photo showing wattage display
Photos: Stan Honda/AFP/Getty Images
Don't Stop:Retrofitted by a company called the Green Revolution, exercise bikes at a New York Sports Clubs gym generate electricity; a display shows the biker's wattage.

Walk into a spinning class at the New York Sports Clubs' facility on Eighth Avenue and West 23rd Street in Manhattan and you'll find 20 sweaty people furiously pedaling their stationary bikes. Look closely and you'll notice something unusual about this workout: Each of the bikes is attached to a black box with wires running out of it. The box is a compact generator that converts the motion of the wheels into electricity, which is then fed into the power grid, offsetting some of the club's energy use. For these gym-goers, it's not just about their cardio fitness; their sweat is helping to make the planet a bit greener.

By adopting power-producing exercise machines in this way, gyms can promote themselves as environmentally friendly and also reduce their electric bills. At least three start-ups in the United States are now selling equipment to retrofit aerobic machines—stationary bicycles, elliptical trainers, and steppers—into electricity-generating gear. These companies have already converted several hundred machines at dozens of U.S. health clubs and university gyms.

The reality, though, is that this technology faces major hurdles before it can go mainstream. For one thing, the economics aren't very enticing. The energy output from a single exercise machine is quite small: Unless you're Lance Armstrong, you might be able to power a ceiling fan while spinning a stationary bike, but not much more. So a gym might have to wait decades to recover the money it spent converting its exercise machines to generate electricity. What's more, the energy output of these machines is so low that the environmental benefits they provide are scant. So don't expect that fitness enthusiasts pedaling stationary bikes are going to free the United States from its addiction to fossil fuels.

Backers of the technology respond by comparing the current cost of these machines with that of technologies like compact fluorescent bulbs or solar and wind power, which many people doubted would ever take off. They claim it's only a matter of time until every exercise machine comes equipped with a generator. And with some 30 000 gyms in the United States, that would mean millions of machines—and many more in people's homes—whose combined energy would then be appreciable.

“Stationary bikes create resistance, and through this friction, heat is produced,” says Jay Whelan, cofounder of the Green Revolution, in Ridgefield, Conn., the company that converted the spinning bikes at the NYSC in Manhattan. “The industrial engineer in me said, 'What a waste! There's got to be a way to capture and use this energy.' ”

stationary bicycle illustration
Illustration: MCKIBILLO

Although the basic idea of attaching a generator to exercise equipment is many decades old, the press started lavishing attention on this concept four years ago, after a Hong Kong gym called California Fitness rigged 18 exercise machines to charge a battery and power fluorescent lights. Since then, three companies in the United States have been working hard to market the technology, each taking a slightly different approach.

In 2007, Hudson Harr, then a 21-year-old graduate of the University of Florida with degrees in electrical and mechanical engineering, spent all the money he had amassing a collection of used elliptical machines and electrical parts. Transforming his mother's house into his laboratory, he began tearing the equipment apart. What he found is that some elliptical models already had small DC generators inside. These power the monitoring console and also serve to increase the amount of resistance the user feels when exercising. That's because the current generated creates a magnetic force that opposes the motion that creates this current. By adjusting the amount of current created, the user can vary the resistance he or she feels.

graphic link to are you a human dynamo

graphic link to people power

Normally, the generator uses a bank of resistors to dissipate the energy it produces. Harr figured that he could get rid of the resistors and put that power to work. “Essentially, we remove the internal resistance the machine has and give it an external load, which is our equipment,” he says. Harr's strategy is to wire each elliptical machine to a central unit containing an inverter that converts the DC power generated to AC. The inverter in turn connects to the building's electrical system and ultimately feeds the grid.

Harr's lab is no longer taking up space at his mother's house. His company, ReRev, based in Clearwater, Fla., has moved into a 1400-square-meter production facility and now employs 15 people. ReRev has wired up 150 machines at more than a dozen gyms. It has installed systems at many colleges, including Drexel University, James Madison University, Oregon State University, Texas State University, and the University of Florida.

Meanwhile, the Green Revolution, the company Jay Whelan and Mark Sternberg founded in 2008, went down a different road. Instead of elliptical trainers, the two entrepreneurs focused on exercise bicycles. They started by taking an ordinary bike and propping its back wheel up on a triangular frame. Then they attached a car alternator to the wheel and hacked it to boost the amount of power it could generate, which also raised the resistance to motion it provided.

Their initial idea was to design and build entirely new exercise bikes with generators attached to them. But the owners of exercise clubs didn't want to buy all new equipment, so the two entrepreneurs decided to build a power-producing modification instead. Their module attaches directly to the bikes, feeding electricity to two 12-volt batteries wired in series. When a user starts pedaling, the batteries charge, and when they're full, the inverter kicks in and sends power to the grid, converting 24-V DC to 110-V AC.

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