THE DIFFERENT TYPES OF SOLAR PANEL

There are two types of solar panel in general use, the flat plate panel and the evacuated tube solar panel.

Flat Plate Solar Panels

The flat plate solar collector consists of a black surface arranged beneath a transparent cover, and insulated to the rear and sides. A heat transfer fluid (often water or glycol solution, sometimes air) is circulated through the panel to transport the heat collected away to where it can be used.


Within this generic format, many, many variations are possible. For example, the cover can be glass or plastic, single or double glazed, with or without anti-reflective coatings. Suppliers have a tendency to wax lyrical about how their particular way of joining the pipes to the black surface is better than all the others. See for yourself how important arguments about small differences in efficiency really are here:
(Diminising Returns in Area and Efficiency).

One technical feature of real note was invented in the 1980s. It is called a "spectrally selective surface", and is generally an engineered film of oxide applied to a metallic surface. Such a surface looks black to visible wavelengths of radiation (sunlight), but is like a silver surface to infra-red wavelengths. A material with a selective surface will absorb light just as well as a surface painted black, but will hold onto its heat much better by being bad at radiating heat away.

Evacuated Tube Solar Panels

Like in a thermos flask, a vacuum can be excellent thermal insulation. It reduces heat transfer by convection to zero. Conduction is not significantly altered under the level of vacuum typically used, but air is already a poor conductor.

The vacuum is held in a glass container (to let the light through). Since glass is very strong under compression but tends to fail in tension, a tube is a good geometry to use.

The tube contains a fin of light absorbing material with a U-shaped pipe for the heat exchange fluid to pass through. A number of these tubes are combined into a manifold to create a larger area or 'panel'. Vacuum tubes are popular as a retrofit option because the array can be taken up to the roof one tube at a time. The additional insulation of the vacuum means that the efficiency at elevated temperatures (>60C) is higher than for flat plate solar collectors, making them more suitable for industrial process heat applications.

Next: The Efficiency of Solar Panels