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378 CHAPTER 12 Concentrating Solar Power

• Its chemical composition is w91.20% of hydrogen and 8.70% of helium, and
less than 2% is accounted for by other elements such as oxygen, carbon, iron,
sulfur, and neon.
• It emits energy in the form of electromagnetic radiation (photons), which
propagates radially in space, traveling at the speed of light in the vacuum
(c ¼ 300,000 km/s). The propagation is in all directions through space or some
medium, although a material medium is not needed (as in the case of conduction
and convection) to propagate, independently of its wavelength (l) and fre-
quency (q), but there is a well-defined relationship between these three ele-
ments: c ¼ l$q. However, the speed, intensity, and direction of its energy flow
are affected by the presence of matter, such as Earth’s atmosphere. A photon
takes only about 8 min to travel from the photosphere to the Earth. Once photon
gets into our planet, its unique atmosphere and peculiar sea and land surface
work out the incoming radiation. This complex pattern of reflection, absorption,
scattering, and reabsorption effects is one of the countless prerequisites that
makes possible life on Earth because, among others, it prevents the highest
energy radiation (ultraviolet and others rays) from reaching the surface and it
warms the planet to a comfortable 15 C average temperature.

• The solar mass that is irradiated per second into space in the form of high-energy
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particles and electromagnetic radiation is approximately 5.6 10 GeV
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(1 eV ¼ 1.6 10 J).
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The average distance between Sun and Earth is 149.6 10 km, known as the
Astronomical Unit (UA), which varies because the Earth’s trajectory is an elliptical;
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presenting extreme values between 147.05 10 km (Perihelio), and
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152.14 10 km (Afelio), the points closest to and farthest from the Sun, respec-
tively (Fig. 12.4).
The Sun’s energy is generated within the core by nuclear reactions, the most
important one involving four protons of hydrogen, which combine to form a nucleus
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of helium. Approximately 650 10 t/s of hydrogen is converted into approxi-
2
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mately 646 10 t/s of helium. According to Einstein’s equation (E ¼ mc ), a po-
wer of about 3.9 10 26 J/s (W) of nuclear energy is delivered, that is, about
1 million times our current global primary energy consumption per year (as a further
comparison, a 1000 MW coal or nuclear power plant converts 0.130 kg of matter
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into energy in 1 year). The Sun’s temperature is about 14 10 K in the core and
5760 K in the surface. For simplified calculations of engineering, it is common to
adopt for the temperature of the Sun the approximate value of 6000 K. The largest
amount of radiant energy in the Sun is concentrated in the visible and near visible
range of the spectrum: visible light 43%, near-infrared 49%, ultraviolet 7%, and
other ranges 1%.
Solar energy advantages aredapart from being freedthe low impact of its
installation because it does not involve gas or acoustic pollution; it is also a simple
installation, which requires minimal maintenance. The energy obtained can be used
directly in numerous applications, from street lamps to satellites. It can be stored in