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8053 clicks; posted to Geek » on 15 Jan 2013 at 11:54 AM (4 years ago)   |   Favorite    |   share:    more»

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Of course... the villagers will probably fill the bag with kerosene.
Wait a sec. The math isn't quite adding up for me.

Imagine a 2 kg bag falling 2 meters over half an hour.

The force on the string would be 2 x 10 = 20 Newtons.

The energy released would be 20 Newtons x 2 meters = 40 Joules.

40 Joules / 1800 seconds = 0.022 Watt.

Does anybody know of a two-hundredth-of-a-Watt LED that puts out any reasonable amount of light?

Ummm.... I'm calling BS on this one.
 1 vote:
I have one of the hand crank lights, and was able to survive in the Grand Canyon because of it when my other light died and I had 2 miles to go...

It isn't the brightest light, but it will let you see where you are going and who you are talking to better.

It is an interesting concept, but I'm not buying the 2m (6ft) drop and 18 minute part. If I hand cranked a light, it would pull up the 2m string in 30 seconds to get a decent amount of light. You would need something like a bicycle gear ratio to slow down a heavy bag over the 18 minutes. Not that I couldn't be proven wrong though, and it is a worthwhile experiment. It is what high school and college physics students should be working on and designing...
 1 vote:

pciszek: LasersHurt: This light has been hotly debated re: whether or not it was physically possible, etc.

People began including calculations of the human power needed to raise the bag a few feet, however, and that's where the sensibility broke down and I stopped paying attention. Still, seems like it's easy enough to understand what goes on here.

I just did a back-of-the-envelope, and it does not look good. If you had a 10kg mass (which is kind of heavy) and 2m drop, you could get one tenth of a watt for half an hour. Whoop-dee-do. And pulleys won't increase the amount of work available from a given mass and a given vertical distance. They would allow a human to raise a heavier weight, but I don't think this gadget is equipped to take advantage of that.

So you're looking at about 4-5 lumens which is in the range of a LED keychain light.- not very bright BUT then entire point is that the alternative for these people is paying 1/3rd of their monthly income for kerosene to make a flame not much brighter.

Yeah I wouldnt be comfortable hanging out in that kind of light but when sitting in the pitch dark is the alternative a 100% reliable and cheap source of light seems like something worth the effort.
 1 vote:

cgs06: These guys use 20kg and say you can get useful light at around 20 milliwatts

so I guess it's plausible... if not terribly practical IMHO.

It's not practicle for us (you wouldnt light your office with one) but if you choice is "sit in the dark" or spend 30% of your monthly income on kerosene to product the same feeble light, it's a slam dunk.
 1 vote:
These guys use 20kg and say you can get useful light at around 20 milliwatts

so I guess it's plausible... if not terribly practical IMHO.
 1 vote:
FTFA: Though it isn't clear quite how much light the GravityLight emits, its makers insist it is more than a kerosene lamp....Talking to Ars by telephone, Therefore's Jim Fullalove was loath to divulge details,

I don't think this is a bad idea, but between this and, you know, the math, it doesn't exactly pass the smell test. Not that the idea won't work, mind you, but it sounds like it's going to be pretty dim, and they're hiding that fact for marketing purposes.

A back of the envelope calculation: Using a 100 kg weight hoisted 2m up gives you 1960J. The article claims that the typical interval between lifts of 18 minutes, so you've got roughly 1.8 W available when the device operational, and 1.44 W if you spot them a very generous 80% conversion efficiency. The luminous efficiency of a cool white LED is approximately 10x that of an incandescent, so with this particular hypothetical setup, you're going to be getting light about as bright as your average 15 W bulb.

This isn't necessarily bad for what they want to use it for, but it may not be something that's going to wow investors. Furthermore, if they want to make it really cheap (i.e. plastic gears, generators without rare-earth magnets, no pulley system for raising the weight, etc.) a 100kg weight is probably out of the question, and the conversion efficiency is going to be much lower - the light output is probably going to be on-par or less than what you get out of one of those mini-Maglites.

Again, reducing dependence on fuel in remote areas is a good goal, but the way that this is being presented is really dodgy.
 1 vote:
Better than a crank-operated flashlight, because those typically still rely on rechargeable batteries, which wear out long before the light itself. I like this idea.
 1 vote:

Tom_Slick: uksocal: Why not also set up a pulley system, bag descends at .2mm/second and gives 2.5 hours of light? Am I missing something here?

Now I want to go get a clock mechanism, crank LED flashlight and start playing.

I think the pulley is replaced by the step up gearing inside used to spin a dynamo - same idea but using gears instead.
 1 vote:
I'll pay \$15 to these hippies just to have one when the tornadoes roll through and I'm without power for a week or so.

Again
twice in less than a year.
 1 vote:
This seems like the next logical step in the evolution of the "hand-crank generators". There have been lights and radios that run on hand cranked electricity for a while now. This is basically just adding a clock type mechanism to let gravity do the cranking.

Simple ideas are usually the best.
 1 vote:
That's actually pretty neat.

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