Emulated Borehole Heat Exchanger for Comparison of Different TRT Units

Abstract

To fulfill the UNFCC-Paris Agreement, shallow geothermal energy will play a major role in the future. For optimal usage of the geothermal potential, a precise estimation of the underground thermal properties is required. The Thermal Response Test (TRT) is a well-established method to measure the relevant underground data. So far, there has been no possibility to check and calibrate the TRT rigs which are mostly built in a self-made version. At the ZAE Bayern, an emulated borehole heat exchanger (E-BHE) to objectively check TRT units under adjustable and comparable underground conditions, has been developed, constructed and experimentally determined. The E-BHE consists of a chiller, a thermal storage tank and two heat exchangers with an electrical heater downstream for fast-acting regulation of the outlet temperature. It uses a regulation concept together with a borehole heat exchanger model. First tests have been conducted with thermal conductivities of the underground between 2.15 and 4 W/mK, borehole lengths between 80 and 100 m and undisturbed ground temperatures between 10 and 12 °C. These varying parameters have been adjusted as default values at the E-BHE. One set of data used corresponds to the values measured at a real borehole heat exchanger (BHE) over the past several years by a high precision TRT constructed at the ZAE Bayern. During the five tests, this TRT unit has been connected to the E-BHE and showed deviations in determined thermal conductivity of only 2.4 % +/- 2.2 % compared to the default values of the E-BHE.