Principal Investigator:Charles E. Bakis (Engineering Science and Mechanics)
Current research in flywheel energy storage in the Composites Manufacturing Technology Center at Penn State University is aimed at developing a cost effective manufacturing and fabrication process for advanced compositerotors. Composites are desirable materials for flywheels due to their light weight and high strength. Lightness in high speed rotors is good from two standpoints: the ultra-low friction bearing assemblies are less costly and the inertial loading which causes stress in the material at high rotationalspeeds is minimized. High strength is needed to achieve maximum rotational speed. Therefore, advanced composite rotors enable the storage of greater amounts of energy on a per unit weight or volume basis, in comparison with other materials. Furthermore, fiber reinforced composite rotors have been shown to fail in a less destructive manner than metallic rotors -- an important factorfor safety reasons. Flywheels offer the potential of higher energy and power density in comparison with conventional energy storage devices such as chemical batteries. There is much interest in flywheels for energy storage in applications as diverse as satellites, stationary diurnal storage,uninterruptable power supplies, and hybrid electric vehicles.
The scope of recent flywheel research includes:
In the figure shown to the left, Chris Gabrys, a Ph.D. graduate in the Engineering Science and Mechanics Department, checks out the electronics on a demonstration flywheel energy battery. The rotor on this flywheel battery consists of concentric rings of glass and carbon reinforced epoxythat are either press fit at the interfaces or separated with compliant polyurethane interlayers for radial stress relief during operation. The rotor is attached to an aluminum shaft that is supported at opposite ends with magnetic bearings and at the bottom end with a pin and jewel bearing for axial stability. Permanent magnets embedded in the glass/epoxy core of the rotor interact with a series of optically synchronized electromagnets to serve as the motor/generator.
The 15" dia. flywheel shown above is a much simpler tabletop demonstration article that has been used many times in classes at Penn State to illustrate the concept of energy storage and conversion. Two spring-mounted electricmotors are used to spin up the rotor and to generate power during spin-down. A 100 W light bulb attached to this rotor will typically remain bright for about 5 minutes, although most of the drag is due to air friction and bearing friction in this particular demonstration flywheel.
The filament winding processes developed for the rapid manufacture of fiber reinforced epoxy and polyurethane rotors is shown in the figure to the left. Dry carbon or glass fibers are drawn through a resin bath and wrapped in the hoop direction on a heated mandrel. Heating power is delivered to the mandrel via slip rings on the mandrel shaft. A thermocouple placed in the core of the mandrel provides closed-loop feedback for the heating power. To avoid thecommon problems of poor consolidation and wavy fibers in the thick rotors, the resin is continuously gelled and cured during the winding process. The temperature of the mandrel and the winding speed are selected to maintain an ungelled thickness of about 1/2 inch on the surface of the rotor. If too much ungelled material is deposited on the rotor at once, two problems can occur: (1) the fibers may "swim" or buckle in the resin as winding tension is lost due to compaction; and (2) a large temperature spike can occur due the large volume of simultaneously curing resin in the part. Researchers at Penn State have developed methods of carefully controlling the winding process parameters to avoid these problems.
Heating of large diameter rings such as the glass/epoxy ring shown to the left is accomplished with external ring heaters on the sidewalls of the mandrel. Typical fiber volume fractions are about 70 percent, while void fractions are less than three percent. Radial deposition rates of up to eight inches per hour have been achieved with this winding technique. Analysis of the residual stresses in rings made by the in-situ curing method developed at Penn State have shown that the non-thermal (eg., winding tension, fiber motion through viscous resin, cure shrinkage) contribution of the manufacturing process to residual stresses is exceedingly small. With low temperature curing resins, large thickness parts are able to be made without cracking upon cooldown.
Polyurethane matrix rotors offer the potential benefits of lower radial stress during operation due to their low radial stiffness and a less catastropic failure mode due to a favorable stress distribution. In typical rigid matrix composites, maxium hoop stresses are at the inside diameter of the composite rings. In elastomeric matrix composites such as those developed at Penn State, the anisotropy ratio is of the order of 1,000 to 10,000, which causes the maximum hoop stress to occur just about at the outside diameter of disk-shaped flywheel rotors. In addition, the maximum radial stress is also near the outside diameter. Since both stress components are maximum near the outside of the spinning disk, it is possible that if the disk is spun too fast, some material may locally fail on the periphery of the disk and be thrown from the rotor without failing the entire rotor. Although this behavior has never been proven in the laboratory, it forms the basis of a potential fail-safe, or safety-fused flywheel. This concept is still under investigation at Penn State. Thick carbon or glass/polyurethane rotors such as the 1 ft. dia. sample shown to the left have been filament wound using an in-situ curing method similar to that described above for epoxy matrix composites. A technique for decreasing the viscosity of the resin and rapidly curing the resin with low void content was developed specifically for this application. Multidirecitonally wound tubes are also being made.
Small flywheels have been tested in the vacuum spin chamber shown to the left. A hydraulic turbine motor capable of speeds up to 70 krpm is used to spin rotors in the vacuum. Rotor rim speeds of 1100 m/s have been achieved to date in this chamber, although higher speeds are expected once a better vacuum pump is installed. Penn State researchers have also collaborated with outside research teams to spin larger rotors to failure in more heavily armored spin pits.
The rotor design which survived the 68 krpm (1100 m/s rim speed) experimentis shown to the left. This design utilizes a highly optimized design in which multiple filament-wound rings have been either press-fitted onto each other or separated by elastomeric interlayers to alleviate the ubiquitous radial tensile stress problem in rotating disks. The light colored rings visible in this photograph are the translucent layers of polyurethane resin which prevent the radial transmission of tensile stresses between the various rings. To obtain the highest speed and minimize costs, high strength carbon fiber (900 ksi) was used in the outermost rings, while lower strength (and cost) carbon fiber (500 ksi) comprised the inner rings. The inner press-fitted ring pair contains a glass fiber ring, visible as the yellow ring in the photo. The simple aluminum hub is contained by a carbon ring and is designed to interface with quill shaft on a hydraulic turbine.
Gabrys, C. W. and C. E. Bakis, "Fabrication of Thick Filament Wound Carbon Epoxy Rings Using in-situ Curing: Manufacturing and Quality," Proc. Am. Soc. Composites, 9th Technical Conf., Technomic, Lancaster, PA, 1994, pp. 1090-1097.
Emerson, R. P., "A Model for Press Force Prediction of Multi-Ring Composite Flywheels," B.S. Thesis, Dept. of Engineering Science and Mechanics, The Pennsylvania State University, University Park, PA, Dec. 1995.
Gabrys, C. W., "Design, Fabrication, and Testing of Advanced Composite Energy Storage Flywheels," Ph.D. dissertation, The Pennsylvania State University, University Park, PA, May 1996.
Gabrys, C. W., and Bakis, C. E., "Design and Manufacturing of Filament Wound Elastomeric Composite Flywheels," J. Reinforced Plastics and Composites, 16:488-502 (1997).
Bakis, C. E., Emerson, R. P., and Gabrys, C. W., "Prediction and Measurement of Press-Fit Forces and Stresses in Fiber Reinforced Composite Flywheel Rotors," VIII Intl. Congress on Experimental Mechanics -- Post Conference Proceedings, Society for Exp. Mech., Bethel CT, 1996, pp. 139-146.
Gabrys, C. W., and Bakis, C. E., "Design and Testing of Composite Flywheel Rotors." Composite Materials: Testing and Design, 13th Vol., STP 1242, S. J. Hooper, Ed., American Society for Testing and Materials, Conshohocken, PA, 1977, pp. 3-22.
Emerson, R. P., Orlet, M. W., and Bakis, C. E., "Fabrication and Characterization of Filament Wound Elastomeric Matrix Composite Materials." Proc. 42nd Intl. SAMPE Symposium, Soc. for the Advancement of Materials and Process Engineering, Covina, CA, 1997, pp. 342-353.
McKnight, G. P., "In-Situ Monitoring of Filament Wound Composites with DC Resistance Measurements," B.S. Thesis, Dept. of Engineering Science and Mechanics, The Pennsylvania State University, University Park, PA, April, 1997.
McKnight, G. P., and Bakis, C. E., "In-Situ Gel Monitoring of Filament Wound Composites with Low Cost DC Resistance Sensors," Proc. American Society for Composites, 12th Technical Conference, Technomic, Lancaster, PA, 1997, pp. 892-901.
Gabrys, C. W., and Bakis, C. E., "Filament Winding of Thick Composite Rings with High Radial Accretion Rates," Proc. Intl. Composites Expo '98, Soc. Plastics Industry, New York, 1998, pp. 17A.1-17A.8.
Emerson, R. P. and Bakis, C. E., "Relaxation of Press-Fit Interference Pressure in Composite Flywheel Assemblies," Proc. 43rd Intl. SAMPE Symposium, Vol. 2, Soc. for the Advancement of Materials and Process Engineering, Covina, CA, 31 May - 4 Jun. 1998, pp. 1904-1915.
Gabrys, C. W., and Bakis, C. E., "Simplified Analysis of Residual Stresses in In-Situ Cured Hoop-Wound Rings," J. Composite Materials, 32(10):1325-1343 (1998).
Bakis, C. E., Weaver, E. J., and Shirey, C. L., "Quasi-Static and Fatigue Behavior of Composite Flywheel Materials in Air and Vacuum Environments," Proc. 8th Japan-US Conference on Composite Materials, G. M. Newaz and R. F. Gibson, eds., Technomic, Lancaster, PA, 1998, pp. 831-840.
Orlet, M. W., and Bakis, C. E., "Viscoelastic Characterization of High Fiber Content Filament Wound Glass/Polyurethane Composites," Rubber Chemistry and Technology, 71(5):1042-1058 (1998).
Shirey, C. L., Bakis, C. E., and Weaver, E. J., "Tensile Behavior of Filament Wound Composites in Vacuum," Proc. 44th International SAMPE Symposium and Exhibition, L. J. Cohen, J. L. Bauer, and W. E. Davis, eds., Soc. for the Advancement of Materials and Process Engineering, Covina, CA, 1999, pp. 2110-2118.
Shirey, C. L., "The Effects of Vacuum Conditioning on the Mechanical Properties of Unidirectional Filament Wound Composites," B.S. Thesis, Dept. of Engineering Science and Mechanics, The Pennsylvania State University, University Park, PA, April, 1999.
Weaver, E. J., "Vacuum Environment Effects On Static And Fatigue Performance Of Unidirectional Composite Materials", M.S. Thesis, The Pennsylvania State University, University Park, PA, Dec., 1999.
Portnov, G. G., and Bakis, C. E., "Estimation of Limit Strains in Disk-Type Flywheels Made of a Compliant Elastomeric Matrix Composite Undergoing Radial Creep," Mechanics of Composite Materials, 36:87-94 (2000).
Shirey, C. L. and Bakis, C. E., "Tensile Behavior of Filament Wound Composites in Medium Vacuum," Journal of Composites Technology and Research, 22:213-223 (2000).
Shiue, F.-W., Lesieutre, G. A., and Bakis, C. E., "Condition Monitoring and Virtual Containment for Composite Flywheels," Proc. 42nd AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics and Materials Conference, American Inst. of Aeronautics & Astronautics, Reston, VA. Paper no. 2001-1441 (CD ROM).
Bakis, C. E., and Emerson, R. P., "Optoelectronic Radial Displacement Measurement on Rotors," Proc. Annual Conference on Experimental and Applied Mechanics, 4-6 June 2001, Portland, OR, Soc. Experimental Mechanics, Bethel, CT, pp. 411-414.
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Last substantial update: 16 Aug 01. Copyright 1999, 2000, 2001, C. E. Bakis.