Solar radiation which is received by an object can be transmitted (τ), reflected (ρ), and or absorbed (α).
τ + ρ +α =1
The equation is valid for light with different wavelengths, as well as to calculate solar radiation and long wave radiation from absorber plate of solar collector. Cover plate of solar collector materials should have high transmissivity values while absorpsivity and reflexivity should minimum. This is because solar radiation which is received by cover plate of solar collector should be forwarded maximally to absorber plate so that radiation energy can be transferred to working fluid as much as possible (reference: Solar Thermal Engineering - Lunde, Peter J 1980).
There are many materials of cover plate that can deliver good solar radiation, but absorb less energy of incoming ray. Glass, air, water and transparent plastic materials are among the materials. In general, glass absorbs 50-10% of incoming ray. Iron oxide content affects absorpsivity and color of glass.
For glass with low iron levels, of 0.05% iron oxide, will lose only about 2.5% of the light that is transmitted to be absorbed by glass with general thickness. For white water glass with very low iron content of 0.01% iron oxide can forward approximately 91.4% of solar radiation.
|Table 1: Cover Plate Properties of Solar Collector|
The table above is important to know heat losses in the cover plate of solar collector. Furthermore intensity of solar radiation which is received and utilized by solar collector can be known.