Solar Panel Types

One of the most common questions asked by students is "Which solar panel type is the best"? Without wishing to be vague, the most accurate answer is that "It depends". We will be looking at the characteristics of various panels and attempting to match panels with their best applicaptions.

For the purposes of the exam, there are 5 types that we need to be aware of;

• High Efficiency Vacuum Tube
• Sydney Type Vacuum Tube
• Selective Flat Plate Collector
• Non-Selective Flat Plate Colllector
• Unglazed Collector (open swimming pool type)

Each of these classes of solar collectors will be dealt with in more detail later, but within this page we will simply give a quick overview. As thermal and optical efficiencies are disscussed, differences and applicaptions will become even more apparent.

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 temperature applications.

Sydney Tube

Each Sydney tube consists of two glass tubes made 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 a the tube vacuum jacket. The insulating properties lie between a good flat plate collector and the high efficiency vacuum tube.

Flat Plate Collectors

Sunlight passes through the glazing and strikes the absorber plate, which heats up, changing solar energy into heat energy. The heat is transferred to liquid passing through pipes attached to the absorber plate.

Absorber plates are commonly painted with "selective coatings," these coatings absorb UV and visible radiation, but are poor emitters of longer wave infrared so that they retain heat much better than ordinary black paint. Absorber plates themselves are made from copper welded to a copper pipe.

Sometimes non-selective coatings may be employed to increase heat losses at high temperatures and prevent a solar panel from boiling, The Solartwin system which deliberatley uses a non-selective coating to prevent overheating. Non-selective panels were more common in the past because of the increased cost of selective coatings. They are relatively uncommon in the marketplace, but may enjoy a renaissance as a cheap non-stagnating panel. However deliberatly decreasing the efficiency of the panel should be compensated for by increasing its collecting area.

Unglazed Collectors

For low temperature applicaptions such as outdoor swimming pool heating, especially in mainland europe plastic mat open collectors have been in use successfully for decades. Because they have no glass cover, they offer excellent optical efficiencies. However they retain heat poorly and efficiency drops off quickly as the panel temperture rises above its surroundings.