22 May 2008
Two new measurement tools have been developed in order to improve the sizing
reliability and the installation quality of geothermal heat pumps with large borehole heat
exchanger (BHE) arrays.
The ground thermal conductivity ? is a decisive property for dimensioning borehole heat
exchangers, and, especially, of systems with BHE arrays. Although ? can be determined on
rock samples from the borehole in the laboratory or in situ by a customary thermal response
test (a BHE circulation experiment), both methods need special equipment and are timeconsuming.
Therefore a small, light and wireless probe with p and T sensors and a
programmable microprocessor has been developed (NIMO-T). The wireless probe is lowered
into one tube of the BHE where it sinks under its adjustable weight and records pressure and
temperature while going down, at preselected time intervals. After the probe has reached the
U-tube bottom it stops there and is flushed back to the surface by a small pump for recovery
and data retrieval. A measurement run in a 300 m deep BHE takes less than 60 minutes.
The depth-temperature-profile allows determination of rock thermal conductivity, identification
of groundwater flow, quality control of the backfilling and advanced analysis of thermal
The second tool is a device to check the leak tightness of borehole heat exchanger tubes, a
prerequisite of safe operation and thus of permitting. The new probe measures automatically
the pressure answer at the BHE outlet tube over time with respect to a defined pressure
signal at the BHE inlet. The temporal variation of the inlet-pressure is adjusted with respect
to the engineering norm SN 805. If the pressure answer at the outlet is not within the limits
given by the SN 805-norm, the BHE fails the leak tightness test. If the pressure answer is
correct, a test certificate is generated automatically.