The all-glass evacuated collector tube is constructed similarly in concept to an elongated thermos flask with the region between the cover glass tube and the inner glass tube being evacuated and sealed off.  The absorber element is coated on outside of the inner glass tube with a selective surface, thermal losses due to conduction and convection are drastically eliminated by the vacuum insulation, and the

 losses due to radiation are minimized by the low emittance of the solar selective coating. The tubes make evacuated tubular collectors achieve high heat extraction efficiencies and provide domestic hot water all year around even in cold climatic areas. The picture illustrate the main reason why the vacuum tube has better thermal performance

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