There was an article on this at The Register a few weeks ago: http://www.theregister.co.uk/2012/12/12/deciwatt_gravity_light/
One advantage of this device is that, other than the LEDs (I think), it is relatively low tech and could be mostly manufactured or at least assembled locally (an economic advantage, not just a hand-out but partly a hand-up) and quite possibly maintained/repaired locally.
I still think that the child-powered (merry-go-round) water well pumps are a little more clever in terms of exploiting an abundant energy source and providing fun.
Well, based on the calculator at http://convert-to.com/159/energy-units.html 100 ft-lb = 135.58 J, and by my calculations a 3V LED powered by a 100% efficient generator that draws 20 mA will stay lit for 37.7 minutes. I would estimate for the 3-LED system shown with a 50% efficient power conversion system would give about 6 minutes of light per 5 foot 100 pound lift. That is some big bag to hold 100 lb. The Gravity Light web page talks about a 20-pound weight, and says 18 to 30 minutes of light output per lift. I agree something seems a little off...
In total darkness, you dont need 3 led's at 20mA.. I'm pretty sure 3 leds at 20mA would blind you. It would be more useful to get this amount of light spread on a large area, but that is a different challenge.
Dont forget to compare it to the current alternative, not to a modern flashlight.
I feel the term Gravity Light is a bit mis-leading. Indeed, this light is powered by burning of bio-mass - the difference being that the bio-mass must comply to human nutritional standards and is burnt inside a human body.
While this may look as an advantage in obesity-ridden countries like USA or Western Europe, I feel this may be look quite different for those countries which are currently using kerosene-lamps or open fires for lighting ..
At first glance, it reminded me of a cuckoo clock and the weight mechanism that drives it. The idea is so deceptively simple that it makes me wonder why I didn't think of it. And it's rechargeable too (in a convoluted lifting soft of way).
Brilliant! While the principle is not innovative, using it for lighting is. Expect some sort of spin-off to take advantage of naturally occurring forces instead of using human energy ... tidal and air pressure changes come to mind, or perhaps the Seebeck effect could be used when day changes to night.
Nothing new, look at this project: http://www.core77.com/competitions/greenergadgets/projects/4306/
It never made it to production, reasons outlined above. Please note, that the "Gravity Light' people won't say anything about the energy generated by the device (other than it won't charge an iPhone). It don't doubt that it will generate some light, but whether this is enough to allow people to read is more than questionable.