Solar panels – Absorbing solar radiation energy and converting it into thermal energy can be done with the help of solar collectors. Generally, due to the design, they can be divided into flat and focused.
Focusing collectors are used in devices, where high temperature of the working medium is required. These collectors use the optical principles of systems focusing the solar radiation on the absorbing element, as a result, the density of the radiation energy flux increases by 1,5 do 10 000 times. These collectors require control devices, so that the focusing system remains constantly perpendicular to the direction of the sun's rays.
Flat-plate collectors are much more widely used in single-family housing, which, due to the heat sinking factors, can be divided into air and liquid.
The most important part of the collector is the absorbing plate (absorber), most often made of steel, copper, aluminum and plastics in the form of a flat plate with channels attached to it or two sheets of appropriately embossed metal, welded together and blackened on the outside.
To increase the efficiency of the collectors, absorbers are covered with selective materials, i.e. such, which easily transmits the sun's rays in the wavelength range from 0,3 do 3 μm, while they reflect radiation above 3 µm. With materials with the so-called. selective properties, there are currently the greatest hopes for improving the efficiency of solar energy reception, min. 50% compared to ordinary black paints. Selective coatings are much more expensive than varnish coatings.
The transparent cover of the collectors should be durable, hard and, above all, highly permeable to solar radiation, and with low transmittance of thermal radiation, with wavelengths over 3μm. The covering should be a barrier to long-wave thermal rays emitted by the absorber plate. Most often, building glass is used for this purpose (usually, hardened or special) or transparent plastics - acrylic (plexiglass, polycarbonate, glass fiber reinforced polyesters). Most of the collectors produced, it has aluminum plates and a construction glass cover. Collectors designed for year-round operation and with a working medium heating above 60 °, have a double cover. Reduction of heat loss from the absorber to the environment can also be achieved by covering the panes with substances that reflect well infrared radiation., while transmitting visible light well, e.g. indium oxide z 5% admixture of tin oxide.