## 03 January 2013

### GravityLight

Their Indiegogo page is here.  The goal was to raise \$55,000; the fund currently stands at \$317,000.

1. The gravitylight is most likely a hoax.

http://sustainabilityworkshop.autodesk.com/blog/importance-calculations

1. thanks for posting this, add a LOT

2. Not necessarily so. I own a flashlight that works off of a crank, and the Russians had a radio that did the same thing. If that can work, then why couldn't something along the lines of a Pendulum Clock?

3. Primarily because the light they're advertising doesn't have enough input energy to provide useful light. Consider how much cranking you need to do in order to get the flashlight to work. You don't get that energy for free.

The link I put earlier has better math, but basically: imagine a 10 kilo weight dropping 3 meters - that's 294 Joules of potential energy from the height (mass * height * g). If it falls that distance in 1800 seconds (30 minutes), that works out to an *absolute maximum* of 0.16 watts of power.

This is assuming 100% efficiency - which is impossible - better to give them the benefit of the doubt and say it's around 65% efficient (still quite good for a small electric generator), which works out really handily to about 0.1 watts of output power.

Your typical little LED flashlight uses about a watt of power. If you wanted to have a light that was about as useful as a little Petzl headlamp, you'd need to either have the weight fall ten times further, use ten times more weight, or have it only last 3 minutes between lifts.