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5 USING OF SOLAR ENERGY
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5 USING OF SOLAR ENERGY
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5.1 Sun - an essential energy source
Sun radiation is the basis of renewable energy sources such as:
- biomass, hydro, wind and solar power.
Facts about the Sun
- star (gas, glowing sphere) with diameter 1,4.10
6
km
- distance of the Sun from the Earth is 149,6.10
6
km
- mass of the Sun is 1,9891 x 10
30
kg, that is 98 % mass of the solar system
(planets, moons of planets, asteroids, meteors, comets, and dust),
http://www.ta3.sk/~zkanuch/apvv/wwwheslar/pdf/1205224343_Slnko.pdf
http://www.astropresov.sk/ss_cisla.html
http://astronomy.wz.cz/info_slnko.php
http://www.youtube.com/watch?v=6Adt2BjZcqA&feature=related
http://www.youtube.com/watch?v=JWcSz6dlZfs
- age of the Sun, as the whole solar system is
estimated to be 4.6 billion years
- composition of the Sun: 74 % hydrogen,
23% helium and 3% of other elements
- Sun's surface temperature is 5780 K
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- Solar energy occurs in its core, in which
thermonuclear reaction
are ongoing,
- Solar core is the "nuclear furnace" with temperatures up to 15.10
6
ºC and a density 160
times greater than the density of water.
- under these conditions, each colliding nuclei of hydrogen atoms combine and turn into
helium nuclei.
- the resulting atom is slightly lighter than the original atom, the remainder material released
through the power.
- every second, the Sun converts 600 million tonnes of hydrogen at about 596 million tons of
helium
Deuterium: isotope of hydrogen (
2
H) Configuration: 1 proton, 1 neutron, 1 electron
Tritium: isotope of hydrogen (
3
H) Configuration: 1 proton, 2 neutrons, electrons 1
Isotopes of the chemical element are atoms of this element with the same atomic number but with different
numbers of neutrons (ie with different nucleon numbers). Nucleon number represents the total number of
nucleons (ie protons and neutrons) in the atomic nucleus.
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5
- thus, every second 4,3.10
6
ton of solar mass is converted to energy radiated
into the surrounding space (according to Einstein's relation E=m.c
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, it is a
energy 3,839.10
26
J)
- therefore, power of this enormous "thermo nuclear reactor" is
3,839.10
26
W
- Earth's surface receive only
1,725.10
17
W
(on the border of the atmosphere)
from the Sun energy
- it is about 10 000 times more than currently humanity needs.
- solar energy comes from thermonuclear reactions converting hydrogen to
helium (nuclear fusion), which will run according to estimates 5-7 billion years
http://www.ta3.sk/~zkanuch/apvv/wwwheslar/pdf/1205224343_Slnko.pdf
http://www.astropresov.sk/ss_cisla.html
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Integral value of the spectrum (the sum of energies of all the spectral elements) on the
border of the atmosphere is called the
solar constant
(about
1 367 W/m
2
)
Solar radiation
is electromagnetic radiation with wavelengths in the range 250 to 2500 nm.
1)
Ultraviolet
radiation (250-380 nm)
2)
Visible
light (380 to 780 nm)
3)
Infrared
radiation (780 - 2500 nm)
Energy spectrum of solar radiation
Spectral characteristics of solar radiation
Calculation of solar constant:
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Size of solar constants slightly change depending on:
http://en.wikipedia.org/wiki/File:Seasons1.svg
http://www.enerpoint.net/photovoltaic_technology_1.php
- the actual distance of the Earth
and the Sun (the movement is
slightly elliptical):
- changes in sunspot activity
(11-year cycle):
152 mil. km
147 mil. km
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- solar spectrum shape of the curve can be approximated by the curve a black body
radiation (5780 K).
Planck law - every body that has a temperature greater than zero (0) K, radiates
energy.
The energy spectrum of solar radiation
The spectrum of a black body radiation
(Planck law)
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http://www.daa.fmph.uniba.sk/index.php?utid=64&id=110
Part of solar radiation is
reflected
by the atmosphere and
absorbed
in the atmosphere by
molecules O
3
, O
2
, CO
2
, H
2
O. Atmosphere has a filter effect - especially in the UV range.
Solar spectrum above the atmosphere and the Earth's surface
-in the ultraviolet part of spectrum is dominated absorption by ozone
molecules O
3
in the ozone layer (absorption bands)
- in the infrared part of the radiation is dominated absorbtion mainly by
water vapor molecules H
2
O and also CO
2
molecules (absorption
bands).
The percentage of the solar
spectrum at the Earth's surface
Thus, on the Earth's surface (perpendicular to Sun) maximum
1000 W/m
2
reaches in good weather around midday.
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Other factors that affect the size of the radiant energy to the Earth:
- altitude (radiation passes through thinner layer of the atmosphere)
- air pollution (causes dispersion or absorbing radiation)
- clouds
- angle of solar rays, which depends on:
- place on the surface
- date and time
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The decreasing of radiation is also dependent on the angle of rays, that is of the thickness
of the atmosphere, which the radiation must pass.
Therefore, Air Mass Factor (AM) is defined.
AM
is optical thickness of the atmosphere,
which indicates –
multiply of the thickness of the atmosphere, which light must pass
.
(Air Mass is the measure of how far light passes through the Earth's atmosphere)
The optical depth of the atmosphere – AM (Air Mass factor)
AM value for the position of the sun in the sky
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AM 0 – Solar radiation on the border the atmosphere (power density is appr. 1367 W/m
2
-
solar constant). (AM0 describes solar irradiance in space, where it is unaffected by the
atmosphere)
AM 1.5 – The spectrum of solar radiation on the Earth's surface acting at an angle of 48.2°
from the perpendicular. Solar radiation passes through the thickness of 1.5xAM.
AM 1 - Sun is perpendicular to the ground, sunlight passes through the thickness of
atmosphere (1xAM).
Energy spectrum of solar radiation
http://www.eyesolarlux.com/Solar-simulation-energy.htm
Note: For PV testing, the Standard Test Condition (STC) is defined as an insolation of 1000W/m
2
at 25 °C
and with a solar spectral distribution equivalent to global AM1.5, per ASTM G173-03 and IEC 60904-3.
For solar simulation performance classification, a spectral irradiance distribution standard has been
established.
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http://www.volker-quaschning.de/articles/fundamentals1/index.php
Definition of angles to describe the position of the Sun
s
- elevation angle - the angle between the horizon
and the center of the Sun
s
– azimuth - the angle measured in a clockwise
direction, between geographic north and a point on the
horizon directly below the sun
Position of the Sun, AM value at the time of noon (i.e.,
when the Sun is during the day highest– max.
s
) for
different days in Berlin (Germany) and Cairo (Egypt)
AM value depends on:
- geographic location,
- the date and time.
Note: It is clear that in Cairo are at the same
time the value of AM always less than in
Berlin.
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Sun path diagram for Berlin
http://www.pvpowerway.com/en/civil-work/shading.html
0
noon time
(midday
)
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- sunlight coming at a different angle to the
Earth's surface at the poles than at the equator.
- consequently, the area covered by the ray is the
smallest at the equator and increases towards
the pole (area b is greater than a).
- therefore the Earth receives more radiation per
unit area at the equator than at the poles.
http://www.boinc.sk/projekty-knihy/zaklady-klimatologie
The average daily value (7/1983-6/2005) of
solar radiation in the world in kWh/m
2
/day
The total annual value of solar radiation in the
world in kWh/m
2
/year
http://www.inforse.org/europe/dieret/Solar/solar.html
http://www.brighthub.com/environment/science-environmental/articles/86226.aspx
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Variation in the amount of solar energy during the year
http://scienceblogs.com/startswithabang/2011/02/10/never-a-miscommunication/
- in the northern hemisphere are: days longer in summer than in winter, the angle of rays
is more perpendicular in summer than in winter
- when we are closer to the equator, there are the smaller differences between winter and
summer half of year
- is caused by inclination of the Earth's axis.
- during circulation of the Earth around the Sun is more irradiated once northern and
once a southern hemisphere
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Variation in the amount of solar energy during the year
http://scienceblogs.com/startswithabang/2011/02/10/never-a-miscommunication/
Daily sum of global irradiation (Aberdeen-Scottland, Upington-South Africa)
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Variation in the amount of solar energy during the year
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Variation of the amount of solar energy during the day
- it is caused by Earth's rotation on its axis
http://vudeevudeewiki.blogspot.sk/2012_01_01_archive.html
- June 21 (summer solstice) - the longest day of the year in the Northern Hemisphere,
the shortest day of the year in the Southern Hemisphere. On the Northern
Hemisphere summer begins, on the Southern Hemisphere winter begins.
- 21.december (winter solstice) - the shortest day of the year in the northern
hemisphere, the longest day of the year in the Southern Hemisphere. The northern
hemisphere winter begins, the southern hemisphere summer.
- 21st March (spring equinox), 23rd September (autumnal equinox). Earth's axis is
perpendicular to the line joining the Earth-Sun
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The total amount of radiation received by Earth from the Sun is given by the Earth's
cross-section (
r
2
).
The average value of the amount of solar radiation received by Earth
As the planet rotates, the energy is distributed over the entire surface (4
r
2
).
Therefore, the average amount of solar radiation
(called insolation) is equal to one quarter of the
solar constant – appr. 342 W/m².
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Due to obstacles in the atmosphere and on the surface we register three basic types
of solar radiation:
1) Direct radiation,
which directly enters the earth atmosphere (its direction is not
changed)
2) Diffused radiation
, which enters the earth atmosphere by reflecting from clouds,
gases, aerosols. It acts from all sides of the sky.
3) Reflected radiation
, reflected from the surroundings (ground, sea or other objects)
The sum of direct, diffused and reflected radiation is
total (global) radiation
,
influencing design and performance of solar energy systems.
Direct, diffuse, global solar radiation
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Note.: In our geographical conditions, the total duration of sunshine without a cloud
is around 1200 - 2000 hours per year, depending on the inverse of mist and cloud.
The intensity of solar
radiation and share of diffuse
radiation in different weather
conditions
The average annual (4/2004-3/2010) value of solar radiation
in Europe in kWh/m
2
/year
- Annual value of solar radiation in our geographical conditions:
appr.
1100 kWh per 1 m
2
- of which 75% in the period from April to October, 25% in the rest of the time
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The share of direct and diffuse solar radiation on the global radiation in Slovakia:
Global Sun radiation in Slovakia
Direct Sun radiation in Slovakia
Diffuse Sun radiation in Slovakia
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The average annual (4/2004-3/2010) value
of solar radiation in Poland in kWh/m
2
/year
The average annual (4/2004-3/2010) value of solar
radiation in Slovakia in kWh/m
2
/year
