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Solar Radiation & Measurement

• Solar radiation reaches the Earth’s surface at a
maximum flux density of about 10kWm−2 in a
Chapter three wavelength band between 0.3 and 25m.
• The spectral distribution is determined by the
6000K surface temperature of the Sun.
Solar energy • . The proportion that reaches a device depends on
geometric factors, such as latitude and on
atmospheric characteristics, such as infrared
radiation absorption by water vapors, carbon
dioxide and other such molecules.

Introduction Structure of Sun

• Solar energy is referred to as renewable because • The Sun is a typical middle-aged star with a
it will be available as long as the Sun continues diameter of 1.39 × 106 km, a mass of 2 × 1030 kg.
to shine. • The Sun is a plasma, primarily composed of 70%
• Solar radiation is the most important natural hydrogen and 28% helium.
energy resource because it drives all • This changes over time as hydrogen is converted
environmental processes acting at the surface of to helium in its core by thermonuclear reactions.
the Earth. • Every second, 700 million tons of hydrogen is
• Although the solar energy source is converted into helium.
inexhaustible and free, it is not the most • The Sun is composed of the core, the radiation
convenient energy source because it is not and the convection zones, and its atmosphere.
constant during the day and not readily
dispatched. • The conditions of the Sun vary greatly along its
radius.
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• The core, with a radius of 0.2R, is the source of all • The solar atmosphere, the exterior of the Sun, is
the Sun’s energy and it contains half of the Sun’s composed of the photosphere, chromosphere,
mass. and the corona.
• The combination of high temperature and high • The photosphere corresponds to the lowest and
density creates the correct environment for the densest part of the atmosphere.
thermonuclear reaction to take place.
• Because the Sun is completely made of gas and
• Two atoms of hydrogen come together to produce there is no hard surface, the photosphere is
one heavier atom of helium, releasing a great usually referred to as the Sun’s surface.
amount of energy.
• Once energy is produced in the core, it travels
from the center to the outer regions.
• The region immediate to the core is identified as
the radiation zone because energy is transported
by radiation and it extends to 0.7R.

• It takes thousands of years for the energy AIR MASS

released by the core to exit this zone. The • The Air Mass is the path length which light takes through
the atmosphere normalized to the shortest possible path
temperature in the radiation zone is about 5 × length (that is, when the sun is
106 K. directly overhead).
• Once the energy has left this zone and its • The Air Mass quantifies the reduction in the power of light
temperature has dropped down to 2 × 106 K, as it passes through the atmosphere and is absorbed by air
and dust. The Air Mass is defined as:
rolling turbulent motions of gases arise; this is
known as the convection zone.
• It takes around a week for the hot material to
bring its energy to the top of the convection
zone. This layer extends from 0.7R to R.
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The mass difference ∆m will be calculated by:

The mass of the neutrinos can be ignored in this calculation

The mass difference is converted into energy ∆E, with the relationship:

C: the speed of light = 2.998•108 m/s,

The sun loses 4.3 million metric tones of mass per second (∆m
=4.3x109 kg/s). This results in the solar radiant power Φe,S of:

This value divided by the sun’s surface area, AS, provides the specific
emission of the sun:

Every square metre of the sun’s surface emits a radiant power of 63.11 MW

• The sun consists of about 80 per cent hydrogen, 20 per

The specific emission at the sun’s surface is much higher than
cent helium and only 0.1 per cent other elements. at the sphere surrounding the sun.
• Nuclear fusion processes create the radiant power of
The Radiant Power through the
the sun. Surface of a Sphere with Radius rSE
• During these processes, four hydrogen nuclei (protons is the Same as through the Surface
1p) fuse to form one helium nucleus (alpha particle 4α ). of the Sun. With
The alpha particle consists of two neutrons 1n and two M e ,S A S  E e.A SE
positively charged protons p. Furthermore, this reaction
and substituting
produces two positrons e+ and two neutrinos and
2
generates energy. A SE  4  rSE
the irradiance at the Earth, Ee,
finally becomes:
AS r2 (rSE = 1.5 • 108 km)
Ee  M e,S  M e,S 2S
A SE r SE As = 6.0874•1012 km2
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• The axis of the Earth is tilted at an angle of 23.458 to the plane of its elliptic path
This determines the extraterrestrial irradiance experienced at
around the sun.
Earth’s orbital distance from the sun. However, the distance • This tilt is the major cause of the seasonal variation of solar radiation available at any
between the sun and Earth is not constant throughout the year. It location on Earth. The angle between the Earth–sun line and a plane through the
varies between 1.47 • 108 km and 1.52 • 108 km. This causes a equator is called solar declination, δ.
variation in the irradiance, Ee, of between 1325 W/m2 and • The declination varies between -23.458 to +23.458 in 1 year. It may be estimated by
1420 W/m2. The average value, called the solar constant I0, is: the relation:

Ee = 1367 W/m2  = 23.45° · sin[360(284 + N)/365°]

This value can be

measured outside the
Earth’s atmosphere

Diffuse radiation: Scattered solar radiation coming from the sky.

The extraterrestrial solar radiation varies throughout the year Direct or beam normal radiation: Part of solar radiation coming
because of the variation in the Earth–sun distance (D) as: from the direction of the sun on a surface normal to the sun’s rays.
Equation of time: Correction factor in minutes, to account for the
irregularity of the Earth’s motion around the sun.
Extraterrestrial solar radiation: Solar radiation outside Earth’s
atmosphere.
Insolation: Incident solar radiation measured as W/m2
Solar altitude angle: Angle between the solar rays and the
horizontal plane.
Solar azimuth angle: Angle between the true south horizontal line
where and the horizontal projection of the sun’s rays.
Solar constant: Extraterrestrial solar radiation at the mean Earth–
and N=Day number (starting from January 1 as 1). sun distance.
Solar declination: Angle between the Earth–sun line and a plane
Example : What is the value of the solar declination on through the equator.
February 15?
Solution: The value of N for February 15 is 31 + 15 = 46
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• For PV systems that have to operate at night or during the

period of bad weather the storage of energy is required, the
batteries for electricity storage are needed.
• The output of a PV module depends on sunlight intensity and
•Direct Solar Energy conversion: Solar radiation can cell temperature; therefore components that condition the DC
be converted into useful energy directly, using various (direct current) output and deliver it to batteries, grid, and/or
technologies. Such as: load are required for a smooth operation of the PV system.
• Photovoltaic
These components are referred to as charge regulators.
• Solarthermal power plant
• Solarthermal collector • For applications requiring AC (alternating current) the
DC/AC inverters are implemented in
•Indirect Solar Energy conversion: Solar radiation can PV systems.
be converted to useful energy indirectly, via other • These additional components form that part of a PV system
energy forms. Such as: that is called balance of system (BOS).
• Wind power
• Hydropower
• Bioenergy
• Wave power

PHOTOVOLTAIC SYSTEMS
• Components of a PV system
The solar energy conversion into electricity takes place in
a semiconductor device that is called a solar cell. array
Cell
• A solar cell is a unit that delivers only a certain amount of
electrical power.
• In order to use solar electricity for practical devices, which
require a particular voltage or current for their operation:
• a number of solar cells have to be connected together to
form a solar panel, also called a PV module.
• For large-scale generation of solar electricity the solar
panels are connected together into a solar array.
module
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Figure The components of a PV system.