STOP PRESS: Pixie dust unsuitable for household lighting

A reader pointed this page out to me, about the recent Greener Gadgets Design Competition

$1000-second-prize-winning Gravia “floor lamp powered by gravity”.

It’s a funky looking thing, which was widely reported around the gadget blogs, and was alleged by its designer, Clay Moulton, to give the equivalent light output of a 40-watt incandescent bulb for four hours from the energy of a weight dropping about four feet, or 122cm. When the weight gets to the bottom, you just lift it back to the top and away you go again.

Now, it stands to reason that a mere 1.2-metre drop isn’t going to give you forty actual watts for four hours unless the weight is incredibly heavy. Ignoring losses, it would by definition take forty watts of power over another four hours to lift the weight back up again, which is 160 watt-hours, which is quite a lot. A normal adult human in reasonable shape can manage about 75 watts of output when pedalling away on a bike connected to a generator; it’d take more than two hours of such pedalling to raise that weight back to the top of the Gravia light’s tube, if the weight were heavy enough to make a constant 40 watts on the way back down.

So I just assumed the lamp’s brightness was greatly overstated, and wasn’t even four-watts-of-LEDs-that-are-sort-of-equivalent-to-forty-watts-of-incandescent. But since they’d clearly actually made the thing and it’d won an award, I presumed it did work, if only as a night-light. Fair enough.

But neither Clay Moulton nor anybody else has, actually, built a Gravia.

The damn thing doesn’t exist.

And Mr Moulton, who apparently designed the thing as part of his Virginia Tech master’s thesis, didn’t even bother to check whether his design could possibly bloody work at all, even if you built it with LEDs from ten years in the future.

Looking at the schematic for the Gravia shows that the falling weight is defined as fifty pounds, which is 22.7 kilos, which is indeed about as much as a variety of humans could reasonably be expected to be able to lift back to the top of the tube every few hours.

22.7 kilograms falling 1.22m in gravity of 9.8 metres per second squared gives you a grand total of 271.4 joules.

That, once again ignoring losses (which are likely to be considerable, seeing as there’s a ball-screw and an electrical generator in the Gravia), will by definition run a one-watt lamp for 271.4 seconds, or four and a half minutes.

If you downgrade the lamp to one tiny 0.1-watt LED night-light, you get three-quarters of an hour.

The maximum possible luminous efficacy for any kind of lamp that will ever exist - if every quantum of energy going into the thing is used to make visible photons that come out - is 683 lumens per watt. And that’s for a lamp that emits monochromatic 555-nanometre green light, not white (the world record for white LEDs in the lab so far is less than 150lm/W), but never mind that for now.

So if your tenth-watt lamp is just such a perfect device that can never actually exist, it will emit 68.3 lumens of light.

There’s no standard lumen rating for an incandescent 40-watt bulb - generally speaking, the ones that last longer have lower output - but something like 400 lumens is in the ballpark. Actually, the Gravia has been alleged to output 600 to 800 lumens, but even if you only shoot for 400, 68.3 lumens is 17% of the target.

So instead of the output of a 40-watt incandescent bulb for four hours, we’ve got the output of a 6.8-watt incandescent bulb for 45 minutes. And that’s with a perfect lamp and no other losses in the system. With the best white-light lamp that humans will actually ever be able to make and million-dollar hardware for the rest of the thing giving the lowest possible losses, I think you’d actually be talking the output of a two-watt incandescent flashlight bulb for about 30 minutes. At best.

Looking at it from another angle, 271.4 joules is 271.4 watt-seconds, 683 lumens per watt is the physical limit, so by definition 271.4 joules of energy can only produce 185,366.2 lumen-seconds of light. Four hours is 14,400 seconds; 400 lumens for four hours is 5,760,000 lumen-seconds. So 271.4 joules into a perfect lamp can only possibly ever give you 3.2% of the required light. Or 1.6%, if you take the 800-lumen ceiling figure for the Gravia’s output.

These facts have not evaded other observers, and have now also been communicated to the Gravia’s designer. That pesn.com page now features, in the comments, about a minute worth of these back-of-an-envelope calculations that anybody with a basic physics textbook could have done, and it also now features an apology from the designer of the Gravia, who now concedes that the thing could not actually be made and that he did not deserve, and will be returning, the prize.

Actually, I reckon he did deserve the prize, since the Greener Gadgets people are clearly a bunch of idiots (see also: The New Inventors) and their prize is therefore worthless.

I hereby propose magical light paint, which glows harmlessly at 200 lux for 500 years (power source: A D battery filled with the blood of saints) and costs a buck a gallon. Tah-daaah! I just won first prize in the next Greener Gadgets Design Competition! Drop me a line, guys, and I’ll tell you where to send the money!

The original press release about the Gravia on the Virginia Tech site now also contains a disclaimer from Moulton, though without any mention of him giving back the prize. I think it’s worth mentioning one line he uses on both pages, though: “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.”

Yes, Mr Moulton, it certainly bloody would be a “conceptual challenge” to make a lamp that produces more than thirty times as much light as the laws of physics say is possible from the energy you put into it. That would be a pretty damn impressive achievement. I propose Virginia Tech not permit you to graduate until you do it. How’s that grab you?

The Gravia is very far from alone, of course. There’s a veritable plague of these entirely imaginary “concept” devices. The gadget blogs are rotten with ‘em. But usually these things have the decency to obviously just be a 3D render of some stupid concept that couldn’t possibly work (image-intensifying sunglasses, say…). Sometimes it looks as if at least a mock-up has been created. Only seldom does an impossible device actually win an award for “design innovations for greener electronics”.

(I suppose a lamp that doesn’t work is, in a manner of speaking, quite “green”. It reminds me of that Goodies episode in which string is a “safer and cheaper” subsitute for electrical wiring, “because it doesn’t work”.)

One bit on the Gravia’s design competition page is particularly priceless: “Gravia is also [a] metaphor for an understanding of social activism.”

Yes, Clay, it is. If you just sit on your arse and make shit up without paying any attention to the actual nature of the world, you will not succeed in social activism or lamp design.