3 History of Solar Energy
The solar industry started in the early 1920s and growth lasted until the mid-1950s when low-cost natural
gas became the primary fuel for heating. Today, people use solar energy to heat buildings, heat water and
to generate electricity. The spectral distribution is determined by the 6000K surface temperature of the
Sun shown in (Sukhatme, 1999) Fig (a).This is an energy flux of very high thermodynamic quality, from an accessible source of temperature very
much greater than from conventional engineering sources. The temperatures of the Earth’s atmosphere,
at about 230 K, and the Earth’s surfaces, at about 260–300 K, remain in equilibrium at much less than the
6000K temperature of the Sun. Solar short wave radiation passes through the Earth’s atmosphere, a
complicated set of interactions occurs. The interactions which include absorption, the conversion of
radiant energy to heat and the subsequent re-emission as long wave radiation: scattering, the wavelength
dependent on change in direction, so that usually no extra absorption occurs and the radiation continues at
the same frequency and reflection, which is independent of wavelength. From the natural heat flux, the
solar collector allows sunlight through receiver glass tube before it strikes the absorber tube. The glass
tube or plate traps most of the solar radiation inside collector using greenhouse effect. The Solar energy
(sun) irradiance is about 63 MW/m2
. However, Sun–Earth geometry dramatically decreases the solar
energy flow down to around 1 kW/m2 on the Earth’s surface. Nevertheless, under high solar flux, this
disadvantage can be overcome by using dissimilar types of concentrating solar systems which transform
solar energy into alternative form of solar thermal energy.