| Abstract |
This paper presents a new evaluation method for the Geothermal Response Test (GeRT, TRT), able to cope with improper response test data, e.g. as a result of instable power. TRT meanwhile is a standard method for determining the thermal conductivity of the underground and the borehole resistance. The test rig is connected to the BHE and water (or brine) is circulated in the system. By injecting thermal energy (heating) the fluid temperatures rise with the time. The slope of the temperature curve is correlated to the thermal conductivity. Temperatures, flow rate and heating load are measured. By evaluating the data of the temperature development, the thermal conductivity of the underground can be calculated. For this evaluation several methods and mathematical models are available, the most common being the analytical model of Kelvin's line source theory, followed by cylinder source and numerical simulation methods (e.g. with finite elements). Whereas the latter are able to cope with different power levels or fluctuations in power, the analytical models assume that heating load stays constant over the whole testing period. Fluctuations of the power supply may lead to misinterpretation and instable results (detectable with the stepwise / sequential evaluation). By using the approach of ESKILSON (1987) for the superposition of the line source approximation all kind of power fluctuations and variations can be handled. With the method the temperature development is calculated using the different heating loads for each time step. The thermal conductivity and borehole resistance are varied within predetermined limits and the resulting temperature curve is compared with the measured temperatures. The parameters of the best fit curve are regarded as the result. A computer program was developed to evaluate GeRT data using the superposition method. Several calculations were performed using test data with showing a stable result in stepwise / sequential evaluation. By comparison of 21 tests, the deviation between standard line source method and superposition method is less than 3%. The superposition method is an easy to use and fast method to find adequate results for test runs with improper power supply or disturbance by environmental influences (solar radiation etc.). |