Over the years I’ve seen numerous claims about devices that generate electricity from human power. The other day I saw that CSIRO, Australia’s national science agency, produced a power generating “Infinity Swing.” And recently, one of my non-engineer friends (yes, engineers have non-engineer friends) sent me a link to an article entitled, “60 Minutes on this Bicycle Can Power Your Home for 24 Hours!” So I thought I’d take a few minutes to explain the physics - and of course, the math - of generating electricity from human power. I’m going to use best case scenarios and assume perfect conversion efficiency for two reasons. First, these are ballpark estimates; exact values vary from one person to the next. (I’ll tell you the second reason later.)
Infinity Swing: Image courtesy of CSIRO
The Infinity Swing demonstrates one way to convert human power into electricity: a person on a swing whose movement causes a generator to spin. The swing wasn’t designed to produce a significant amount of electricity, but to show how a lot of human exertion translates into such a small amount of electrical power. How much electricity can it produce?
The primary force on a swing is the force of gravity pulling down on the weight, but gravity always pulls straight down, so the force on the chain equals the weight only when the swing is at its bottom; the rest of the time it’s somewhat less. The force on the chain depends on the angle of the swing, and since there are an infinite number of infinitesimally small locations for the swing, trigonometry and calculus are needed to calculate the energy exerted from one oscillation of the swing. Or, you could use an online pendulum calculator like I used:
http://www.calctool.org/CALC/phys/newtonian/pendulum
Energy = 667 Joules
Period (Time) = 3 seconds
Power = Energy / Time = 667 J / 3 s = 222.2 J/s = 222.2 Watts.
Assuming a generator with 100% conversion efficiency, a 68 kg (150 pound) person on that swing can generate about 222 Watts of power. That’s just enough to light up a few LEDs on the chain and power the lighted letter above the swing. The Infinity Swing has eight individual swings. Assuming each one is manned 24/7, the swing could generate about 300 kWh of electricity per week.
So if you have 24 friends who are all willing to swing for eight hours per day, you could save about $143/month on your electric bill! Of course, you’ll have to feed them, and they’ll burn a lot of calories. Which leads to the Free Electric Machine...
The Free Electric Machine: Image courtesy of Billions in Change
Improve your health! Eliminate your electric bill! (One of those statements is true.)
The Free Electric Machine promises to power your house for 24 hours after just an hour of pedaling. How much is that?
A typical person (68 kg, 150 pounds) pedaling at a moderate rate (19 km/hr, 12 mph) burns about 500 calories in an hour. One calorie is equivalent to 0.0012 kWh, so that hour on a bike will generate 0.6 kWh of electricity (again, assuming perfect conversion efficiency). If the only electrical appliance at your house is a 25 watt light bulb, then the claim is true. In reality, this device is intended for developing nations where they don't use much electricity. One kWh can easily light a few LED bulbs for a night and charge a cell phone, which is pretty much what they need in those locations. However, a typical home in a developed nation uses forty times that much energy. That’s a lot of pedaling!
One thing that's amusing: the guy who created the Free Electric Machine also owns the "5-Hour Energy" product line. When I first saw news of this device, I thought, "What a brilliant way to sell more of his energy drink!" (Maybe he’s shooting for perpetual motion: chug a 5-Hour Energy drink, pedal like crazy. Chug another one, pedal some more. Repeat.)
So why did I assume best case scenarios and perfect conversion efficiency? Well, as you can see from the calculations, even under the best of conditions the electricity produced from people power is feeble by first world standards. The human body is a remarkably efficient machine. It only uses about 2000 calories each day - roughly the equivalent of a 100 watt light bulb. Our appliances and machines require considerably more energy; no generator is going to produce that from human kinetics.
Hand-crank flashlights and emergency radios use very little energy, so it’s quite feasible to power them with human motion. But the next time someone tells you that you can power your whole house by riding a stationary bike for an hour, you can tell them that they’re just spinning their wheels. And if anyone tells you about a “miracle power source,” please send the information to me via LinkedIn, Twitter, or Sustainable Energy Today. If it’s interesting enough, I’ll do the analysis and write about it on ENGINEERING.com.