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.
Energy stored E = mgh (300 J)
Energy stored E = 10 (kg) (9.8 m/sec) 3 (meters)
2940 J
In reality, you will be lucky to get 50% back, and that's with a very good generator.
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.
http://www.wolframalpha.com/input/?i=20+kilogram%E2%80%90for...
So probably a hoax.
(Wolfram fun fact: thats roughly the energy of the weight of a typical snowflake in oil (~4mg))
(http://www.who.int/mediacentre/factsheets/fs292/en/)
> Around 3 billion people still cook and heat their homes using solid fuels in open fires and leaky stoves. About 2.7 billion burn biomass (wood, animal dung, crop waste) and a further 0.4 billion use coal. Most are poor, and live in developing countries.
> Nearly half of deaths among children under five years old from acute lower respiratory infections (ALRI) are due to particulate matter inhaled from indoor air pollution from household solid fuels (WHO, 2009).
etc etc
This isn't to say that the device is any good.
They do if you're comparing it to a kerosene lamp; kerosene has certainly not been "replaced" in parts of the world that don't have easy access to batteries or electricity in general. See the numbers elsewhere in the thread. And it apparently does use an LED light.
False, and the authors should be ashamed of themselves for posting this lie in a page meant to attract investors:
http://naldc.nal.usda.gov/download/38761/PDF
Also, a wound-mainspring arrangement would be much more effective as a low-cost, practical third-world product. It would be lighter, less costly to produce, and would require much less space. This gravity angle looks like a gimmick, possibly to avoid infringing someone's patent.
Good idea, but the physics doesn't work yet. A heavier weight might fix the problem, though. (I use a 5W LED lamp in my apartment that I keep on all the time. It's almost enough for reading and it's certainly enough for walking around at night.)
And/or more height. The numbers are comparable to the low end of kerosene lamps as given.
This thing talks about a 1:600 duty cycle (3 seconds of charging for 1800 seconds of lighting), so, at the very best, it could use a 1.5W LED. Looking a a more realistic 100W human power output (Tour de France riders do 6W/kg or so sustained), it's gets down to .1W or so.
A heavier weight, more height, or more frequent charging all will increase the amount of light produced, but it also means less time to enjoy it.
In the end it all boils down to the fact that a kWh is a lot of energy for a human to produce.
Why does every crackpot "revolutionary" energy gizmo make this same kind of nonsense claim? "Minute" is not a unit of energy, luminous intensity, or any other measure that is actually useful in evaluating the practicality of this.
The reason I know this is crackpottery is that, as others have pointed out, the physics just doesn't add up. Assuming perfect efficiency, you'd be looking at somewhere on the order of 1% of the light output from a standard 40 watt light bulb.
A better way to go would be to validate the concept and find a way to produce the lamp and as much parts of it in the target communities, helping them to become self sustainable. The current idea just replaces local economy with earnings for a {american, european, chinese} manufacturer, effectively funneling funds away from the people that you want to help. So the goal is laudable, but I can't support the approach they're taking.
[1] other examples include food help which drives the local farmers out of business or donating clothes. Clothes often get sold for cheaper prices than the locally produced ones. Both may be useful in very specific circumstances, but are harmful in large scale.
For example I live in Europe, and when I get an iPhone... the money goes to the US, but this is not a problem as its technology and its an extra in my life.
But see when you have to buy basic things for living, like food, fuel etc... from non local companies its a problem.
Especially the poorness of Africa is deeper subject and i don't think that tools like that are the cause of it.
This thing can be applied my many more places that lacks electricity and its rather good than bad to have it.
Yes, about a tenth of a watt (see the numbers upthread). However, that gives an LED light output that's comparable to a kerosene lamp. So it could be worth it for the target users.
I think that'd be easy to fix though.
This has me wondering how much energy a larger scale application could store and what its efficiency would be.
Also this is very poor energy density each kg moved 1m results in 9.8 joules or ~2.7e-6 kWh. So one metric ton moving 1km would generate 2.7 kWh.
I don't remember all the economics, but the cost of kerosene for x months would pay for the entire setup. Also, the light was an order of magnitude brighter than a kerosene lamp. But the interesting part (again, there's nothing highly technical here) was the business model.
It had a social/micro-finance bent in that he would source investors, who'd invest in shipping containers of these systems, which they'd sell on consignment to local entrepreneurs in poor areas. Those entrepreneurs would pay a fixed interest rate, so the investment was for-profit, and the local entrepreneur would profit from the rest. I remember him talking about building a Kiva-like interface where you could track your investment, right down to the shipping container, village and entrepreneur.
The best thing about this guy moving in was that my co-founder (of Flightfox) was just preparing for an expedition across the Gobi desert and needed solar power for her Macbook so she could blog about the expedition from the desert. All went well and she (Lauren) spent 52 days, trekking 1,000 miles, with full power and ability to blog over satellite the entire time. All using solar panels small enough to fit in a backpack.
That said, love the concept of the gravity light. For anyone who's spent a lot of time off grid, even dim light makes all the difference.
Edit: this system cost about $20.
Hell, they should combine this thing with clock: a clock with weights that is also a lamp when needed.