Capturing wasted energy in our cities
Think about anything that moves, a runner sprinting, a car driving, or a train rolling down the tracks. All that movement requires energy. Scientists call the energy of motion kinetic energy. The faster something moves, and the heavier it is, the more kinetic energy it carries.
Now picture a massive and powerful train accelerating to full speed. It builds up a tremendous amount of kinetic energy. But when it needs to slow down or stop, whether for a station or a signal, what happens to all that energy?
Traditionally, braking systems relied on friction or braking resistors, converting that kinetic energy into heat that simply dissipates into the air. It’s like the sprinter coming to a sudden stop, and all their momentum just vanishing as warmth. On busy metro lines, where trains stop frequently, this adds up to a staggering amount of energy, built up, then wasted.
In today's world, you read a lot in the news about the urgent need to reduce energy consumption. This global push comes from several critical places: the need to combat climate change by lowering carbon emissions, the desire for greater energy independence and security, and the simple economic reality that energy costs money. Finding ways to do more with less energy is therefore not just an environmental goal, but a strategic and economic imperative for cities and rail operators worldwide. In this context, preventing wasted energy is paramount.
Rail is already one of the most energy-efficient ways to move many people and carry heavy cargo.
Since the 1990s, the introduction of regenerative braking has taken that efficiency even further. Instead of wasting kinetic energy as heat, trains can now use their motors to brake and convert motion back into electricity. To be used by another train accelerating nearby. But what if there is no train nearby? Does the valuable energy slip away?
This is where smart engineering comes in, with systems designed to recover this surplus energy. A leading example is Alstom's Hesop technology. Think of Hesop as a two-way street for electricity in the rail network. Standard electrical substations only allow power to flow in one direction: from the grid to the train to make it go. Hesop is different; it is reversible. When there is more braking energy available than needed by other trains nearby on the same line, Hesop can receive that power and send it back to nearby rail stations or to the main electricity grid for broader use.
"Our reversible substation technology essentially turns the train into a temporary power source during braking. Instead of dissipating that energy as heat, we convert it and send it back"
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136Alstom is the world leader of reversible substations with 136 units sold globally and 110 in commercial service
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20%less substations for a new line
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99%of recoverable energy captured
"Our reversible substation technology essentially turns the train into a temporary power source during braking," explains Alstom’s Master expert for Energy Management Carsten Söffker, Ph.D., “Instead of dissipating that energy as heat, we convert it and send it back. We're seeing recovery efficiencies that were previously unachievable for this type of application, capturing close to 100% of the available energy during braking and putting it back into the system or the grid where it can be used."
The practical results of implementing this kind of technology are significant and provide concrete examples of progress. In Hamburg, Germany, where Hesop has been helping the metro since 2020, the system recovered an impressive 1042 megawatt-hours (MWh) of energy in just one year. To give you a sense of scale, it is roughly equivalent to the annual electricity consumption of around 300-400 average European household, of course this number can vary depending on house size, climate, and heating types but it gives you a general idea of powering a small neighbourhood.
Reclaiming this energy means less power needs to be generated elsewhere, cutting carbon emissions in cities. Plus, HESOP minimizes braking dust, creating a fresher, healthier environment for metro passengers.
Beyond the environmental benefits and the savings on electricity bills for operators, this smart energy management also helps make the rail network more reliable by contributing to the stability of the power supply, even when the system is under heavy load. The successful deployment of this technology in a range of diverse rail environments globally, such as the metros of London, Panama, Riyadh, Hamburg, and Dubai, and the tramways of Sydney and Hanover — highlights its effectiveness and potential
Supporting the UN Sustainable Development Goals
Technologies such as regenerative braking and energy recovery improve rail efficiency and contribute to supporting the UN Sustainable Development Goals. By reducing emissions and optimizing energy use, smart mobility solutions can help cities meet global sustainability targets.
"Reclaiming energy is not just an operational benefit; it is a core component of building the resilient, low-carbon transit systems that our growing cities need to meet global environmental targets. It's about embedding sustainability into the very way we design and operate rail."
"Looking at solutions like this, you see the future of infrastructure taking shape," states Philippe Bertrand, Infrastructure solutions director. "It’s intelligent, interconnected, and fundamentally built around sustainability. Reclaiming energy is not just an operational benefit; it is a core component of building the resilient, low-carbon transit systems that our growing cities need to meet global environmental targets. It's about embedding sustainability into the very way we design and operate rail."
Innovation in the rail industry, like the development and implementation of systems that reclaim energy, offers tangible solutions to the pressing need for greater energy efficiency. It transforms what was once simply wasted into a valuable resource, displaying how smart technology is helping rail play a role in building a more energy-efficient and sustainable future for transportation, one recovered megawatt at a time.
