Coaxial Borehole Heat Exchanger: Modeling, Thermal and Hydronic Analysis

Nowadays, the use of low-carbon energy sources results fundamental to achieve an energy transition towards a cleaner energy production in the European Union. Among today’s accessible and efficient renewable energies many of them are constrained by geographical availability, intermittent operation or large required areas for installation. Geothermal energy can overcome some of these withdraws being available almost anywhere, supposing a constant supply of energy without interruptions and not needing a large area, as geothermal systems are often aimed for small cross-sectional areas and long depths. Geothermal energy systems as the ground source heat pumps (GSHP) are positioned as reliable systems which main aim these days is covering part of the heating and cooling energy demands in several countries. These systems exchange heat with the surrounding ground by means of a borehole heat exchanger (BHE). Among different types of BHE, the U-pipe configuration has dominated the geothermal market for many years. More recent designs as the coaxial pipe-in-pipe configuration are still mainly used for research purposes. The coaxial borehole heat exchanger shows some interesting advantages over the U-pipe design: much lower pressure losses and in some cases even a lower borehole thermal resistance. Therefore, the coaxial heat exchanger, which constitutes the case study of this work, has still a long path of investigation and improvement to walk. In this work the thermal and hydraulic performance of the coaxial borehole heat exchanger are studied by implementing an analytical heat transfer mathematical model in Matlab® and a 2-dimensional axisymmetric computational domain in COMSOL®. Both are used to conduct parametrical studies in which the influence of geometrical factors, different circulating fluids and the thermal conductivity of the pipe are examined. The analytical model implemented in Matlab® shows to be an effective tool to compute the temperature profiles with a short required computational time. The CFD-CHT simulation allows a deeper study of the fluid flow and it provides detailed results in which how the heat is transferred inside the flows can be assessed, though at relatively higher computational effort. When comparing the results from both models, very similar findings are obtained in terms of thermal and hydraulic performance. Nevertheless, different sources of uncertainty are identified and they must be investigated in further studies. The results of the parametrical studies of both models are put together to compare and discuss them, with the aim of extracting common learnings when variating certain borehole and fluid flow parameters. The influence of the borehole length and the flow rate inside the coaxial channels results to effect drastically on both the total exchanged heat and the obtained pressure losses. Though other parameters, as the outer pipe diameter or the inner pipe thickness, show to have a slight effect on the thermal performance in the coaxial configuration, their variation have a significant impact on the hydraulic behaviour. Furthermore, diverse circulating fluids are simulated, as water and different antifreeze solutions. Even if they exchange a similar amount of heat with the ground, the water results to offer the lowest pressure loss in the heat exchanger