Sydney Type Vacuum Tube

Each evacuated tube consists of two glass tubes made from 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 (Al-N/Al) which features good solar radiation absorption and minimal infrared radiation properties.

The top of the two tubes are fused together and the air contained in the space between the two layers of glass is pumped out while exposing the tube to high temperatures. This "evacuation" of the gasses forms a vacuum, which is an important factor in the performance of the evacuated tubes.

A vacuum is important because once the evacuated tube absorbs the radiation from the sun and converts it to heat, convection and conduction losses are minimised.The insulating properties lie between a good flat plate collector and the high efficiency vacuum tube.

In order to maintain the vacuum between the two glass layers, a barium getter is used (the same as in television tubes). During manufacture of the evacuated tube this getter is exposed to high temperatures which causes the bottom of the evacuated tube to be coated with a pure layer of barium. This barium layer actively absorbs any CO, CO2, N2, O2, H2O and H2 out-gassed from the evacuated tube during storage and operation, thus helping to maintaining the vacuum. The barium layer also provides a clear visual indicator of the vacuum status. The silver coloured barium layer will turn white if the vacuum is ever lost. This makes it easy to determine whether or not a tube is in good condition.

The big advantage of the Sydney tube is its ability to passively track the sun. This gives it a more consistent output than any other collectore over the whole day. This effect is called the Incident Angle Modifier and is dealt with in more detail later. Word of warning, (maths involved!)..

CPC variation

This is a variation on the Sydney type where a curved reflector is added to the back of the collector. The CPC stands for compound parabolic collector. This shape of reflector is capable of reflecting the sun's rays onto the central tubular absorber, even as the sun tracks across the sky. The aperature area of the collector is now the area of the reflector, but it can only lose heat through the vacuum tube and the manifold. For this reason, the thermal characteristics of this type of collector are excellent.

Further discussion is available; CPC Collector

Extracting heat

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

Manifold Close-up

Bracket

The panel frame can be adjusted to line up with the rafter underneath.

Outputs

A 30° roof angle is very common in the UK and Ireland and was chosen to give the most relevant results.

Likewise to compare like with like, the daily consumption was fixed at 250 litres of 45°C water per day. Changing the water consumption both in volume and profile can lead to large changes in system output which overwhelm other system parameters.