While it is feasible to “harvest waste energy,” says Addison Killean Stark, Ph.D. candidate in the Department of Mechanical Engineering, the real question is “whether it even makes sense to go to human power,” given the cost of producing energy this way.
Stark figures that a robust workout on an elliptical trainer or treadmill that is connected to a device that can convert that output into electricity might deliver 10 Calories per minute, which translates to 700 watts—or the power consumption of seven good light bulbs. While pumping away, an athlete could brightly illuminate a room. A dedicated athlete who commits to an hour on the machine every single day could produce 255 kilowatt hours per year—not a negligible number when the average single-family U.S. home consumes north of 600 kilowatt hours per month.
But, cautions Stark, before pulling on the training togs and chasing what appear to be serious energy savings, remember to factor in the costs of home-brewed electric production. Decent exercise machines go for a $1000 or more, and the conversion equipment runs extra. This method of producing electricity works out to around 65 cents per kilowatt hour, far more than the eight to 15 cents per kilowatt hour charged by your local electric company. Even a coalition of the willing at a local gym, running round the clock, couldn’t lower costs much beyond 10 cents per kilowatt hour. Stark figures that his own typical home energy consumption would require him to work out for at least four hours daily. “I’d rather buy electricity from the grid at one tenth the price than buy the equipment and exercise so much, although I’m sure I’d look great.”
Battery packs for energy storage and integration into the electrical system of home or grid would raise prices even further. Also, don’t forget that people convert chemical energy (food), into mechanical energy, in order to perform energy-producing work on exercise machines. “My rough estimate of the cost of electricity doesn’t include the cost of food people eat—whether a kid who eats ramen all day, or the guy who eats only foie gras,” Stark says. So even if you assume that you are “utilizing a waste energy—that people want to burn those calories anyway,” the costs of generating exercise-machine electricity outweigh the costs of conventionally produced energy—at least until carbon commands much higher prices.
For Stark, who studies biomass energy conversion, the ultimate barrier to utilizing exercise machine for electricity involves efficiency. This former president of the MIT Energy Club suggests that since several types of energy conversion are required to enable exercise machine-based electricity (solar energy to chemical energy, chemical energy to mechanical, then mechanical to electrical), it might “make the most sense just to create a solar cell in the first place.”—Leda Zimmerman
Thanks to Peter De La Cruz of New York City for this question.