Page 407 - Advances in Renewable Energies and Power Technologies

380 CHAPTER 12 Concentrating Solar Power

FIGURE 12.5
Monthly insolation values in a year.
Courtesy M. Kaltschmitt, W. Streicher, A.A. Wiese, Renewable Energy: Technology, Economics and Environ-
ment, Springer, Berlin, Heidelberg, 2007.

known as solar peak hour or simply pick hour, whose generalized units are as fol-
lows: 1 h solar peak ¼ 1000 Wh.
The radiation received at the Earth’s surface consists of direct and scattered (plus
reflected) short-wavelength radiation plus long-wavelength radiation from sky and
clouds, originating as thermal emission or by reflection of thermal radiation from
the ground. Direct radiation is defined as the radiation that has not experienced scat-
tering in the atmosphere, so that it is directionally fixed, coming from the disk of the
Sun. Scattered radiation is, then, the radiation that experienced scattering processes
in the atmosphere. In practice, it is often convenient to treat radiation that has expe-
rienced only forward scattering processes together with the unscattered radiation,
and thus direct and scattered radiation are defined as radiation coming from or
not coming from the direction of the Sun. A fraction of the incoming solar radiation
is reflected back into space. This fraction is called the albedo of the Earthe
atmosphere system. Its medium value is about 0.35. This is composed of about
0.2 from reflection on clouds, 0.1 from reflection on cloudless atmosphere (particles,
gases), and 0.05 from reflection on the Earth’s surface. If there were no air, obviously
all the radiation would be direct, and our eye would receive no radiation (the sky
would be black). Clouds diffuse solar radiation more strongly than dry air; therefore
on a cloudy day all the radiation that we get will be diffuse radiation. On a typical
clear day, the direct radiation is several times greater than the diffuse one.