About 1/2 of of the radiation is withinside the seen short-wave a part of the electromagnetic  
spectrum. The different 1/2 of is in most cases withinside the near-infrared part, with a few withinside the ultraviolet  
a part of the spectrum. The quantity of radiationis the  achieving the earth is much less than what entered  
the pinnacle of the surroundings we classify it in surroundings. It has  categories :  
1) Direct Radiation : Solar radiation that reachesis  the floor of earth with out being  
subtle is known as direct beam radiation. 
2) Diffuse Radiation : As daylight passes via is the surroundings, a number of it is  
absorbed, scattered and pondered through air molecules, water vapour, cloud, dust, and  
pollution from electricity plants, woodland fires, and volcanoes. This is known as subtle  
radiation. 
radiation obtained from the solar in the world is nearly parallel.A sun thermal collector collects warmth through soaking up daylight. The collector acquire the  
warmth from sun rays and provide it to the warmth delivery fluid. Solar collector floor is  
designed for theexcessive absorption and occasional emission. So, the usage of sun power  
calls for the sun creditors. These creditors are categorized into  types. They are follows,  
 Non-concentrating or flat plate sun collector  
 Concentrating kind sun collector.  
In non-concentrating creditors, the aperture place (i.e., the place that gets the sun  
radiation) is more or less similar to the absorber place (i.e., the place soaking up the radiation).  
Concentrating creditors have a miles larger aperture than absorber place (additional  
mirrors consciousness daylight at the absorber) and most effective harvest the direct aspect of  
sunlightFlat Plate Solar Collector  
Flat plate sun collector (Non- concentrating) are handy mainly for area and  
carrier water heating in which temperature beneath approximately 90 °C are adequate. The Flat plate can  gather and soak up each direct and diffuse sun radiation. They are therefore partially  
powerful even on cloudy days while there's no radiation.  
The Flat plate creditors are especially divided into  
 Liquid heating collector  
 Air heating  
 Evacuated tubular collector.  
 3.3.1.1 Liquid Heating Collector  
The majority of the flat plate creditors have 5 components :  
 Transparent is the cowl of glass, Teflon, marlex.  
 Black cease absorber plate generally of copper, aluminium or steel.  
 Tubes, channels or passages in thermal touch with the absorber plate. They are  
soldered, brazed or clamped to the lowest of the absorber plate.  
 Thermal insulation generally of foam, improved polystyrene or glass wool typically  
5-10 cm  .