Emulated borehole heat exchanger – current state of development of test rig for the assessment of TRT units

Shallow geothermal energy already is and will be playing a major role in achieving the climate goals of the Paris Agreement. When designing geothermal energy systems, it is crucial to know the thermal conductivity of the underground as well as the thermal borehole resistance, besides further parameters. A thermal response test (TRT) is the method of choice for determining these values. Therefore, two teams in Sweden (Gehlin) and the USA (Spitler) developed TRT rigs in the 1990s. Since then, there have been numerous instructions and recommendations on how to execute testing, including the correct equipment, procedure, and evaluation of results, for example ISO 17628:2015, EN 17522:2023, VDI 4640 Blatt 5. However, currently there is no established method for testing TRT units and procedures to assess their functionality and quality. Therefore, an emulated borehole heat exchanger (E-BHE) for TRT unit quality assurance was developed and built at ZAE Bayern. It emulates the thermal behavior of a real borehole heat exchanger (BHE). Different borehole lengths, single and double U-pipes, as well as various underground properties, can be set. This allows for testing different TRT units within a short time span and with reproducible boundary conditions. The E-BHE uses the so-called EWS model by Huber. First tests underline the importance of using consistent models for TRT evaluation and BHE system design.