Speeds and Feeds

As an engineer, I hate to see bad engineering treated like a good idea.

I've written recently about questionable proposals for human power generation, electric vehicle recharging and fuel cells. In some cases, there's nothing really wrong with the underlying technology, but it's being implemented and promoted using bad math and misleading promises.

The latest example of innumeracy comes from Shawn Frayne, an independent inventor here in Silicon Valley. Frayne's Windbelt is a low-cost wind-power generator that uses a fluttering membrane instead of rotating blades to convert wind power into mechanical motion; a simple linear generator turns the motion into electricity.

The Windbelt idea won a "Breakthrough Award" in the November issue of Popular Mechanics, but not because Windbelt is actually useful or innovative. It looks like the magazine was a bit too eager to jump on the alternative-energy bandwagon, so nobody bothered to run through the numbers.

The numbers are awful. According to Frayne's estimates, for "a few dollars" you could make a version of the Windbelt capable of generating 40 milliwatts of power in a 10-mph wind. In a 5-mph wind, if the things works at all, it'll produce less than 5 milliwatts.

In a stronger wind, it's liable to break entirely, since the power of wind varies as the cube of the wind speed. In a 40-mph wind, the Windbelt could potentially extract more than 2.5 watts...but in practice, the membrane would simply tear. This failure is exactly what caused the famous 1940 failure of the Tacoma Narrows bridge in Washington.

Frayne wants the Windbelt to power LED lighting, replacing kerosene lamps in Haitian homes, but that's not going to happen. I did some simple research through Google--the kind of research the staff of Popular Mechanics should have done--and the numbers just don't work out. It would take at least 5 watts of power to produce enough LED light to replace a kerosene lamp. Generating that much power would require 125 of Frayne's small Windbelt generators.

Current Windbelt prototypes are small, so one might suppose they can simply scale up to produce more power. Unfortunately, they can't scale very far because of limits to how strong the membrane can be. There's an inherent conflict between making the membrane thin to maximize flutter and making it strong so it can tolerate more fluttering.

Conventional propeller-bladed wind turbines are not commonly made with outputs below about 50 watts, but there's no reason that can't be done. It isn't done because it isn't worthwhile. For low-power applications--especially in sunny places like Haiti--the appropriate technology is solar power. Solar panels have no moving parts, require little maintenance and when properly protected, are pretty much immune to high wind.

And there's nothing to be developed for this situation. Home Depot sells truckloads of solar-powered lighting equipment. I checked the company's Web site just now, and for a mere $26.97, you can get a solar-powered LED floodlight that includes the solar panel, a battery and a three-LED lamp. Such a product would require some repackaging for the home-lighting market, but all the parts are there.

I suspect someone's already done that work, but I didn't turn up any commercial products intended for this kind of use in my brief search. If you're aware of such products, add a comment! Thanks.