Response of an urban bus flywheel battery to a rapid loss-of-vacuum event

Abstract

The University of Texas at Austin Center for Electromechanics (UT-CEM) is developing a 2 kW-hr flywheel battery for energy management on a hybrid electric urban bus. The battery will recover braking energy and store excess energy generated by the prime mover. The flywheel rotor, fabricated from high-strength composites, spins at 40,000 rpm at full charge (~900 m/s tip speed), and is housed in a vacuum enclosure to minimize windage drag. Also integrated into the enclosure is a composite containment system that has been proof-tested to provide additional safety. Ensuring flywheel safety is a major issue that must be addressed in using flywheels for transportation applications. A large leak caused by a service failure of the vacuum system could damage the flywheel before the energy dump system has time to act. A rapid loss-of-vacuum test on a rotor similar to that planned for the urban bus flywheel was conducted. Instrumentation, during the flywheel spin test recorded increasing flywheel surface temperature (>316°C or 600°F) following an intentional and abrupt loss of vacuum. No severe damage was noted on the surface of the flywheel, which was later retested to a higher speed to assess structural integrity. This paper provides an analysis of the data from that test and discusses the experimental results as they pertain to safety of the bus flywheel.