Application of a thermal transient subsurface model to a coaxial borehole heat exchanger system

We present an application of a thermal transient model to a coaxial borehole heat exchanger system. We compare two numerical methods. First, a model with a prescribed formation temperature (PFT); secondly, a method with a modeled formation temperature (MFT). In this comparison, several parameters are analyzed, such as the transient temperature profiles, the heat flux along the wellbore, the overall heat transfer rate, the thermal conductivity of the formation, and the type of flow inside the pipe and annulus - laminar or turbulent. The description of the system by the MFT method is more physically consistent. Then we proceed validating this method against two experimental setups, thereby showing good agreement. We perform a sensitivity analysis to the MFT method, varying the direction of the flow, regular and reversed, and the center tube material, with a high (steel) or low (polyethylene) thermal conductivity. It is shown that the reverse circulation has a better heat extraction, while regular flow performs better in the case of heat injection. For the center tube material, polyethylene shows a better thermal performance when compared to steel.