IEEE Spectrum July, 2011 - 53

are You
a HuMan DYnaMo?

mckIbIllo

cal 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.
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.
Although the Green Revolution decided not to build custom
equipment, that's exactly the approach another entrepreneur
in this field has taken. Mike Taggett attended the University
of Arizona, where he majored in Latin American studies. He
worked as a river guide and started a business making an
eyeglass retainer that he'd invented himself. At a trade show
sometime in the late 1980s, he had the idea of attracting visitors to his company's booth by bringing a converted exercise
machine that would generate electricity and power lights and
kitchen appliances. "We made smoothies," he recalls.
spectrum.ieee.org

If you're in good shape, you can generate
about 100 watts on a stationary bike.
What's that good for? It depends on how
long you can keep going. Below, see how
much time you'd need to pedal to power
each of these devices for an hour.

18 MinuTeS

30 MinuTeS

Fluorescent light

Laptop

1 Hour

10 HourS

Fan

Cof feemaker

14 HourS

40 HourS

Va c u u m c l e a n e r

Clothes dryer

Assumes electricity produced is stored in a battery. Wattages used: compact ﬂuorescent lightbulb,
30 watts; laptop, 50 W; fan, 100 W; coffeemaker, 1000 W; vacuum cleaner, 1400 W; electric clothes dryer,
4000 W. Sources: Energy Efficiency & Renewable Energy/U.S. Department of Energy; Wikipedia

july 2011 * iEEE SpEctrum * NA

http://spectrum.ieee.org

Table of Contents for the Digital Edition of IEEE Spectrum July, 2011