Regenerative braking system lowers carbon footprint of trains

A new flywheel-based regenerative braking system has shown it can reduce the carbon footprint of rail travel on Britain’s diesel-powered trains, with the potential to also increase capacity on crowded lines.

The DDflyTrain project has resulted from a collaboration between Ricardo, who designed the flywheel, digital hydraulics firm Artemis Intelligent Power and rail technology company Bombardier Transportation.

It combines Ricardo’s TorqStor high speed flywheel energy storage system and the Artemis Digital Displacement hydraulic pump-motor transmission system to store and reuse energy using flywheels spinning at 45,000rev/min.

The consortium’s aim was to build a demonstration rig to show that the technology could work in a diesel environment on multiple unit trains.

‘Every train journey involves a lot of stopping and starting, and so also a lot of braking and acceleration,’ said David Rollafson, vice president, global innovation at Ricardo. ‘If we can harvest some of the energy from braking and use this when the vehicle is gathering speed again, we can save a lot of diesel and so a lot of money for train operators.’

There would also be benefits in terms of reducing pollution. ‘When pulling out of a station, diesel engines are noisy and pump out a lot of pollution,’ said Rollafson. ‘Adding stored energy to help acceleration would reduce this.’

Compared to their electric counterparts, diesel trains are also slower to accelerate. However, using energy from the flywheel would improve this, and could therefore increase capacity on existing rail lines.

The system could also be used on third rail, live rail electric trains to improve their efficiency. Whereas electric trains with an overhead power supply can harvest energy from braking and put this back into the network without much loss, the losses are too great with a third rail to make this efficient. Instead, the flywheel-based system would capture energy at its source and immediately put this back into the vehicle’s power system, meaning that around 80 per cent of this captured energy could be reused.

The DDflyTrain system is easy to retrofit, and is also modular, meaning that operators could scale up their energy storage when finances allowed. The consortium are now in talks with a number of UK train operators concerning application of the technology within their existing rolling stock.

The technology was acknowledged in November 2014 when it was awarded the ‘Most Interesting initiative in safety and sustainability’ award at the Rail Exec gala dinner.