Effect of buoyancy-driven natural convection in a rock-pit mine air preconditioning system acting as a large-scale thermal energy storage mass

Underground mining is among the most energy-intensive industries and ventilation comprises a significant portion of the energy demands of this important industry. Using the vast volume of broken rock, left in a decommissioned mine pit, as a thermal energy storage mass has enormous potential to lower ventilation-related energy costs in deep underground mines. This approach facilitates moderating seasonal air temperature variations. Seasonal thermal energy storage is a cost-effective solution to improve cooling and heating process efficiencies, thereby reducing associated costs. Temperature gradients observed in the proposed storage system suggest the presence of a natural convection heat transfer mechanism that is buoyancy-driven. The effect of natural convection and a variety of heat transfer mechanisms were modeled and simulation results and field-data measurements were compared. The conjugate heat transfer and fluid flow model that was developed considers the porous rock mass in the rock-pit along with the air (i.e. fluid) blanketing the top surface. The effects of rock size, permeability and porosity were studied. It was observed that, for the range of porosities (from 0.45 to 0.20), these parameters have a small effect on the outlet air temperature and the performance of thermal storage phenomenon. The novel model compares forced (from ventilation fan) and natural (result of buoyancy) convection. Further, it incorporates the effect of design factors, such as air trench positions and flow rate of ventilated air, on energy savings.