For centuries, spinning wheels have been used to store energy. That's how potter's wheels work, with the energy in the disk helping maintain a consistent velocity. Flywheels are also key components of steam engines, where they convert energy from pistons into consistent rotational motion. Even toy cars with friction motors make use of flywheel technology.
But it is only recently that large cylindrical flywheels have been used to quickly store and release electric power, as a way to keep complex power grids in balance.
One Canadian company, Temporal Power Ltd., is on the leading edge of this technology, making flywheel storage devices that are now being used to add stability to Ontario's electricity system. In a windowless two-storey building just outside of Harriston, Ont., 150 km northwest of Toronto, 10 of the company's 4,000 kilogram solid-steel flywheels are sunk into cement vaults in the ground. Linked to the province's power grid, they rapidly charge and discharge to help maintain the right balance of power as demand fluctuates.
These kinds of installations could become commonplace in the coming years, because every power grid in the world – and especially those with high proportions of renewable power – will likely need more means of fine-tuning the balance between electrical production and demand.
Temporal's flywheels are, essentially, mechanical batteries that store power in the form of kinetic energy as they spin – unlike normal batteries that hold power as chemical energy.
A Temporal flywheel is "charged" using an electric motor that turns a huge rotating steel cylinder. The motor converts electric power to the mechanical momentum of the rotor, and the rotor speeds up. To release this stored power, the motor switches to become a generator, as the slowing rotor transfers its energy back into electricity.
A flywheel can almost instantaneously switch back and forth between loading up on power and releasing it – a characteristic that makes the technology attractive to electrical system operators that constantly need to balance supply and demand.
"We respond to a grid-wide imbalance of energy," said Temporal chief executive officer Cam Carver. "Sometimes you have too much and you want it off, sometimes you have too little and you want more. It's a constant balancing act." Flywheels are ideal for this delicate task, he said, and they operate will little power loss.
Each of Temporal's flywheels is an impressive example of precision engineering. Forged from a single piece of steel, each rotor is a massive cylinder weighing
4,000 kg. When spinning at full tilt, just under 12,000 r.p.m., the speed at the surface of the cylinder exceeds the speed of sound.
At the Harriston facility, which has been up and running since July, the Temporal flywheels are being tested to see how well they work in keeping the province's electricity system stable.
The facility, owned by energy storage developer NRStor Inc., receives a signal from Ontario's Independent Electricity System Operator (IESO) every four seconds, telling it whether to send electricity to province's power grid, or to absorb some. Up to two megawatts of power can be sent out or absorbed.
Leonard Kula, IESO's director of operations, said that in theory flywheels should be "perfectly suited" do this job more effectively than the older techniques, which usually involve getting certain hydro power plants to start and stop water flowing through turbines. But the IESO wants to make sure flywheels work well over the long term before it commits to more, or larger, installations. IESO is also testing a large chemical battery site to see how it performs the task.
With more and more intermittent renewable energy being added to power grids – from wind and solar farms – the need to control power fluctuations will likely become even more acute, Mr. Kula said.
Temporal's flywheels are also being installed at another site near Tillsonburg, Ont., where Ontario power distributor Hydro One will soon begin testing them to help smooth out regional fluctuations caused by the large number of wind farms in that area.
"No one has taken a piece of steel this large, and run it this fast," said Temporal Power president Jeff Veltri, an engineer who created the company's proprietary technology.
His key invention is the magnetic levitation system that suspends the cylinder from above, to reduce energy loss through friction. Each rotor is also in a vacuum, to further reduce drag, and a cooling system allows it to run continuously without heating up.
One thing the flywheels can't do effectively is store large amounts of electricity for long periods of time. Other storage technologies – such as systems where water is pumped into a reservoir and then released through a power turbine when it is needed – are more cost effective for that purpose, Mr. Carver said.
But with more intermittent renewable electricity being added around the world, the market for Temporal's flywheel's is growing sharply, Mr. Carver said, and every power grid will need more short-term power balancing. Island power systems – such as those in Hawaii and the Caribbean – can't trade electricity with adjacent grids, so their need for this type of technology is even more acute.
And Temporal has few competitors. The only company with a similar technology is Beacon Power, a Massachusetts-based firm that has installed flywheels at power-regulating facilities in Stephentown, New York and Hazle, Pennsylvania. Its devices are made from carbon-fibre composite materials, unlike Temporal's steel rotors.
Uses for flywheels
Flywheels have many uses beyond the power grid. Because they can store power and then release it quickly, they are used in some uninterruptible power supplies (UPS). In a power failure, the spinning cylinder can instantaneously generate emergency electricity until a bigger back-up generator is started up.
Flywheels have also been developed for use in racing cars. Like electric hybrids, these systems store energy generated during braking and then release it later when extra power is needed.
But using them in vehicles is not a new idea. In the 1950s the Swiss experimented with "gyrobuses" that ran on flywheels. These vehicles had huge 1.5 ton flywheels, located in the middle of the bus, which were spun to 3,000 r.p.m. by a motor at a central depot. The power was then released by the flywheel to propel the bus along its route.