Approximately 80 percent of total energy consumed by trains is for traction, and this energy is generally given off as waste heat as the train decelerates into a station. Light rail transit operators usually space stops about every quarter mile to keep walking distances short, meaning they have the energy-zapping task of stopping and propelling their trains 10 or more times every hour. Considering the kinetic energy of a typical two-carriage metro car is about 3 to 4 kilowatt hours (kWh), that means that in just 10 stops, a train has released enough braking energy to power the average American household for an entire day!
Over the last few decades, there has been a strong drive for technologists and engineers to reduce electricity waste, which leads to lower electricity bills for transit authorities. The first major improvement came with the adoption of AC drives so that braking energy can be regenerated and sent to the overhead catenary lines for use by other nearby trains. This enabled systems to recapture braking energy about 70 percent of the time, with the remaining 30 percent still requiring the use of large braking resister banks to burn off the excess energy (see Thomas Heilig et al.).
Starting in the mid-2000s, the idea of using energy storage to capture this remaining energy gained momentum, with companies such as Bombardier, Siemens and ABB taking the lead. The challenge was to build an energy storage system capable of capturing 3 to 4 kWh in the 10 to 20 seconds that it takes for a train to stop, and then to repeat it reliably every 15 to 20 minutes for 15 years or more. The energy storage system would need nearly 1 megawatt (MW) of power capability and a cycle life of over 250,000 cycles.
If such a system were based on batteries, which typically have a practical life of 5,000 cycles or less, the system would have to be oversized by 50 times to meet the cycle life, or it would need replacement every three to four months. This would make it either too large to be placed at most metro stations, or an impractical maintenance headache. However, through the use of ultracapacitors, the system size is such that it could easily be placed on or alongside the station platform – this has been demonstrated by the Bombardier EnerGstor and ABB ENVILINE energy storage systems.
As explained by Todd Hollett and Salwa Fouda of Bombardier Transportation in their paper Wayside Energy Storage System Modeling, these types of wayside energy storage systems connect to the overhead DC line and achieve their energy savings with a remarkably simple control scheme that monitors only the DC line voltage. When the line voltage rises above a set threshold (typically 600V) due to a local train regenerating more power than can be absorbed by the other nearby trains, the energy storage system charges at a rate that maintains the line voltage at a fixed 600V. Then, when the same train accelerates out of the station, drawing power enough to sag the line voltage below some other fixed threshold (typically 530V), the same energy storage system provides the recuperated energy back to the line to maintain line voltage at a fixed 530V. This type of wayside energy storage system can achieve an additional 15 to 20 percent energy saving over AC drive regeneration alone, and has already been in operation for nearly a decade.
More recently, companies such as CAF and American Maglev have extended this idea by placing the energy storage on the train itself, enabling not only braking energy recuperation but also operation over limited distances completely free of the overhead catenary lines. This is particularly valuable in cities that do not want the “clutter” of catenary lines visually disrupting the skyline. American Maglev’s system for the Tri-County Metropolitan Transportation District in Portland, Oregon, had the goal of catenary-free operation to enable train operation during harsh winter conditions when lines would completely ice over in sections of the track, as explained by Thomas Heilig and Jason Grohs in a their paper On Board Energy Storage for Light Rail Vehicles. This system has been in operation since 2011 and has achieved energy efficiency improvements of about 15 percent or about 35,000 kWh a year per train.
CAF, on the other hand, had much larger ambitions with its Freedrive system in Seville, Spain. This system, in addition to regenerating braking energy, operates on catenary lines installed only at the station stops. The ultracapacitors charge in about 20 seconds while passengers are loading and unloading and then the train travels for up to 1.4 kilometers (km) completely catenary-free to the next station. This may sound futuristic, but this system has been operational since 2010 and was operating flawlessly when I had the opportunity to visit and ride on it in 2016. In my humble opinion, one of the CAF system’s greatest benefits, beyond saving literally tons of carbon emissions, is that it also saved the majestic 16th century skyline of old-town Seville. Not a bad way to stop a train at all.
The CAF Freedrive system in operation in Seville, Spain