Batteries on tracks could power trains and save Metro money

Metro trains can sometimes generate energy when they brake, but right Metro doesn’t have a way to capture and use it. A recent WMATA study says the agency would need to buy less power from Pepco and Dominion if it installed battery systems along its rails.

In 2010, WMATA staff presented the idea for an “energy storage demonstration project” to the agency’s board of directors. Metro spends nearly $50 million dollars per year on power to make trains move, so any savings could be substantial. The study was performed and tested around 2013, but the FTA only recently published the results.

A brief background on train braking

Metro trains have two types of braking systems: dynamic and friction brakes. Dynamic braking on a train car is similar to what hybrid cars do when the driver steps on the brake: A motor turns when the brake pedal is pushed, and essentially acts as the brake to slow the car down while also generating an electrical current. That current gets fed back to the car’s battery, which lets you drive farther.

Dynamic braking is the focus of Metro’s demonstration project. When it’s used, the energy generated gets “dumped” back into the third rail, which is what transmits power for trains up and down the tracks. Usually, that energy put back into the system is lost and rendered useless, except when another train nearby is accelerating at the same time.

This is where batteries come in. Instead of wasting this energy, the testing system uses it to charge a large bank of batteries nearby. This energy can then be stored for a later time for when other trains are passing through and need to accelerate.

If Metro could save a significant amount of money with the battery system, then the agency could be able to pay less each year for electricity and put its money to better use elsewhere.

Metro tested how large-scale batteries might integrate with the train systems

In order to test and see if batteries would work, Metro bought and installed a Battery Power System (BPS) from Kawasaki. The system, made up of over 550 smaller battery cells, is rated at 2 megawatts, or 378 kilowatt-hours. This isn’t incredibly huge — the larger Metro power substations are rated up to nearly 10 megawatts — but it’s large enough to get some results, which is what the study aimed to do.

The system was activated and installed in June 2013 outside of the West Falls Church station. The left and right-most rows of equipment shown here include all the battery cells making up the overall unit and the center row has extra equipment needed (circuit breakers, cable terminations, etc.).

The battery system was tested in a few different scenarios

To test a few different real-world configurations, the battery system was tested with and without a 6 megawatt transformer (Scenario A), a 3 megawatt one (Scenario B), and none at all (Scenario C). Scenario C represented a case where the battery system is installed at a Tie Breaker Station, which are typically located between the numerous transformer substations along the railroad. The TBS’ are used to help balance how much power is available to all the nearby tracks.

Testing in each scenario consisted of running trains near where the battery system was installed, and measuring how much power was saved by having the batteries active. The scenario’s success was measured by comparing how much power was consumed with the battery system installed, versus how much was consumed without it. The three scenarios tested are shown on the left, and the amount of power used is graphed.

The chart below shows a modified version of the one above; that is, the power saved by having the battery system installed is what is graphed here (to calculate this, subtract the “Peak power with BPS” bar above from the “Peak power without BPS one”).

As shown in the two graphs, the most power was saved in Scenario C, followed by Scenario A and then Scenario B. Scenario C’s results make the most sense: a TBS is located between power substations, meaning any sort of power draw at that location would be the most “expensive” since it’s furthest from a power substation. Also the fact that the TBS doesn’t produce its own power means that any saved by the battery system would be the first used when a train needs some, which helps increase the amount of power “saved.”

When applying the power-saved numbers to moving trains, the impacts found in the study start coming into focus. The 2MW BPS could reportedly move 25 empty or 19 fully-loaded trains 2800 feet - a little over half a mile. The same BPS could move 17 empty and 13 fully-loaded trains 4000 feet (75 percent of a mile). If the BPS is fully-charged, then all of this power could be coming from what you’ve reused. This starts adding up for some serious savings.

If WMATA was able to save $888,289 over 10 years in the first scenario and $1,863,229 scenario C as the study claimed, what gives? Basically, if the cost of installing and maintaining the battery system is lower than what the agency could save over the life of the system, they come out ahead and are able to use that saved money in other ways for other things that need to get done.

Possible Benefits

The FTA gave WMATA a $300,000 grant to study this one method of capturing and storing the energy created by trains, and to figure out what the savings and benefits of such a system could be.

The installation of one of these Battery Power Systems can provide a number of benefits with seemingly few if any downsides. One of the big possibilities it provides is to act as an emergency power source (like a UPS) if there’s a commercial power outage. If the commercial power feed goes down in the area, there wouldn’t be nearly as much a worry of stranding trains, and passengers, on the tracks. The batteries could keep the trains moving until they reached a station where passengers could get off safely. Currently if power were to fail, a train in the middle of a tunnel would be out of luck and couldn’t move.

With the study complete, WMATA doesn’t have any public plans to expand the usage of batteries to supplement the power system. But the results of this study suggest that the battery system could save WMATA some money, which would be beneficial in today’s financial environment.