News and analysis about energy in California with an eye toward renewables.

The Flywheel: A New Spin On Renewable Energy Storage

Artist's rough version of Velkess' flywheel storage concept | Velkess image

As ReWire readers remember, one of the main obstacles to greater use of renewables is the fact that the cheapest forms of renewable energy, solar and wind, don't offer power 24/7. Without an effective way of storing surplus energy when the sun's out and the wind's blowing, those readily available renewables don't help much on windless nights.

A San Francisco-based company hopes to offer a practical way out of that problem by using high tech to fine-tune a technology we've used since the Stone Age: the flywheel.

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Flywheels are, simply put, heavy wheels that store energy as momentum. Get a flywheel moving and it will spin for some time, offering the possibility of recapturing that energy for longer-term use. It's the principle behind the potters' wheel: with a few pumps of the treadle to get a heavy stone wheel going, you can get steady output of energy to make the work surface turn.

Flywheels have been used as energy storage and regulation tools for a long time, and it's not much of a surprise that they've been eyed as a way of storing electrical power. But flywheels do have certain disadvantages. Friction's a big one. Flywheels that use mechanical bearings can lose as much as half their stored energy to friction in two hours. Unless the flywheel is housed in a vacuum, the turning wheel causes friction with air. Magnetic bearings offer almost no friction and work fine in a vacuum, but they make it harder to support a heavy flywheel.

Instability is another issue: any kind of perturbation can cause a spinning flywheel's "orbit" to wobble out of control, and perturbations range from small earthquakes to the precession caused by the earth's daily rotation. When a flywheel starts wobbling, it loses energy -- and also presents a potential safety hazard.

We do have the technology to make flywheels store energy efficiently, and to make them relatively stable as well. NASA's been examining the possibility of using flywheels to store energy on the International Space Station, for instance. But for those without a NASA-sized budget, flywheels have been an interesting but largely irrelevant tech when it comes to storing electricity.

San Francisco-based Velkess hopes to change that. The company has been building prototypes of flywheel energy storage systems designed, as Velkess CEO Bill Gray tells ReWire, to offer a plug-and-play replacement for conventional lead acid battery systems.

Velkess is working on a prototype flywheel storage unit that's small enough for distributed generation applications. The unit would have about the footprint of a typical home refrigerator, says Gray. The unit would be able to store 15,000 kilowatt-hours of electricity, losing only about 2 percent of its stored power to friction each day. Gray expects the unit to be competitive, price-wise, with currently-available lead acid batteries -- especially as the flywheel would far outlast those batteries' two to four year life expectancy.

Most interestingly, Velkess has approached the instability problem by creating a flywheel that's flexible: as wobbles develop, the flywheel itself absorbs the inconsistent motion by changing its shape. The concept is demonstrated in this radically slimmed-down desktop demo model:

Velkess' existing prototypes include 25-pound flywheels. In order to store 15,000 kilowatt-hours they'll have to scale up to 750 pounds, and using the much more powerful magnetic bearings those flywheels will require presents a logistical challenge which Velkess has launched a Kickstarter campaign to help them tackle:

Our challenge is with the magnetic assembly. The magnets to float 25lbs are easy to get on the internet and easy to work with by hand. Magnets strong enough to float 750lbs, are a different story. They need to be custom made and are too powerful to safely work by hand.

This Kickstarter will fund the development of a new magnetic motor/bearing assembly that can support the new 750lb flywheel and run with high efficiency.

If Velkess gets over that hurdle, one goal for the heavier prototypes is to put them to work storing electrical power on islands such as Puerto Rico and Oahu, where the majority of conventionally fueled electrical power comes from diesel. "Hawaii's installing lead-acid batteries like crazy right now," Gray tells ReWire, "because solar's so cheap compared to diesel that the grid operators have said there's too much solar in the mix now, and people can't add more solar capacity without adding storage. We hope to be able to offer an alternative to importing more lead batteries."

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About the Author

Chris Clarke is a natural history writer and environmental journalist currently at work on a book about the Joshua tree. He lives in Joshua Tree.
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