A Thermal Response Test or synonymous a Geothermal Response Test (TRT or GRT) is a field method to investigate thermo-physical properties of a borehole heat exchanger (BHE) drilled into soft or hard rock (fig. 1). With a TRT thermal energy is injected into a BHE with a steady state impulse. Typically a BHE contains one or more loop type PE-HD heat exchanger pipes where a heating carrier fluid is pumped in a circuit with a constant rate. The heater of the TRT device is heating the fluid with constant power. The input and output temperature and the circulated volume flow is measured continuously. The heat transfer mechanism from the working fluid via the wellbore completion into the geological formation is conductive. Evaluating these data leads to heat losses caused by the geological formation and the BHE construction itself. The obtained heat losses are leading to integrative data over the whole drilled length. The measurement of single geological layers is typically not utilized. The TRT method is in opposite to the newly developed eTRT method (enhanced TRT or eGRT) economically feasible and very useful in existing BHE with no equipment needed for an eTRT.

The standard application of a TRT is collecting data for the design and model process of a larger BHE field. Furthermore with a TRT the technical integrity of built BHE can be evaluated. Conventional evaluation (following the line source approach) requires tests durations typically exceeding 50 h to obtain quasi-steady state heat flow conditions. Applying the cylinder source theory enhances the precision of evaluation and reduces the test durations (and therefore cost) because the non-steady state is evaluated. The impact of the GRT-method will increase in the future because real estate business with properties already equipped with borehole heat exchangers need to be valued. The examination of a BHE field in a real estate deal could be a part of a geothermal due diligence. The resulting TRT temperature graph is demonstrating the relation between the injected heat energy and the thermal rock properties. The determination of rock parameters is conventionally done by a line source theory based simplification (Hellström, 1991). Kelvin (1860/61) developed in a theorem of the heat transfer from buried electrical wire lines into the soil his line source concept. Ingersoll & Plass (1948) related Kelvin`s theory to the thermal dynamics for a single U-type borehole heat exchanger and presented a mathematical model. While the mathematical line source model is based on an infinitesimal small line source (Carslaw & Jaeger, 1959). The simplification on the other hand requires a curve fitting regarding the geometry and conditions of a BHE.

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