Capturing energy from braking

Many modern metro or tram networks use technologies to harness energy generated by braking trains. These are known as ‘regenerative braking systems’.

One of the aims of these systems is that energy from braking trains can be passed to another train in the network which is accelerating at the same time. This allows up to 40% of the network’s energy to be recovered, bringing strong environmental and cost benefits.

However, these systems traditionally run from electricity substations where power only flows in one direction – to the transport network, but not back from it. Whenever there are no other trains nearby to harness regenerated power, other solutions are needed to deal with the excess energy. Generally, this means that the energy is stored in heavy on-board batteries which weigh the trains down, or burned in resistors which produces extra heat.

Going beyond traditional systems

The Hesop system introduces ‘reversible power substations’ which can be retrofitted to existing systems or built into new networks. These substations allow energy to flow in both directions – to the transport network and back to the grid.

Hesop captures more than 99% of recoverable braking energy which can then be sold back to the grid or used in other parts of the transport network to power escalators or lifts, lighting or air conditioning.

Under the LIFE RE-USE project, a new 1 500V version of the Hesop system was developed and tested, and is now being brought into service on metro and suburban systems around the globe. This 1 500V version builds on lower voltage systems which had previously been developed by the project coordinator Alstom SA.

22% in energy saving

Alstom, a specialist in sustainable power generation and rail infrastructure, manufactured and prototyped Hesop at increasing power levels - up to 12MW peak power - and worked with Azienda Trasporti Milanesi, the Milan metro operator, to test the Hesop 1500V on metro line M3. By testing this way, the project demonstrated the technology under commercial service conditions.

The project team’s objective was to show that Hesop could achieve at least 15% lower energy consumption. But after 10 months they were able to reach 22% energy savings when tested on the Milan metro system. This is equivalent to

2MWh (megawatt hours) per day saved

171 fewer tonnes of CO2 during the trial period between October 2017 and July 2018

“Thanks to all the achievements on this project, Hesop has been adopted by other transport networks worldwide,” said product manager François Maurin. “It has benefits on infrastructure costs, on energy consumption, on operation and on the environment.”

An international impact

These results are an extra boost to the project, enabling Alstom to bring this technology to more and more cities around the world. While a lower voltage 600V system was implemented on the London Underground, Hesop has been extended to other rail systems in Australia, Panama, Saudi Arabia and Dubai.

See also

Video on Hesop 1500V in Milan (watch on Youtube)