Flywheel Energy Storage
Innovative flywheel offering low cost, high energy density and inherent safety
An innovative new flywheel system design employing laminated steel to reduce cost, improve energy density and safety
There are now a number of commercially available flywheel solutions, some of which have been developed from Formula-1 motorsport, Kinetic Energy Recovery System (KERS) technology. These employ high-strength carbon-fibre composite materials to achieve very high operating surface speeds. However, composite-materials are not isotropic and consequently their failure-modes are variable and statistical by nature. Because the failure of a high-speed flywheel can be catastrophic, many of these composite-material flywheels are ‘designed not to fail’, which means that they cannot fully utilise the potential of the high-strength composites used, due to incorporating a high safety margin to cater for the statistical variations in composite properties. The cost of manufacture of composite materials nevertheless remains high and this leaves room for an alternative. The Low-Cost, Laminated Flywheel proposed here has the benefits of being constructed of steel – a material that is isotropic and well understood – and due to the patented innovation of its construction is able to safely achieve much higher operating speeds compared to previous steel designs. The flywheel consists of three main aspects: the combined flywheel and magnetic rotor; the bearing and cooling arrangement; and the outer housing. The main inertial element of the flywheel comprises a series of circular laminations pinned together into a stack and sandwiched between two cheek plates. The pins perform a dual role of locating the laminations and also providing tension to hold the stack together. The stack of laminations, cheek plates and locating hardware form a sub-assembly that is bolted directly to the magnetic rotor of the drive/braking electrical machine. At each end of this assembly – so on the uppermost cheek plate and at the bottom of the magnetic rotor a ball bearing is fitted that supports the whole spinning assembly. Location of the discs is achieved through 8, shape-optimised, locating holes on the low-stress periphery. This provides a safe, highly reliable, design, capable of high performance as a flywheel. The patented laminated design is a fail-safe design in that even if a rotor failure were to occur, it can be fully contained since only a small portion of the energy is released. Failure of a single laminate will only occur if a substantial crack grows in that laminate due to fatigue and it is statistically improbable that cracks can grow in two or more different discs simultaneously to the same critical length. By means of careful design and limited maximum stresses, failure of even one laminate will be prevented within the life of the flywheel but in case the unexpected happens, full containment is guaranteed. The spinning assembly is mounted in a two-piece casing that houses the two rotor support bearings, the electric machine’s stator and also the rotor containment element. The casing forms a semi-hermitically sealed shell that in operation is evacuated to a medium vacuum of a few Pascals. A further sleeve is fitted around the bottom casing to form a series of sealed water cooling passages. Heat is removed by the water flow from both the electrical stator and also the oil returning to the lower sump on its journey through the machine. The bearings of the machine comprise two precision ball bearings with spring preload. The uppermost bearing is relatively small and is grease packed. The lower bearing is larger as this takes the net downward thrust due to the mass of the flywheel and this is oil lubricated/cooled in vacuum. Since the axis of the flywheel is vertical, the net weight of the rotor acting on the lower bearing can be reduced substantially by means of a passive magnetic thrust system. This is relatively easy to implement given the rotor is made of magnetic steel and has been used on other flywheel systems. The magnetic rotor also needs cooling; any heat generated by electromagnetic eddy currents is trapped in the rotor due to the lack of gas cooling in the vacuum conditions inside the machine. To remove this heat a novel, passive, self-pumping, oil-based cooling system that exchanges heat to the outer machine cooling jacket has been developed. The fail-safe design has been successfully proven by experiment with an artificially-induced crack in one of the laminates. There was no distortion nor damage to the casings and no damage was sustained to the other laminate discs in the rotor, which remained totally contained within the casing. A high-speed camera operating at 50,000 frames per second was used to capture the instance of the artificially-induced failure. In addition some preliminary performance tests in a specially developed laboratory have been undertaken to give confidence in the specifications for this novel design of flywheel.
A laminated flywheel offers significant benefits for short term high power energy storage solutions where frequent cycling of energy storage and release is required without any degradation in performance. In the first instance a laminated flywheel construction using readily available sheet steel provides known characteristics that allow high speed operation and therefore increased energy storage density. The benign failure mode of the laminated steel construction means that the flywheel does not have to be heavily encased leading to a much lighter and more compact system at lower cost than alternative solutions. In addition this will allow for a very long service life compared with battery storage, typically decades and the materials can be easily recycled to provide a green solution to temporary energy storage needs.
The flywheel energy storage system has wide application where high power electrical energy needs to be stored and released quickly for relatively short periods of time, usually several minutes. Specific applications include: Grid stabilisation, providing steady electrical energy under fluctuations in supply and consumption Port cranes, recovering energy when lowering and raising containers Hybrid vehicles, storing energy under braking and recovering under acceleration Rail locomotives, storing energy under braking, providing power on acceleration and when stationary Bridging the energy gap in short term power outages