| Slashdotters Pwn Gravity Lamp Claims |
| Written by Hank Green | |
| Friday, 22 February 2008 | |
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But when the Slashdotters actually did the math, they discovered a few flaws. First, the inventor assumed 100% efficiency. Much worse, is that they seem to have put a decimal in the wrong place, and ended up with completely erroneous numbers:
It's sad news, but it is nonetheless the case. The inventor has admitted his mistake and offered the Greener Gadgets honor to the runner-up below him. Really, this makes you appreciate how much work needs to be done even to power the lowest-wattage light bulbs. Sometimes, it's good to realize how really remarkable and efficient our current system is...even if it comes at the cost of an idea that, at first glance, looked quite appealing. Via Slashdot Comments
(5)
So no one at greener gadgets did the mat
written by nickjohnson , February 23, 2008
This is a major problem with greener gadgets- they don't know how to do the math to analyze efficiency claims. How can we take any of their awards seriously?
alternative math guide
written by Mark Bartosik , February 24, 2008
A human is able to output in the order of 100W when doing a strenuous workout. Now there may be a factor of 2 there depending on your level of fitness.
The time to lift the weight would be a couple of seconds. So the work lifting the weight would only likely power the light for some tens of seconds.
hey you!
written by Reg , March 19, 2008
While it would take 925 lbs on a 4 foot drop, why not think bigger?
How about a 40 foot drop and 200 or 300 (or more) lbs? It would be easy to lift 200 lbs to a height of 40 feet with the proper winch. It would be a matter of spending a few minutes to winch up the weight in return for an evening of light.
Serious workout
written by DM , March 20, 2008
200-300 lbs 40 ft up in the air is a serious workout. You would probably need a pulley system to get the force down to what most people could handle, but then the pull distance increases.
Also, 200-300 lbs 40 ft up in the air, unless the structure is properly stress-analyzed, is potentially a very dangerous contraption. Not to mention moving and erecting. And maintanance... | |
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In a display of the awesomness of the Internet, users of the website Slashdot have thoroughly debunked claims made by the inventor of the Gravia gravity lamp. The inventor claimed that the lamp could use a 50 lb weight dropping five feet to power a 700 lumen LED for 4 hours.
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The Gravitational Potential Energy (GPE) of the 50 lb (22.7 kg) mass lifted to a height of 48 inches (1.2 m) is:
22.7 kg * 9.8 m/s^2 * 1.2 m = 267 J.
I found a "warm white" LED (7000 mcd at 35% viewing angle) that consumes 120 mW of power.
Assuming 100% efficiency converting GPE to electric power, a 120 mW LED could stay lit for (267 J) / (0.120 J/s) = 2220 s (37 min) So the design goal of 400-600 lumens for 4 hours is definitely beyond reach of the design. That doesn't mean the idea is completely without merit. Even a single LED could penetrate the darkness in an off-grid cabin. I like the idea of consuming the electric power immediately as it is needed. Use GPE and the calories in your diet as the battery.
I'd like to see the "high efficiency ball screw" turning a generator.