A Numerical Approach to Assessing Thermally Interacting Multiple Boreholes with Variable Heating Strength

The use of geothermal energy systems is widespread but, having had a revival in the 1980\'s and recently, both the sustainability and impact of these systems on the environment are now being questioned. Due to its efficiency, the use of geothermal energy is advantageous in many cases. However, little research is available to guide regulatory agencies and industry towards designs and installations that maximize their sustainability. One potential hindrance to the sustainability of these systems at their design efficiency is the thermal loss from the system itself, which can affect adjacent systems and the surrounding ground. Studies show that interference effects are present in some installed geothermal systems. The influence of these systems on each other implies that they have a spacing that is smaller than the threshold spacing for such systems to avoid thermal interactions, and indicates that there is a limit to the density of geothermal development that can occur in a given region of the ground. Many studies in the area of geothermal energy have focused on modeling single ground boreholes. The potential existence of thermal interaction among multiple boreholes is identified in the literature, but not formulated, and the affecting parameters have not been assessed in detail. In order to model interacting borehole systems, Koohi-Fayegh and Rosen (2011) evaluated the temperature response in the soil surrounding multiple boreholes in a numerical study. They assumed that the heat flux from the borehole wall is constant and, therefore, that heat conduction in the direction of the borehole length is negligible for a major part of the solution domain. In the current study, the assumption of constant heat flux along the borehole wall is examined by coupling the problem to the heat transfer problem inside the borehole. A numerical finite volume method in a three dimensional meshed domain is used to model the conduction of heat in the soil surrounding boreholes. In order to determine the heat flux boundary condition, the analytical quasi-three-dimensional solution to the heat transfer problem of the U-tube configuration inside the borehole (Zeng et al, 2003) is used. This solution takes into account the variation in heating strength along the borehole length due to the temperature variation of the fluid running in the U-tube. Thus, critical depths at which thermal interaction occurs can be determined. References Koohi-Fayegh S., Rosen M. A., Examination of thermal interaction of multiple geothermal storage and heat pump systems, Proc. 3rd International Conference on Applied Energy, 16-18 May 2011, Perugia, Italy, pp. 3473-3486. Zeng, H. Y., N. R. Diao, Z. Fang, 2003, Efficiency of vertical geothermal heat exchangers in ground source heat pump systems. Journal of Thermal Science 12(1):77–81.