SEP 08

When a subway train pulls into a station, it produces two things:  a loud screeching sound and lots of kinetic energy.  The Philadelphia subway is putting that second thing to good use by capturing the kinetic energy produced when trains put on the brakes.

A 1.5-MW regenerative braking system will be installed along the Market-Frankford line, which has the highest ridership in the city.  A huge battery will capture the kinetic energy that will then be used by trains accelerating out of the stations, stored for future use or fed to the grid.  The Southeast Pennsylvania Transportation Authority will have the option to either use the power produced or sell it to the local utility.

The trains already use regenerative braking on a smaller scale but half of the energy is lost as heat.  This pilot program is aiming to dramatically increase the power captured.

The project should be completed by next spring and could save the transit authority $500,000 in energy costs.  If all Philadelphia stations were outfitted with the system, energy consumption could be cut by 40 percent.

via Wired Autopia

Comments (6)

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40% energy savings seems high.
written by Alessandro Machi, September 08, 2010

40% energy savings seems high to me, unless we are talking that the trains brake every mile or so??? If the regenerative system adds weight to the trains, that has to be factored in as well.

Why about the tailwinds that get generated, do those end up pushing the train along, or could some type of wind generation system be set up as well?

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What if wind stopped the train?
written by Alessandro Machi, September 08, 2010

So, what about a vacuum system that allowed air to stop the train? As the pressure built up it would spin turbines that would generate power.

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3 year payback
written by Matt, September 08, 2010

Cost $1.5m
Saving in NRG per year $0.5m
Assuming constant NRG cost and no interest on loan then a 3 year payback. Not bad, and if they do the buy cheap night time power (see full story) and use it during the day; payback is faster.

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What is the cost of this system?
written by Mike, September 09, 2010

Matt: 1.5 MW means 1.5 megawatt, not $1.5 million I believe. But you do bring up a very important question that was not addressed in the article, how much did this cost to install and maintain. If we knew it was cost effective, we would have evidence to push transit officials everywhere to implement this system. What culd be better, clean energy that is cost effective.

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...
written by Nik, September 16, 2010

I've ridden the El, what we natives call the Market Frankford Line, and it does stop pretty frequently. Downtown it's every few blocks.

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Kinetic Energy
written by pj, October 29, 2010

Not wanting to be too pedantic, but given this is a self styled eco-"geek" site, I felt it worth mentioning that kinetic energy is not produced when brakes are applied. The kinetic energy is the energy embodied in the motion, and it is built up as the train accelerates. When it comes into the station and the brakes are applied, the existing kinetic energy is converted to thermal energy if old fashioned brakes are used, or converted to electricity if regenerative braking is used.

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