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Spinning flywheels boost subways

About the author

Jon is the presenter of Science in Action on the BBC World Service. He trained as a mechanical engineer (with automotive and aeronautic design) before becoming a journalist. He has worked at the BBC for over a decade and has reported from areas as diverse as war zones and technology shows.

LA Metro (Copyright: Getty Images)

LA Metro (Copyright: Getty Images)

A technology normally associated with the steam age could make a comeback in tomorrow's trains.

Public transportation systems are usually viewed as the “green” alternative to private vehicles, but that does not mean there is not plenty of room for improvement.

Take electric subway trains, for example. After they have filled up with passengers they need to draw on a huge amount of energy to accelerate away from the platform. In fact, a typical train might draw 4 megawatts (MW) of power for 30 seconds to get up to speed – that is the same peak demand as around 1,000 average US homes. Yet, just a few minutes later, the train sheds all of that kinetic energy when it brakes to pick up its next load of passengers.  

Over the years, various schemes – from the very simple to the complex - have been devised to make this process less wasteful. In some stations the approach to the platform has an incline to aid braking, while the exit has a downhill slope to help acceleration. In others, so-called “regenerative braking” is used, where the electric motors are run as generators to slow the train. The recovered energy is then put back onto the electrified rail. But that energy is not always put to use.

“If there’s a train close enough it can use it, but otherwise it is thrown onto the rail where it dissipates out, or if the voltage goes too high it’s burnt onboard with resistors. But 95% of the time it’s wasted,” says Patrick McMullen, the chief technical officer at Vycon, a company that aims to solve the problem.

McMullens solution is to use gigantic spinning wheels – known as flywheels - that act like a battery to temporarily store the excess energy. It has just signed a deal with the Los Angeles Metro in California to trial its system.

Power boost

In their simplest form, these devices are just a heavy disc mounted on a shaft that store rotational energy.  Spin the disc up, and it gains momentum and continues to rotate.  Spin the disc back down and it releases all that energy. This is all done at high-speed – up to tens of thousands of revolutions per minute.

“What the flywheel energy storage does is it captures that braking energy. It holds onto it while the train is at the station, and then when it goes to leave it uses that energy to help get the train out of the station,” says McMullen.

Flywheels are nothing new, of course. They have been used for decades to store energy from steam engines, while companies like Vycon already install them in shipyard cranes, where power is stored each time a load is lowered, ready to be used for the next raising.  And in this column, we reported on their use in the car which won this year’s Le Mans 24hr race.

They have even been trialed in public transport - back in the 1940s Swiss firm Oerlikon introduced the Gyrobus, while established underground and overground systems in New York and London have trailed various systems from at least 1974 onwards .

Despite some successes, the technology has never taken off in a big way. In New York the company supplying the units went bust, while earlier trials of onboard systems were hampered by inefficiencies and the weight of the units which negated any saving in energy. That is beginning to change, thanks to lightweight materials such as carbon fibre, which can be used to make the spinning discs. For example, Formula 1 team Williams is developing its GyroDrive technology, while there are trials onboard trains and buses everywhere from London and Tokyo to Cologne.

But Vycon’s systems are different. Unlike many existing transport systems, it does not use onboard flywheels. Instead, the flywheels are used to store energy trackside at each station, meaning there are fewer limitations on size, and the weight of the device does not have to be transported.

“It reduces the overall energy consumption of the station, and also the peak power demand,” according to McMullen.

The LA trial, for example, will collect power generated by braking trains that enter the Westlake and MacArthur Station. It will store 2MW, with flywheels designed to fit into the existing electricity substation. The power storage unit consists of 4 modules, each containing a high-speed steel flywheel and a motor/generator. Each one is about 0.9m by 1.2m (3ft by 4ft).

Over the next year, the company hopes to work with other transport agencies around the world to install other demonstrations of its technology.  McMullen sees a future where flywheels are not only installed in existing public transport systems, but designed into it from the start.

“When you build out new systems you look at putting a substation every mile. Now, you can alternate between putting a substation and a flywheel, so that you can end up having a reduced overall cost. You don’t have to drop your power lines every mile.”

Retrofitting existing tracks is likely going to be an expensive business, meaning the first full-scale use of flywheels will be in those emerging economies – like China and India – where there is the room and money to build new transit systems.

If and when they get built, passengers may not know – or care - that their train is running on steam age technology. But its effects will be seen. As well as saving money for the public purse, flywheels have one last trick up their sleeve – they could allow trains to run more quickly, regularly and smoothly.

Currently, train departures are – in part – managed by their power demands. Less frequent or staggered trains do not put unnecessary loads on the system. Add a flywheel and you have a cheap energy supply that can provide a boost when it is most needed, potentially allowing more trains to run simultaneously and pull out of the station more quickly. 

So, in the future, that rush hour crush may be just that little bit more bearable. And it could be thanks to a technology that has been in use since the steam age.

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