EVACUATED SOALR TUBE WATER HEATERS
Solar evacuated tubes AKA solar vacuum tubes are pyrex (borosilicate) glass that does not crack or break when exposed to radical temperature changes. This glass is crystal clear, very strong and have been used for years in China. Evacuated tubes offer an inexpensive method for efficiently transferring the sun's power to heat water. These tubes have a clear outer layer with a dark inner absorber layer (selective coating) containing a vacuum between the glass walls.
Just like the internal double glass walls of a traditional glass thermos, the vacuum (lacking air molecules) in the solar tubes prevents heat from escaping into the atmosphere. Unlike z glass thermos, solar tubes are clear and not mirrored allowing sunlight can penetrate the first layer with a 90%+ efficiency as sunlight is absorbed on the dark inner layer.
The entire unit requires the tubes to be exposed to pressurized water in a traditional setup. Because the glass cannot withstand pressure exceeding 20 PSI, these heaters incorporate a copper spiraled core that is placed inside the length of the tube connected to a nipple (heat pipe condenser). Only this copper nipple, which gets very hot, is exposed to water pressure. These copper nipples can withstand several hundred PSI. This setup of heat transfer reduces the units overall efficiency.
For the backyard scientist, the copper core is not needed. By filling these tubes with water and setting them out in the sun, water boils with no additional Reflectitve Film, Fresnel Lens, or Parabolic Mirror in about an hour. Adding any of the above increases boiling times dramatically.
Our video with a Parabolic trough "Solar Water Heater Steam Parabolic Mirror Trough"I used a black steel pipe as the absorber. Light refracted can be seen throughout the video due to heat loss into the atmosphere dramatically reducing efficiency. Touching this pipe with your hand would be an instant mistake! By replacing the black pipe with a solar vacuum tube in the same design, the outer glass tube would be warm to the touch "Solar Water Heater Steam Parabolic Mirror Trough".
Solar tubes come in a variety of lengths and diameters. They are a great tools for the solar backyard scientist and are used in many professional jumbo scale installations. The concept can boost efficiency of your project by 50% saving you time, money and unit footprint dimensions. These tubes are also a great teaching tool for science teachers.
One thing to remember; NEVER leave these tubes in the path of a Fresnel Lens, Parabolic Reflector or in the sun EMPTY. The tubes can build extreme heat exceeding the tolerance of the black coating and adding water to a 300c tube will shock even the best borosilicate glass.