Common (High Efficiency) Tube
A clear single tube contains a selective absorber plate. Once assembled, the tube is evacuated
down to a very low pressure level. The virtually perfect vacuum elimates the convection
heat losses, the conduction heat looses can only take place at the bottom and top of the tube, so
these are virtually elimated also. Radiation heat losses are in turn minimised through the use of
selective coatings which as we have seen earlier hinder the emission of infra-red heat radiation.
This makes this the vacuum tube very efficient at retaining heat and hence it is very good for high
There are two primary methods of extracting heat from the vacuum tube.
One, use a heat pipe or secondly pump the anti-freeze mix down into the vacuum tube.
The second method is konwn as primary flow, or primary fluid. Using a heat pipe introduces
an extra heat exchange surface but in practice there seems to be very little difference
in efficiency between the two extraction methods.Beacuse these methods of extracting heat are common across all types of vacuum tube panels.
It is dealt with in more detail seperately. Heat Extraction