Why vacuum tubes?
Vacuum (evacuated) tube works like well-known, a thermos flask, which uses vacuum to provide thermal insulation; once the evacuated tube absorbs the radiation from the sun and converts it to heat; the vacuum helps keep the heat.
The vacuum insulation makes evacuated tube collector excel the traditional flat plate collector in performance.

What are the main advantages of evacuated tube over flat plates collectors?
1- Increased efficiency: Due to the tabular shape of the evacuated tubes, the sun is always perpendicular to the surface of the glass for most of the day. Flat plate reaches maximum efficiency only at noon time.
2- Minimal heat loss: Since air is evacuated from the glass tubes to form a vacuum, it reduces the heat loss from the tube. This means that wind and colder temperatures have less effect on the collectors’ efficiency.
3- Freeze protection: Evacuated tube collectors can be used in freezing conditions -15C° without the system being damaged.
4- The system is not affected by scale.
5- High benefit: the developed absorber materials of the tubes can absorb direct or indirect heat spread in the air.
6- Needs less space than the flat plate.
7- Cheaper in cost than the flat plate.