Light rail lines' use of flywheels to store rotational energy that could later be converted to electricity would accrue significant power and cost savings to such transit systems, according to University of Alberta mechanical engineering professors Pierre Mertiny and Marc Secanell.

The two recently calculated that the use of flywheel technology to assist light rail transit in Edmonton would produce energy savings of 31% and cost savings of 11%.

University of Alberta mechanical engineering professors Marc Secanell (l) and Pierre Mertiny. Image credit: University of Alberta. A flywheel is exactly what it sounds like: a disk, also known as the rotor, spins and increases its rotational speed as it is fed electricity. This rotational energy can then be turned back into electrical energy whenever it is needed. It is, in a sense, a mechanical battery. The system loses very little energy to heat or friction because it operates in a vacuum and can even use magnetic bearings to levitate the rotor.

Although flywheels are an older technology, they are used on many modern high-end applications, such as the International Space Station and race cars built by Audi and Porsche. In the U.S., high-capacity flywheels are also used in areas of high population density, including parts of New York, Massachusetts and Pennsylvania, to buffer electricity to prevent power outages.

Secanell and Mertiny examined the possibility of using flywheel technology to store energy generated when Edmonton’s LRT trains decelerate and stop. Trains such as the LRT are designed with so-called dynamic braking, which uses traction motors on the train’s wheels for smooth stops. But the deceleration generates energy, which needs to go somewhere.

“Electric and fuel cell vehicles already implement regenerative braking in order to store the energy produced during braking for startup, so why would trains not be able to do so?” asks Secanell.

Currently that electricity is considered to be "dirty" because it is intermittent and thus difficult to use. Conventional systems simply send the braking electric power to resistors on the train that convert the electrical energy to heat, which is then released into the air. A flywheel system would take that electrical energy and store it as mechanical energy. This energy would then be converted back to electrical energy when the train is ready to leave the station again.

“It’s difficult to use a conventional battery for this purpose,” explains Mertiny. “You need to recharge and discharge a lot of energy very quickly. Batteries don’t last long under those conditions.”

According to Mertiny, the city of Hanover, Germany is already testing flywheel technology for just this purpose. The city has installed banks of flywheels at each station to capture energy as trains arrive for use later. Locating the flywheels at each station meant that Hanover’s trains did not have to be retrofitted for the development.