Performance of open borehole thermal energy storage system under cyclic flow regime

Abstract

Thermal energy storage can be accomplished through the installation of an array of vertical boreholes. Coupled hydrogeological-thermal simulation of the storage system is essential to provide an optimized configuration of boreholes and operation schedule for the thermal storage system on the site. This paper presents numerical investigations and thermohydraulic evaluation of open borehole thermal energy storage (BTES) system operating under cyclic flow regime. A three-dimensional numerical model for groundwater flow and heat transport is used to determine the annual variation of recovery temperature from the borehole thermal energy storage. The model includes the effects of convection and conduction heat transfer, heat loss to the adjacent confining strata, and hydraulic anisotropy. The operation scenario consists of cyclic injection and recovery after holding interval and four periods per year to simulate the seasonal temperature conditions. For different parameters of the system, performances were compared in terms of extraction temperature. The calculated water temperature at the producing pipe remains relatively constant within a certain range through the year. Heat loss, injection/production rate, and geometrical configuration of boreholes and aquifer used in the model are shown to impact the predicted temperature profiles at each stage and the recovery water temperature. However, injection temperature and hydraulic anisotropy have a less significant effect on the performance of BTES systems. Absolute permeability does not affect the temperature, but is inversely proportional to the injection pressure.