Analog Angle Article

Put pedal power in perspective

Generating electricity via a bicycle-driven arrangement is a fairly old way of turning human power into electrical power. Despite its age, the proliferation of low-power products means that this concept has resurfaced again as a “green” or healthy alternative power source. Recently, for example, a company used a team of cyclists to power its new series of low-power, high-performance computers, as a public-relations stunt; it looks like the stunt worked, since even EETimes covered it (“Firm demos computer powered by bicycles,” click here)!

But the reality of pedal-power is what engineers should look at. A typical adult in good condition can produce one-fifth to one-quarter horsepower for 30 minutes or so. As a benchmark, Bryan Allen, the pilot-engine of the human-powered Gossamer Condor (winner of the Kremer prize in 1977) produced one-third horsepower for the seven-minute flight, and he was in outstanding shape with a very high power/weight ratio; there's more information here.

Do a basic, back-of-the-envelope calculation (does anyone use that phrase anymore?). A horsepower is about 750 watts (there are actual several standard values, but this is close enough), so an average adult can source between 100 and 150 watts. Then take into account the mechanical and electrical losses, and you might get 50 to 75 watts from the hard-working cyclist—for a while.

This amount of power is not trivial, but it is not a lot either; it's about the amount that a laptop or small TV requires. You also have to factor in the cost of a reliable, mechanically rugged generator, charger/controller, and battery to make a workable, safe system as well. So while the concept of “free and green” power may sound good, and may be a good way to stay in shape and lose the fat, it's probably not a practical solution for most needs, except for emergency power or special situations. It's a tough way to generate power to store and then use in your evening illumination, as well, unless your super-efficient LED reading lamp only needs a few watts.

I like to read about these power-generation ideas and stunts as much as the next person, but it would also make sense to stop, and go through the numbers to see where they lead. Otherwise, we risk falling to the sway of the constant sensationalism of today's news.

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