Richard C. Thompson

Design and Testing of a Flywheel Battery for a Transit Bus

The University of Texas at Austin Center for Electromechanics (UT-CEM) has designed and tested a flywheel energy storage system conventionally referred to as a flywheel battery (FWB) for power averaging on a hybrid electric transit bus. The system incorporates a high speed (40,000 rpm) 150 kW permanent magnet motor generator with magnetic bearings to levitate a 2 kWh composite flywheel. This paper summarizes: design goals, required operating parameters, system design, analysis completed prior to fabrication, and initial performance testing completed in the laboratory. The paper includes information on the motor/generator, power electronics, magnetic bearing sensors and controls, and FWB subsystems (including containment). Finally, recommendations for continued testing are made along with recommendations for improvements to the existing design.

Flywheel batteries for vehicles

Energy storage flywheels are useful in power conditioning applications, i.e. when there is a mismatch between the power generated and the power required by the load. Two examples of this mismatch are a temporal mismatch and a mismatch in magnitude. The use of a flywheel in a hybrid vehicle, for example, permits the engine to be designed to provide only the power needed to overcome steady-state losses and not have the inefficiencies that result when the engine must also provide power for maximum acceleration. While power management has provided the opportunities for flywheel batteries in vehicles, advances in technology have made the systems more practical. The key advance is the development of very high strength, long-life composites. These materials have significantly improved the energy density in the system over what could be achieved with a steel wheel by permitting much higher rotational velocities. So smaller, lighter wheels can store energy in the range from less than a kilowatt-hour to more than 100 kilowatt-hours. Other important advances have been in magnetic bearings that allow reliable high-speed operation and in power electronics to control the output power. Vehicular operation does produce new issues that were less significant in the more traditional stationary applications. One of the most obvious among these is torque management. In charging or discharging a flywheel, the rotational velocity is changed and a torque is produced. For example, systems intended for the International Space Station, where torque management is critical, the initial plan is to cancel torque by using two counter-rotating flywheels. Once confidence is gained in this mode of operation, energy will be distributed among various flywheels to produce the net torque needed for stable operation. For terrestrial vehicles, the flywheel is in a gimbled compliant mount with the axis of rotation orthogonal to the plane of vehicle motion. This orientation permits torque to be compensated by the magnetic bearings and the mount. Tests show that the mount and bearing system can accommodate the shock and vibrations, as well as traveling up or down grades, expected under on-road operation.

Performance Testing of a Vehicular Flywheel Energy System

The University of Texas at Austin Center for Electromechanics has designed and integrated a 40,000 rpm, 150 kW, 1.93 kWh flywheel energy storage system into a hybrid electric transit bus as a demonstration of the technology. The flywheel stores energy recovered during braking and returns it to the power train during acceleration, reducing the peak power requirements and size for the prime power unit. Additionally, the system provides a longer life energy storage alternative to the more traditional chemical battery bank. While the flywheel system was demonstrated on a transit bus, similar improvements are possible on other terrestrial or marine mobile applications. This paper presents the results and information learned during several multi-thousand-cycle duration tests, composite flywheel tests, terrain-simulating shaker tests, and on-bus road tests.

Design and analysis of passive homopolar null flux bearings

Electrodynamic Maglev systems (EDSs) and passive magnetic bearings (PMBs) generate magnetic forces by induction through the movement of permanent or superconducting magnets past conducting coils. Nearly all EDSs and PMBs use a null flux coil exposed to changing magnetic fields in the rotation or travel direction. An unusual variant eliminates discrete coils by using field structures that do not vary in the direction of rotation. We discuss the design and analysis of this homopolar variant in this paper. We believe that this is the first combined passive damper/axial support bearing, and we present here the first generalized solution of its governing equations. Actuated brushes during motoring and generating simplify energy exchange when the device is used for flywheel energy storage.

Lightweight containment for high-energy rotating machines

Developed a lightweight containment system for high-speed composite rotors. The containment device, consisting of a rotatable, composite structure, has been demonstrated to contain the high-energy release from a rotor burst event and is applicable to composite rotors for pulsed power applications. The most important aspect of this design is that the free-floating containment structure dissipates the major loads (radial, torque, and axial) encountered during the burst event, greatly reducing the loads that pass through the stator structure to its attachments. The design results in significant system-level weight savings for the entire rotating machine when compared to a system with an all-metallic containment. Of equal interest to the containment design, the experimental design and instrumentation was very challenging and resulted in significant lessons learned. This paper describes the containment system design, rotor burst test setup, instrumentation for measuring loads induced by the burst event, and a detailed explanation of the successful containment test results and conclusions.

End-of-life design for composite rotors [flywheel systems]

The University of Texas Center for Electromechanics (UT-GEM) is developing flywheel energy storage systems for combat and commercial vehicles and also leads the major US Flywheel Safety and Containment Program, a consortium effort of several leading flywheel developers. Safety for high performance composite flywheel systems on combat vehicles presents special challenges that impact the design of all flywheel components, especially the composite rotor and the bearings. This paper presents an overview of the issues and discuss design strategies and solutions applicable to the combat vehicle environment, using the flywheel energy storage system design recently completed under the Defense Advanced Research Projects Agency (DARPA) Combat Hybrid Power System (CHPS) Program as a case study. In particular, the paper will trace basic design and safety strategy, fatigue cycle development, lifetime design approach, and the resulting design margins.

Advanced flywheel technology for space applications

For spacecraft applications, energy storage sources are required to produce a high yield with minimum size and mass. Flywheel systems have the potential to fill this need while also providing attitude control for the guidance of the craft. Other advantages include on-board peak power management, extended service life (as compared to chemical batteries), and provisions for redundant systems with minimum effect on the projected payload of the craft. This paper reviews the results of flywheel design projects carried out at The University of Texas at Austin Center for Electromechanics (UT-CEM), discusses the role of composites in design development, and presents a detailed discussion of a flywheel design currently under study.

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

The University of Texas at Austin Center for Electromechanics (UT-CEM) has developed a 2 kW-hr flywheel battery for energy management on a hybrid electric urban bus. The battery recovers braking energy and stores excess energy generated by the prime mover (e.g., internal combustion engine). 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 prooftested 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.

Modeling of multiple liner containment systems for high speed rotors

High speed composite rotors typically require containment structures to protect personnel and equipment from high energy rotor material and fragments that result from rotor burst events, which may occur during rotor overspeed. Loading to the containment structure during a burst event is a function of rotor design and containment geometry. A containment system proposed by The University of Texas at Austin Center for Electromechanics uses graphite-reinforced composite cylinders to dissipate radial kinetic energy from the rotor debris and reduce torque loads transmitted to the rotor housing and mounting hardware. Using an analogous mass-spring-damper system, a model was developed with bond graph techniques to estimate containment loads and response. The bond graphs, state equations and simulation results are compared with experimental results. The model is able to predict the general trends observed in experimental data and is used as a design tool for containment systems.

Energy Storage Analysis to Increase Large Ship Fuel Efficiency

The marine transportation industry is a significant contributor to global emissions of CO2 and other pollutants. Although marine emission standards have become increasingly stringent, increasing fuel efficiency remains the primary objective in terms of further reducing emissions and overall marine energy use. In this paper, a hybrid powertrain is investigated as a means of increasing fuel efficiency for a modern, 100 m class, passenger vessel. The hybrid powertrain includes an Energy Storage System (ESS) based on sodium sulfur (NaS) batteries and commercially available Caterpillar diesel engine-generator sets. The ship’s power load profile is based on annual averages for similar vessels. A control strategy and simulation models are developed and implemented in Simulink to analyze the power and energy flows in the hybrid powertrain. The Simulink model is used to compare the base scenario of a ship without energy storage to a hybrid scenario employing a 7.5 MWh NaS battery pack with related control strategy. Annual fuel consumption is the primary measure that is used to assess efficiency. Unlike hybrid powertrains for light-duty surface vehicle transportation, which achieve efficiency gains on the order of 10–20% [8, 9, 10], the hybrid powertrain for a large ship is estimated to lower annual fuel consumption by approximately 2%. The surprisingly small level of fuel savings is explained largely by the granularity of marine power systems, which include multiple generators that can be switched on and off to maximize fuel efficiency.Copyright