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The phrase “everything old is new again” certainly applies to today’s flywheel technology. Forget the mechanical bearing, standard atmosphere 5,000 RPM steel behemoths of the past, many of today’s flywheel designs feature compact carbon fiber composite rotors on magnetic bearings, turning in a vacuum at up to 60,000 RPM.

As flywheel technology continues to improve, flywheel energy storage (FES) systems are gaining in use across a wide variety of applications, from frequency regulation in power utilities to energy recovery in trains and industrial equipment to rack-mounted uninterruptible power supplies. With demand rising for reliable, cost-effective, and environmentally friendly energy storage, especially to support the growth of green power solutions like wind and solar, FES is quickly coming into its own.

Compared to other energy storage solutions, FES systems have long lifetimes with minimal maintenance requirements, high energy densities (~ 200 kJ/kg), and large maximum power outputs. The round-trip efficiency (ratio of energy out per energy in) of flywheels can be as high as 90%, with power output capacities ranging from 2 kWh to 133 kWh, and an FES system can typically reach full charge in as little as 15 minutes. Compared to batteries with low capacity, long charge times, heavy weight, and short usable lifetimes, FES presents a bright spot in tomorrow’s clean energy solution.

These new generation flywheels are made possible by advances in material science for rotor technology, as well as the application of magnetic bearings running in a vacuum environment. While rotating a flywheel in a vacuum is an obvious way to get rid of the windage friction losses, mechanical bearings alone won’t stand up to operating in a vacuum, nor to the high speed requirements of the new flywheel designs. With the advent of magnetic bearings and magnetic-mechanical hybrids, FES engineers gained bearing solutions with very low and predictable friction, the ability to run without lubrication, and the capability of high performance in a vacuum – the ideal bearings for high-speed, vacuum applications.

However, with the requirement for operation in a vacuum comes one of the critical design challenges facing today’s FES engineer - ensuring the vacuum integrity of the flywheel housing, while meeting the needs for noise-free monitoring and high power inputs and outputs. Any breach in the vacuum environment of the rotor could lead to FES failure, making hermetically sealed feedthroughs a critical engineering component for FES development. FES designers often try to significantly reduce the system size, making it as small as possible, while taking into consideration the co-location of associated electronic and control systems and how the essential feedthroughs will be successfully situated. Thus, control and power feedthroughs that fit into tight areas, turn corners, and still maintain vacuum, require custom housing designs, often with unique geometries and specialty materials.

Massachusetts-based Beacon Power uses hermetic vacuum feedthroughs to optimize the performance of its Smart Energy 25 FES systems, which are being deployed on the utility grid to provide frequency regulation. The feedthroughs provide transfer of power and signal data from the control system on the atmospheric side to the internal volume of the vacuum-sealed flywheel chamber.

Beacon Power is currently building the nation's first full-scale flywheel energy storage plant in Stephentown, New York. A portion of this planned 20 MW plant is expected to begin earning revenue by providing frequency regulation, an essential grid-balancing service, on the New York State electricity grid in the fourth quarter of this year. The company has achieved a number of milestones on the road to building the 200-flywheel Stephentown plant, including more than 18 months of successful operation on the New England grid; receipt of a $43-million conditional loan guarantee commitment from the U.S. Department of Energy (DOE); and the award of a$24-million DOE smart grid stimulus grant for another 20 MW plant.

“Our Gen4 flywheel design relies on hermetic feedthroughs in order to reliably maintain vacuum inside the chamber during operation,” says Dick Hockney, chief engineer at Beacon Power. “This capability is critical for reducing windage, which increases efficiency and prevents the high-speed rotor from overheating.”

For control systems, speed, temperature and vibration all need constant monitoring via numerous thermistors and other sensors, often incorporating shielded and/or twisted wires to maintain signal integrity. For power transfer, copper post studs or heavy gauge wire feedthroughs must be accommodated, depending on current requirements. In all cases, small and high density feedthroughs provide less risk of leakage than multiple connectors. In the case of flywheel chambers that are submerged in a heat transfer fluid, these feedthroughs must also be leakproof and resistant to whatever fluid is in use.

Oftentimes the vacuum environment and heat transfer fluid requires that special attention be paid to material selection. Understanding parameters such as outgassing, permeability and material compatibility is critical in developing solutions that will perform as desired over the 20+ years of operation that most of these units require.

While the potential for FES solutions is tremendous, these projects are often at risk when the challenge of getting signals and power into and out of the vacuum environment is underestimated. Consulting hermetic feedthrough experts during the design phase can ensure that these small, but necessary, components do not become the failure points for an otherwise successful project.