Gravity Lamp Grabs Green Prize

eldavojohn writes "A lamp powered by gravity has won the second prize at the Greener Gadgets Conference in NYC. From the article, "The light output will be 600-800 lumens — roughly equal to a 40-watt incandescent bulb over a period of four hours. To "turn on" the lamp, the user moves weights from the bottom to the top of the lamp. An hour glass-like mechanism is turned over and the weights are placed in the mass sled near the top of the lamp. The sled begins its gentle glide back down and, within a few seconds, the LEDs come on and light the lamp ... Moulton estimates that Gravia's mechanisms will last more than 200 years, if used eight hours a day, 365 days a year." The article contains links to the patents and the designer/inventor Clay Moulton's site." I think my laptop would require a slightly larger weight to pull this off.

Comments

Where does the energy come from?

[gdr]

Energy for 50lb weight (22.7 kg) dropping 4 ft (1.22 m), g = 9.8 m/s/s:

E = mgh = 271 J

Assume LEDs 100% efficient and standard lightbulb 2% efficient: 2% of 40W is 0.8W.

0.8W * 4 hrs * 3600 s/hr = 11520 J.

Have I made some horrible miscalculation or are they claiming over 4000% efficiency for this device.

Doing the maths

[abigsmurf]

seems a bit suspect to me so I'm going to try going through the maths

4ft = 1.2m, the LEDs, being slightly generous I'll say consume 5W. 5*60*60*4 (total power he claims it uses) = 72000J. Work = force * Distance so 72000/1.2 = 60,000N. translating that into Kg that's a 6100kg weight needed for that much energy (assuming 100% efficiency)!

Is my science/maths flawed or has the guy in question simply not done his figures?

Conservation of Energy

[psmears]

OK, can someone tell me where my calculation is wrong:

The designer's diagram shows 50lbs of weight falling 58 inches. Google tells me in metric that's about 23kg falling 1.5m; under the force of gravity (9.8N/kg), that gives a total potential energy of 23*1.5*9.8 Joules—call it 350J to be generous.

Now, the claim is that this thing outputs 600-800 lumens of light. Let's assume that LEDs can put out 200 lumens per watt of electricity delivered—this is apparently quite generous. That means the LEDs will need at least 3 watts of electricity to give out that amount of light.

As everyone here knows, 3 watts is 3 joules/second—meaning our total of 350J will last slightly less than two minutes; this is substantially less than the claimed four hours!

Either my number-crunching is wrong (in which case I'd be delighted to be enlightened—excuse the puns), or this device ain't ever going to do what it claims...

Gear ratios, people...

[JMan1865]

Why is it that the fastest runner in the world cannot hold a candle to the fastest cyclist? This concept shows, if you would bother to read it, a gear in the base with a 1:160 ratio. Thus, at perfect efficiency, the weight is not falling 48-54 inches, rather the motion would be transferred 160-fold - thus 7680-8640 inches (640-720 foot equivalent)

This seems to be in line with what you are all arguing. Change your math(s) from a drop of 48" to 7680" - I'll bet it becomes more feasible.

I'm not saying that this is possible today, or should be rushed into mass production. But don't blindly hate on it, or dismiss it simply because you do not bother to read all about it first.

Wait a minute - this is Slashdot. Flame away, since you did not think of it first.