A reader writes:
So is the GravityLight a sensible design that conforms to the laws of physics, or is it also powered by Pixie Dust?
There's nothing inherently wrong with the basic idea of converting the energy of a falling mass into electricity. (That's how hydoelectric power stations work, after all.) It may even be possible to do it quite efficiently, and cheaply, on a small scale now.
In the olden days you would have needed to gear up your dynamo a lot from the pulley your falling weight (or flow of stream water) was turning, but today super-powerful magnets are very cheap and so efficient lower-speed dynamos are easier to make. Especially if you're only trying to light one high-intensity LED to a brightness that'll let someone read a book at close range.
The GravityLight people claim thirty minutes of light per lift of the weight, and the tape connecting the weight to the light looks to be, being generous, about 1.5 metres long. They don't say exactly what the weight is, but they do say you can fill the GravityLight weight bag...
...with use "anything weighing about 20lbs"; let's again be generous and say you get ten kilos of stuff in there, which is a perfectly liftable weight for a wide range of humans.
Split over thirty minutes, which is 1800 seconds, that gives us only 0.082 joules per second. That means a power of 0.082 watts, or 82 milliwatts.
A single standard white LED will have spec-sheet power numbers of about 20 milliamps at 3.6 volts, which is 72 milliwatts. It'll work fine - and more efficiently - at rather lower current, though, which is just as well because this falling-weight system is certain to be a long way short of 100% efficient at turning the weight's gravitational potential energy into light.
Even if it's only 50% efficient, though, you've still got 36 milliwatts to play with, which is plenty to light one LED to a useful, though far from room-filling, brightness. Even if you pare off some of the above generous assumptions about weight and fall distance, you'll still be easily above 20 milliwatts, which is also usefully bright.
The GravityLight people say their invention is intended to replace kerosene lanterns, but it's definitely not going to have the room-filling brightness of a kerosene lamp with the wick turned well up. Given the numerous downsides of kerosene lighting, though, and the fact that a lot of poor people probably don't turn their lamps up to max very often, a GravityLight or two could well replace one.
The ridiculous Gravia concept thing had a heavier weight falling a similar distance, for a total of about 271 joules. But the designer idiotically claimed that the thing would have the light output of a forty-watt incandescent bulb for four hours. This was so impossible that it couldn't come anywhere near being done even in Physics Experiment Land, with a perfectly efficient dynamo and a perfectly efficient lamp.
With real-world hardware, in contrast, the GravityLight can work. I'm not totally convinced that for household lighting you wouldn't be better off with a couple of conductive objects a reasonable distance from each other in the galvanic series, some damp earth as an eletrolyte, and a Joule Thief to boost the output to run an LED. That sort of improvised battery can run for a very long time at the very low power a Joule-Thiefed LED requires, and its poor portability doesn't matter if you're using it as a hoursehold light (and is an advantage if you want to avoid your light being stolen...).
But the physics, at least, checks out for the GravityLight.