Led by Vehicle Projects LLC, an international consortium has developed a 160-kW, 23-tonne fuelcell mine loader, a key production element of underground mining (see illustration). The project, commencing in 2002, is nearly complete as an innovative technical development project. We expect the fuelcell loader to be demonstrated in an underground gold mine in late 2007. Based on the empirically determined duty cycle, its powerplant (see illustration) is a fuelcell-battery hybrid using 90-kW (nominal) PEM fuelcell stacks (manufactured by Nuvera Fuel Cells Europe, Milan, Italy) supplemented by a 70-kW transient-power nickel metal-hydride battery. A fuelcell-battery hybrid powerplant design was chosen because of high but narrow power peaks in the duty cycle (see illustration) and the ability to recover some of the vehicle kinetic and potential energy as it descends ramps. With parasitic losses estimated to be no more than 20 kW (77% efficiency), the powerplant will be capable of delivering 70 kW of continuous net power and 140 kW peak net power. This compares favorably to the diesel version of 123-kW peak net power.

Because the loader is an underground vehicle, its powerplant must be compact, and its high power density is a significant achievement. Contributing to the power density is an innovative electrically powered centrifugal blower operating at 150,000 rpm to provide cathodic air. The blower is small enough to hold in one hand. Being a hybrid allows the powertrain to meet the high but narrow power peaks of the duty cycle with relatively small fuelcell stacks. Also, the battery can absorb regenerative braking energy as the loader descends mine ramps. Regenerative braking increases energy efficiency and reduces wear on the mechanical brakes. Unlike the conventional diesel loader in which the diesel engine mechanically drives both the hydraulic and traction systems, the fuelcell powerplant allows the hydraulic and traction systems to operate independently. Hydraulic loads are very high, and the fact that high traction loads will not drag down hydraulic power will increase productivity of the loader. The reversible metal-hydride storage system can be refueled in 10-15 minutes, which we believe is a record for large vehicular metal-hydride storage systems.

Reversible metal-hydride storage (see Technology Tutorial) offers two paramount advantages as a hydrogen storage medium for underground vehicles: safety and compactness. Because of the stringent safety requirements of underground mining, we believe metal-hydride storage may be the only method acceptable to mine health and safety regulatory agencies. Two such agencies were involved in safety analysis of our earlier metal-hydride fuelcell mine locomotive, which was permitted to operate underground in a working gold mine in Canada. Compactness is critical in underground vehicles because they must operate in space dug from rock or coal that is therefore minimized to reduce operating costs. Theoretically, metal hydride-material can store on the order of five times the mass of hydrogen in a given volume as can compressed hydrogen at 350 bar (5,1000 psi). As discussed in the Technology Tutorial section of this website, the practical ratio is about two. Even if compressed hydrogen were allowed underground, there is insufficient volume onboard the loader to package the amount of hydrogen, at reasonable pressures, we are able to store as a metal hydride at an equilibrium pressure of 10 bar. Because waste heat from the fuelcells desorbs the hydrogen from the hydride bed for use by the fuelcells, metal-hydride storage is more energy efficient than compressed-hydrogen storage, which consumes a substantial amount of energy to provide the PV-work of compression.

The disadvantages of metal-hydride storage, compared to compressed-gas storage, are high weight and high cost. Weight is not a problem for locomotives, underground or on the surface, and generally it is not a critical issue for rubber-tired counter-balance vehicles such as loaders. However, a limit on weight of rubber-tired vehicles is imposed by the strength of the tires. Metal-hydride storage is presently several times more expensive per mass of stored hydrogen than compressed-gas storage, but we expect the cost ratio to diminish if a large market, mining and other underground vehicles, opens for the technology.

Due to safety requirements, recharging of the metal hydride for this prototype vehicle will take place on the surface and will utilize two possible methods: (1) the loader is driven to the surface (in ramp mines) and the metal hydride is recharged while remaining onboard or (2) the metal-hydride storage unit is removed from the loader underground and taken to the surface for recharging. Offboard heat exchangers, using cool water as a heat sink, will remove the heat generated by the absorption of hydrogen into the metal hydride. Planned future work will address the issues associated with refueling underground.

In conclusion, an international consortium has developed and will soon demonstrate a 160-kW fuelcell-battery hybrid underground mine loader that is fueled by an innovative onboard metal-hydride storage system. The system, rechargeable in 10-15 minutes, safely and compactly stores 14 kg of hydrogen at low pressure. Zero emissions of the fuelcell powerplant, plus operational advantages leading to higher productivity, and the safety and compactness of reversible metal-hydride storage, place this technology in a unique position for commercialization in underground applications.

Bibliography

G. Desrivieres and M. Betournay, “Duty Cycle Evaluation Project.” Final report MMSL 02-036(CR), CANMET Mining and Mineral Sciences Laboratories, prepared under contract to Vehicle Projects LLC, Denver, USA, 22 July 2002.

A. R. Miller, Least-cost Hybridity Analysis of Industrial Vehicles. European Fuel Cell News, Vol. 7, January 2001, pp. 15-17

A. R. Miller, D. L. Barnes, Brian D. Hoff, Omourtag Velev, Lindsay Sheppard, Prashant Chintawar, and Mark Golben, Fuelcell-Battery Hybrid Mine Loader. Proceedings of 2004 Fuel Cell Seminar, San Antonio, USA, 1-5 November 2004

A. R. Miller, D. H. DaCosta, and M. Golben, Reversible Metal-Hydride Storage for a Fuelcell Mine Loader. Proceedings of the Intertech-Pira Conference “The 2006 Hydrogen and Storage Forum,” Vancouver, Canada, 11-13 September 2006

Acknowledgements

We thank the following funders for their generous support of this project: US Department of Energy (contracts DE-FC26-01NT41052, DE-FC36-01GO11095, and DE-FC36-05GO85049); Government of Canada (Action Plan 2000 on Climate Change contract 23440-0310202-001); subcontractors to Vehicle Projects LLC who contributed project cost-share. Disclaimer: Funding support from the US Department of Energy, Natural Resources Canada, or Government of Canada does not constitute an endorsement by same of the views expressed in this website. Caterpillar participation in this project was undertaken pursuant to an agreement with the United States in connection with settlement of disputed claims in an enforcement action under the Clean Air Act.

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