The impact of fleet electrification on productivity in heat-constrained underground mines

Underground mine planning uses production schedules to determine a (near-)optimal sequence of activity execution to maximize net present value while considering resource limitations and spatial precedence. At the time of this writing, the mining industry relies heavily on the use of diesel-powered equipment, which accounts for heat accumulation and exhaust emissions that, if not managed, can create hazardous conditions in the work environment. Sustainable mining practices call for the transition from diesel- to battery-powered equipment. This study presents a large-scale production scheduling model that (i) prescribes activity start times in a medium-term schedule at daily fidelity, taking into account ventilation and refrigeration; and, (ii) determines a fleet composition, relative to a diesel-only fleet, that improves productivity. The authors implement an enumeration technique, embedded in the optimization model, whose special structure they exploit, to generate schedules within an operationally feasible amount of time. The findings show that the need for refrigeration, i.e., lowering the air temperature, is delayed by weeks to over a year, and exhaust emissions can be virtually eliminated as more battery-powered equipment is introduced, demonstrating the utility of battery-powered equipment to improve underground work environments while lowering infrastructure costs. Under the assumption that procurement and maintenance costs are sunk, this study shows a 1% to 26% improvement in net present value over an all-diesel fleet. Soot and NO${}_x$ emissions are reduced as battery-powered equipment is infused. The greatest average reduction in cumulative emissions is 31%, which occurs at the transition from a fleet composition of five battery-powered equipment pairs to six. These reductions range from 7% to 59% and are associated with five of the twelve scheduling cases presented. The results provide schedules and fleet compositions that enhance the integration of battery-powered equipment into an operation while assessing overall productivity and net present value to effect more environmentally friendly strategic decisions.