Low Cost Flywheel Energy Storage for a Fuel Cell Powered Transit Bus

Traditionally, the energy storage requirement for conventional and fuel cell hybrid buses has been satisfied by chemical batteries.  This is due in part to the low initial purchase price, the high energy density, and the high level of familiarity that engineers possess compared to newer technologies.  However, batteries have a number of disadvantages, such as limited cycle life, maintenance and conditioning requirements, and modest power densities, which have been mitigated by newer technologies such as ultracapacitors and high-speed flywheels.  Flywheels in particular offer very high reliability and cycle life without degradation, reduced ambient temperature concerns, and construction free of environmentally harmful materials.  Integration of these technologies is well suited for urban bus applications due to the traditional bus route cycles which contain large amounts of deceleration and acceleration for potential energy recovery and re-usage.  In addition, buses have high public visibility, providing an attractive platform for demonstrating safe and effective implementation of alternative energy technologies.

In 2005, the Center for Electromechanics developed and tested a high speed composite flywheel for an Advanced Technology Transit Bus.  The flywheel operated at 40,000 rpm and could deliver 844 Wh at a power rating of 150 kW.  Road testing of the bus revealed acceleration time to 75 km/h was reduced by a factor of two with a simultaneous reduction in engine power of 25%.  Although the flywheel demonstrated marked improvement in bus performance, the use of magnetic bearings, an all composite flywheel, and a large gimbal, made it unappealing from a commercial aspect with respect to cost and size.

Recently, the Center for Electromechanics designed a new flywheel that utilizes steel rotors and rolling element ball bearings to reduce cost and complexity.  Emphasis was placed on size reduction for vehicle packaging advantages.  Simulations were used to help formulate energy storage and power requirements for the new design.  The paper, “Low cost flywheel energy storage for a fuel cell powered transit bus,” which describes the design of the compact flywheel energy storage solution for a fuel cell powered transit bus, was presented at the 2007 IEEE-Vehicular Power and Propulsion Conference (VPPC 2007) in Arlington, Texas, September 9-12, 2007.  The paper, which was co-authored by Clay Hearn, Mark Flynn, Mike Lewis, Richard Thompson, Brian Murphy, and Raul Longoria, describes the simulations used to size the flywheel requirements, a comparison of three different design concepts to meet the flywheel requirements, and development of the final flywheel design.

For further information, please contact Clay Hearn.

Complete flywheel energy storage system

Prime Mover and Energy Storage Considerations for a Hydrogen-Powered Series Hybrid Shuttle Bus

U.S. Environmental Protection Agency reports show that internal combustion engine-powered vehicles using hydrocarbon fuels account for 75% of national carbon monoxide emissions, 45% of nitrous oxide emissions, and nearly 40% of the volatile organic compound emissions.  Larger vehicles, such as urban transit buses, can be among the worst offenders; however, buses that consume a non-hydrocarbon fuel, such as hydrogen, can achieve ultra low or even zero emissions with a simultaneous 30-66 % reduction in fuel usage, compared to conventional diesel powered buses.

The paper, “Prime mover and energy storage considerations for a hydrogen-powered series hybrid shuttle bus,” co-authored by Clay Hearn, Mark Flynn, Mike Lewis, Richard Thompson, and Raul Longoria, provides the results of computer simulations of a 6.7 m long fuel cell-powered plug in hybrid shuttle bus over an urban transit route, as well as the performance of the actual vehicle for comparison and model validation.  The paper, which was presented at the 2007 IEEE-Vehicular Power and Propulsion Conference (VPPC 2007) in Arlington, Texas, September 9-12, 2007, includes simulations that were leveraged to evaluate the performance of the bus while using various prime mover and energy storage combinations. For more information, please contact Mike Lewis.

Fuel cell shuttle bus modeled in simulations

Modeling and Simulation of Electric Ships’ Power System Components and their Interaction

Prime power generation on board all-electric ships presents several options that affect fuel consumption, power density, operational effectiveness, and survivability.  A study aimed at understanding the effects of some of these options was conducted and the results were reported in the paper, “Modeling and simulation of electric ships’ power system components and their interaction,” co-authored by Shazib Vijlee, Abdelhamid Ouroua, Lori Domaschk, and Joe Beno and presented at the IEEE Electric Ship Technologies Symposium in Arlington, Virginia, May 21-23, 2007.

It was found that direct coupling of gas turbines to permanent magnet generators reduced system mass and volume significantly, as compared to electric power generation systems installed on present-day navy ships.  Furthermore, it was found that a significant benefit of this topology was a reduction in gas turbine air duct volume if the compact gen-set units were relocated on or near the ship’s upper decks.  In addition, a combinatory analysis revealed that the number of generating units and their respective power levels has a significant influence on overall efficiency. For further information, please contact Hamid Ouroua.

80 MW directly-coupled gas turbine-PM generator set in future electric ship