Product Design & Development

Next-gen Of Flywheel Energy Storage

Friday, March 13, 2009
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Next-gen Of Flywheel Energy Storage

Hermetic feedthroughs are critical components for FES solutions

By Ed Douglas, President of Douglas Electrical Components

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 60,000 RPM. 

Power plate assembly.

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 60,000 RPM. 

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As flywheel technology continues to improve, flywheel energy storage systems (FES) are gaining in use across a wide variety of applications, from peak shaving in power utilities to energy recovery in trains and industrial equipment to rack-mounted uninterruptable power supplies.

With demand rising for reliable, cost-effective, and environmentally friendly energy storage, such as in 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 (~ 500 kJ/kg), and large maximum power outputs.

The energy efficiency (ratio of energy out per energy in) of flywheels can be as high as 90 percent, with power output capacities ranging from 3 kWh to 133 kWh, and an FES 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 as part of tomorrow’s clean energy solution.

These new generation flywheels are made possible by advances in material science in rotor technology, as well as the application of magnetic bearings running in a vacuum environment.

While the movement of the rotating flywheel into a vacuum is an obvious way to get rid of the windage friction losses, mechanical bearings don’t stand up to operating in a vacuum nor to the high speed requirements of the new designs.

With the advent of magnetic bearings, FES engineers gained a bearing solution with very low and predictable friction, the ability to run without lubrication, and high performance in a vacuum – the ideal bearing 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.

Hermetic feedthrough with plastic seal.

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.

The main cause of FES failure is a breach in the vacuum environment of the rotor, making hermetically sealed feedthroughs a critical engineering component for FES development. FES designers often try to significantly reduce the system size, making it at small as possible, not 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.

For control systems, speed, temperature and voltage all need constant monitoring via numerous Hall Effect 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 which 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 we pay special attention to material selection,” says Chris Rempel, technical sales manager for Douglas Electrical Components.

“Understanding events like outgassing, permeability and material compatibility is critical in developing solutions that will perform as desired over the 20+ years of operation 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 are underestimated. Consulting hermetic feedthrough experts during the design phase can ensure that small, but necessary components are not the failure points of an otherwise successful project.

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