Modelling and evaluation of groundwater filled boreholes subjected to natural convection

Abstract

One of the challenges of designing ground source heat pumps systems is the calculation of the effective thermal borehole resistance which affects the heat extraction rate and thereby the required length of the borehole. Calculating effective borehole resistance is especially demanding when the boreholes are filled with groundwater. The natural convection in the groundwater affects the heat transport from the borehole heat exchanger to the borehole wall. The objective of this paper is to present the results of the modified Pygfunction software in which recently developed correlations for calculating influence of the natural convection in boreholes has been incorporated. The calculated results were validated by using results of thermal response test (TRT) from four adjacent groundwater filled borehole in the central parts of Sweden. The numerical results reveal that the original and unmodified Pygfunction can be used for accurate calculation of fluid temperatures in borehole heat exchangers by using an effective thermal conductivity of the filling material. However, that requires access to thermal response test data. The modified software calculates the effective borehole resistance with a deviation of about 15–30% compared to the effective borehole resistance from TRT. This deviation is an improvement compared with the standard assumption of neglecting natural convection in boreholes. However, for large installations with more than 4000 m of borehole heat exchangers a thermal response test is still recommended in order to ensure a more correct sizing of geothermal system.