REGENERATIVE BRAKING


“The Market-Frankford subway line in Philadelphia is part of a regenerative braking experiment”

WAYSIDE ENERGY STORAGE SYSTEMS
https://researchgate.net/publication/338541009_White_Paper_on_Wayside_Energy_Storage_for_Regenerative_Braking_Energy_Recuperation_in_the_Electric_Rail_System
https://grist.org/transportation/barcelona-is-turning-subway-trains-into-power-stations/
Barcelona is using regenerative braking subways to power trains, stations, and EV chargers. Could New York do it too?
by Natalie Donback   /  Sep 24, 2024

“Most of the passengers emerging from the station in Bellvitge, a working-class neighborhood outside Barcelona, have no idea just how innovative the city’s subway system is. Using technology not unlike the regenerative braking found in hybrids and electric vehicles, the trains they rode generated some of the power flowing to the EV chargers in the nearby parking lot, the lights illuminating the station, and the escalators taking them to the platforms. Every time a train rumbles to a stop, the energy generated by all that friction is converted to electricity, which is fed through inverters and distributed throughout the subway system. One-third of that powers the trains; the rest provides juice to station amenities and a growing network of EV chargers.

The ultra-fast charger outside the Bellvitge station is among four electrolineras — Spanish for “electric gas stations” — that went up in July. The city’s main transit operator, Transports Metropolitans de Barcelona, or TMB, plans to add three more as the project, called MetroCHARGE, expands. “We’re trying to take advantage of the power that’s already in the metro system and use that spare energy to feed EV chargers on the street,” said Marc Iglesias, head of sustainable mobility at Àrea Metropolitana de Barcelona, a regional agency working with TMB on the project. Each year, residents and tourists take 440 million trips on Barcelona’s subway system, which includes 165 stations linked by 78 miles of track. The transit agency has installed three inverters so far; 13 more are in progress. Once they’re all in place by the end of September, it expects regenerative braking to provide 41 percent of the energy needed to power the trains, a renewable source of energy it says will save about 3.9 metric tons of CO2 emissions annually.

Although many cities, including ViennaPhiladelphia, and São Paulo, use regenerative braking to some degree, Barcelona is among the few to use it so extensively and the first to tap it for electric vehicle charging infrastructure. Utilizing energy that is otherwise lost as heat when a train slows can significantly reduce a transit system’s energy consumption. (Other efforts, such as optimizing the settings for Barcelona’s semi-autonomous trains and using AI to optimize the ventilation in each car, have further reduced energy needs by double-digit margins.) With the adoption of MetroCHARGE, 33 percent of the energy used by the trains comes from regenerative braking, or enough to power 25 subway stations, said Jordi Picas, who leads the project and is director of metro systems at TMB. In subway systems that don’t deploy regenerative braking, “there’s so much energy that’s not being used, and not only is it lost, it also generates heat that spreads inside the tunnels and increases the temperature,” he said. Since implementing regenerative braking, the temperature in Barcelona’s subway system has decreased by 1.8 degrees Fahrenheit.

Last year, transportation electrification surpassed renewable energy as the world’s largest category of energy transition investment, receiving about $634 billion globally. Although implementing MetroCHARGE has cost about $8.6 million (7.8 million euros), TMB expects to recoup that in four to five years through energy savings and revenue from the charging stations, where drivers pay about 33 cents per kilowatt-hour that flows into their cars. Metro systems worldwide already have the electric infrastructure needed to adopt this approach, but not all of them use trains outfitted with regenerative braking — and retrofitting it is expensive, Picas said. All of Barcelona’s trains have featured the technology since the 1980s. Given that a single train costs about $6.6 million and has an average lifespan of 35 to 45 years, it’s essential that transit operators include them in medium- and long-term planning, he said.


“red lines represent power flows during propulsion, whereas
green lines represent power flows in regenerative braking”

There are other challenges beyond cost, like finding optimal spots for the inverters and charging stations in a dense metropolitan area. “The hardest challenge was reaching an agreement with the various city halls to get access to the public space to set up the chargers,” said Picas. Other cities have expressed an interest in replicating MetroCHARGE, and TMB recently met a delegation from New Delhi. It has also shared information with officials from Vienna and an international consortium of 45 transit systems called The Community of Metros Benchmarking Group.


“Overall power delivered according to the operative modes over the entire route”

Cities like New York — which has the world’s fifth-largest metro system, with 472 stations and 665 miles of track — could see significant energy savings from regenerative braking due to the sheer scale of its subway network, said Ahmed Mohamed, director of graduate studies at the Department of Electrical Engineering at the City College of New York. In a 2018 study, Mohamed and his team found that the Metropolitan Transit Authority, or MTA, which runs New York’s subways, could cut its energy consumption by 35 percent if it adopted regenerative braking system-wide and used the electricity it generates to power trains and station amenities.

As of 2022, just half of the city’s trains use the technology, according to the New York State Energy Research and Development Authority, although any new trains are required to have it. However, “there’s not necessarily a strategic plan for how they can be used for energy saving,” Mohamed said, adding that the MTA, which would not make anyone available to comment, favors regenerative braking because it requires less maintenance than conventional friction brakes.

One of the main barriers to implementing regenerative braking is the lack of data outlining just how much energy, and money, might be saved with its adoption, Mohamed said. “When you’re not quite sure about the savings, it’s hard to run a cost-benefit analysis, so decision-making is not very easy,” he said. “It’s important to fund pilot projects to get the real numbers.” Learning from projects like MetroCHARGE could help other cities understand the benefits of regenerative braking, Mohamed said. Improved modeling and encouraging transit systems to share the details of their systems so others may learn from them would help, too. Another challenge is the number of stakeholders involved — including technology and electricity companies — makes determining things like who should pay for the project difficult. “Who’s running it? Who’s controlling it? And what’s the role of different stakeholders?” Mohamed said. He’d like to see the MTA take the lead in adopting this energy-saving technology so the trains rumbling into its stations might one day power cars on the busy New York city streets above.”

HYBRID SUBWAY TRAINS
https://spectrum.ieee.org/supercapacitor-enhanced-hybrid-storage-could-earn-cash-for-subways
https://extremetech.com/philadelphia-unveils-new-hybrid-subway-system-that-uses-prius-like-regenerative-breaking-to-feed-energy-back-into-the-grid
Philadelphia unveils new hybrid subway trains that use Prius-like regenerative braking to feed energy back into the grid
by Joel Hruska  / April 16, 2014

“The Southeast Pennsylvania Transit Authority is preparing to deploy a new hybrid energy system that combines regenerative braking, supercapacitors, and new lithium-ion batteries to provide power instantly to trains as well as the rest of Philadelphia. The regenerative braking applications are similar in concept to what you’ll find on the Toyota Prius — the idea is that brake energy from subway trains is fed back into a storage system rather than dissipating as simple friction. While it’s unusual to find this technology in the United States, it’s actually common in other parts of the world — the first trains with regenerative brakes were deployed in England in the early 1900s.The new Philadelphia program is unusual in two respects. First, braking trains don’t just recover energy for their own use — the motor-generator in each train channels its power down a third rail and into a centralized bank of batteries. Second, these batteries can sell power back to the larger energy grid. The entire system is designed to hold 1.5 – 2MWh of power, generated over an entire day. Unlike a gas-fired turbine, which takes several minutes to come to full power, banks of Li-on batteries can flip to full in about a second.


The lithium-ion batteries appear to be stored in a shipping container That’s important because under new Federal Energy Regulatory Commission guidelines, power providers who can supply energy quickly can charge a higher premium than slower services. While it’s only designed to provide power for minutes at a time to deal with peak demand, Philadelphia projects that the system will earn between $150,000 – $200,000 a year. For a city that’s struggling under massive budget cuts and decreased tax revenue, any municipal system that can turn a profit is a rare bright spot.So where do the supercapacitors fit? They’re being deployed as buffers between the trains and the batteries. Instead of directly charging the battery, the regenerative braking system will be modified to charge the supercapacitors first. A quick look at the difference between batteries and supercapacitors (also called ultracapacitors) should help explain why the two systems complement each other.

Supercapacitors have very low energy per unit weight (specific energy), but very high energy per unit volume (specific power). They can withstand up to a million cycles, compared to 500 full charge cycles or less for lithium, and they don’t have the associated heat or expansion problems caused by fast charge/discharge cycles on lithium-ion. In short, you can recover energy more efficiently from a hybrid system than you gain from using either technology alone. Research into new types of supercapacitors that would allow them to take over some of a battery’s duties is ongoing, but for now they occupy fairly distinct niches.

A story in IEEE Spectrum makes it clear that policy changes are a critical part of why the system makes sense — if the city of Philadelphia couldn’t earn a profit on the entire system, it wouldn’t have been cost-effective to implement. Still, this highlights the manner in which governments and markets can encourage the growth of more efficient power grids — the new rule, which took effect in 2011, was a regulatory change that allowed producers of fast power including flywheels and electric vehicles to complete effectively with ancillary power services from coal and natural gas that take minutes to ramp up. In other words, under the old rules, being able to deliver power within one second of increased customer demand was treated identically to being able to deliver power within five minutes of demand. Now, that’s no longer the case, and we’ve seen a commensurate surge in businesses and commercial enterprises adapting battery or flywheel storage technology for increased efficiency and on-demand power.”

PREVIOUSLY

FREE PUBLIC TRANSIT
https://spectrevision.net/2020/01/15/free-public-transit/
FIRST ELECTRIC CARS
https://spectrevision.net/2022/09/09/first-electric-cars/
SOLAR POWERED ROADS
https://spectrevision.net/2014/05/16/solar-powered-roads/

Leave a Reply