Metro explores new green energy options: placing a wind turbine in a subway tunnel

Photos: Evan Rosenberg/Metro

You’re standing on a subway station platform, waiting for the train. Suddenly, the wind picks up. You know this means the train is coming. Many of you may also know why there’s wind: it’s displaced air being pushed through the tunnel by the fast moving train. And some of you — including Tom Kefalas, Metro Environmental Compliance and Services Manager — may have wondered if there was a way all that generated wind could be utilized as a renewable energy source.

Thanks to Tom Kefalas and Cris Liban, Director of Metro Environmental Compliance Services, we now know the answer is yes. From August through September 2013, Metro conducted a one-month pilot program to see if wind energy could safely and effectively be captured and used. The project involved working with engineers from WWT Tunnel, LLC, a subcontractor to Arcadis U.S., to create and install a unique 10-foot multi-blade mass airflow collection equipment (MACE) in the Red Line tunnel. To our knowledge, this is the first time a transit agency has tested the effect of having a wind turbine in a subway tunnel.

The MACE was installed between the North Hollywood and Universal City stations, a segment of the tunnel that sees trains reaching speeds of up to 70 mph. Each time a train left the station, the MACE fan blades would start spinning, thus capturing energy up to a minute before the train actually passed by. The blades would continue to spin up to 2 minutes after the train passed, and exceeded 1,070 revolutions per minute (RPM). The amount of electricity produced by these train initiated events was nearly double the amount that had originally been anticipated.

During the run of the pilot program, the MACE generated an average of 77.7 kilowatt hours (kWh) per day. It is projected that the 10-foot MACE section could generate more than 28,000 kWh per year–enough to power about 12 homes in California for one year, or turn out approximately $6,000 per year in electrical production.

In addition, a single 10-foot MACE unit could potentially help Metro avoid using electricity that would have generated more than 17 tons of CO2 emissions annually if created from natural gas, or more than 30 tons of CO2 emissions annually if created by a coal-powered plant.

This is great news for Metro as the agency strives to become ever greener. The MACE can safely and efficiently collect clean electrical power from wind that is already being produced by passing trains. Due to the regularity of the speed and schedule of Metro trains, the power generated is far more reliable than above-ground wind and solar power, and the electrical power generated can be used in various ways. It can be stored for use to avoid power spikes in usage during flex alerts or high demand periods. In AC format, the power could also be stored and used to provide power for electric vehicle charging stations, station and tunnel lights, escalators and more. And because the project is entirely underground, no CEQA environmental clearance process was required.

Since the end of the pilot program, Metro staff has been analyzing the data captured. Over the next year, Metro staff will follow up with a report evaluating the best use of MACE generated power and the feasibility of installing MACE into existing and new rail line tunnels.

All current trademarks and patents are currently held by MACE Energy Inc.

28 replies

  1. Of course, the $64 question is whether we’re really just harvesting energy that is otherwise wasted on blowing loose items around, or whether we’re increasing the aerodynamic drag on the trains.

  2. This is great — but why isn’t there cell service in the tunnels like Chicago, NYC, SF, or Boston…? What if someone needs to dial 911 (or just play Candy Crush)?

  3. It’s clear that it is generating power. The question is the cost of the MACE unit and their maintenance and the cost of power being produced. This test area will produce the most energy due to the higher speed of the trains and the length between stations. It would be interesting to use the Gold Line as the next test site under East First Street. The MACE unit as tested produced more power than anticipated, what would a refined unit produce? Could rail become self sufficient? I’m not sure about light rail but the Red Line already reclaims some of it’s power due to up hill/down hill configuration of the system. Going down hill the brakes become generators of power. This is an example of how in house innovation can produce great results for the MTA. Both the Pacific Electric and Los Angeles Railway, predecessors private companies, were leaders in the industry. From rebuilding streetcars, including lengthening them, to L. A . Railways building an entire fleet in house. In house expertise was allowed to grow old and disappear. It’s great that perhaps the MTA will reclaim it’s success from the past.

  4. This is an ingenious and creative way to generate renewable energy. Go Metro! Is this something that Metro created and could potential sell or license to other subway systems around the world? Is it possible to place the ten-foot MACE generators all along the entire Red/Purple Line route (minus the stations)? Even at just ten miles (52,800 feet) could generate approximately $31M per year in electrical production or enough to power 63k homes. That is not insignificant at all. How much does it cost to build a ten-foot segment?

    • Hi Craig,

      Have passed on your questions to the Environmental and Maintenance Services staff, hope to have an answer for you soon.

      EDIT: And the response is: Metro is not at the stage yet to be able to consider licensing the MACE. That answer may be included in later reports down the line. The cost to build/install a MACE would be significantly reduced going forward as Metro staff learns from this initial process. Thanks!

      Anna Chen
      Writer, The Source

  5. Hi Anna Chen,

    This is a really exciting pilot project!

    Craig raised some interesting questions, thanks for checking with E&MS.
    I’d also love to get a question answered: Given the size of the tunnels and other space considerations, how many rows of fan arrays can we reasonably pack into each subway tunnel?

    I’m imagining one or two sets below the tracks, and one each on the upper right and upper left of the subway tunnel ceiling.

    • Hi Dayle,

      I will pass that question along as well, but I’m fairly sure that question will not be answered until later on–it will likely be part of their final report. I can let you know that safety comes first, so chances are unlikely we’ll have multiple MACE set up in one tunnel segment. I’m sure there would also be additional research needed on how having multiple MACE units in close proximity would affect their effectiveness!

      Anna Chen
      Writer, The Source

  6. I agree with Craig. You have to show all the variables and cost is a huge one. How much is this going to cost? You can’t blinding jump into a project to go “Green” without looking at how much it’s going to cost tax payers. Also what about upkeep, maintenance, etc.

  7. How about installing wind turbines between Hollywood/Highland and Universal City? (lots of them for both directions) Will these turbines be installed on the roofs of the transit stations in the middle of the freeways? That’s a lot of wind over there.

    • Hi Ivan,

      That is also another location being studied. However, no turbines will be installed above ground – the project covers underground locations only.


      Anna Chen
      Writer, The Source

  8. Regarding the question from “mike dunn,”
    > Could rail become self sufficient?
    Well, certainly not on power harvested from tunnel wind alone. That would make it a perpetual motion machine of the first kind, in direct violation of the laws of thermodynamics.

    The aforementioned $64 question remains:
    Does the energy harvested from the tunnel wind exceed the energy lost to additional aerodynamic drag on the trains, caused by the tunnel wind harvesting apparatus itself? And that can only be answered by very careful measurements under controlled conditions.

    Certainly the harvesting of waste energy makes sense, where it actually works. We harvest waste heat from our automobile engines whenever we open the heater-core valve. Multi-story buildings harvest the waste heat from the lower floors to help heat the upper floors. And of course, fully-regenerative dynamic braking has already been mentioned (and something like it also happens on cable cars, whether of the funicular variety [e.g., Angels Flight], or the kind Andrew Hallidie invented in San Francisco: in both cases, the descending cars help pull the ascending ones).

    But on the other hand, consider the opposite extreme taken ad absurdum: sure, you could stick a model airplane propeller mounted on a generator shaft out your car window, but the laws of thermodynamics tell us that you could never hope to generate enough electricity to make up for what you’re losing in additional aerodynamic drag.

  9. Can you address James’ question about whether this is only increasing drag on the trains? As an engineer, I would think this would only be harvesting extra power the trains need to overcome drag.

    • Hi Em,

      That’s a question Metro staff will work on answering as they continue to study the date they’ve collected. Preliminary reports indicate the MACE does not increase significant drag on trains heading through the tunnel due to its position, which is something that will be taken into consideration if the MACE is used in other rail tunnels.

      Anna Chen
      Writer, The Source

  10. Metro has the money to spend for this, but it will not spend money like adding in retail space that will create jobs, help the economy, keep the stations safer, bring in more sales tax revenue and create extra additional revenue in form of rental income to Metro.


  11. I assume there’s a reason they cannot be on top of the rail cars themselves – likely due to clearance/space issues…. but if that *was* possible, it would be great. Could power the train’s lights/Air circulation/etc. Generate power directly onto the rail car.

  12. […] San Francisco’s Metro is figuring out how to harness wind power from subway trains as they fly through tunnels and use it for electric car charging stations, lights in the stations, tunnels, and escalators and more. (The Source) […]

  13. Perhaps a MACE unit could be installed in the U.S. Senate chamber, lots of hot air being circulated there

  14. CEQA exempt? In that case, issue a press release proclaiming plans to seek recognition in the Guinness Book of World Records for the largest MACE installation … to be constructed in Beverly Hills.

    (April Fool!)

  15. The idea that the units do not add drag because of their location is completely invalid, because if they were in a location that did not add to drag then they would not spin the turbines. The locomotives on the LA Metro rail make over 5000 hp from what I can find. The amount of electricity this unit generates (77.7 kWh/day) is equivalent to 3kW or 4 hp. That is less than 0.1% of the engine’s output. That drag addition would be immeasurable I would think, but it is there. You’re wasting lots of money on this technology. You are essentially using the trains to generate electricity VERY, VERY inefficiently and are BY NO MEANS green. If this technology gets scaled up it will cause problems for the rail cars as well as cost you lots and lots of wasted money. You will also be completely ridiculed.

    Seriously, if you would like some consulting I am a professor at a University in Los Angeles and I can consult with you and save you lots of time and effort. Feel free to have someone e-mail me.

  16. Also, this unit would not power 12 homes. It would maybe power 4. The numbers you used to calculate with were per capita, not per household. That line is a complete lie.

  17. Will Metro be able to install MACE in the Purple Line while it is being built? Will the Purple line be able to travel 70 MPH along any part of the line? What is the cost vs the benefit of $6,000?

    • Hi Warren,

      Metro staff is looking into the possibility of including the MACE in the Purple Line Extension and will be able to answer that more definitively in their final report.


      Anna Chen
      Writer, The Source

  18. The wind you are harvesting here is caused by the fact that a train moving through a tight tunnel is highly unaerodynamic. The train car’s tight fit to the tunnel forces air to move in front of the the train rather than passing around it. This increases the energy required to drive the train forward.

    Has Metro (or any other transit agency) looked at ways to reduce the drag on cars through passive design features in the tunnels that would allow them to pass more efficiently through the air? Perhaps bypass air vents that allow the air trapped in front of a moving train to pass around behind it more smoothly. This would reduce the initial energy usage instead of just trying to recapture some percentage of it after the fact.

    That all said, it is exciting to see Metro think creatively about increasing efficiency. If a final version of the device could be developed that is low cost it could be a great retro fit for huge numbers of similar tunnels aroune the world.

  19. Em I is correct. This is a foolish waste of money. But I must give KUDOs to the turbine salesperson for rewriting the Law of Conservation of Energy! As far as publishing an article on this, well… it was very nicely written.