Moving trains carry a significant quantity of energy which is often lost when they slow down and stop at stations.
Researchers have found a way for trains to capture this valuable energy in banks of batteries and sell it to the local grid operator.
Trains help capture and store energy
The Philadelphia subway has caught onto this and is expected to capture even more energy this year and earn additional income from grid operators as it plans to upgrade the system with a hybrid of both lithium ion batteries and supercapacitors.
The Southeastern Pennsylvania Transit Authority (SEPTA) stores energy produced by braking railway cars. This is similar to the way in which a hybrid car charges its battery when slowing down. The spinning wheels turn a motor-generator to charge a bank of batteries via a third rail system. The battery is located at the Lettery substation, which powers a portion of the Market-Frankford Line in Philadelphia. The upgrade will be installed on the same line approximately 5 kilometers away.
The process of capturing energy from braking railway cars, which can reduce energy use by to 30%, is relatively new technology, according to Jacques Poulin, director of energy storage for public transportation at ABB, which is the systems integrator on the SEPTA project. However, energy storage is still new to authorities.
Battery helps balance the grid
The Philadelphia project is unique in that the battery also provides balancing services to the local electricity grid.
"We connect the energy storage to the frequency regulation market locally in order to use that asset 24 hours a day and make money for the train company over and above recovering the braking energy of the train," explains Mr Poulin.
Grid operators need to maintain a balance between power supply and power demand to ensure the grid's frequency remains within a narrow band.
Normally, natural gas power plants supply power into the grid to match demand and maintain the right frequency. However, batteries are beginning to fulfill this role and they are able to provide bursts of power very quickly.
In the case of SEPTA, the current lithium-ion battery can generate 1.5 MW of power within one second, while a thermal power plant can take a few minutes.
The local grid operator PJM has implemented a new Federal Energy Regulatory Commission order that pays more to assets that can provide fast frequency regulation services. That makes the economics of the system compelling: SEPTA projects it will earn between US$150 000 and US$200 000 annually from its battery.
Using software from Viridity Energy, the battery will receive signals from PJM and either supply or absorb power for a few minutes at a time.
Supercapacitors will help capture more energy
The addition of the supercapacitors (or, ultracapacitors), which store charge electrostatically rather than chemically, means the trains will be able to capture more energy during braking.
Supercapacitors can rapidly store or discharge power, but are less energy-dense than batteries. This is what makes supercapacitors well suited for capturing the energy of a braking train or hybrid bus, explains John Meaney, an account executive at Maxwell Technologies, which is supplying supercapacitors to the SEPTA project. Supercapacitors will also reduce the strain on the battery and extend its life.
The existing energy storage system, which has been in place since 2012, can capture between 1.5 megawatt-hours and two megawatt-hours a day. "That amount of energy is equivalent to a solar system that would cost at least twice as much as the cost of our system," notes Mr Poulin. He adds that if several energy storage units are put in place, they could provide backup power as well.
The earning potential of the project is receiving a great deal of attention from other municipalities. While the project can be replicated in other areas, it is essential to have a grid operator which is very progressive in adopting new rules for frequency regulation.