Monitoring of Borehole Heat Exchangers Using Wireless Temperature Measurements

Abstract

Vertical temperature profiles (T-logs) of U-pipe borehole heat exchanger (BHE) can be measured with wireless measurement techniques. One innovative technique to perform wireless temperature measurements (WTM) is the spherical GEOsniff measuring probe. The main field of application of the measurement sphere is shallow BHEs with common depths of up to 100 m. The sphere is a miniaturized and wireless sensor system and passes narrow and strongly twisted parts as well as the basis of a BHE. Due to a small diameter of 20 mm and a density of 1,700 kg/m3, the sensor unit automatically navigates through the fluid containing pipes. The measurement sphere can be used for initial as well as long-term measurements to generate continuous T-logs used for monitoring and quality assurance of BHEs. In this work, we present various fields of application of the GEOsniff and discuss measurement results of the undisturbed ground temperature. In addition, wirelessly collected T-logs of the heating and cooling period of an enhanced geothermal response test (EGRT) are shown. Furthermore, the plausibility and quality of the generated T-logs are compared with those created with common stationary measurement technologies such as Pt100-sensors and hybrid cables using distributed temperature sensing (DTS). The field measurements show a strong correspondence between the WTM and the Pt-100 sensors with a maximum deviation of 0.14 K. The DTS measurements have an average offset of 0.74 K compared to the T-logs measured with the GEOsniff. The T-logs of the heating and relaxation procedure of the EGRT show an almost equidistant spacing and allow conclusions to different flow velocities of the groundwater. Due to a fast and easy measurement procedure, and a high precision of the T-logs, WTMs using the GEOsniff are an alternative or support for commonly used measurement technologies.