Thermal response tests : influence of convective flow in groundwater filled borehole heat exchanger

Sammanfattning: The main objective of this doctorial thesis was to investigate how thermally induced movements in the groundwater (natural convective flow) may influence the heat transport in borehole and surrounding bedrock in a groundwater filled borehole heat exchanger system. The purpose was also to determine if thermal response tests could be used to detect the convective influence and the effect on evaluated heat transfer parameters, effective bedrock thermal conductivity and borehole thermal resistance. In order to increase the knowledge about the natural convective influence in groundwater filled borehole heat exchangers, numerical 3D simulations in the computer fluid dynamic (CFD) software Fluent were conducted. It was shown that the thermally induced convective flow influenced the borehole thermal resistance independently of bedrock characteristics (solid or fractured). A larger convective heat flow (dependent on density gradient) resulted in a lower resistance. The density gradient and thereby the convective flow are affected by the water temperature level and the used heat injection or extraction rate. At a water temperature around 4ºC (maximum density), the borehole thermal resistance had its maximum value resulting in values close to stagnant water. In other working conditions the heat transfer could be up to 2.5 times greater than that of stagnant water. This was further investigated and confirmed by in-situ thermal response tests in two boreholes at the campus of Luleå University of Technology. Several multi-injection rate thermal response tests were performed, which is a test protocol where several test periods are performed in a row using different heat injection rates. With this protocol it was shown that natural convective flow may be seen to affect both the borehole thermal resistance and effective bedrock thermal conductivity. For the bedrock thermal conductivity it was shown that the convective influence was seen only for fractured bedrock. A larger convective heat flow resulted in a higher effective bedrock thermal conductivity. The numerical 3D simulations were also used to study some common approximations when modelling grouted boreholes to see if these would also be suitable for groundwater filled boreholes. The purpose was to find approximations that would allow for a simpler model for evaluation of thermal response tests and design of borehole heat exchanger systems. It was shown that using an equivalent radius model (one single pipe in the middle of the borehole) instead of the more complex u-pipe geometry was a good approximation, if the appropriate equivalent radius was used. For the total heat transfer, including the convective heat flow, the total heat transfer area should be the same as for the u-pipes. Another approximation that was tested was to use a boundary condition at the outer pipe wall instead of simulating the fluid flow inside the pipe and the heat flow through the pipe wall. It was shown that the two common boundary conditions, constant temperature and constant heat flux, gave similar results for total heat transfer calculations but quite different results for only conductive heat transfer calculations. Performed investigations showed that the convective influence could give large differences in evaluated borehole thermal resistance and effective bedrock thermal conductivity. It is therefore strongly recommended that thermal response tests are performed using similar heating or cooling conditions as the planned borehole system. In Sweden, most systems use heat extraction during part of the year. For that reason, heat extraction thermal response tests in groundwater filled boreholes were studied. It was shown that ordinary evaluation methods did not work due to the large variations in the value of the borehole thermal resistance during the test. Instead a new evaluation method was proposed, where the measurement time was divided into intervals, where each new interval allowed for a new borehole thermal resistance. The same numerical model was used as in the ordinary parameter estimation evaluation used for the other tests. The model was run manually, and each new borehole thermal resistance was chosen so that the calculated mean fluid temperature for that period matched the measured values. The intervals were recommended to be chosen between 4 to 10 hours depending on how fast the mean fluid temperature changed.