This type is the second type of solar thermal collector, and is made up of multiple long glass tubes that are evacuated of all air. In those tubes are heat absorption panels with small tubes running through them that also have a liquid that gets heated by the solar energy. That liquid is usually water or an antifreeze mix. Because of the lack of air in the tubes, this type doesn’t lose nearly as much heat to natural convection and conduction, which allows it to reach much higher temperatures.

This system works much better than flat plate collectors in colder climates because it relies on available light instead of the temperature outside to heat the fluid. Evacuated tubes also lose much less heat to the environment as flat plate collectors.