Did Metro build a perpetual motion machine?

Metro’s turbine installation. Courtesy of LA Metro

As Deputy Executive Officer for Environmental Compliance Services for Metro, Cris Liban’s job is to find ways to save energy in the subway. “My staff and I were standing one day on the platform and somebody lost his toupee,” he explained. “We realized the wind in the subway is strong. Maybe we can harness it.”

His colleagues decided to test a wind turbine—a device that turns wind into electricity in the tunnel to see if it could power lights, escalators, and perhaps electric vehicle recharging stations. A ten-foot prototype was installed last summer. The construction and installation cost $600,000.

Metro said the unit grossed enough power to run about 12 houses for a year.

But not everyone likes the idea. Kim Aaron, who has a PhD in Aerodynamics at the California Institute of Technology, and now designs spacecraft with the Jet Propulsion Laboratory says there are some problems with the physics. “Clearly it’s air that’s driven by the train. It’s not as if the tunnel system has natural wind blowing through,” he said. “What they apparently don’t understand is that installing a wind turbine puts back pressure against that air which ultimately pushes back against the front of the train…which slows the train down or requires you to step a little bit more on the gas, so to speak.”

In fact, he said Metro’s turbine is essentially a perpetual motion machine: that’s an historic idea for a machine—a physics scam is another way to describe it—that supposedly runs without energy or seems to generate surplus power, but is really just robbing energy from someplace else. Physics professors use perpetual motion machines to teach a basic principle: mainly, that energy can’t be spontaneously created. It always comes from somewhere.

Emilio Graff is Manager of the Lucas Wind Tunnel at Caltech. He agreed the turbine must cause trains to consume more electricity. However, he thought it was at least theoretically possible to recoup some wind energy that would otherwise be lost as heat. Fathi Tarada, founder and managing director of Mosen Ltd., a British company that specializes in studying the behavior of air in train tunnels, said it’s possible to find places where the back-pressure on the train would be minimized. But, he explained, the best places to generate power from the turbines are exactly where they put the most drag on the trains.

Either way, according to the aerodynamic experts, the turbine is generating such a small amount of power relative to the train that the back pressure is too small for subway drivers to notice. And, unfortunately, Metro only looked at the turbine’s gross output. “We didn’t measure any on the train side,” said Metro’s Liban, conceding that Aaron could be correct that the turbine wastes more energy than it produces.

A Metro representative said that back-pressure on the trains is something it will study moving forward.