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".

Gravia lamp

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 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.

25 Responses to “STOP PRESS: Pixie dust unsuitable for household lighting”

  1. robzy Says:

    When I first heard of this (it was very popular on StumbleUpon) I knew something smelt fishy, but figured that "They won an award, they must be right".

    Lesson learnt.

    Thanks Dan :)


  2. kamikrae-z Says:

    As an Industrial Design student with basic grasp of physics (rarer than you would think) it shi*ts me no end to see product concepts released by students which give absolutely no consideration to the laws of physics, or even just plain common sense. However, the fact that this guy is a Masters student is really perplexing, because it was quite likely that he would have had to complete a well-reasearched and sustained project over the course of his final undergraduate year before this (unless of course he did not do an undergraduate product design degree). Worse than this is the fact that his teachers would have let him complete this without any kind of formal proof-of-concept - I wouldn't have believed it myself had I not encountered the "Oh I'm sure it would work" attitude myself.

    To restore some of your faith in Industrial Design I give you the winning entry for the Australian Design Award:
    While not an electronic gadget it does fall somewhat under the eco design banner, and I am told that the figures given (500% faster growth with 99% less water) are derived from proper research (although of course I remain optimistically skeptical).

  3. cjdmi Says:

    Thanks for posting this Dan.

    When I first saw Gravia press release, I was amazed they managed to get any decent efficiency out of the basic design (reverse driving a screw?). As Dan showed, the numbers in the press release don't work, but I assumed the error was due to press office and not Clay's.

    In Clay's defense, it does look pretty, even if it won't work.

  4. Says:

    La lampara que funciona mediante la gravedad, es falsa

    Hace aproximadamente 2 semanas, salio la noticia de que un estudiante del Instituto tecnologico de Virginia habia inventado una lampara que obtenia energia mediante un peso que caia por la accion de la gravedad (

  5. arteitle Says:

    Dan, thank you for this excellently researched article.

  6. beita7 Says:

    I'm not much of a physics person per say but I noticed something a bit off in your assumption of the device and how it functions. I take by what you say here, "22.7 kilograms falling 1.22m in gravity of 9.8 metres per second squared gives you a grand total of 271.4 joules" implies that the weight is falling from top to base directly. Now looking over the schematics, it most certainly says that the weight is descending over a period of 4 hours and that the energy is from the torque of the weight on the ball-screw. Now, forgive me if I am jumping the gun here but I do not think the energy of a free walling weight and a descending spinning weight is the same. Now as said I am not a physics person but I think there is more energy to be accounted for that was neglected.

  7. shusky Says:

    beita7-- whatever energy got from the spinning would be converted from potential gravitational energy anyway. the maximum energy that could be derived from a mechanism powered by gravity would be a free fall. spinning has the effect of slowing the fall (so less energy is generated over longer period of time) and, in practice, would while away a lot more energy on account of the friction.

  8. pulsar Says:

    #6 - Potential energy is conservative, which means that no matter which way you get from point A to point B, the result is the same. Therefore, the total amount of energy you could get out of a 22.7 kilogram weight falling 1.22m in Earth's gravity is m*g*h, or about 271J.

    See for more information.

  9. beita7 Says:

    Thanks for the explanation but I'd rather have the math backing it so I can see it for myself

  10. tantryl Says:

    beita7 - check the wiki link in pulsar's post. You cannot create more energy by slowly spinning a falling object, only disperse it into other things (friction) and reduce the total amount of usable energy. The maths would require a lot of background information and also would be based on... the conservation of energy and the calculations would simply be on how much power was lost to friction during the spinning versus it's maximum potential in freefall.

    Basically it's a bullshit trick, by making the object fall slowly it's trying to convince you that it's better than dropping straight away. Which is magical fairly land nonsense.

    That being said:

    Dan - you didn't tell us anything more about this Pixie Dust and why it's unsuitable for household lighting? It sounds promising...

  11. todd Says:

    this is a real lesson learned for me. i came across that site and thought "cool" and then never gave it another thought. i just assumed it worked because a) it got an award and b) it looked professionsal. well, thanks, man, for my lesson learned is to never, ever take anything at face value, especially all these new fangle gadgets that i come across in the blogosphere. btw, i am not a physicist, just a wannabe. it's my dj name.

  12. Chouette Says:

    Thank You, bookmarking for the next time a Stumble! on to the articles about the "Innovative New Lamp Design". Ha I feel bad for the inventor but you would think that he would do the math or have someone do it for him before entering it into a competition.

  13. andiem Says:

    This is exactly the attitude that made me ditch 4 years of Industrial Design study and take up a career in IT.

    The emphasis was always on presentation rather than substance. I can't count how many crap designs I saw that could not work. Most earned the student high marks based only on superior presentation. I even remember one that was patently impossible to manufacture by any method that got one of the highest marks in the class! Apparently a turd ceases to smell like a turd if you tie a pretty bow around it.

    It's annoying when the marketroids peddle lies to make a buck. When the attitude becomes entrenched in institutions that are supposed to train the people who actually make things it becomes intolerable.

  14. reyalp Says:

    The fact that he claimed a very specific number is what really gets me. If he hadn't done his homework, and just expected it make some light, that would be somewhat more excusable (although still pretty pathetic for masters student)

    The claim that it will give out "600-800 lumens" gives the impression that he arrived at this number for some reason, rather than just pulling it out of his posterior.

    The weasel worded retraction shows that he's still clueless, or lying. Ugh.

    Or maybe he's a neutron star dwelling alien living among us ?

  15. chrisovenden Says:

    I'm sorry - your calculations are quite wrong, as you are failing to take into account the massive increase in the universal gravitational constant which will occur in time it takes to develop these super-LEDs. Haven't you read "Slapstick" by Kurt Vonnegut?

  16. Alex Whiteside Says:

    Let's not be too hard on industrial designers. The guys behind things like the ZX Spectrum and the original Gameboy knew what they were doing, and had a good deal of engineering/manufacturing sense. It's when people couple industrial design with technological crystal ball gazing that it goes horribly, horribly wrong.

  17. tantryl Says:

    I don't want to promote a hate crime against me... but the Spectrum and original Gameboy looked like what you'd imagine an AIBO shits.

  18. tristram Says:

    I concur with Alex - ease up. I study architecture, and we like to pursue emerging technologies and explore the way they may possibly influence our designs, and how we can make them work for us. Whilst some of this stuff may still be pie in the sky, it's nice to see what it can do. I understand your frustration directed at the student, but hey, without people pushing the boundaries exploring, where would we be today?

  19. rcousine Says:

    Tristram: few boundaries have ever fallen to devices that push the boundaries of the second law of thermodynamics. This device is the proverbial pretty-but-stupid, and it serves no purpose.

  20. jtwing Says:

    Ok, I'm not a physics god. But -- what if the motion of the weight was captured by a gearing mechanism that stepped up the slight motion of a heavy weight into very rapid rotations of a small generator? I mean, LED flashlights exist that you can shake to charge, or turn a crank...I opened one of those up (it stopped working) and saw the gears myself. The gear stepping transferred the mechanical energy to a small generator that stored its energy in a watch-sized rechargable battery. I fixed the flashlight, by the way. It's pretty bright -- bright enough to read by, which is all you'd really need in a dark room anyway. Maybe it doesn't last 4 hours, and maybe the artist's concept render is hopelessly impractical, but I'm not convinced the concept itself is dead. It certainly doesn't take me 2 hours of vigorous pedaling to recharge the LED flashlight -- just a couple dozen energetic cranks of the handle.

    I think something marketable is still there, in the concept. Maybe not at the level the designer envisioned, but still...

  21. shimavak Says:

    I've figured it out. All that it has to do to work correctly is to cause the laws of physics (more specifically the action related to gravitation) to be slightly variant in time. If it manages that tiny trick, it need no longer abide by petty laws such as the conservation of energy. Problem solved. I'll take my Nobel prize now.

    Wait. What's that you say? We can't seem to find any way to generate a system where our functional mapping our configuration space onto our manifold is not invariant under time? Well, maybe we just need a master's student from Virginia Tech engineering school to help us come up with a system which could do just that.

    If it is all right with you, I will just keep working on other methods while we wait, though...

  22. Alex Whiteside Says:

    jtwing: Let's say that your "energetic crank" is about 10N (i.e. enough to lift 1kg) of force over 5cm. That means that a 20kg weight over 200cm is equal to 800 cranks. At a vigorous crank per second, that's 800 seconds of cranking. These torches seem to get about a 1:15 cranking:lighting ratio, or to put it another way, 4 seconds of cranking gives 1 minute of light. So we're talking 800/4=200 minutes, or about 3 hours, of hand-torch-grade light from a device which is two metres tall and weights 20kg, and which is dispersing its light over an entire room, rather than in a neat little beam. If you want to boost the light up from hand-torch level (say 4W candescent) to comfortable room lighting (say 40w candescent) you're going to need to burn up that energy 10 times faster. So you'd have to turn this light over every 20 minutes. Now, looking at Dan's rigorous analysis, I'm clearly being generous here, but it just doesn't add up however you look at it.

  23. A late arrival Says:

    kamikrae-z: my favourite few sentences from the Project Eden "it's good because it's good" bumf:

    The way in which the system applies the technology makes small-scale farming in a confined space economically sustainable, generating the equivalent weight of produce to a ground farm that occupies ten times its horizontal footprint. It therefore presents an entrepreneurial business opportunity to sections of the urban poor who may find difficulties in finding normal wage employment and also in this way suits the lifestyle of the target demographic.

    so the guy's not just smoking it, he's helping others to grow and sell it! what a fantastic human being!

  24. hye Says:

    What the designer neglected to mention was how a tiny fraction of the weight is shaved off for every drop and thrown into a cold-fusion reactor which then produces the energy for the light. Rough back of the envelope calculations indicate that the 50-lb mass would last quite a while if it was only outputting 800 lumens for 4 hours/day (even for white light), so the lifetime of the product WOULD be limited to the lifetime of the LED's. Of course, if you DID have that kind of cold-fusion technology, it would probably be easier to pluck a hair and drop it in than lift a 50-lb weight, but then you wouldn't get the nice pretty weight spinning slowly down a 1.2 m acrylic cylinder, would you?

    See, didn't even need to break the laws of physics: simply needed to leave out a very-patentable component. The press release should have read: "The news release should have said: 'based on future developments in LED^H^H^H cold fusion technology.'"

  25. pvolk Says:

    Actually, it's completely feasible, as long as the weight is made of Plutonium (maybe with some Tungsten shielding to reflect neutrons). Then it's a Godiva device.

    Of course, let it alone, and it will heat itself to incandescence.

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