thermal conductivity were estimated using graphical and analytical interpretation methods. The results of interpretation indicated the improved (reduced) thermal resistance of the ungrouted GHEs in comparison with grouted GHEs, due to the effect of natural convection of groundwater between the U-tube and the well wall. The interpreted vertical distribution of thermal conductivity showed a good agreement with the results of graphical interpretation, the local geological information and the laboratory measurements of core samples, indicating the reliability of the interpretation method of vertical temperature profiles measured by optical fiber thermometers in ungrouted GHEs.
