Analysis of long-term changes in temperature and heat flow rates caused by the operation of a BHE for a typical suburban setting

The utilization of geothermal energy through borehole heat exchangers (BHE) is growing in significance for a sustainable energy and heat supply in urban areas. Therefore, a comprehensive knowledge of the subsurface heat balance and the governing heat transport processes influenced by urban infrastructure is essential when operating multiple BHEs with a high spatial density in city districts.

The aim of this study is to investigate the long-term changes in the subsurface heat balance and subsurface temperatures due to the operation of an individual BHE and to determine the temporal evolution of the heat capture zone of the BHE depending on the presence of streets and buildings in the near surroundings. For this purpose, a numerical model for one plot of land with a standard single-family home from the 2000s and a 3-meter-deep basement next to a street was developed. A BHE is placed in the front yard between the street and the house in accordance to the required minimum distances. Operation of this BHE was simulated for 30 years using a standard load profile based on German guidelines. The model also includes seasonal temperature variations for the street and land surface as well as heat transfer from the building to the ground.

At the beginning of the simulation, the extracted heat originates from the plot subsurface itself. The street as well as the heated basement of the building transfer heat into the subsurface, which is partially extracted by the BHE since it is located in their immediate vicinity. During the 30-year simulation period, the heat capture zone of the BHE increases. After only two years, about 50 % of the extracted heat stem from the subsurface outside the plot. After five years this fraction increases above 65 %, after 30 years above 80 %. In simulations without accounting for heat transfer from the street or the building to the subsurface, this fraction increases to about 90 %. As a consequence, also subsurface temperatures near the BHE as well as BHE return temperatures are reduced by up to 0.87 K and 0.33 K, respectively, compared to the original scenario.

Overall, the results show that the major fraction of the heat extracted by BHEs originates from heat stored in the subsurface, and that after 30 years most of the extracted heat is replaced by heat from neighbouring plots. This indicates, that even if minimum distances to neighbouring BHEs are maintained, these might be significantly affected with consequences for subsurface and BHE return temperatures.