Utility companies have long known how to capture the braking energy from electric trains, but they've usually found it difficult to get any large-scale benefits from such projects -- until now.
An energy storage project in Philadelphia promises to capture braking energy and provide a voltage boost for electric trains, while offering so-called "frequency regulation" for utilities. The companies behind the project hope that it will reduce the need for fossil fuel generation, thereby creating economic and environmental benefits. "The central aspect of the Philadelphia system is that it doesn't just do regenerative braking and power-assist," Jim McDowall, business development manager for Saft, told us. "It helps with the demand side of the equation."
The new system could help utilities use regenerative braking power more intelligently. In the past, regenerative braking energy from train motors was typically pushed back into the electric train's third rail, raising the voltage. When the voltage rose too high, however, excess energy was diverted through coils and dissipated as heat.
Saft's Max20 lithium-ion energy storage system can push power back into the third rail when system voltage drops. (Source: Saft)
The Recycled Energy and Optimization project takes a different approach. Using a dc/dc converter from Envitech Energy, it senses when track voltage is too high (about 800V). It then sucks DC power out of the third rail and pushes it into a giant lithium-ion battery pack provided by Saft, until it's needed. "When the third rail's voltage drops too low, they can bring the power out of the battery and then put it back in the third rail," McDowall said.
Saft's batteries are located in a substation that serves five or six stations on the Southeastern Pennsylvania Transportation Authority's (SEPTA's) electric elevated train line. Using lithium-ion batteries with a well-known nickel-cobalt-aluminum (NCA) chemistry, the substation offers about 1.5MW of charge and discharge capability. Saft employed a high-power (rather than high-energy) battery configuration, largely because big electric trains on the line can capture 3MW during a 15-second period.
The company's Max20 lithium-ion energy storage system is roughly equivalent in size to 280 Toyota Prius battery packs. It employs 290 modules, each with 14 cells rated at 30 A-h.
The big potential benefit of the new system, however, goes beyond its ability to capture braking energy. By employing software from Viridity Energy and piggybacking a frequency regulation signal atop the power flow information, the system can modulate the loads at the substation in response to signals from the grid operator. As a result, it can change the amount of power that gets fed into the substation, and therefore eliminate the need to dissipate excess power as heat. "So a megawatt of storage might displace a megawatt of fossil fuel generation," McDowall said. "That means less cost for the market and less cost for society."
Engineers who are part of the pilot project believe that strategy could provide a return on investment that wouldn't otherwise be available by simply capturing braking energy and pushing it back into the third rail. "If we can prove the value proposition, then other cities will look at this for their substations, as well," McDowall said. "This is a very high-visibility project."
i worked in an open pit copper mine and the trucks we used had a gross wt 0f 300 tons loaded uphill and 150 tons empty yet their diesel-electric system included 100% retarding i.e. electric to resistance braking which was effective up to 40mph downhill to a 2-5mph stop. then you'd use the hydraulic brake. pretty cool eh ?
beautiful, jerry ! i love the ge loco part with the molten salt battery. per a previous comment i've heard of stationary batteries deployed in japan, presidio, tx and now philly.
Good point, Beth. It would be good to know where the funding is coming from and whether there is a reasonable return on investment with this technology.
Certainly transit systems, Metro's in particular, are huge power users. The shorter the vehicle headways, the greater the potential benefit & ROI, methinks. Couple this with more electrically efficient aluminum/stainless steel 3rd rail (compared to the steel rail still in use on many systems) and you could reap some significant benefits.
There may also be an opportunity for this technology with ship-to-shore cranes or intermodal facility cranes where high duty cycle, repetitive lift & drop motions of heavy containers would make good use of energy storage & retrieval. Slab handling or ladle cranes in a steel mill may also be good candidates for this technology.
The amount of energy turned into heat in a standard braking system is very large. Most regenerative systems that store it in batteries are unable to recover very much of it because the battery can only accept so much charge. How much energy? consider that a freight train may spend ten minutes getting up to speed, and yet do a fast stop in thirty seconds or less. Think about a passenger car, possibly ten seconds for zero to sixty MPH, but in a panic stop, sixty to zero in much less time. The limitation is always in the energy conversion process, it appears. Asking an inverter that delivers up to 10KW for an acceleration to convert 50KW back into electrical power is asking for component failure unless the system is built for much more than the driving loads would ever be.
Forcing power back into the grid is a similar situation, in that each element is only sized for driving power peaks which are usually much smaller.
Railroads have used regenerative braking with energy recovery for years. The Northern Pacific used it going over the Rockies: the downhill train was pushing energy into the overhead wire which was used by a train coming uphill. The high cost of maintaining the overhead wire was the eventual demise of this operation. Other locations tried this but found the ROI did not justify the expense. Currently, I don't recall any major railroads or shortline freight railroads that use electric power as their principle energy source.
In the Northeast, the electric trains are for passenger operations. These are either 600 Vdc for subways and other third rail operations or about 11,000 Vac, 25 Hz, for the overhead wires. In the subways, the grades as slight and the trains relatively light. Trains are frequent. This may help with using regerative braking with energy recovery. The problem is that many of the old subway cars are not designed and built to provide regenerative operation. With the units operating on 25 Hz. power, the trains are more widely distributed. For these trains, regenerative braking with energy recovery must have either energy storage for the recovered energy or needs the converter stations to be able to change the 25 Hz. energy to 60 Hz. Most of the electric locomotives use dc motors and do not have inverters for regenerative operation. The point this leads to is that the locomotives and/or infrastructure will need some substantial modifications to use energy recovery systems.
One last point, many freight railroads do use regenerative braking without energy recovery. It is dynamic braking where the recovered energy is converted to heat and dissipated into the air.
I have a friend who works with heater strips that place them on rails to keep ice from causing wheels to spin in train stations. I could see this little bit of accumlated energy created by braking being used to power this system at the stations. Or maybe use this energy to keep the lights on and heat/AC going in the cars while sitting in train stations rather than using the engine power. The list goes on and on and the needs are everywhere!
How about piezoelectric generators under the tracks for when these hugh masses move over them? It probably doesn't meet the ROI, but like capturing energy from the sun, waves, water, and oil, it starts somewhere!
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