Performance Testing of a Vehicular Flywheel Energy System

2005-01-0809 SAE TECHNICAL PAPER SERIES Performance Testing of a Vehicular Flywheel Energy System M. M. Flynn, J. J. Zierer and R. C. Thompson Center for Electromechanics, The University of Texas at Austin Reprinted From: Advanced Hybrid Vehicle Powertrains 2005 (SP-1973) 2005 SAE World Congress Detroit, Michigan April 11-14, 2005 400 Commonwealth Drive, Warrendale, PA 15096-0001 U.S.A. Tel: (724) 776-4841 Fax: (724) 776-5760 Web: www.sae.org

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2005-01-0809 Performance Testing of a Vehicular Flywheel Energy System Copyright 2005 SAE International THIS DOCUMENT IS PROTECTED BY U.S. AND INTERNATIONAL COPYRIGHT M. M. Flynn, J. J. Zierer and R. C. Thompson Center for Electromechanics, The University of Texas at Austin ABSTRACT 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 multithousand-cycle duration tests, composite flywheel tests, terrain-simulating shaker tests, and on-bus road tests. INTRODUCTION The use of flywheel energy storage systems in hybrid electric vehicular applications provides numerous benefits in terms of improved fuel economy with a simultaneous increase in acceleration, as well as reduced vehicle maintenance by way of reduced mechanical brake wear. This paper describes the results of the laboratory and on-bus testing of these components based on several multi-thousand-cycle duration tests, composite flywheel tests, terrain-simulating shaker tests, and on-bus tests. A summary of previously published road testing that yielded improved vehicle acceleration, improved fuel economy, and energy recovery is presented as well. FLYWHEEL CONSTRUCTION The UT-CEM FWB is shown in Figure 1; the various components are briefly described below. Further detail of these components and associated references are provided in the laboratory testing section of this paper. MOTOR The motor is a 3-phase, permanent magnet (neodymium-iron-boron), 2-pole synchronous machine capable of 250 kw continuous operation. The electronics used to control the motor however, are limited to 110 kw continuous and 150 kw peak. The University of Texas at Austin Center for Electromechanics (UT-CEM) has developed a flywheel energy storage system for use as a load-leveling device onboard a 12,725 kg (28,000 lb) hybrid electric Advanced Technology Transit Bus (ATTB) developed by Northrop- Grumman under funding from the Federal Transit Authority. The flywheel system provides on-vehicle energy storage in much the same way chemical batteries do and is thus often referred to as a flywheel battery (FWB). Compared to chemical batteries however, FWBs offer increased power densities, reduced life cycle costs, life spans comparable to the host vehicle, and are free from potentially environmentally harmful materials [1]. The UT-CEM FWB utilizes a composite flywheel and permanent magnet motor-generator that rotates in a vacuum, levitates on active magnetic bearings, and is enclosed in a gimbal mounted containment system. Figure 1. Bus flywheel battery

FLYWHEEL The flywheel consists of a titanium hub supporting a series of tapered carbon composite rings which have each been press fit onto the hub or preceding composite ring to achieve a calculated amount of preload. The overall wheel has a nominal diameter of 431 mm (17 in.) and thickness of 152 mm (6 in.); see Figure 2. The motor and flywheel are mounted on a common shaft to form the entire rotor which is installed inside a vacuum housing to minimize windage losses. The overall mass of the rotor is 58.2 kg and at a maximum speed of 40,000 rpm (outer rim tip speed of 902 m/s) the stored energy is 6.949 MJ (1.93 kwh). MAGNETIC AND BACKUP BEARINGS Figure 2. Composite flywheel with titanium hub To minimize friction losses the rotor is supported on active magnetic bearings which have a 3 gee dynamic load capacity. Mechanical roller bearings are used as backup bearings for times when the rotor is at rest or when the capacity of the magnetic bearings is exceeded such as during unusually abrupt road conditions. CONTAINMENT AND GIMBAL The flywheel spins within a lightweight rotatable composite structure that has been demonstrated to contain the energy released from the unlikely event of a flywheel burst. The flywheel rotor and containment assembly are mounted on a 2-axis gimbal to counter gyroscopic forces as the host vehicle rolls and pitches. LABORATORY FLYWHEEL TESTING Initial magnetic bearing, rotor heating, and vibration tests were conducted in the laboratory using an all-titanium flywheel. After these tests were completed the composite flywheel rings were installed. This development path minimized risk during early testing of the FWB systems and allowed the development of the composite flywheel to proceed concordantly. MAGNETIC AND BACKUP BEARING TESTING Two magnetic bearings of permanent magnet bias homopolar construction are employed on the FWB. The bearing mounted near the top of rotor shown in Figure 1 provides 2-axis radial control, while the lower bearing provides a combination of radial and thrust control. Each bearing is composed of wound stators mounted inside the vacuum housing with steel lamination stacks mounted on the rotor as shown in Figure 3. Further details on the magnetic bearing construction and operation can be found in [2]. In the laboratory the magnetic bearings were tested to determine their ability to maintain the end of the rotor shafts within ± 0.254 mm (0.010 in.) of the geometric center. Throughout the 0 40,000 rpm speed range the bearings performed well, keeping the end of the rotor shafts within the prescribed limit. Figure 3. All-titanium flywheel rotor and magnetic bearing lamination stacks Testing was performed up to 42,000 rpm (5% overspeed) with successful bearing response. Beyond levitating the rotor appropriately, a major focus of the laboratory testing concentrated on reducing the power consumed by the magnetic bearing stators. Baseline power consumption of the magnetic bearing stators versus flywheel rpm is shown in Figure 4. To reduce this consumption UT-CEM and the bearing manufacturer Calnetix focused on the elimination of synchronous magnetic bearing forces. Synchronous forces (radial forces synchronized to the rotation of the rotor) result from commanding a rotor with a finite imbalance to spin about its geometric axis rather than about its center of gravity. During the early stages FWB testing, it was felt that forcing the rotor to spin about its geometric axis afforded the best performance by way of offering the largest clearance between the rotor and the backup bearings. However, doing so requires a synchronous force input from the magnetic bearings and thus additional current into the bearing stators.

Fig. 4. Comparison of magnetic bearing stator losses with baseline and improved compensators As FWB testing progressed, the rotor imbalance was deemed acceptable for UT-CEM and Calnetix to implement an adaptive control strategy that essentially eliminated synchronous response to imbalance in the rotor, thereby eliminating synchronous power consumption in the bearing throughout the applied speed range [3]. The adaptive control strategy, referred to as open loop cancellation (OLC) in [3], is disabled when the rotor traverses a mode since synchronous forces are needed to counteract imbalance forces. The reduced power consumption of the magnetic bearing actuators is shown in Figure 4 where it can be seen that a majority of the power usage of the baseline magnetic bearing system was used to generate synchronous forces. The mechanical backup bearings were also tested with much success. Several events were conducted where the magnetic bearings were turned off in one or more axes until zero rpm was reached. Additional events were noted when the rotor momentarily touched the backup bearings during testing of the magnetic bearings. Moderate to no wear was noted on the FWB backup bearing components during these tests as summarized in Table 1. ROTOR HEATING TESTING The FWB rotor is sensitive to heating since the strength of the permanent magnets used in the motor decreases with increasing temperature and risk permanent demagnetization at temperatures above 150 C (302 F). Table 1. Backup bearing tests Speed (rpm) Test Wear 32,000 5-Axis Shutdown Moderate 18,000 Upper radial bearing 2-axis shutdown Minimal 8,000 Combination bearing thrust axis shutdown Minimal 2,000 to 37,000 Momentary backup bearing touch None Figure 5. Rotor temperature comparisons Additionally the preloaded composite materials in the flywheel are temperature sensitive and may experience accelerated stress relaxation or creep effects leading to an imbalance condition when operated for prolonged periods above 95 120 C (203 248 F). It should be noted that this imbalance condition is a benign failure of the flywheel as it can be readily detected and stopped without incident. A third temperature sensitivity arises from the possibility that thermal growth of the magnetic bearing rotor laminations can appreciably decrease the air gap clearances in the magnetic bearing flux paths and thereby increase the magnetic circuit gain leading to potential bearing instability. During operation the rotor is heated from three main sources: 1. Motor induced rotor losses 2. Magnetic bearing induced rotor losses 3. Windage losses Because the rotor is levitated on magnetic bearings and operates in a vacuum, radiation is the only means of rotor heat removal. The physics of radiation heat transfer coupled with the relatively low temperature limits of the rotor materials given above means that the only way to achieve acceptably low rotor temperatures is to carefully manage the rotor losses from the above three sources [4]. During initial laboratory testing of the FWB a baseline rotor temperature measurement was recorded (see Figure 5) by using an infrared thermocouple to monitor the hottest part of the flywheel this location was determined to be near the magnets at the very top of the rotor. This test was conducted with the flywheel continuously charging and discharging at a constant power of 30 kw between 15,000 and 25,000 rpm. The motor was powered by a 2-level, hard switched, PWM inverter operating at a switching frequency, f s, of 9 khz. Vacuum levels for all flywheel tests were approximately 6 mtorr.

From Figure 5 it can be seen that the rotor temperature at the top of the shaft (effectively the magnet temperature) reached the 150 C limit within 90 minutes. For this flywheel system, which was intended to operate on a transit bus for up to 16 hours at a time, the baseline 90 minute operating duration was unacceptably short. Motor Induced Rotor Losses Analysis of the rotor heating problem revealed that a chief factor was the very low inductance of the motor at the switching frequency harmonics. This low inductance due both the permanent magnet, toothless design which incorporates no iron on the rotor and the use of a copper eddy current shield was determined to be approximately 17 µh. With such limited inductance, the motor is unable to present a large enough impedance to the harmonic voltages developed by the inverter. This in turn means that large harmonic currents flow through the motor which then create eddy currents and heating on the rotor. To lessen the rotor heating due to the motor, the inverter switching frequency was raised to 12 khz in an attempt to present a larger impedance to the switching frequency voltage harmonics. This change resulted in a modest decrease in rotor heating. To obtain a more significant decrease a 22 µh filter inductor was added in-line with each phase to greatly increase the impedance present at harmonic frequencies. The results of the above two modifications are shown in Figure 5. With the addition of the filter inductors, rotor heating was low enough such that the steady state temperature of the FWB rotor was below permissible limits at all locations. For completeness, the motor induced rotor losses for the baseline, increased switching frequency, and filter inductor tests were calculated to be 85 W, 63 W, and 17 W respectively. Magnetic Bearing Induced Rotor Losses While the above rotor heating improvements allowed the FWB to be operated continuously, even lower temperatures were obtained by reducing the magnetic bearing induced rotor losses. This reduction was accomplished when the magnetic bearing OLC strategy discussed above was implemented. Recall that the OLC strategy essentially eliminates synchronous forces produced by the magnetic bearings. When the synchronous forces are eliminated, the synchronous stator currents that developed these forces are also removed. In turn the air gap flux decreases and thus so do eddy current, hysteresis, and tooth permeance losses in the rotor magnetic bearing laminations. The effectiveness of the OLC strategy in terms of reducing magnetic bearing induced rotor losses is shown in Figure 6 where a 30-hour rotor thermal test without the OLC is compared to that of a 32-hour test using the OLC. Both tests operated the motor continuously between 25,000 30,000 rpm at a constant power of 40 kw. The switching frequency of the inverter for these tests was raised from 12 khz to 14.8 khz while the 22 µh inductor continued to be used. Because the only difference in the two tests of Figure 6 is the use of the OLC strategy, its impact on the rotor losses can be easily computed. To do this, a FWB radiation model was developed which is able to estimate the total rotor losses given the steady state rotor temperatures from Figure 6. For the 30-hour, non-olc test, the fitted steady state temperature of 134.6 C (274 F) at 48-hours is used. It should be noted that the difference between the predicted steady state temperature and the measured temperature at 30 hours differ by only 1.7 C (3 F). The steady state temperature for the OLC test at 32 hours is measured as 109.1 C (228 F). For both tests the temperature of the oil-cooled FWB housing was measured as 33.3 C (92 F). The radiation model thus calculated 103 W of rotor losses for the non-olc test and 69 W for the OLC test. The difference between these loss values is the average reduction afforded by the OLC strategy over the 25,000 30,000 rpm speed range, which is 34 W. The OLC strategy provides similar rotor loss reductions throughout the FWB speed range of 25,000 40,000 rpm. Windage Losses For the FWB, windage losses were minimized by maintaining the vacuum level at low values, typically 6 mtorr. Detailed rotor loss calculations and reduction techniques for all three sources are discussed in [4]. SHAKER TABLE TESTING OF GIMBAL AND MOUNT To increase the FWB s tolerance to excessive loads beyond the 3 gee dynamic capability of the magnetic bearings, a number of additional precautions were taken. First, elastomeric shock isolators were installed between the flywheel vacuum housing and the gimbal mounting system. Figure 6. Rotor temperature comparison with and without the OLC strategy

This attenuates the vibration transmitted to the flywheel by approximately 50% and is most important during the type of axial shock loading that occurs when hitting a pothole, curb, or other suspension bottoming event. Second, by using a gimbal mount with appropriate damping, the gyroscopic forces transmitted to the magnetic bearings during pitching and rolling of the ATTB are minimized as well. Yawing (turning) maneuvers of the vehicle do not require a reaction force from the magnetic bearings since the rotor is oriented vertically. In order to validate that the flywheel isolation, gimbal and magnetic bearings could act together to prevent backup bearing impacts during the full range of expected motion, the FWB was tested on an existing UT-CEM terrain simulator. The terrain simulator setup is shown in Figure 7. The FWB is at the top of the figure, supported in the two-axis gimbal which is mounted to a support skid through the elastomeric isolation mounts. The skid, which was later mounted to the frame of the ATTB, is bolted to the table top of the terrain simulator. The terrain simulator system utilizes three hydraulic cylinders to simulate the pitch, roll and shock seen on a transit bus. Each of the hydraulic cylinders has a 0.254 m (10 in.) stroke and a 13,380 N (3,000 lb) capacity. A programmable controller directs the table motion, and inputs and responses were measured with a Zonic signal analyzer and digital data recorder. Liquid dielectric inclinometers were used as pitch and roll sensors on both the gimbal table and on the FWB. This type of sensor has the advantage of providing an absolute angle (an angle relative to an inertial reference frame), but they have low bandwidth compared to other available angle sensors that measure relative angles. The tests included spin speeds from rest to 37,000 rpm and a variety of amplitudes and rates. The nominal input for the terrain simulator was developed based on information provided by Northrop-Grumman, the DOT White Book, and bus frame vibration measurements conducted by UT-CEM. To verify operation under more aggressive conditions, the nominal amplitude and rate values were increased by 50%. The laboratory tests verified the suitability of the flywheel battery skid system for field testing in a transit bus. The gimbal support reduced the flywheel bearing loads by approximately 65%. The shock isolators reduced the transmitted axial shock by 65%. Most importantly, the control of the flywheel while levitated by the magnetic bearings is maintained for shock and vibration levels well in excess of the values that are expected during transit bus operation [5]. COMPOSITE WHEEL TESTING Structural Tests To study the structural performance of the composite flywheel, several tests were conducted that simulated the stresses encountered during the typical on-bus duty cycle speed range of 27,000 to 36,000 rpm. During a stationary test, a single composite ring was hydraulically loaded to stress levels encountered in the typical speed range approximately one-half million times. No indication of structural degradation was observed. A series of tests were also conducted with a fully assembled multi-ring transit bus flywheel in a spin pit facility. The first of these was a single cycle overspeed test which was intended to document the failure mode of an as built flywheel. The resulting failure pattern served as a baseline for comparison against the failure mode of an identically fabricated flywheel subjected to cyclic fatigue loading. Figure 7. Flywheel battery system mounted on the terrain simulator The single cycle overspeed test revealed as predicted, that the failure mode was not a hoop fiber burst, but a controllable loss of mass balance. The flywheel maintained excellent mass balance up to and past 36,000 rpm. At about 42,000 rpm, the flywheel began to show slight balance changes, which was expected, since some of the flywheel s internal rings had transitioned from radial compression to radial tension and had begun to physically separate. At about 47,500 rpm (1120 m/s), the flywheel showed a rapidly increasing mass balance shift due to further ring-to-ring and rim-to-metallic hub separation. Driving the test to higher speeds, at this point, would have resulted in increased vibrations leading to failure of the quill shaft that supported the flywheel during the test, thereby

dropping the flywheel to the bottom of the spin test pit. UT- CEM thus decided to stop the test so the flywheel could be safely spun down to rest, and available for post-test inspection. In conclusion, this test successfully characterized the as built overspeed performance of the flywheel. During the second spin test, UT-CEM completed fatigue cycling of an identically fabricated flywheel. This flywheel was tested through speed excursions representative of the transit bus s duty cycle. Over a period of six months, the flywheel underwent speed excursions from 27,000 rpm to 36,000 rpm, with a peak tip speed of 825 m/s representing about a 50% depth of discharge. The test was performed at elevated temperatures of approximately 60 C (140 F). Testing was complete after accumulating 112,000 cycles with only very minor mass balances changes. Lastly, this flywheel was subjected to an overspeed test during which, it performed nearly identically to the as built flywheel. This close comparison from the two overspeed tests infers that no structural degradation (i.e., fatigue fiber damage leading to reduced composite modulus) was identified between the fatigue cycled flywheel (tested to 112,000 cycles) versus the as built wheel. Loss of Vacuum Test 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 composite flywheel similar to that used for the ATTB FWB was conducted [6]. Instrumentation monitoring the flywheel surface temperature during the spin test recorded values as high as 316 C (600 F) following an intentional and abrupt loss of vacuum at 40,625 rpm (925 m/s tip speed). No severe damage was noted on the surface of the composite flywheel, which was later retested successfully to a higher speed to verify structural integrity. CONTAINMENT SYSTEM TESTING During a six year membership in the DARPA/DOT Flywheel Containment Program, engineers from UT-CEM designed, built, and tested several full-scale composite flywheel containment systems for use in mobile applications. The DARPA consortium adopted a defense in depth safety approach. The first line-of-defense is a properly designed flywheel verified by testing to safely operate over its service life. The second is adequate instrumentation for health monitoring to assure proper flywheel operation (i.e. speed, temperature). Finally, the third line-of-defense is a test-verified containment system in the unlikely event of a flywheel burst. A commissioned study agreed with adding the containment system for added operational safety [7]. The containment system described in previous publications is an energy absorption device with a design that stems from an in-depth investigation into the type of faults that are most likely to occur in mobile applications. The most important aspect of the containment device is the free-rotating composite liner intended to absorb the energy of a flywheel failure [8, 9]. The final laboratory test was conducted in August 2002 and a detailed description of the mounting configuration, test setup, and data acquisition is presented along with results [10]. Of particular interest to the design team was torque on the aluminum containment housing, axial and hoop stresses in the housing, and acceleration. The test was successful in that the composite debris was contained and all metallic structures remained fully intact. CONSTANT POWER DURABILITY TESTING A series of durability tests were performed during which the flywheel was charged and discharged repeatedly between two speed set points at a constant power. These tests served as a general measure of the robustness of FWB components and characterized FWB performance over the operating temperature range. Durability tests were scheduled for the typical power levels and speed ranges that would be required on the ATTB while in operation, rather than absolute maximum powers and speeds. Table 2 summarizes the durability tests, note that the number of cycles and minutes presented for each speed range and power level represent aggregate results from a group of tests. The majority of the individual tests that constitute these aggregate results were longer than 6 hours; several of the tests spanned multiple speed ranges and/or power levels while operating the FWB continuously for more than 47 hours. The durability testing aided the development of the FWB system and gave assurance that the rotor thermal situation would be compatible with the composite flywheel rings that were installed subsequently. Table 2. Constant power durability test summary Speed Range (krpm) Power (kw) Cycles* Time (min) 15-20 20 8,146 4,535 30 404 409 15-25 50 311 232 60 476 222 20-25 20 2,869 2,135 20 410 325 25-30 40 9,192 3,765 60 1,777 535 80 203 76 90 1,073 252 25-35 30 388 440 Total 25,249 12,926 *One cycle represents a combined charge and discharge event

ON-BUS TESTING With the composite flywheel mounted, the FWB was installed onto the ATTB. Simple vibration tests were performed to verify that the magnetic bearings responded favorably to events such as passenger on/offloading and ATTB engine starting and operating vibrations. Before electrically connecting the FWB to the ATTB, the flywheel was powered from a laboratory based supply and operated over a range of sustained power levels up to 130 kw and speeds of 31,000 rpm. These power and speed values were deemed sufficient for the ATTB road testing that was to follow. Laboratory based power supply testing was completed with no anomalies in the FWB s expected performance and thus the FWB system was electrically connected to the ATTB. A series of road tests were undertaken to verify the viability of the magnetic bearings in a vehicular application. During these tests the ATTB was driven at speeds less than 32 km/h (20 mph) around a service road owned by the University of Texas that has a relatively flat asphalt surface. The first test was performed with the rotor levitated but not spinning. As successful bearing operation was verified, further road tests were conducted with the flywheel speed increased to 5,000, 15,000, and finally 31,000 rpm. With the magnetic bearing examination completed, the FWB was brought online as a fully functional energy storage system for the ATTB. Maximum ATTB acceleration tests and regenerative braking tests then commenced. These tests successfully met design goals and are reported in detail in [11]. An example of the improved performance of the ATTB is shown in Figure 8 where it is apparent the FWB system doubled the acceleration rate of the ATTB to 72 km/h (45 mph). Furthermore, this was accomplished with a simultaneous 25% reduction in the power required from the ATTB s engine with similar estimated savings in fuel consumption. Figure 8. ATTB acceleration performance with and without FWB installed CONCLUSION The UT-CEM FWB represents a collection of advanced technologies which each presented challenges that had to be solved in order to achieve a successful integration into a vehicular application. Through innovative design and systematic testing the FWB was developed into an advanced and robust energy storage device proven by more than 100,000 fatigue test cycles of the composite flywheel with additional overspeed and loss of vacuum tests; aggressive terrain simulating shaking tests; tens of thousands of continuous power cycles over a range of flywheel speeds; and successful on-bus testing. The results of the on-bus testing successfully demonstrated the design goal of improved ATTB performance by doubling acceleration while simultaneously using 25% less engine power. Furthermore, the FWB is able to provide this enhancement unaffected by the number or depth of discharge cycles for the life of the host vehicle a benefit not possible with traditional vehicular energy storage systems. ACKNOWLEDGMENT The authors would like to acknowledge the support of the following individuals and organizations who provided funding and/or support on this project: Dan Raudebaugh at The Center for Transportation and the Environment; Bob Rosenfeld at the Defense Advanced Research Projects Agency; Jeff Arndt and James Blocker at the Houston Metropolitan Transit Authority; and Larry Hawkins at Calnetix. The publication of this work was funded by the Office of Naval Research through the Electric Ship Research and Development Consortium. REFERENCES 1. R.J. Hayes, J.P.Kajs, R.C.Thompson, and J.H.Beno, Design and Testing of a Flywheel Battery for a Transit Bus, SAE Publication No. 1999-01-1159, Society of Automotive Engineers, February 1999. 2. P. McMullen, C. Huynh, and R. Hayes, Combination Radial-Axial Magnetic Bearing, in the Proceedings of the Seventh International Symposium on Magnetic Bearings, 2000. 3. L. A. Hawkins and M. Flynn, Influence of Control Strategy on Measured Actuator Power Consumption in an Energy Storage Flywheel with Magnetic Bearings, in the Proceedings of the 6 th International Symposium on Magnetic Suspension Technology, Turin, Italy, 2001. 4. M.M. Flynn, A Methodology for Evaluating and Reducing Rotor Losses, Heating and Operational Limitations of High-Speed Flywheel Batteries, Ph.D. Dissertation, University of Texas, Austin, 2003. 5. L. Hawkins, B. Murphy, J.J. Zierer, and R.J. Hayes, Shock and vibration testing of an AMB supported energy storage flywheel, Eighth International Symposium on Magnetic Bearings, Mito, Japan, August 26-28, 2002.

6. R.C. Thompson, J. Kramer, R.J. Hayes, Response of an urban bus flywheel battery to a rapid loss-ofvacuum event, Accepted for publication in SAMPE Journal of Advanced Materials, April 2003. 7. J.M. Kramer, R.J. Page, C.A. Blomquist, and W.C. Lipinski, Urban Bus Flywheel Battery Interim Safety Assessment," Argonne National Lab, May 1996. 8. E. Sonnichsen and R. Thompson, Final Report for Phase I, II, III, IV of the DARPA Flywheel Safety Program, Center for Transportation and the Environment, September 2002. 9. J.L. Strubhar, R.C. Thompson, T.T. Pak, J.J. Zierer, J.H. Beno, and R.J. Hayes, "Light-Weight Containment for High Energy, Rotating Machines," 11th EML Technology Symposium, St. Louis, France, May 14-17, 2002. 10. J.J. Zierer, J.H. Beno, R.J. Hayes, J.L. Strubhar, R.C. Thompson, and T.T. Pak, Design and proof testing of a composite containment system for mobile applications, SAE 2004 World Congress, Detroit, MI, March 8-11, 2004. 11. R.J. Hayes, D.A. Weeks, M.M. Flynn, J.H. Beno, A.M. Guenin, J.J. Zierer, Design and Performance Testing of an Integrated Power System with Flywheel Energy Storage, presented at SAE Future Transportation Technology Conference, June 23-25, 2003, Hilton, Costa Mesa, California and published in SAE Publication SP-1789. CONTACT M.M. Flynn, Research Engineer The University of Texas at Austin Center for Electromechanics 1 University Station, #R7000 Austin, TX 78712 Telephone: 512-232-5713 Fax: 512-471-0781 Email: mm_flynn@mail.utexas.edu