There are certain things you get used to if you have to ride the subway every single day. People diving at closing doors to avoid waiting two minutes for the next train, mysterious sticky patches on the floor, handrails clearly designed only for people over six feet tall, and the constant shuddering and shaking as the train slows, stops, starts, and speeds up again — these are evils that are all unlikely to go away anytime soon. However irritating, the trembling and wobbling may promise a vast capacity for cutting costs and shrinking carbon footprints, according to a recently completed trial on the London Underground transit system.
The annoying side-to-side rattling of a moving train occurs as it modulates its speed, and every shake or extra scrape against the rails sacrifices kinetic energy to the surrounding environment in the form of friction and heat. The piloted technology adopts the long-extant concept of regenerative braking (a general term for recovering energy from mechanical brake components) and couples it to the substations that provide power to the entire Underground system to create a mechanism for “inverting” that previously wasted energy back into the power grid. It’s a scale of regenerative braking technology that has never been witnessed before; despite recent heavy use in the electric car sector for range extension, this is the first time that it has made the jump into mass transit.
Long before the introduction of regenerative braking for electricity recovery, the London Underground pioneered a simpler system of regenerative braking for energy efficiency. To minimize energy usage in the deepest tunnels of the subway network, stations and track were aligned such that a slight upward slope led into each station and a slight downward slope led out — a conversion of kinetic energy into gravitational potential energy that permits partial deceleration or acceleration without external energy input. However, these “platform bumps” only contributed a minute reduction in the electrical energy needed to operate the train system. Trains still had to accelerate and decelerate during numerous time intervals, losing costly energy through the heat and friction of braking.
The recently-piloted program, which was limited to one transit line and one substation, differs from these primitive techniques in two key ways. First, it is motor-driven — operation of the brakes triggers a switch in the circuit between the train engine and electric motor, flipping the direction of the motor so that it produces negative torque and functions as a generator. The energy generated is channeled to an inverter system at a substation, which feeds it back into the power grid.
The Underground is not the first rail system to begin to exploit the benefits of motor-driven regenerative braking. The Transcaucasus Railway implemented a similar system in the early 1930s to minimize electrical infrastructure needed in dangerous elevated mountain passes, and some Nordic freight lines use electrical energy conserved while traveling downhill to power the return uphill trip. However, the London project is the first to ambitiously attempt to turn the energy wasted by its trains into a source of energy for its stations.
Though it may seem like only miniscule amounts of electrical energy are actually recovered, the brake-pumping adds up. Across the pilot program, one week’s worth of operation conserved enough energy to power to run one of the Underground’s larger stations for at least two days a week. All together, up to one megawatt hour of power was recovered per day — without yet implementing the program across the entire system.
Politically, of course, the most immediately appealing aspect of the energy-recycling plan is the financial one. With the Underground currently facing a upgrade plan for four lines that carries a £760 million ($1.16 billion) price tag, a 5% energy bill reduction can only help in keeping the world’s oldest subway system fiscally solvent. It might also open up the opportunity for funding one of the even more radical suggestions proposed for revitalizing aging rail infrastructure: replacing trains with moving walkways or turning older tunnels into bike thruways. At the very least, we can hope that this regenerative braking/power inversion combination becomes a model for transit agencies around the world as they look to cut their energy intakes and slash their carbon footprints.