Wednesday, June 3, 2009

Solar Lightbulbs

Solar energy, particularly free passive solar energy, has been a passion of mine for 30 years or more. I became convinced years ago that there is lots of free solar energy that we could use, if only we knew how.

Recently a friend introduced this YouTube video on a forum I follow on sustainability. It demonstrates a means of bringing the equivalent of a 50 watt light into dark interiors, using not much more than a clear 2 liter soda bottle, and water. Of course it only works when the sun is shining, but for the price, who cares?
(By contrast, a Solatube costs about $300. The only big advantage I see is that it can be installed through a home insulated attic space thanks to the long attached tube.)

The concept is so simple I immediately smacked my head and thought, "DUH!!" The concept is the same as the deck prisms used on wooden sailing ships for centuries. The deck prisms were flat on one end and installed flush with the decking for safety. The prism end hung below deck in all sorts of dark nooks and crannies; it magnified the sunlight and refracted it for a flood of natural light. A plain cylinder of solid glass would not have magnified and refracted the light, although it would have provided a spot of bright light directly below it.

In the case of the soda bottle light... the water it contains acts as a prism and refracts the sunlight all around the circumference of the plastic bottle. Making it is simple, and since the video is in Portuguese, I'll walk through the steps here. Remember, the soda bottle needs to be clear, not green or opaque. Put 2 capfuls of household bleach in the bottle, and fill it with water.

The bleach keeps algae from growing in the water and making it cloudy; cloudy water will not refract light. In the video, they put a used film canister over the top of the bottle, to keep the bottle cap from deteriorating. With the advent of digital cameras, film canisters are hard to find. I think I'd try a short length (2 inches or so) of plastic pipe (but not PEX which I know breaks down in sunlight) and fill it with the same silicone adhesive I'd use to seal around the hole in the roof for the bottle. The water-filled bottle will have some weight and it may be necessary to support it until the sealant cures.

I haven't made one yet but I plan to put several in my old barn, which has a dark interior and no electricity run to it. Perhaps I will also put a couple in my garden shed. I don't go into either structure at night, so darkness is no problem. In fact, they'd do great in a chicken house!

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