this seems a lot like http://www.museumofhoaxes.com/hoax/weblog/comments/gravity_l...
Quick back of the envelope calculation, using what few numbers are given in the article.
Ballast: m = 10 kg (1)
Height: h = 3 m (2)
Energy stored: E = mgh = 300 J
Time: 30 min = t = 1800 s (3)
Wattage available: P = E / t = 0.17 W (4)
Notes:
(1) Article says you can hang anything weighing about 20 lbs.
(2) No numbers given in the article, but the pictures make it look like ceiling height (8 to 10 ft., I took the larger)
(3) Article says light for half an hour.
(4) With current LEDs this is the equivalent of about a 2 W incandescent bulb, i.e., pretty dim. The light in the pictures in the article looks like the equivalent of about a 40 W incandescent bulb, so the numbers come up short by a factor of about 20. That would indeed be "super-efficient" for an LED; I'm not aware of any even on the drawing boards that are that efficient.
[Edit: The numbers actually are not too low compared to kerosene lamps, which are what this light is supposed to replace. See exchange downthread with xd.]
Human eyes can adjust very well to low light levels.
The GravityLight article does say that it's meant to replace kerosene lamps, which according to Wikipedia range from 20 to 100 lumens:
http://en.wikipedia.org/wiki/Kerosene_lamp
This is about the equivalent of a 1.6 W to 8 W incandescent bulb, so you're right, I was too quick to dismiss the numbers as being too low. They're certainly low compared to what first world citizens are used to, but they are roughly equivalent to what the target users are used to.
Kickstarter is overflowing with game projects raking in the millions collectively with unverifiable claims altogether, like "awesome multiplayer experience". But this stuff? "It's a hoax, they're trying to rip the public off for 55k!" You simply cannot make this shit up.
Energy stored E = mgh (300 J)
Energy stored E = 10 (kg) (9.8 m/sec) 3 (meters)
2940 J
AND I have to correct mine too since I used 180 for time (30 * 6 rather than 30 * 60)
In reality, you will be lucky to get 50% back, and that's with a very good generator.
However, in commercial power generation, the mechanical energy usually comes from some sort of heat engine, the heat for which is provided by fossil fuel combustion or nuclear fission. There is a big haircut in that step. Heat engines are generally only 35 to 60 percent efficient.
Hydro power does much better, but it doesn't use a heat engine; it's just a much larger scale example of converting gravitational potential energy into electricity.
I think the reason this contraption is valuable is that LEDs don't need much power to produce enough light to please someone used to kerosene lamps.
> I think the reason this contraption is valuable is that LEDs don't need much power to produce enough light
Actually, that contraption cannot produce enough light, unless you have a crazy weight, a lot of elevation, and a magical efficient system that converts energy to electricity, and then transforms it to whatever voltage/amperage appropriate for you LED lights.
As far as voltage/amperage is concerned, I would expect the generator to be low voltage DC, matched to some voltage in the range the LED light could support. AFAIK LED lights are fairly tolerant of a range of low DC voltages, so I don't see this as a major issue.
I'm guessing that's pretty reversible - if you spun that motor backwards with the right speed and torque and loaded the output up in just the right fashion, I think you'd get 94% of the energy you put in out as electricity.
The problem I see is that a slowly falling weight is unlikely to provide "the right speed and torque" without some sort of lossy gearbox in between - there's the physical equivalent of an impedance mismatch there. If I had to imagine a motor/generator that'd be likely to work on a direct-drive to a falling weight it'd probably be several feet in diameter. (For model plane motors, the diameter of the motor is a significant factor in the kV constant and the rpm at max efficiency. Small diameter motor spin fast, large diameter motors spin slow.)
If you have a 50% efficient generator, just double the size of the ballast; 20kg is still reasonable. Or double the height, though that would be somewhat harder; you would need some kind of pulley arrangement. Or some combination of the two.
Also, I think low voltage DC generators can do considerably better than 50%; the numbers for this kind of application can be quite different than those for the kind of large scale AC generators we're used to.
First we'll assume that the weight changes altitude by 2 meters, next we'll assume the "weight" is 5 kG (about 10 lbs) it could be more than that if you used denser material but it looks like they are expecting you to fill a sandbag to weight it dry sand is about 1600 kg/m^3 [1] so a sandbag that was 15cm/side would be about .0033 m^3 or 5.3 kg.
The force exerted by that sandbag, 2 meters up is 5 * 2 * 9.8 or 98 Newton-Meters. Now the campaign says it runs the light for 30 minutes so to find the power in watts we take 98 Newton-Meters divide by 1800 seconds (bag goes from 2m to 0m in 30 minutes) and get .0544 watts per second. Assuming the generator is 50% efficient (that is a really good generator) that is about .025 Watts to run your LED. So can you get decent light from an LED with 25 mWatts? At a forward voltage drop of 4V (White LED) that is 6.2 mA of current. (updated to be a decimal order of magnitude smaller)
Given that current LEDs are seeing something like 50 lumens/watt you might see 2 - 5 lumens from such a light. Not nearly as bright as I originally estimated.
EDIT: The time was wrong 1800 not 180
Hang it in the shed or make it into a great porch light, you can clip on a hanging basket or anything weighing about 20lbs.
So that's ~2x your estimate, which doesn't seem impossible.
I do wonder what they're suggesting using for the ballast though, that avoids excessive bulk. Another consideration is going to be finding a suitable mounting point in a typical shack/slum type dwelling.
Their claim is a 20 pound weight raised 6 feet (or so) in the air will generate light for 30 minutes:
20 lbf * 6 ft / 30 min = ~90 mW
For reference, the LED indicators on your keyboard use about 15 mW each. So that's about two keyboards worth of light.
However maybe I'm being stingy -- because of persistence of vision (a characteristic of the human eye), you can run LEDs (or any light) on a low duty cycle and still produce the same apparent brightness. So if we say they run it at a 10% duty cycle (I'm not sure how accurate this estimate is), they might be able to get closer to 1 W of LED light, which is enough for reading.