Two makers of energy-storage devices recently announced projects involving the capture of energy from train braking that otherwise would be lost as heat. Saft is working with several partners, including the Southeastern Pennsylvania Transportation Authority (SEPTA) on an “energy and optimization” project, while Maxwell Technologies is working with Bombardier Transportation.
The scope of the projects is different, as is the energy-storage technology employed.
The more expansive of the two projects is Saft’s, which involves integration of braking-derived energy into the local power grid. One of its partners in the project is Viridity Energy, a Philadelphia-based smart-grid technology firm that specializes in electric market integration.
“In a smart grid world, two-way digital information exchange opens up new horizons,” said Viridity CEO and President Audrey Zibelman. “This project truly showcases the potential of that smart-grid world, particularly as it applies to the transportation industry. By harnessing the regenerative braking power of the trains and empowering SEPTA to become a virtual power generator that can provide valuable and environmentally responsible service to the electric grid, we can fulfill the promise of interconnected systems on the grid and behind the meter responding dynamically to reliability and economic signals to strengthen the grid.”
Saft’s lithium-ion battery technology supplies megawatt-level energy storage for the project. It has provided the project with one of the first dual-purpose track-side energy-storage systems in the United States.
“As a fully integrated, containerized lithium-ion solution, the Saft system provides efficiency of greater than 95% and maximizes system availability, as well as helps to manage power flows,” said Thomas Alcide, President of Saft America.
Jim McDowall, Business Development Manager for Saft’s Energy Storage Business Unit, explained to SAE Off-Highway Engineering that the 95% efficiency cited by Alcide means that 95% of the energy that is received and stored can be delivered. He said brake energy is converted to electric energy on board the trains and delivered via the third rail to a Saft stationary energy-storage unit serving a half dozen substations on a certain stretch of the SEPTA system.
The containerized unit is 20 ft (6.1 m) long and rated at 1.5 MW and 420 kW·h. The basic building block of the unit is a 30 A·h high-power cylindrical cell using Saft’s nickel cobalt aluminum (NCA) electrochemistry—the same used in several passenger-vehicle applications including Mercedes-Benz and BMW hybrids, except in a larger form factor (for cars, cells of 6 A·h are used). Fourteen cells are packaged into a module, which is the smallest field-replaceable item in the unit, according to McDowall. The modules—29 of them—are packaged into one of 10 “strings.”
McDowall characterized the train application of Saft batteries to “a Toyota Prius on steroids.”
Saft makes several versions of its NCA cells. The “high power” nature of the cells in the SEPTA application allows energy to be stored and delivered more quickly than could be with “high energy” cells.
Because of limitations associated with funding for the project, the system is capable of capturing only 1.5 MW of energy—not enough to capture the full peak brake energy of a single train, let alone several in the event that multiple trains brake at the same time on the stretch of rail line covered by the project, McDowall said.
Still, he noted, even with that limitation and with train regenerative braking capability not yet fully built out, the recently initiated project is saving 9 MW·h per week.
The Maxwell Technologies’ energy-storage technology is the ultracapacitor, which discharges energy at a much faster rate than batteries and so is a good solution for trains using regenerative braking, Michael Everett, the company’s Chief Technology Officer, told SOHE.
Bombardier earlier this year announced that it had selected Maxwell to supply capacitors for the former’s EnerGstor wayside energy storage system. Maxwell also supplies product for onboard energy storage.
Each stationary "wayside" EnerGstor unit incorporates an ultracapacitor array that is capable of storing up to 2 kW•h of electrical energy generated by a rail vehicle's braking energy recuperation system. The EnerGstor system offers multiple benefits to rail system operators, according to Maxwell, including:
• 20-30% reduction in grid power consumption
• Improved regulation of line voltage across a multistop rail system
• Significant reduction in brake maintenance expenses
• Backup power to enable vehicles to reach a station in the event of a grid power failure.
Unlike batteries, which produce and store energy by means of a chemical reaction, ultracapacitors store energy in an electric field. This electrostatic energy storage mechanism enables ultracapacitors to charge and discharge in as little as fractions of a second, perform normally over a broad temperature range (-40 to +65° C [-40 to +149°F]), operate reliably through 1 million or more charge/discharge cycles, and resist shock and vibration, according to the company.
Everett noted that ultracapacitors have additional advantages over batteries in terms of cycle life (millions for ultracapacitors) and thermal management. They can’t compare with batteries in terms of energy density, however. Maxwell’s largest cell is about the size of a can of soda and contains 3.0 W·h of energy; by comparison, Everett said, the energy density of a Li-ion battery of that size is about 100 times that.