This radiation intensity is called as the solar constant or air mass zero (AM0) radiation.
Solar intensity at the earth’s surface
Sunlight will reduce at least thirty percentage when it go through the earth’s atmosphere. The most important parameter of determining the total incident power is the length of the light through the atmosphere. The ratio of actual path to this minimum value is referred as optical air mass. When the sun has an angle θ to overhead, the air mass is equal to1/cosθ . When other atmospheric variables constant, the energy reaching the earth will reduce with the air mass increasing. However, the terrestrial sunlight varies greatly both in intensity and spectral. It is necessary to make a standard value which is AM1.5.
Direct and diffuse radiation
The terrestrial sunlight comprised by the direct radiation from sun and indirect, diffuse caused by atmospheric scattering. When it is not the sunny day, most of the radiation is diffuse. If the total radiation is only covered one-third value in the sunny day, the most radiation is diffuse. As the spectral composition of diffuse sunlight is different with direct sunlight spectral, it will cause the variability in the spectral composition of light received by the solar cell …show more content…
This is a device that absorbs the incoming solar radiation, converts it into heat, and transfer the heat to a fluid (usually air, water, or oil) flowing through the collector. The solar energy either directly to the hot water or space conditioning equipment or a thermal energy storage tank to use at night or cloudy days . The following will introduce the flat-plate collect as an example.
A typical flat-plate solar collector is shown in Figure 1. The solar energy will be absorbed in the flat-plate collector by a spectrally selective surface that is covered by glass plates for reducing the heat loss. As at the back of the panel is thermally insulated, it can be used as the part of the roof of a building. The tube containing air, water or oil is usually used to attach on the back of the absorber to transfer the heat . There are large-diameter header tubes connected at the both end of the tubes. It is the typical design for the flat-plate collector to have the part of header and rise collector shown in Figure 1. Another design is the serpentine shown on the right-hand side of Figure 2. The advantage of flat-plate collectors are inexpensive to manufacture and they can obtain both the diffuse and bean radiation, and the position is stable, no need to track the sun