Electric generators can get up to 95% efficiency, meaning electrical output as a percentage of mechanical energy input. In the current case, the relevant input is mechanical energy produced by a slowly dropping weight, so there would be losses from friction. But overall, I'd expect much better than 50% in the conversion of gravitational potential energy to electrical energy.
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 posts upthread have shown, the light output will be comparable to a kerosene lamp, which is what these lights are supposed to replace. It doesn't take a "crazy weight" or "a lot of elevation". And the difference between 50% efficiency and 100% efficiency is only a factor of two; that's well within the range of making reasonable adjustments in the weight and/or height.
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've not seen a generator (quite likely 'cause I've never gone looking), but I've seen model plane motors claiming almost 95% efficiency (a quick hunt found this one claiming 94% max efficiency: http://www.astroflight.com/index.php?main_page=product_info&... )
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.)
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.