Evacuated Glass Solar Tubes - Brief Introduction

The use of evacuated glass solar tube technology appears to be one significant progress in solar industry all over the world and  evacuated tube is the key component to all the thermal heaters and systems. It’s similar to a conventional Dewar flask and consists of two borosilicate glass tubes, resistant to high chemical and thermal shock. The outer side of the inner tube is coated with a sputtered solar selective surface. This coated inner tube is closed at one end and sealed at the other end to the outer tube. The annular space between the outer tube and inner tube is evacuated to avoid heat loss in conduction and convection.

The key element determines that the function of tube is the technology of the film coating, our chosen tube production company  uses the patented technology  to produce  vacuum tubes with the best efficiency in the world with the highest absorption rate of 96% compared with common rate of 93% and lowest emission rate near 4% compared with common rate of 7%.   At this efficiency, it has been very hard to improve the function any more, even very little extent. And the  coating layers can be divided into  four layers:   metal infrared reflector layer, HMVF cermet, LMVF cermet and anti reflection layer.  When the tube is placed into the coating machine, pumps draw the air outside and form a vacuum at about 1x10-3. Later, some nitrogen is sent into the coating machine and form a film of Al/N-Al. At this stage, the main percentage of the film is AL since there is only very few Nitrogen entering the coating machine.

So this stage film is called High Metal Value Film (HMVF). And little by little, more and more nitrogen is released into the coating machine, and form Al/N-AL film, but at the stage, the percentage of Nitrogen increases and forms Low Metal Value Film (LMVF). At last, a black chrome anti-reflection layer are coated on the three previous metal layers  to reduce the emission rate of the film. Thus the coating procession is finished. During the whole process, with continuous inputting of nitrogen, films with different content of AL/N-AL formed. Tubes made by that  process called  Three target layers vacuum tubes  and that process is  time consuming and expensive.....    Because of that, not all  the tube production companies can afford to have such expensive equipment and  used cheep technology and machinery for  coating  only  two layers on the tube- one metal Al  and black  antireflection layers.  Tubes produces by that cheep process called One target layer vacuum tube and  such tubes has a shorter life time as well much lower efficiency to compare with Three target layers vacuum tubes.

Below the example of  ordinary one target layer vacuum tube Al ( left) and innovative high efficiency three target layers vacuum tube Al/N-AL (right).

On today's market  in South Africa you will find a lot of different vacuum tubes (dimensions, glass thickness, number of layers and ect. ) and all retailers will praise their product.  As mentioned above  not all  tubes are good quality...  and when you choose a tubes for your solar installation, please have a look inside of the tube to see the color of  infrared reflection layer. If it's red as on picture above, that tube has three layers on it and can withstand hard working conditions (temperature inside the tube reaches 250C⁰).  If it's silver color  - it's better to cancel from such product or if you decide to have such product, efficiency  and life expectancy will be much lower. Also take into consideration glass thickness of the tube. Ask your supplier to provide you with piece of tube glass (all retailers have such glass due to brakeage). 2,0mm glass will withstand heavy storm and  35-40mm hail stones.  It's enough for South African weather and it's not necessarily to have 2,3-2,5mm tubes. It's good only for Northern Hemisphere to withstand  heavy   snowfall and freeze.

We supply only the vacuum tubes with Three target absorption layers AL/N-AL  to achieve best  absorption and  heat transmitting  efficiency. Also, all our solar system are fitted by  solar tubes  with dimensions:         Diameter-58mm. Length-1800mm. Glass thickness-2,0mm which  are the best for African climate conditions. We promise, in  just clear and sunny day one our vacuum tube will  easily heat about 8,5 L of water  from 20^oC up to 70-80^oC per day and because of that, all our solar system are fitter with right number of tubes  for the  each size of the geyser. (For example - 100L solar system has a 12 tube collector, 150L solar system has a 18 tube collector, 200L solar system has a 24 tube collector and so on)