Thermal performance evaluation of integrated solar-geothermal system; a semi-conjugate reduced order numerical model

Abstract

In cold climates, a borehole thermal storage system is key to supplying renewable energy year-round for heating applications. For such systems, coaxial borehole heat exchangers are getting more attention in recent years due to their superiority over other types of borehole heat exchangers. As such, more research is needed to understand the heat transfer mechanism and enhance the heat transfer process. In this study, an efficient reduced-order numerical code is developed to solve the heat transfer phenomenon in the coaxial borehole heat exchanger system for application in solar borehole thermal energy storage system. This concept assists in transferring heat between the subsurface and the Heat Transfer Fluid. In addition, the numerical solution integrates the building thermal load, thermal energy originated from the solar collector system, and heat loss to the adjacent strata and atmosphere. The developed numerical model is validated against the field-test experimental data performed with a coaxial borehole heat exchanger. The accuracy and computational speed of the model are compared its corresponding three-dimensional full-scale finite volume based model. Finally, the verified numerical code is employed to estimate the efficiency of solar-borehole thermal energy storage system for a two multi-family residential building in Ontario, Canada. The proposed model offers a novel holistic approach for estimation of the solar heat collection, geothermal heat storage/extraction, and heat loss phenomenon in a solar-BTES system accurately and efficiently. Moreover, it can serve as the basis to design solar-borehole energy storage systems of any size and at any location.