LaunchPoint Technolgies Design for Ring Type Levitated Flywheel
Engineeing design group LaunchPoint Technologies has proposed a design for a hubless magnetically levitated composite flywheel which has the potential to be scaled up to very large sizes. Here are some details from the presentaion called Third Generation Flywheels For High Power Electricity Storage by O.J. Fiske and M.R. Ricci:
ABSTRACT: First generation flywheels of bulk material such as steel can mass tens of tons, but have low energy storage density. Second generation flywheels of composite materials have higher energy storage density but limited mass due to structural and stability limitations. LaunchPoint is developing high energy third generation flywheels – "Power Rings" – using radial gap magnetic bearings to levitate thin-walled composite hoops rotated at high speed to store kinetic energy. Power levels exceeding 50 megawatts and electricity storage capacities exceeding 5 megawatt-hours appear technically feasible and economically attractive. Power Rings can be used to decrease the peak power requirements of electric transportation systems by supplying intermittent high power for vehicles such as maglev trains. They can also store braking energy, isolate the power grid from surges and spikes, reduce the incidence of transportation system power outages, and provide
back-up power in case of blackouts.
As rotational velocity increases, the rotor experiences increasing radial force causing it to expand faster than the shaft. The spoke or hub assembly must compensate for this differential growth while maintaining a secure bond with the rim. High-speed carbon composite rims can expand by more than 1% in normal operation, E-glass even more. Hoop stress is highest at the inner boundary of the rim and causes a common failure mode in which the rim separates from the spokes.
Hoop stress decreases rapidly from the inner boundary of the rim to the outer boundary. The fibers used in the construction of the rim are extremely strong along their length, but are held together in the radial direction only by relatively weak epoxy binder. This results in another common failure mode in which the rim delaminates or fractures due to radial stress, which peaks at a point part way between the inner and outer edges. The longer the rim radius the higher the forces become.
Many methods have been proposed to alleviate these problems, but a fundamental limitation remains in all present designs – the rotating mass is far from the axle while the stabilization system (bearings and actuators) operates directly on the axle. If the arbor or spokes are flexible enough to expand as rpm increases, then the stabilization system must transmit control forces to the rim through a “floppy”structure – an impossible task – but if the structure is rigid it will delaminate under high radial stress. The only way to resolve this conflict, so far, has been to restrict composite flywheels to small diameters.
In a permanent magnet Halbach array (Halbach 1985), the field produced by each magnet reinforces the fields of all the other magnets on the “active” side of the array, and cancels them on the other side. The result is, in essence, a one-sided permanent magnet with an intense field. When two identical Halbach arrays are placed with their active sides facing
each other, they produce powerful repulsive, attractive, or shear forces, depending on alignment. As compared to simple opposed pole faces, a 5-element Halbach array provides more than three times as much force per unit volume of magnet. This provides the basis for the “shear-force levitator”, shown in Figure 3. Here two Halbach levitation arrays are arranged vertically with the “static” array attached to a stationary support. If the “moving” array is now offset, from the initial position shown, upward to the correct operating point, it will be subject to a large, stable upward force. This shear-force levitator is laterally unstable and must be actively stabilized. The stabilization actuator configuration and operation is illustrated in Figure 4.
Positive current through the coil in the direction shown causes a negative lateral force, i.e. attraction, on the magnet array. Negative current causes repulsion. Vertical forces produced by the upper and lower sides of the coil are equal and opposite so no net vertical force is applied to the magnet array. When driven by an active feedback control system operating in conjunction with a position sensor, the actuator force can be used to balance the lateral
forces on the magnet array to achieve stable levitation. In this design, a thin composite ring spins around the vertical axis in a toroidal vacuum chamber. The stationary arrays of two shear-force levitators, upper and lower, are embedded in the stator wall opposing the inside face of the ring. The moving arrays of the levitators are embedded in the inside face of the composite ring itself. Both the stationary and moving arrays are continuous around the ring and provide sufficient force to levitate a large mass. As ring height is increased for larger capacity, the levitator heights can also be increased. Actuator voice coils, upper and lower, are embedded in the stator wall where they interact with stabilization arrays on the inside face of the ring. These actuators keep the ring centered and prevent stator wall contact.
A magnet array mounted at the vertical center of the inside face of the ring interacts with stator windings to form a motor/generator, which is used to spin up the ring to store energy and to extract that energy by generating electricity. A containment vessel prevents the ring or any components from escaping the chamber in case of ring failure.
In my previous post about Pentadyne's magnetically levitated composite flywheels I mentioned their power consumption rate of 0.8%/minute of the total stored energy which is inadequate for night/day load shifting. Although Launchpoint's bearing design is new it uses conventional magnets, voice coils, etc., and it is not clear to me what aspect of the design would allow them to achieve loss rates that are much lower than Pentadyne's. In the paper from which the above abstracts were taken the authors briefly discuss the economics of this flywheel design, and they conclude that applications with short storage times and frequent cycling of the stored energy such as frequency regulation or regenerative braking for urban trains are more likely to have good economic returns than night/day load leveling.
On LaunchPoint's web site they write this concerning the development of the Power Ring:
LaunchPoint’s goal for the Power Ring has been to design, construct, and demonstrate a small-scale third generation electricity storage flywheel using a revolutionary architecture scalable to megawatt-hours per unit. We have currently completed design and construction of a 3 kWh flywheel motor and are assembling the rim and housing units, estimating the completion of a final small scale prototype for testing in May 2007.
It is now September of 2009 and I can detect no sign that such a prototype has ever been built and tested. However, LaunchPoint has also published this statement:
Santa Barbara, CA, August 5, 2009 -- LaunchPoint Technologies Inc. has won National Science Foundation (NSF) American Recovery and Reinvestment Act (ARRA) funds to develop advanced control techniques for the operation of high-speed, high-efficiency, energy storage flywheels. Energy storage flywheels, such as LaunchPoint's 'Power Ring', provide bursts of power over short time periods and are currently being developed as a more cost-effective method for maintaining electric power stability in the face of outages, surges, and sags that typically last up to a few seconds.
So the Power Ring design has not yet been abandoned, but I am not holding my breath waiting for it to become a commercial product.