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3. Concentrating Solar Power Plants 381

In the characterization of the solar radiation incident on the Earth, to estimate the
solar potential, several factors intervene, which includes climatic conditions (which
influence the degree of cloudiness, atmospheric turbidity, prevailing wind, etc.), time
of the year, the latitude of the place, and orientation of the receiving surface. Vari-
ations of the sunburst are relatively small, compared with other factors that deter-
mine it to a greater extent, such as the transparency of the atmosphere (defines
albedo, direct light, and diffuse light), the duration of light day (determines the en-
ergy reaching a particular point during the day), and the angle with which the sun’s
rays fall on the Earth (the more perpendicular the rays fall on a surface, the greater
the energy uptake). Of course, the distribution of the solar energy that arrives at the
Earth is not uniform. The abovementioned factors lead to the solar irradiance on the
2
surface of the Earth being between 900 and 1000 W/m .
6
The solar energy that arrives at the Earth is distributed as follows :
• Reflected by the atmosphere into outer space: 30%, 0.52 10 14 kW;
• Used in heating the atmosphere: 47%, 0.80 10 14 kW;
• Used in the evaporation of the oceans: 23%, 0.40 10 14 kW;
• Used to generate atmospheric disturbances, such as wind: 0.0037 10 14 kW;
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• Used in photosynthesis: 0.0004 10 kW.
Thus, 47% of the incident solar energy reaches the Earth’s surface (31% do so
directly and the other 16% after being diffused by dust, water vapor, and air mole-
cules). The rest of the solar energy, 53%, does not reach the surface of the Earth,
because 15% is absorbed by the troposphere (water, ozone, and clouds), 23% is re-
flected by clouds, 7% is reflected by the soil, 2% is absorbed by the stratosphere,
mainly by ozone, and the remaining 6% is the energy diffused by the atmosphere
that is directed toward the sky. 7
Solar radiation data are supplied to the public in the form of tables, maps, and
computational tools. The SWERA (Solar and Wind Energy Resource Assessment)
Project of the United Nations Environmental Programs Group, on the http://en.
openei.org/apps/SWERA/ website, offers free information.

3. CONCENTRATING SOLAR POWER PLANTS

To convert solar energy into power, the radiation may concentrate on a point or on a
line, where thermal energy is transferred to the heat transfer fluid (HTF). Consid-
ering these two types of concentration (on a point or on a line), CSP plants can
be classified in two main designs: parabolic trough and power tower.

6
See footnote 5.
7
P. Ferna ´ndez Dı ´ez, Procesos Termosolares en Baja, Media y Alta Temperatura [WWW Document], n.d.
URL: http://www.marioloureiro.net/ciencia/EnerSolarTermica/SolarPROCESOSTERMO/Solar1.pdf.