The flywheel has fallen off many people’s radar since the industry’s leader, Beacon Power, filed for bankruptcy in 2011. Though the company was revived shortly after—and other competitors joined the market since—the flywheel’s comeback to the mainstream hasn’t quite happened yet. But one startup is hoping to change that.
A flywheel battery stores electric energy by converting it into kinetic energy using a motor to spin a rotor. The motor also works as a generator; the kinetic energy can be converted back to electric energy when needed.
While the interest in flywheels soared in the late 1990s and 2000s, it had shortcomings. These early flywheel batteries were bad at storing energy for long periods. So flywheels at the time were used more for short-term energy storage, providing five-to-ten-minute backup power in data centers, for example. And Beacon Power, before its bankruptcy, focused largely on using flywheels as frequency regulators for power grids.
But Ben Jawdat, the founder and CEO of Revterra, a flywheel startup based in Texas, thinks that his company has overcome the shortcomings, making flywheels capable of long-term energy storage for renewable energy.
Revterra’s technology is made possible by developments in three key areas: rotor materials, motor-generators, and bearings. Improvements in metal alloys and composite materials enhanced rotors’ strength, enabling them to spin at high speeds more reliably. And the new generation of motor-generators reduces system energy loss by switching its magnetic reluctance (analogous in a magnetic circuit to electric resistance in an electrical one) to stop energy leaks while idling and to make power input and output more efficient.
But the most important technological development is in the bearing, Jawdat says. Previous flywheel storage systems used either mechanical bearings, such as ball bearings, where the bearing physically touches the rotor, or active magnetic bearings, which eliminate friction at the cost of complex and power-hungry control systems. Both of those options ultimately result in a significant amount of the rotor’s mechanical energy being lost as waste heat.
Revterra uses passive magnetic bearings that can hold a rotor in equilibrium without an external control that consumes the additional energy, which improves the energy efficiency even further by removing the energy consumption of the bearing itself.
Revterra's flywheel spinning inside the vacuum chamber while magnetically levitated, as seen through a vacuum tight glass windowPhoto: Revterra
The secret is to use the high-temperature superconductor as a bearing. This trick not only allows the bearing to lift a very heavy rotor—Revterra’s commercial-scale rotor will weigh seven tons—but it also cuts energy losses thanks to the bearing’s inherent ability to trap the magnetic field that holds the rotor in place. Revterra’s 100 kWh flywheel system will lose only 50 Watts when idling. In comparison, many flywheels consume over 1000 Watts, according to Jawdat. So if you charge the flywheel battery all the way and discharge completely, you would only lose about 10% of the energy, he adds.
Improvements in superconductor manufacturing have made them more practical for commercial applications. And, Jawdat says, Revterra’s design only requires a small amount of superconducting material, kept at a temperature around -196 C, or 77 K, by an off-the-shelf cryocooler—which, being cryogen-free, doesn’t require liquid nitrogen. Meanwhile, the vast majority of the flywheel system is kept at room temperature.
Flywheels are now coming back at a time when the push for renewable energy is soaring in the U.S. A key Biden administration climate goals is to make the U.S. a 100% clean energy economy with net-zero emissions by 2050. And California—the fifth largest economy in the world if it were a country—made it a state law to hit a 100% renewable energy goal by 2045.
All that renewable energy will need grid storage, too. For which there are many contenders. But each leading grid-scale storage tech is not without its drawbacks, says Jawdat. Chemical batteries degrade over time—and lithium-ion’s cobalt problems and other sourcing challenges don’t make the challenge of affordable, grid-scale batteries any easier. Another popular technique, compressed air energy storage, is cheaper than lithium-ion batteries but has very low energy efficiency—about 50%.
Here is where Jawdat sees a market opportunity. Compared to lithium-ion batteries, flywheel batteries essentially last forever. “You can charge and discharge all day every day for 30 years, and your [flywheel] battery will still have 100% capacity,” Jawdat says. “With chemical batteries, you have to keep replacing them every five to ten years,” which drives up the cost for long-term usage.
With the help of funding from the National Science Foundation, Revterra built and tested a working prototype 1 kW flywheel system. And Jawdat and his team have been working on a commercial scale 100 kWh system.
“We need a way to store vast amounts of energy without creating a lot of additional negative environmental impact if we are going to really transition to low emission renewable power sources,” says Jawdat. “I think there is a lot of work being done on this, and [flywheels have] a lot of promise as a clean energy storage solution.”
Note: This story has been updated (7 April, 5:30 p.m. EST) to reflect additional information and context provided by Revterra on superconductors and magnetic levitation in the flywheel storage industry.