Numerical and analytical study of a geo‐exchange borehole using conventional grout and bentonite‐based backfilling material

Summary Ground source heat pumps (GSHP) have been used in various types of residential and commercial buildings due to their high efficiency. Numerical models are useful to predict the overall performance and ground temperature response of these systems. This paper presents a hybrid model that contains a modified finite element model and an analytical solution for a single conventional vertical borehole system. In this modified finite element model, turbulent heat transfer equations were solved for the ground heat exchanger and the actual building load variation and heat pump performance variation were considered. The present model was then used to explore an emerging geo‐exchange technology, which involves the use of a bentonite slurry enhanced with graphite flakes in the vicinity of a borehole heat exchanger. The results revealed slight increases in the mean average ground temperature in the vicinity of the borehole by 1.1°C over 4 years. Furthermore, the analytical solution of the ground temperature response was in good agreement with the results obtained using the finite element model within a maximum relative error of 3% (0.5°C). The results revealed that the 40 m depth bentonite‐based borehole achieved better performance than the conventional design by 5% to 13% in the monthly average COP.