Power Engineering International
May 2009 - The following article originally appeared in Power Engineering International (http://www.powergenworldwide.com/index/display/articledisplay/361774/art...)
An article by Ed Douglas on New Generation Flywheels
The phrase “everything old is new again” certainly applies to today’s flywheel technology.
Unlike 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 being used in 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. And with increased demand 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 percent, with power output capacities ranging from 2 kWh to 133 kWh. An FES system can typically reach full charge in as little as 15 minutes.
The 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 or to the high speed requirements of the new flywheel designs. With the introduction 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, operating in a vacuum presents one critical design challenge for an 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 also frequently try to make the system size as small as possible, 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 are required, as are custom housing designs for them, 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. Hermetic feedthroughs reliably maintain vacuum inside the chamber during operation, 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.
Often, material selection for the vacuum environment and heat transfer fluid requires special attention. Understanding parameters such as outgassing, permeability and material compatibility is critical in developing solutions that will perform as desired for the 20 or more 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.
Author: Ed Douglas is president of Douglas Electrical Components Inc. (DECo). A third-generation owner, Douglas has more than 15 years of engineering and management experience in the semiconductor, vacuum deposition systems and electrical component manufacturing industries. Douglas earned B.S. and M.S. degrees in engineering from Stevens Institute of Technology in New Jersey.