Geothermal response test (GRT): more than just effective thermal conductivity

According to VDI Guideline 4640 Part 2, the design of a properly sized ground source heat pump requires the performance of a GRT to determine the thermal properties of the ground. Specific parameters to be obtained are the undisturbed mean temperature and the thermal conductivity of the lithologies present. With the first practical tests carried out in Sweden in 1995 and in the USA in 1996, the method is now well established worldwide.

The effective thermal conductivity of the ground and the thermal resistance of the borehole are estimated from the measured temperature evolution in the borehole for a given energy input/output, according to Kelvin's line source theory. While thermal conductivity values for different lithologies published in the literature and VDI Guideline 4640 Part 1 provide rough estimates, only the GRT can provide real scenario values as parameters other than mineralogical and chemical composition can affect the effective thermal conductivity.

In reviewing the GRT reports from across Schleswig-Holstein, we observed very high effective thermal conductivities in some locations that could not be explained by lithology or incorrectly performed measurements or installed geothermal probes. Groundwater flow and, in some coastal areas, even tidal currents were found to influence thermal conductivity.