Temperature of energy boreholes accounting for climate change and the built environment – A new model for its estimation

Abstract

Changes in the ground surface temperature, as it can occur in built-up areas or due to climate change, affect the temperatures of geothermal boreholes. Analytical models for the thermal simulation of boreholes and considering this factor have been proposed. However, they all impose a uniform heat extraction boundary condition along the borehole walls. This boundary condition overestimates the temperature change in the underground caused by the borehole heat extraction and underestimates it in case of rejection. More accurate results are most often obtained by imposing a uniform temperature boundary condition.In this paper, we propose a new model to calculate the boreholes wall temperature taking into account both the heat extractions/rejections from all the boreholes in the area and the change in ground surface temperature. The model is tailored for areas with independent ground source heat pumps and imposes a uniform temperature boundary condition along the borehole walls, overcoming the limitation of the existing models.We apply the new model to a real Swedish neighbourhood and show that existing systems may already be significantly affected by the increased ground surface temperature due to urbanization.We also compare our new model with an existing similar model and show that while the two models provide similar results for smaller areas, their difference tends to be relevant for bigger areas – including the real Swedish neighbourhood analysed - thus making the application of our model important for neighbourhood- and city-scale studies.