Energy in the Sun.
By Emil Bedi, CANCEE and Hakan Falk, "Energy Saving Now".
SOLAR RADIATION
 |
| Solar radiation is electromagnetic radiation in the 0.28...3.0 µm
wavelength range. The solar spectrum includes a small share of ultraviolet
radiation (0.28...0.38 µm) which is invisible to our eyes and comprises
about 2% of the solar spectrum, the visible light which range from 0.38 to
0.78 µm and accounts for around 49% of the spectrum and finally of
infrared radiation with long wavelength (0.78...3.0 µm), which makes up
most of the remaining 49% of the solar spectrum. |
HOW MUCH SOLAR ENERGY STRIKES THE EARTH?
The sun generates an enormous amount of energy -
approximately 1.1 x 10 E20 kilowatt-hours every second. (A kilowatt-hour is the
amount of energy needed to power a 100 watt light bulb for ten hours.) The
earth’s outer atmosphere intercepts about one two-billionth of the energy
generated by the sun, or about 1500 quadrillion (1.5 x 10 E18 ) kilowatt-hours
per year. Because of reflection, scattering, and absorption by gases and
aerosols in the atmosphere, however, only 47% of this, or approximately 700
quadrillion (7 x 10 E17 ) kilowatt-hours, reaches the surface of the earth.
 |
In the earth’s atmosphere, solar radiation is received directly
(direct radiation) and by diffusion in air, dust, water, etc., contained
in the atmosphere (diffuse radiation). The sum of the two is referred to
as global radiation. | The amount of incident
energy per unit area and day depends on a number of factors, e.g.:
 latitude
local climate
season of the year
inclination of the collecting surface in the direction of the
sun.
TIME AND SITE
The solar energy varies because of the relative motion of the sun. This
variations depend on the time of day and the season. In general, more
solar radiation is present during midday than during either the early morning or
late afternoon. At midday, the sun is positioned high in the sky and the path of
the sun’s rays through the earth’s atmosphere is shortened. Consequently, less
solar radiation is scattered or absorbed, and more solar radiation reaches the
earth’s surface.
 The amounts of solar energy arriving at the earth’s
surface vary over the year, from an average of less than 0,8 kWh/m2 per day
during winter in the North of Europe to more than 4 kWh/m2 per day during summer
in this region. The difference is decreasing for the regions closer to the
equator.
The availability of solar energy varies with geographical
location of site and is the highest in regions closest to the equator. Thus the
average annual global radiation impinging on a horizontal surface which amounts
to approx. 1000 kWh/m2 in Central Europe, Central Asia, and Canada reach approx.
1700 kWh/m2 in the Mediterranian and to approx. 2200 kWh/m2 in most equatorial
regions in African, Oriental, and Australian desert areas. In general, seasonal
and geographical differences in irradiation are considerable (see the table
bellow) and must be taken into account for all solar energy applications.
Variations of solar irradiation (tilt angle South 30Deg.) in
Europe and Caribbean region in kWh/m2.day.
| |
Southern
Europe |
Central
Europe |
North
Europe |
Caribbean |
|
January |
2,6 |
1,7 |
0,8 |
5,1 |
|
February |
3,9 |
3,2 |
1,5 |
5,6 |
|
March |
4,6 |
3,6 |
2,6 |
6,0 |
|
April |
5,9 |
4,7 |
3,4 |
6,2 |
|
May |
6,3 |
5,3 |
4,2 |
6,1 |
|
June |
6,9 |
5,9 |
5,0 |
5,9 |
|
July |
7,5 |
6,0 |
4,4 |
6,0 |
|
August |
6,6 |
5,3 |
4,0 |
6,1 |
|
September |
5,5 |
4,4 |
3,3 |
5,7 |
|
October |
4,5 |
3,3 |
2,1 |
5,3 |
|
November |
3,0 |
2,1 |
1,2 |
5,1 |
|
December |
2,7 |
1,7 |
0,8 |
4,8 |
|
YEAR |
5,0 |
3,9 |
2,8 |
5,7 |
CLOUDS
The amount of solar radiation reaching
the earth’s surface varies greatly because of changing atmospheric conditions
and the changing position of the sun, both during the day and throughout the
year. Clouds are the predominant atmospheric condition that determines the
amount of solar radiation that reaches the earth. Consequently, regions of the
nation with cloudy climates receive less solar radiation than the cloud-free
desert climates. For any given location, the solar radiation reaching the
earth’s surface decreases with increasing cloud cover. Local geographical
features, such as mountains, oceans, and large lakes, influence the formation of
clouds; therefore, the amount of solar radiation received for these areas may be
different from that received by adjacent land areas. For example, mountains may
receive less solar radiation than adjacent foothills and plains located a short
distance away. Winds blowing against mountains force some of the air to rise,
and clouds form from the moisture in the air as it cools. Coastlines may also
receive a different amount of solar radiation than areas further inland.
The solar energy which is available during the day varies and depends
strongly on the local sky conditions. At noon in clear sky conditions, the
global solar irradiation can in e.g. Central Europe reach 1000 W/m2 on a
horizontal surface (under very favourable conditions, even higher levels can
occur) whilst in very cloudy weather, it may fall to less than 100 W/m2 even at
midday.
POLLUTION
Both man-made and naturally occurring
events can limit the amount of solar radiation at the earth’s surface. Urban air
pollution, smoke from forest fires, and airborne ash resulting from volcanic
activity reduce the solar resource by increasing the scattering and absorption
of solar radiation. This has a larger impact on radiation coming in a direct
line from the sun (direct radiation) than on the total (global) solar radiation.
On a day with severely polluted air (smog alert), the direct solar radiation can
be reduced by 40%, whereas the global solar radiation is reduced by 15% to 25%.
A large volcanic eruption may decrease, over a large portion of the earth, the
direct solar radiation by 20% and the global solar radiation by nearly 10% for 6
months to 2 years. As the volcanic ash falls out of the atmosphere, the effect
is diminished, but complete removal of the ash may take several years.
POTENTIALS
Solar radiation provides us at zero cost with 10,000 times more energy
than is actually used worldwide. All people of the world buy, trade, and sell a
little less than 85 trillion (8.5 x 10E13 ) kilowatt-hours of energy per year.
But that’s just the commercial market. Because we have no way to keep track of
it, we are not sure how much non-commercial energy people consume: how much wood
and manure people may gather and burn, for example; or how much water
individuals, small groups, or businesses may use to provide mechanical or
electrical energy. Some think that such non-commercial energy may constitute as
much as a fifth of all energy consumed. But even if this were the case, the
total energy consumed by the people of the world would still be only about one
seven-thousandth of the solar energy striking the earth’s surface per year.
In some developed countries like in the United States people consume
roughly 25 trillion (2.5 x 10E13 ) kilowatt-hours per year. This translates to
more than 260 kilowatt-hours per person per day - this is the equivalent of
running more than one hundred 100 watt bulbs all day, every day. U.S. citizen
consumes 33 times as much energy as the average person from India, 13 times as
much as the average Chinese, two and a half times as much as the average
Japanese, and twice as much as the average Sweden.
Even in such heavy energy consuming countries like USA solar energy
falling on the land mass can many times surplus the energy consumed there.
If only 1% of land would be set aside and covered by solar systems (such as
solar cells or solar thermal troughs) that were only 10% efficient, the sunshine
falling on these systems could supply this nation with all the energy it needed.
The same is true for all other developed countries. In a certain sense, it is
impractical - besides being extremely expensive, it is not possible to
cover such large areas with solar systems. The damage to ecosystems might be
dramatic. But the principle remains. It is possible to cover the same total area
in a dispersed manner - on buildings, on houses, along roadsides, on dedicated
plots of land, etc. In another sense, it is practical. In many countries already
more than 1% of land is dedicated to the mining, drilling, converting,
generating, and transporting of energy. And the great majority of this energy is
not renewable on a human scale and is far more harmful to the environment than
solar systems would prove to be.
SOLAR ENERGY UTILISATION
In most places of the world much more solar
energy hits a home’s roof and walls as is used by its occupants over a year’s
time. Harnessing this sun’s light and heat is a clean, simple, and natural way
to provide all forms of energy we need. It can be absorbed in solar collectors
to provide hot water or space heating in households and commercial buildings. It
can be concentrated by parabolic mirrors to provide heat at up to several
thousands degrees Celsius. This heat can be used either for heating purposes or
to generate electricity. There exist also another way to produce power from the
sun - through photovoltaics. Photovoltaic cells are devices which convert
solar radiation directly into electricity.
Solar radiation can be converted into useful energy using active systems
and passive solar design. Active systems are generally those that are very
visible like solar collectors or photovoltaic cells. Passive systems are defined
as those where the heat moves by natural means due to house design which entails
the arrangement of basic building materials to maximize the sun’s energy.
Solar energy can be converted to useful energy also indirectly, through
other energy forms like biomass, wind or hydro power. Solar energy drives the
earth“s weather. A large fraction of the incident radiation is absorbed by the
oceans and the seas, which are warmed than evaporate and give the power to the
rains which feed hydro power plants. Winds which are harnessed by wind turbines
are getting its power due to uneven heating of the air. Another category of
solar-derived renewable energy sources is biomass. Green plants absorb sunlight
and convert it through photosynthesis into organic matter which can be used to
produce heat and electricity as well. Thus wind, hydro power and biomass are all
indirect forms of solar energy.
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