Novel Instrument for Temperature Measurements in Borehole Heat Exchangers

Abstract

The thermal response test (TRT) is the standard method for characterizing the thermal properties of the ground and those of a borehole heat exchanger (BHE). During the TRT, the inlet and outlet temperatures of the BHE are monitored. However, this test typically considers the ground as a homogeneous, isotropic, and infinite media, and therefore, it only determines the bulk and effective parameters, such as effective thermal conductivity and thermal borehole resistance. Hence, the enhanced TRT protocols are necessary where the depth-dependent temperatures are measured to estimate depth-specific thermal properties. Thus, a novel instrument with a data logger to automatically obtain the temperature measurements along the BHE is introduced. This device is based on a Zynq-7000 all programmable system on a chip. It has a dual-core central processing unit and a field-programmable gate array on one chip, thus providing a versatile architecture that reduces cost and improves efficiency in comparison with other systems of similar characteristics. This paper describes the implemented hardware and software developments that range from user interface application to a free-distribution operating system based on an embedded Linux. The proposed instrument can be easily incorporated throughout a TRT, and the nonspecialized staff can remotely manage or visualize the results through a menu-driven interface. The device is tested in a specific BHE installation and validated with standard Pt100-temperature-sensors. The results are comparable and, therefore, demonstrate the applicability of this novel instrument called Geowire.