While poking around looking into the DST-doesn’t-save-energy story, looking for something that didn’t just link back to the WSJ story, I ran across this: a new lamp being hyped by some sites with a “green” tint, called Gravia. (a second story is here at treehugger)

The lamp took second place in the Greener Gadgets Design Competition. It’s described as being gravity-powered, which is wrong. It’s human-powered — you lift 50 lbs, and the weight falling back down supplies energy to some LEDs, and is supposed to supply 600-800 lumens, or the equivalent of about a 40-W light bulb, for four hours. Something about this immediately struck me as being wrong. You aren’t going to power the equivalent of a 40-W light bulb with that, not even with really efficient LEDs. 50 lbs, lifted a bit over a meter, will require 250 Joules of work. Over 4 hours, that’s 17 milliWatts of output. That didn’t add up — even the best LEDs are only 5 to 10 times more efficient than incandescent bulbs. There’s no way this can work.

And sure enough, that’s what I found

While many people want to know when the lamp will be available, many others point out that it won’t actually work.

The criticism is that a great deal of weight –- tons — would be required and current LEDs are not sufficiently efficient.

Designer Clay Moulton acknowledges that the current state of the art isn’t sufficient to actually build the lamp. The news release should have said: “based on future developments in LED technology.”

Moulton said: “I was told it was not possible given current LED’s, but given the rapid pace of innovation in low powered lighting, it would be a conceptual challenge. The mechanism itself is the novelty. I hope everyone understands that this criticism and even failure is all part of a process, and that my job as a designer is to take this feedback and work on.”

That the device was purely conceptual didn’t make it into those other stories. Neither did the fact that it can never work with anything close to this design.

It’s not just that current LEDs are insufficient. It’s literally impossible. The lumen is a unit of luminous flux, that is, how much power there is that can be perceived by the human eye — how bright things look. Contrast that with the radiant flux, which is the total power, or how bright things actually are — luminous flux corrects for the response of the eye, using 555 nm as the peak response point. The luminous efficacy gives you a conversion, and at 555 nm, that’s 683 lumens/Watt. So if we’re at that wavelength, we get the best-case scenario: all of the light we are getting is at the point where the eye is most sensitive, so it’s as bright as it’s going to look. 683 lumens, right in the target band, still requires a Watt of power. The device comes up a factor of 60 short, in addition to the fact that the output of current LEDs is somewhere between a factor of six and ten less than the ideal value. So the minimum weight for this device to work, using 100% efficient devices, is going to be 300 lbs (or alternatively it will run for 4 minutes as currently configured), and with today’s LEDs it would require (as the VT news story says) more than a ton to run for 4 hours. Remember to lift with your legs.