GEOTHERMAL RESPONSE TESTS USING CONTROLLED MULTI-POWER LEVEL HEATING AND COOLING PULSES (MPL-HCP): QUANTIFYING GROUND WATER EFFECTS ON HEAT TRANSPORT AROUND A BOREHOLE HEAT EXCHANGER

Classical Geothermal Response Testing depends on applying a constant energy forcing to a Borehole Heat Exchanger and analysing the temperature response in terms of ground thermal conductivity and borehole resistance using the line source or similar analytical approach. In the test the only heat transport mechanism accounted for is conduction, and the principle aim of the test is to measure accurately the ground thermal conductivity. Although the test is very useful in practice, there are several drawbacks. First of all, only the conductivity and borehole resistance can be estimated. Other parameters (borehole geometry, heat capacity etc) remain unknown. Secondly, to achieve high accuracy in a limited amount of time, a good idea of the thermal response of the ground is needed beforehand. If this information is not available, the test may saturate too quickly (too large energy forcing) or the thermal forcing may be too small, leading to an insufficient temperature response. In both cases the required accuracy of the test is not achieved. Thirdly, other heat transport mechanisms, such as ground water effects, are not considered. However, ground water flow affects and even may invalidate the test results, as has been shown in experiments performed by us where ground water flow conditions could be controlled. We recently developed a new test protocol that is based on the idea of using a numerical model and parameter estimation procedure to obtain estimates of any parameter of interest. In this test protocol the thermal pulse is modulated to achieve different energy levels. Pulses of about 24 – 40 hours are used, and both heating and cooling pulses are combined. The analysis procedure can be carried out by basically any model capable of calculating the energy transfer between a Borehole Heat Exchanger and the ground. In this case we employ a model based on TRNSYS with SBM, which was specifically adapted for this purpose. To develop the data analysis procedure we carried out a reference experiment, where ground water flow is virtually absent, and in exactly the same conditions an experiment where ground water flow was forced. In this paper we will present the results of this experiment and develop the methodology to quantify ground water effects using a Type III MPL-HCP geothermal response test.