DAVID NUNUK / SCIENCE PHOTO LIBRARY DAVID NUNUK / SCIENCE PHOTO LIBRARY
Solar water heater on the roof of a building. It comprises two rectangular plate collectors and, above those, a cylindrical water storage tank. Inside each collector are a series of small tubes that pass over a black absorber plate. The plates absorb sunlight and warm the water passing through the tubes. Warmed water rises to the storage tanks by convection as it is heated. Colder water sinks down into the collector. This system, known as a thermosiphon solar water heater, is cheap to install and maintain, and produces no waste products. Photographed in Barbados.
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