The International Space Station(ISS) has rechargeable Ni- H2 batteries, which are charged from solar arrays whilst the ISS is in direct sunlight and discharged when the station is in shadow. It is hoped that they can be replaced by a flywheel energy storage system, when the flywheels will be able to recover as much as 80% more energy than when using electrochemical batteries and additionally, would weigh less. Magnetic bearings suspended in vacuum are fitted with a control system, with an extremely rapid response time, of the order of a fraction of one millisecond, to correct for shaft deviations and stabilize the rotating shaft, permitting flywheels to rotate at 60,000 rmp. The University of Texas Center for Mechanics are planning to supply a 10% glass/90% carbon fiber flywheel to US flywheels that will be able to withstand operating at 60,000 rpm and would use a composite housing to provide protection and contain a sealed vacuum prior to launch into space.
The University of Texas Electromechanical center, in conjunction with Allied Signal, are fabricating a flywheel assembly weighing some 9980kgs, with a flywheel about 125cm diameter made primarily of IM carbon fiber/epoxy with some glass. The flywheel is made by filament winding 12 concentric rings each 76cm long and bonded together.
Penn State University has used concentric rings of glass and carbon that are either press fit at the interfaces, or separated with a compliant polyurethane interlayer to provide radial stress relief during operation. The outermost rings are fabricated from HS carbon fiber, whilst lower strength carbon fiber is used for the inner rings, with a glass fiber ring innermost. The rings are fabricated by wet filament winding, wrapping in a hoop direction onto a heated mandrel. To overcome the problem of wavy fibers and poor consolidation in the thick rotors, the epoxy resin is continuously gelled and cured during the winding process, carefully controlling the conditions to maintain just about 12.5 mm of uncured resin on the rotor surface.
At this stage of development, it is doubtful whether a flywheel incorporated in an electrical highway vehicle can replace lead acid batteries due to problems with vibration and safety, but the performance of a lead acid battery deteriorates in hot wet conditions, requires frequent maintenance and has a life expectancy of only about 4years. The aim of US flywheel systems, however, is to drive a car with a flywheel system. It is reckoned that some 16 units would be required to fulfill this objective and adequate protection would have to be supplied, since, in the event of a wheel disintegrating, it would dissipate its energy into hot fluff and high speed dust. A very sophisticated computer control system is used to filament wind each wheel with a high fiber content of 86% w/w.
At the need for uninterruptible power supply systems increases, there is a developing market here for flywheel systems using carbon fiber rotors. The faster a flywheel spins, the greater is the energy storage. A 275kgs steel rotor rotating at 8000 rpm would generate 900 W h of energy, whilst a cfrp flywheel weighing only 68kgs can spin at 22,500 rpm and store about 2700 W h of energy. Increasing the speed of rotation to 100,000 rpm would increase the storage capacity by a factor of 10.