Thermal response test numerical simulations and analyses

Abstract: When constructing large borehole heat exchanger (BHE) systems, bedrock and borehole thermal properties are vital for a good design. Today's design programs presume conductive heat transfer in both borehole and bedrock. In groundwater-filled boreholes, however, convective flow will be induced in the groundwater due to the occurring temperature gradients. The resulting more efficient heat transfer lowers the borehole thermal resistance. A 3 m long borehole was numerically studied to investigate the effect of heat injection on natural convection in a groundwater-filled borehole heat exchanger in impermeable bedrock. A convective flow with rising water close to the U-pipe and descending water at the borehole wall was induced. The flow rates in the groundwater are determined by the temperature gradient in the borehole. A higher injection rate results in a larger convective heat transfer, lowering the borehole thermal resistance. An equivalent radius model was also constructed in order to examine possible model simplifications. Using an annulus instead of a more complex U-pipe geometry may radically decrease the required computer capacity and calculation time. The result shows that for a solid bedrock model, borehole mean heat transfer patterns are similar for both models. Therefore, it may be possible to use the simpler equivalent radius model to simulate the convective heat transfer in borehole heat exchangers. Thermal response tests in boreholes were also conducted to investigate the effect of different power and temperature levels on convective heat transfer. A decrease in borehole thermal resistance is seen for higher fluid temperatures. A cold injection test was also performed. The resulting lower temperatures in the borehole increase the borehole thermal resistance, and leading to the formation of ice in the borehole. These tests indicate the importance of using different borehole thermal resistances in BHE design calculations, if the system should operate under several power levels. Thermal response testwhile drilling was investigated as an alternative method to the standard thermal response test. With this new method, bedrock conductivity would be continuously determined along the borehole. Therefore, bedrock anomalies such as fractures may be detected. The method is investigated for water driven down-the-hole hammers. A numerical model was developed to investigate the thermal response to heat release during drilling. The results show that by providing measurements of high accuracy and precision, occurring small changes in conductivity may be detectable. This licentiate thesis is the first part of a PhD thesis. It summarises the results of the study on the effect of natural convection on BHEs, as well as theoretical investigation of a new thermal response test method. To fulfil the PhD, the influence of groundwater movement on thermal response tests will be further studied with numerical models and field tests. The goal is to implement the result in BHE design calculation programs and TRT analysis. This licentiate thesis includes two submitted journal articles and one conference paper.

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