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 and very strong. Solar vacuum tubes have been used for years in Chinese Solar Water Heaters and offer a great inexpensive tube for efficiently transferring the sun's power to heat water. These tubes have a clear outer layer with a dark inner 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 for the most part. Unlike the glass thermos, solar tubes are not silver so sunlight can penetrate the first layer with a 90%+ efficiency and is absorbed on the black layer. A glass thermos would make a perfect solar collector if it were NOT coated with a silver mirrored finish and was modeled exactly like a solar tube.
Ironically, the Chinese Water Heaters have not made a big splash in most parts of the world. The main reason is shipping. During the importing process, these large, fragile units usually suffer some breakage. While many are available in the USA, they are expensive compared to their cost in China. The bright side to this is the tubes. Because of the high breakage, the solar tubes are over manufactured as replacements and are very inexpensive.
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".
How Do You Get Pressure?
For steam production, I am currently testing some options incorporating an oil surrounding a steel boiler. For household hot water the answer is MUCH simpler. I took a tube and made a mold to produce a cap that has a short copper pipe and a longer copper pipe running the length of the collector. Water (or anti-freeze exchange solution) is pumped into the long copper pipe, exits the pipe at the bottom of the tube and flows the out the short one. This forces water to travel the length of the tube two times. The pressure is low because this fluid is moved with a solar circulating pump. The fluid then travels to a copper tube loop that sits inside an insulated pressurized water tank. Only the exterior of the copper tube loop is exposed to household water pressure. This tube loop gets very hot because it is a good conductor. The heat is lost into the large body of pressurized water to be used by your house. This is considered a passive system.
There are many challenges to this design for the do it yourself enthusiast , primarily, making the tank. There are some short cuts we will be exploring in future videos and this entire project will soon be posted.
Sunlight area is sunlight area. Having many black tubes collecting the same solar values as a Fresnel Lens of the same area will deliver the same heating value. Fresnel lenses and parabolic reflective shapes do not AMPLIFY light, they CONCENTRATE it. The Fresnel or parabolic option produces temperature gains that are much greater. This setup is much more efficient requiring a smaller collection area.
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.
Dan Rojas
Greenpowerscience.com
