"Vacuum” in Solar Collectors Explained
Instead of insulation solar thermal evacuated tube collectors use a vacuum to stop the transmission of outdoor temperature to the collector.
Only matter can have a temperature, which is the speed of its molecules. The faster a molecules in a material move, the warmer it is. By definition a vacuum is a void therefore it cannot have temperature as it has no molecules. This makes the vacuum in a solar thermal evacuated tube the perfect insulator to protect the heat pipe from the outdoor temperatures.
In reality an absolute vacuum does not exist. Even out in space there are a small amount of molecules spinning around, but their density is incredibly low so that in practical terms, space is a vacuum. Space is Earths insulator for convection heat from the sun since there are not enough molecules to pass the heat. Radiation can pass through a vacuum. This allows us to get light and warmth from the sun.
Both solar thermal and PV collector panels are powered by the radiant energy from the sun.
A solar thermal collector panel that uses even high quality insulation will not perform as well as one protected by a vacuum.
The design of an evacuated tube has the slim copper heat 
pipe enclosed in a vacuum within the glass tube, or in an air chamber blanketed with a double wall of glass with a vacuum inside the two layers (see left). The vacuum protects the heat pipe from the outdoor air temperature. It also prevents the heat generated from escaping. The inside of an evacuated tube can be 150°, but the glass will be cool to the touch.
In the case of a solar thermal flat plate collector, a series of collector pipes are located under a glass or polycarbonate sheet. Some of the heat gene
rated within the pipe will radiate back to the atmosphere. One of the sales pitches for flat plates is that they clear themselves of snow. This can be true. The escaping heat will melt off snow. With evacuated tubes no heat escapes, but since the glass remains cold, snow normally blows right off. With flat plates you are constantly loosing some of the heat that is generated. The greater the temperature difference, the greater the heat transfer, thus the increased degradation in performance during winter.
In the manufacture of an evacuated tube the removal of matter raises the insulation value. The more complete the vacuum, the better the performance. Since there is no way for humans to remove every speck of matter a Barium coating is placed inside the tube to absorb any remaining matter. If a tube ever fails, you can tell because the Barium turns white.
Comparing the vacuum standards is one way to determine tube quality. Due to the machinery and production requirements, it costs more to create a higher vacuum. In addition, high quality tubes are made of soda/lime glass, it is impermeable to gasses found in our air.
As far as we know, all tubes made in China are borosilicate glass. Borosilicate glass allows helium and hydrogen to permeate the glass and degrade the vacuum. Over the first 4.5 years you can see as much as a 20% performance loss. From then on the vacuum is stabilized with the atmosphere and there is no further loss. Under these circumstances it seems logical to say, just add some more tubes. The issue becomes the original tubes can no longer create the high temperatures they once did. If the system is used just for hot water, this may not be an issue. If you are using it for space heating you need the water as hot as possible, especially in the short days of December. The advantage of Borosilicate glass is price.
A larger barium getter area will maintain a vacuum in the tube for a longer period of time under outgassing conditions