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 the technology improves, flywheel energy storage (FES) systems suit more applications such as frequency regulation in power utilities, energy recovery in trains and industrial equipment, and rack-mounted uninterruptible power supplies.

Compared to other energy storage setups, FES systems have relatively long life cycles, require minimal maintenance, have high energy densities (~ 200 kJ/kg), and large power outputs. The round-trip efficiency (ratio of energy out per energy in) of a flywheel can be as high as 90%, with power output capacities ranging from 2 kWh to 133 kWh. Typically, an FES system can reach full charge in approximately 15 min.

The new generation flywheels are a result of advances in material science for rotor technology and the application of magnetic bearings running in a vacuum environment. While rotating a flywheel in a vacuum is one way to eliminate windage friction losses, mechanical bearings will not necessarily withstand operating in a vacuum or new flywheel speed requirements. With the advent of magnetic bearing and magnetic-mechanical hybrids, FES designers created bearing configurations with low predictable friction. The new systems can operate without lubrication and are said to perform well in a vacuum.

However, there are some challenging issues for FES designers. For instance, they must ensure the vacuum integrity of the flywheel housing while meeting requirements for noise-free monitoring and high power I/O. A breach in the rotor vacuum environment could lead to FES failure. As a result, making hermetically sealed feed throughs is an important step for FES development. When FES designers reduce system size, they should consider the co-location of associated electronic and control systems and how the essential feed throughs will be successfully situated. Thus, control and power feed throughs that fit into tight areas, turn corners, and maintain vacuum, require custom housing designs with special geometries and materials.

Tyngsborogh, Mass-based Beacon Power uses hermetic vacuum feed throughs to improve the performance of its Smart Energy 25 FES systems used on the utility grid to provide frequency regulation. The feed throughs transfer power and signal data from the control system on the atmospheric side to the internal volume of the vacuum-sealed flywheel chamber.

Control systems’ speed, temperature, and vibration need constant monitoring using numerous thermistors and sensors. Often, these devices require shielded or twisted wires to maintain signal integrity. For power transfer, copper post studs or heavy gauge wire feed throughs must be accommodated, depending on current requirements. In all cases, small and high density feed throughs provide less risk of leakage than multiple connectors. Flywheel chambers that are submerged in a heat transfer fluid must be leak-proof and resistant to whatever fluid is in use.

Material selection is important for the vacuum environment and heat transfer fluids. Understanding parameters such as outgassing, permeability, and material compatibility can be critical in developing systems that will perform efficiently for more than 20 years.

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For sales and marketing information, contact Cindi Telep, director of customer relations, DECo, by phone at (973) 627 8230, or by e-mail at

About Douglas Electrical Components
Founded in 1945, Douglas Electrical Components, Inc. (DECo) is a third generation, family-owned company that pioneered the development and practical application of epoxy technology to electrical wire feedthroughs.

DECo offers proven solutions and works with customers to develop high quality products for their conductor sealing needs.

DECo is an ISO 9001 registered manufacturer of UL- approved electrical and fiber optic feedthroughs, hermetically sealed connectors and cables, explosion proof wire assemblies and turnkey subassemblies for semiconductor manufacturing equipment. Major markets served include automotive, magnetic bearing, power generation and distribution, semiconductor processing, space simulation, military, alternative energy, air conditioning and refrigeration, X-ray, explosion- proof, oil and gas exploration, and glovebox.