Environmental Engineering Reference
In-Depth Information
2.4 SOLAR RADIATION PROCESSES
2.4.1 Conversion of solar radiation into heat
2.4.1.1 Calculation the exergy of solar radiation
Solar energy is the most important renewable source of energy on the Earth. Solar
energy is a high temperature source, however it's harvesting occurs inefficiently due to
extensive degradation of the energy. The degradation of solar energy is well demon-
strated by exergy analysis. Therefore the engineering thermodynamics of thermal
radiation addresses mainly exergy analyses of diversified problems of utilization of
solar radiation.
Generally, the solar radiation passing through the atmosphere is absorbed, scat-
tered and reflected not only by air molecules but also by e.g. water vapor, clouds,
dust, pollutants, smoke from forest fires and volcanoes. These factors cause diffusion
(called also dilution) of solar radiation. The portion of solar radiation which reaches
the Earth's surface without being diffused is called direct beam solar radiation. Thus,
the global solar radiation (global irradiance) consists of the diffuse and direct solar
radiation. For example, during thick cloudy days the atmosphere reduces direct beam
radiation to zero.
Only a part of scattered sunlight reaches the Earth because some sunlight is scat-
tered back into space. Also some radiation from the Earth, together with sunlight
scattered off the Earth's surface is re-scattered to the atmosphere. This effect can be
significant e.g. when the Earth's surface is covered with snow.
Solar radiation energy is difficult to calculate because, as discussed, the radiation
energy reaching the surface on Earth, is composed of direct and diluted radiation
components, and depends on geographic location, time of day, season of year, local
weather and even on local landscape. The relatively effective method of determining of
solar radiation is by carrying out spectral measurement and application of the obtained
results in the formulae derived in paragraph 2.2.6.
A certain basis for the evaluation of solar radiation reaching the Earth's surface
can be the energy of solar radiation incident outside the Earth's atmosphere which is
called extraterrestrial radiation, and its average value is about 1367 W/m 2 . The exergy
of extraterrestrial radiation is determined with the following examples.
Example 2.4.1.1 The exergy of the extraterrestrial solar radiation, when recog-
nized as non-polarized, black, uniform and propagating within solid angle ω , may
be approximately calculated by means of equation (2.2.66). The required exergy b b
of emission density can be calculated from (2.2.65) for the Sun's surface temperature
T
=
6000 K and for the environment temperature T 0 =
300 K as follows:
10 8
5 . 6693
×
6000 4
300 4
6000 3 )
68 . 5MW / m 2
b b
=
(3
×
+
4
×
300
×
=
(a)
3
Approximately, the radius of the sun is R
=
695,500 km and the mean distance
from the Sun to the Earth is L
=
149,500,000 km. The integral in formula (2.2.66)
 
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