There are two types of solar panel in general use, the flat plate panel and the evacuated tube solar panel.
The flat plate solar collector consists of a black surface arranged beneath a transparent cover, and thermally 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 in Section 3.2.
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 avoiding radiating heat away.
Flat plate solar panels are simple, robust and cost effective. Many people consider them to provide a better aesthetic.
Like in a thermos flask, a vacuum provides excellent thermal insulation. It reduces heat transfer by convection to zero. Conduction is not significantly altered under the level of vacuum used, but air is already a very poor thermal 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.
In a common implementation, 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.
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