A one-material cylindrical model to determine short- and long-term fluid-to-ground response factors of single U-tube borehole heat exchangers

A new cylindrical model is proposed, suitable to determine both the short-term and the long-term fluid-to-ground thermal response factor of a single U-tube borehole heat exchanger (BHE). In the model, a BHE is represented by an equivalent cylinder, with the same radius and heat capacity as the BHE. The cylinder is made of a homogeneous material and contains a heat-generating cylindrical surface with an equivalent radius, req, optimized by repeated 2D finite-element simulations. The thermal resistance of the layer between req and the BHE radius equals the BHE thermal resistance. A correlation yielding the optimized values of req is provided.