When investing in solar water heaters, it is important to ensure the "engine" you select is the most powerful and the longest lasting made. This if why we use SunRain's patented "3-Hi" solar vacuum tubes. The absorbing coating found on the inner layer determines how efficient the vacuum tube will operate. The method and material of this coating will also define the life expectancy of the solar vacuum tube.
Most vacuum tubes use a hi-tech reactive sputtering process to coat the inner tube. Most commonly this coating is made from an aluminum and aluminum-nitrogen coating (Al-N/Al) which is the primary absorbing coating.