Multi-stage hybrid drives for traction applications

Recent increases in fueling costs have created new interest in energy storage technologies for transportation applications. Railroad locomotives often have high peak-to-average demand for power, which leads to poor fuel efficiency from diesel prime movers, or arcing and pitting of overhead cantenary. One approach to this problem has been the development of diesel-battery hybrid locomotives. A large bank of lead-acid batteries provides peak instantaneous power, while a constant speed, high-efficiency diesel-electric generator set provides average power to the battery bank. This arrangement has proven practical in the Green Goat and other hybrids, although sizing of the generator set for a particular application, such as switching a railyard, becomes crucial. In addition, battery wear must be accurately predicted for peak performance, and the battery bank itself must be replaced after a few thousand cycles, increasing cost and creating a lead disposal hazard. Advances in capacitor design have created a new family of devices, known as super- or ultracapacitors. These devices have a mass-energy density which approaches lead-acid batteries, but with dramatically lower internal resistance and potentially limitless cycle life. However, the volumetric-energy density is much lower than batteries, thwarting a direct application in a practically sized rail vehicle. In this paper, a multi-stage drive is proposed, which takes advantage of the best properties of each energy storage device - capacitor, lead-acid battery, and diesel prime mover. The capacitor provides high peak instantaneous power, absorbing the cyclic nature of the locomotive's power demands. The battery bank acts as a buffer stage, with fewer full discharge cycles and longer life. The diesel prime mover provides longterm sustaining power for the system. In electric railway applications, adding an ultracapacitor stage reduces peak current through the wire-pantograph interface, reducing wear and improving lifetime of the entire system. By increasing the size of the ultracapacitor stage to practical limits, a traction system may be constructed which reduces or eliminates the need for the cantenary/third-rail supply and interface, substantially reducing the cost of electrified rail transportation.