GravityLight, an idea from two British designers, is something completely different. It gets its energy from gravity: a 22-pound bag of sand that gradually cranks a gear-train attached to a D.C. motor. One lift is enough for 30 minutes of light, and recharging is as simple of pushing the weight up again.
Martin Riddiford and Jim Reeves are crowd-funding the prototype on Indiegogo, and have already raised almost $100,000. They plan to distribute 1,000 units to villagers during a test stage, before developing and commercializing further. They estimate the current cost at $10 per machine, but reckon they can halve that by scaling up, and finding better materials.
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http://www.scientificamerican.com/article.cfm?id=a-5-light-for-the-developing-world-2012-12
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