In the early age 80 of the last 20th century, Tsinghua University Professor Qin Zhiqiang Research invented the "Gradient Al - N / Al solar selective absorbing coating" technology, in 1985, obtained national patent. Tsinghua University use this patent to produce China's first all-glass vacuum tube solar collector, and in 1997 formulated the national standard "all-glass vacuum tube solar collector", it is also the world's first type of product standards!

Each evacuated tube consists of two glass tubes made from extremely strong borosilicate glass. The outer tube is transparent allowing light rays to pass through with minimal reflection. The inner tube is coated with a special selective coating named AIN/ALN-SS/CU or Al-N/Al which features excellent solar radiation absorption and minimal reflection properties.

Why need to be vacuum? Because vacuum space have a great ability to keep the heat as much as possible. It is the excellent insolator. this is one of the key point for the whole process of absorption of solar energy.