Technical and economic potential of energy recovery and re-use on board surface mining haul trucks

Abstract

This thesis investigates the technical and economic feasibility, as well as emissions related benefits, of incorporating energy recovery systems (ERSs) on board mine haul trucks (MHTs) used in surface mining. More specifically it aims to answer the research question: “What practical combination of hybrid drive topology, and energy storage technology, capacity and use on board mine haul trucks, will maximise the economic benefit of energy recovery and re-use, considering variation in haul route characteristics and the time value of money through the life of a surface mining project?”To answer this question, a simulation program was developed and progressively expanded to conduct each of four main stages of the research. In Stage One, the simulation program was used to determine the potential of various technologies to reduce the fuel consumption per tonne mined for each of a range of pit depths. Appropriately informing the simulation program required identifying and quantifying representative input values for truck, haul route, and ERS characteristics. Truck and haul route characteristics determine the energy recovery rate and amount of energy recoverable to the trucks’ DC-links. ERS characteristics enable evaluating the potential and practical implications of each ERS technology to capture this recovered energy. ERS technologies considered include ultra-capacitors, lithium-ion capacitors, chemical batteries, and electro-mechanical flywheels (EMFWs).In Stage Two, the simulation program was expanded to incorporate cost models for the various haulage cost elements including: truck; fuel; labour, maintenance and repair; tyre; driver; and ERS costs. This enabled evaluating the haul cost reduction potential for different system sizes of each of the technologies, for truck elevation changes ranging between 15 and 600 m. Two energy re-injection strategies (a fuel replacement (FR) strategy, and an engine power augmentation (PA) strategy), were considered. EMFWs and fast charging lithium iron phosphate (LiFePO4 or LFP) batteries using the PA strategy, shows the greatest potential to enable practical haul cost reductions. However, practical mining may not always allow using the PA strategy. If so, the FR strategy should provide a minimum level of benefit primarily achieved through fuel savings.Recoverable potential energy changes as a function of vertical lift, larger ERSs may be needed to capture the increased amount of energy available from greater ramp descents. However, larger systems have a greater impact on payload, and cost more. In the context of a mining project spanning several years, technology selection and system sizing are complicated by operational demands changing over time and by economic factors such as inflation, depreciation and the timing of capital and operational expenses.To explore the life of mine impact of ERSs in Stage Three, two block models were generated to define two hypothetical mining projects in terms of material quantities, depth, timing and project duration. The modified simulation program used input from the block models to define the haul routes required at different stages of each project, in order to determine annual trucking requirements, fuel consumption and amount of energy stored. Incorporating the timing of acquisitions and expenses with appropriate economic factors, the simulation program can determine - for each combination of technology, energy re-injection strategy, and ERS size - the net present haulage cost (NPHC) for each project. For the mine reaching a maximum 600 m depth, using 257 t empty vehicle mass trucks, EMFW results are better for the first 10 to 14 years, depending on the ERS mass considered. However, an LFP system mass of about 6300 kg is anticipated to provide the best NPHC reductions of 1.4% and 6.2% respectively for the FR and PA strategy. For the maximum 240 m depth mine, using 141 t empty vehicle mass trucks, an EMFW system mass of about 3000 kg is expected to achieve the best NPHC reductions of 2.5% (FR strategy) and 5.5% (PA strategy).The primary task in Stage Four was to assess NPHC sensitivity to variations in system parameters. The two mining projects considered previously were used as baselines, each with the most promising combination of technology and system mass. Having considered an extensive range of parameters including ramp grade, rolling resistance, recovery efficiency, storage system efficiency, and all cost elements, the study highlighted energy recovery efficiency, storage system return efficiency, fuel cost and ERS cost as some of the more important parameters to the success of ERSs.The thesis results strongly suggest that on-board ERSs could enable a meaningful reduction in fuel consumption and CO2 emissions, and improvements in productivity resulting in significant NPHC reductions. The extent of benefit achievable will depend heavily on the extent to which the PA energy re-injection strategy could be used in practice. As the cost of fuel increases, the cost of energy storage technologies continues to reduce, and the technologies advance especially in terms of mass required and cycle life, the economic and environmental benefit enabled through ERSs is only expected to increase.