Comparative temperature measurements in an experimental borehole heat exchanger

Abstract

Temperature measurements in vertical borehole heat exchangers (BHE) are now common practice in the building industry. They are used for the design of BHE installations, as well as for quality control purposes, for instance in detecting the presence of groundwater circulations. In the city of Zollikofen (CH), a BHE-array is planned for heating and cooling a retirement home. In the design phase, a pilot BHE was drilled. For technical reasons, the pilot BHE will not be connected to the final system. The opportunity to rehabilitate the pilot BHE has therefore been seized for the purpose of installing an experimental BHE that can be permanently accessed for measurements and experiments. The SwissEnergy Programme initiated some projects to increase the execution quality of BHE in Switzerland. In this framework, the experimental BHE has been used to compare seven commercially available temperature logging devices. The results indicate that all seven measurements show a high level of congruency all along the profile. Deviations are mainly explained by an inadequate measurement speed with respect to thermal reactivity of the sensor. Current commercially available devices are suitable for temperature measurement in BHE only if the operator respects the recommended measurement speed. These devices are very reliable and provide accurate information about the subsurface temperature that could be used either in the design of BHE or for quality control purposes.