Vacuum Tube Collectors
There are two main types of construction, the first is a clear single-walled tube with a conventional plate absorber inside, and the second is a double walled glass tube with the absorber coating sprayed onto the inside wall. Because this type of collector was developed in the University of Sydney, it is commonly referred to as the 'Sydney' vacuum tube or occasionally as the 'Phillips' tube after the manufacturer.
Common (High Efficiency) Tube
A clear single tube contains a selective absorber plate, the tube is evacuated down to a vacuum-level normally encountered only in outer space. This virtually eliminates the conduction and convection heat losses, leaving only the radiation heat losses. These are minimised through the use of selective coatings which hinder the emission of infra-red heat radiation. This makes this type of collector very efficient at retaining heat and hence it is very good for high temperature applications.
However, because there is only a single wall, the vacuum seal must be between the glass and metal surfaces. This has proved problematic in the past, with some models showing poor reliability. The glass-metal sealing arrangement is a more complex manufacturing process than making a double walled complete glass vacuum tube, which adds to their expense. The single glass surface and full tube vacuum generally gives it better a better optical and thermal efficiency than the Sydney tube. Because the glass is curved reflective looses are generally slightly higher than for a flat plate collector (i.e. the optical efficiency is slightly lower than for a flat plate collector).
Each Sydney tube consists of two glass tubes made normally from borosilicate glass. The outer tube is transparent, the inner tube is coated with a selective coating (Al-N/Al) which absorbs the solar radiation and turns it into heat.
The top of the two tubes are fused together and the space between the two layers of glass is evacuated, giving vacuum-tube jacket, similar to a Thermos flask. The insulating properties (thermal efficiency) are generally not as good as the high efficiency vacuum tube but are a lot better than the flat plate collector.
One advantage of the Sydney tube is its ability to passively track the sun. This gives it a more consistent output than any other collector over the whole day.