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Flywheels are back on track

The return of the flywheel, an old device known to store electricity quickly and for a long time

by Stefano Bevacqua
12 February 2020
5 min read
byStefano Bevacqua
12 February 2020
5 min read

Invented by Oerlikon in Switzerland and built in Freienbach, the Gyrobus was a cross between a bus and a trolleybus. Two Gyrobuses linked the towns of Yverdon-les-Bains and Grandson on Lake Neuchâtel, travelling back and forth over the few kilometres that separated them, meeting each other halfway.

The Gyrobuses were powered by flywheels. Charging rods were located in the two termini, supplying electric power to the flywheel motors which in turn rotated the steel disks, 1.6 metres in diameter, weighing one and a half tonnes and reaching their optimum speed of 3 thousand revolutions per minute.

Once the Gyrobus disconnected from the grid, the flywheel in turn provided electricity to two motors on the axles of the rear wheels. This meant you could travel six kilometres, with almost no noise, zero emissions and without the need for batteries onboard the vehicles. After the ten-minute journey the Gyrobus returned, ready for its 5-minute charge, just like a normal bus stopping at its terminus.

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A Gyrobus loading up the flywheel at the electric loadpoint (Smiley.toerist.Wikimedia)

Oil makes its way

But this was the 1950s and even in mountainous Switzerland, the oil era was being ushered in. This new low-cost fuel source meant investment in such a heavy vehicle no longer seemed viable. So, in 1960 the short history of the Gyrobus ended when it was replaced by a normal bus powered by a diesel engine.

At the same time, a similar scheme was launched in the current Democratic Republic of the Congo. Twenty Gyrobuses crossed the streets of the centre of Kinshasa, at that time called Leopoldville, capital of the Belgian Congo. In June 1960, the country became independent and the Gyrobuses were abandoned in favour of diesel buses. Only the year before, the same fate had befallen the line that connected the centre of Ghent to the suburb of Marelbeke in Belgium.

Storing kinetic energy in significant quantities with very short charging times was actually a great idea, so why was it abandoned? Mainly because of the relatively high costs combined with the technical constraints of the time.

The revenge of the flywheel

But today, faced with the need to reduce global emissions of climate-changing gases and, at the same time, further improve air quality in cities, the idea of the flywheel is having something of a renaissance in the Formula 1 laboratories where cars of the future are tested. Flywheel devices can be found in all current competition models, where they store the energy produced during braking to be then used during acceleration. Of course, the application for this is limited, but according to current tests, 60 kW more power is provided to the racing cars, even if only for a few minutes.

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Cut-out of a dual mass flywheel for cars (Cschirp, Wikimedia)

But research into other possible applications for these rotating parts is growing. Some car manufacturers are experimenting with their use as an alternative to the electrochemical batteries in rechargeable hybrid cars, while in the field of electricity production, they are being used in generation plants to balance grid fluctuations.

American Beacon Power in Tyngsboro, Massachusetts is one such example. Here a system has been put in place with a few dozen modules, each with a flywheel of about one metre in diameter and weighing over a tonne, rotating at 16 thousand revolutions per minute. Traditional support systems are no use here because of the weight and speed of the flywheels, so they are supported by an electromagnetic levitation system in vacuum chambers, eliminating the wear and tear that would result from the friction of direct contact. The energy stored by each of them is somewhere around 30 kWh and the available power can reach 180 kW for a few minutes and at least 50 kW for over half an hour.

The flywheel has two advantages over the large batteries of hybrid cars: much faster energy availability and a longer life span, with a higher number of possible charge and discharge cycles.

A long way to go

However, its main drawback is its weight. To produce similar power levels, we are talking about tonnes rather than the tens of kilograms of batteries, so of course, right now using a flywheel in a hybrid car would make no sense.

So, technology still has some way to go. The energy storage capacity of flywheels is not just about their mass but also their rotation speed. A flywheel of a few tens of kilograms would be sufficient to power a small car if it could run at 20 or 50 thousand revolutions per minute. But at those speeds, materials start to become an issue. Those that have sufficient tensile strength at the rotor will fracture on the outer edge, while materials that can maintain their solidity become fragile where force is applied. The solution is innovative materials that combine steel, rigid and light metals and carbon fibre. And this brings us full circle back to the Formula 1 track, where Flybird flywheels made of steel, titanium and fibre turn at 64,500 rpm without breaking.