Yglesias links to a new blog about “sustainable mobility”. As a bike triumphalist, this is right up my alley. But the post at the top of the page is… unfortunate. Entitled “There Must Be a Catch, Right?”, it discusses a student’s proposal to attach power-generating systems to the fleets of bikesharing programs, collect riders’ spare energy, sell it back to the grid and pass the savings on to the consumer. It sounds great! Until you start doing math!
There’s some confusion about whether the power would come from regenerative braking or is siphoned off during pedaling. For a moment, let’s keep this in the realm of the plausible and stipulate that it’ll be from regenerative braking (anybody who’s used a generator-powered bike light knows that they make pedaling unpleasantly difficult). How much energy could be harvested from a cyclist coming to a complete stop? Well, let’s specify an implausibly heavy average cyclist (100 kg/220 lbs), an implausibly heavy bike (20 kg/45 lbs), an implausibly fast speed (48 kph/30 mph). Plug it into this equation and you’ll get 10,773 joules per stop. Now let’s specify an also-implausible 100 stops per mile — you’d be accelerating to 30mph and stopping every 53 feet. How much energy would you generate for every mile traveled?
The depressing answer: about 0.2 kilowatt hours. Which, using these figures, works out to about 2.25 cents’ worth of electricity.
And of course not only are the above figures unrealistically optimistic, but the impracticality of having everyone drag along extension cords introduces new problems: the battery system will cost you a lot of energy. The conversions from kinetic energy to electrical energy, from electrical energy to chemical energy, and from chemical energy back to electrical energy will all be far from perfectly efficient. I wouldn’t be surprised to see this eat up half of the energy generated.
Sadly, this doesn’t look like a compelling case for spending money to outfit bikes with regenerative braking systems. You’ll save much, much more energy by simply avoiding motorized transportation than you will by trying to squeeze more energy out of your bike.
But although this is bleak news for this particular instance of innumerate ecothusiasm, I still find the situation kind of inspirational: it’s a reminder that the forces we casually harness are incredibly vast when compared to the relatively meager capabilities of our biology. That’s not bad for a species that watches as much Law & Order as we do.
Well, I guess that’s the catch, then. Wisdom of the internet, right?
Anyway, hope you like some of the other, better-informed posts better.