Flywheel systems have been used for many years for storing energy in a system, and then releasing that stored energy back into the system when it is needed. Flywheel systems also provide a smoothing effect to the operation of internal combustion engines and other kinds of power equipment. More recently, flywheel systems are being used in electrical applications for uninterruptible power supplies (UPS) by storing and releasing energy. Flywheel energy storage systems (FESSs), which include a flywheel rotor and an attached motor/generator, convert electrical energy to mechanical energy by using the motor to accelerate the flywheel rotor to rotate at high speed. The energy is stored kinetically in the motion of the flywheel rotor. Mechanical energy is then later converted back to electrical energy when required by using the rotational inertia of the spinning flywheel rotor to drive the generator. Using FESSs instead of conventional electrochemical batteries for electrical energy storage offers the advantages of potential higher reliability, longer life and much higher power capability if desired.
Flywheel rotors constructed of steel were used in flywheel UPS system due to simplicity in early days, however the performance of such flywheels is low with tip speeds typically limited to around 400 m/sec or less. In comparison, a flywheel rotor having a composite flywheel rim can be operated at much higher speeds (700-1000 m/sec). The higher performance is the result of the increased strength to weight ratio by applying and tailoring high strength glass and carbon fibers in flywheel rims. Because the energy stored for a given flywheel design is proportional to the square of the tip speed but is only linearly proportional to the mass of the flywheel rotor (rim and hub), researchers have fervently pursued the much the higher speed composite flywheel rims.
To be competitive in the power quality and power reliability industry, flywheel based UPS systems must compete against conventional electrochemical battery based UPS systems which has low reliability and life, but has low cost. To compete effectively, high performance composite flywheel rims must be manufactured at a high rate and low cost. Among the many composite part fabrication methods, it is generally accepted that filament winding offers the greatest potential for composite flywheel rims. Filament winding can be a highly automated process that is capable of high material deposition rate and very high part quality, which is inherently needed for highly stressed flywheel rims. Filament wound flywheel rims are typically designed as thick, predominately hoop-wound composite rings that can be spun to very high speeds and are hence very effective for energy storage. Wet filament winding, where a thermoset resin is impregnated into the raw fibers during the filament winding operation, is the preferred fabrication method for a composite rim. Composite quality for very thick structures such as flywheel rims has also been shown to be much higher (lower void content and better fiber alignment) by wet winding than by filament winding using prepreg (previously impregnated and partially gelled) tows. The higher quality is the result of the much lower viscosity of wet winding resins allowing for entrapped air to be squeezed out of the part as the fiber is laid onto the part, as opposed to high viscosity prepreg resin that is not designed flow as freely during the winding process.
Filament winding of thick parts has been done by winding the part in stages, or staged winding, in which the part is wound in annular layers having a radial thickness of less than about xc2xe inch, which layers are successively cured. Subsequent layers are wound and cured repeatedly on top to allow fabrication of parts several inches thick. Staged winding has been done for several reasons that include; production of straight fiber alignment and the prevention of excessive heat generation during exothermic resin reaction that could damage the part. Producing straight fiber alignment, or the lack thereof which is know as fiber buckling, can be explained as follows. During filament winding, fibers are initially wound onto the mandrel with some amount of tension. Successive layers are wound on top, each increasing the part thickness and also inherently adding some compression to the layers below. As a part becomes greater than about xc2xd inch thick, the combined superposed compression of all of the outer layers causes the inner layers to loose tension and they actually go into compression. Because of the extremely small diameters of the individual carbon or glass fibers (5-20 xcexcm), the fibers easily buckle under compression and this buckling spreads radially outward forming an unacceptable kinkband in the cured composite. The buckled or wavy fibers reduce the hoop strength of the structure and would hence reduce the maximum speed and energy storage capability of a flywheel rim.
Besides allowing for straight fiber alignment in thick filament wound parts, staged winding has also been done to prevent excessive exothermic reaction of the resin during curing. If parts were cured at one time with thickness greater than about xc2xd inch, the heat generated in the part center becomes excessively high due to reaction of a large amount of resin all at once, and the inability for heat to quickly disperse due to the poor thermal conductivity. Staged winding has allowed for fabrication of high quality thick filament wound parts.
The problem with staged winding is that the process is extremely time-consuming and costly. In many cases, a filament wound composite flywheel rim has a radial thickness of as much as 6 inches. Such flywheel rims would need to be wound and cured in 12 separate stages taking approximately 12 days in the case of stage winding. Further increasing the manufacturing time is the staged winding requirement to wrap the rim with porous release tape after winding each stage to improve adhesion with the next stage to be wound after curing. The tape is removed and the rim is sanded and coated with wet resin prior to winding the next stage. Considering that the expenses for equipment use time are one of the largest composite fabrication costs in many cases, staged winding would appear to be an undesirable method for composite flywheel rim fabrication.
In-situ curing filament winding is a relatively new process that allows for fabrication of very thick filament wound parts in less time by curing the part progressively and continuously while it is being filament wound, thereby reducing the manufacturing time and cost. In-situ curing has been the subject of experimental projects but is still in its infancy and heretofore has not been developed to the degree in which it could be considered to be a robust, reliable, repeatable industrial process.
Some researchers have pursued flywheel fabrication methods by filament winding with thermoplastic prepreg with in-situ consolidation by addition of very high heat ( greater than 200xc2x0 C.) at the point of fiber contact with the rim. However, using a thermoplastic matrix further exacerbates the problems with using prepregs by having an even higher raw material cost and also a very slow manufacturing rate due to the much more high resin viscosity that slows consolidation. The cooling of flywheel rim from the high manufacturing temperature also causes unacceptably high thermal residual stresses.
Accordingly, this invention provides an improved in-situ curing filament winding process for making high quality flywheel rims at low cost and the rims made by the process, and also includes an apparatus for performing the process. The method uses in-situ curing to continuously cure the resin during the filament winding process. A lower winding process temperature is used along with a lower cure temperature and inherently higher toughness epoxy resin system allows for the rim to be wound continuously. Multiple types of fibers can be used in a rim where they best serve the strength and stiffness requirements of the structure. Press-fit assembly of rims containing both glass and carbon fibers is no longer required as they can be wound together without causing the rim to crack under thermal stress. This is accomplished by using relatively low cure temperature epoxy along with simultaneously using a conservative radial deposition rate of approximately xc2xc-xc2xd inch per hour. The winding temperature can be kept at approximately 60xc2x0 C.-80xc2x0 C., providing low thermal residual stresses. Aliphatic amine or ether amine curing agents with epoxy resin generally work well to reduce the winding temperature and allow low viscosity, but other resin systems that provide similar characteristics can be used.