It may sound like an impossible dream, but two designers in London have built functioning prototypes of GravityLight, a cheap way for people in developing countries to light homes, recharge batteries, or power a radio.
Martin Riddiford and Jim Reeves have spent four years developing GravityLight, which uses the Earth’s gravity to generate enough power to light an LED bulb for half an hour — no electrical grid, batteries, or any external generator required.

Using the GravityLight simply requires removing the small white lamp from its bag, hanging it up, filling the bag with about 20 pounds of dirt or rocks, and attaching the bag to bottom of the device.
Gravity powers a generator, light fills your room, and every 30 minutes, you hoist the bag back up.

The goal is to provide clean, efficient light for the 1.5 billion human beings on this planet who still do not have reliable access to electricity and use kerosene-powered lamps. According to Riddiford and Reeves, the use of kerosene results in vastly higher cancer rates due to smoke inhalation, and 2.5 million burn victims due to dropped or jostled lamps every year in India alone. Not to mention the cost: 10 percent to 20 percent of a household’s income in the developing world can go to fuel for lighting.

The initial run of 1,000 GravityLights will be distributed for free to villagers in Africa and India, the designers say. Based on the results, they will tweak the product and then seek NGO and non-profit organizations’ help in distributing it even more widely.

[b][/b]How does it really work?

Potential Energy
GravityLight is installed to provide a 6ft/ 1.8m drop of a 12kg weight. This weight is lifted and on release starts falling very slowly (about 1mm / second).

Kinetic Energy
This movement powers a drive sprocket, which rotates very slowly with high torque (force). A polymer geartrain running through the product turns this input into a high speed, low torque output that drives a DC generator at thousands of rotations per minute.

Electricity and Light
This generates just under a tenth of a watt, a deciwatt, to power an onboard LED and ancillary devices. Given the ever-increasing efficiency of LEDs, this produces a light over 5 times brighter (lux) than a typical open-wick kerosene lamp.

Once the weighted bag reaches the floor, which depends on how high it was installed, it is simply lifted to repeat the process.

Gravity Light specification.
Power Output (12.5kg Load)

High: 0.10W
Med: 0.075W
Low: 0.05W

LED

Power (max): 124mW
Operating Temp: -30 ~80 °C
Storage Temp: -40 ~100 °C
Luminous Intensity: 13.8 cd*

Product Dims

Weight: 0.8kg
H:160mm, W:103mm, D:78mm

Load Weight

Minimum: 8.0kg
Maximum: 12.5kg

Source- http://gravitylight.org/how-it-works
http://venturebeat.com/2012/12/26/this-5-lamp-is-powered-by-gravity-and-just-destroyed-its-funding-target-on-indiegogo/

amazing

Hello,
Please is the gravity light available for sale in Nigeria?