Recycle a clean, clear plastic, one-liter beverage bottle by filling it with water and three tablespoons of chlorine bleach, secure the bottle in a hole in a metal roof, and you have all the technology needed to illuminate the dark interiors of thousands of homes of the world’s poorest people. In an ingenious use of optics and chlorine chemistry, and for very little money, 55 watts of solar energy are streaming into formerly unlit homes in communities in Brazil and the Philippines. Many of the homes being outfitted with the device were perpetually dark due to the close, side-by-side construction of rudimentary dwellings. Now the spirits of residents are brightening along with their interior dwellings: Daylight is penetrating their living quarters.
The solar bottle bulb was invented by a group of resourceful Massachusetts Institute of Technology students; bulbs take approximately an hour to install. Solar bottle bulbs are positioned to rest partly above and partly below the roof surface, protruding from the interior ceiling. During the daytime sun rays stream into the water bottle, bending (refracting) and internally reflecting to produce a bright light source that does not depend on an electrical connection. As an example of the physical phenomena responsible for this lighting technology, the brilliance and “fire” of diamonds are caused by light refracting and reflecting throughout the crystal.
Chlorine Bleach Helps Light up the Darkness
Good sunlight refraction and reflection depend upon a clear water medium, just as a diamond’s brilliance depends on its clarity. Chlorine bleach plays the role of destroying the microorganisms that could proliferate inside the bottles, reducing the clarity of the water. As for maintenance, water and bleach must be replaced, but only every five years. It is not clear how long the beverage bottles hold up in this role (caps are protected from cracking with sealant—see the video), but replacing them should not be too great a hurdle.
Solar bottle bulbs are a wonderfully safe, cheap, energy-efficient lighting technology being supplied to those who need it most! Kudos to the inventors and the installers!
Bruce Bernard, PhD, is President of SRA International, Inc. and Associate Editor of the International Journal of Toxicology.