Sustainable Energy Technologies

Unit-I
Solar Radiation

K. Naga Suresh
Assistant Professor
Department of Mechanical Engineering
Introduction to Energy Sources:
Energy is the primary and most universal measure of all kinds of work by
human beings and nature. Every thing what happens in the world is the expression of flow
of energy in one of its forms. Most people use the word energy for input to their bodies or
to the machines and thus think about crude fuels and electric power

Types of Energy Sources: I) Primary Energy sources

II)Secondary Energy Sources
III)Supplementary sources
I) Primary Energy sources: Primary energy sources can be defined as sources which
provide a net supply of energy. Coal, oil, uranium etc. are examples of this type. The
energy required to obtain these fuels is much less than what they can produce by
combustion or nuclear reaction. Their energy yield ratio is very high
II)Secondary Energy Sources: Though it may be necessary for the economy, these may
not yield net energy. Intensive agricultural is an example wherein terms of energy the
yield is less than the input.
III) Supplementary sources: This is defined as those whose net energy yield is zero and
those requiring highest investment in terms of energy Insulation (thermal) is an example
for this source.
Coal, natural gas, oil and nuclear energy using breeder reactor are net energy yielders
and are primary sources of energy. Secondary sources are like solar energy, wind energy,
water energy etc. Solar energy can be used through plants, solar cells and solar heaters.
Solar tower is another emerging technology. Solar drying and solar heating are
economical applications when passive methods are used. Because of the dilute nature of
solar energy it is difficult to classify the source as a primary one. Better sources are wind,
tide, wave and hydroelectric applications. Geothermal and ocean thermal are the other
sources which may well prove worthwhile. It may be necessary in future to develop the
secondary sources like solar, wind etc.
Energy Sources and their Availability:
Today, every country draws its energy needs from a variety of sources. We can broadly
categorize these sources as commercial and noncommercial. The commercial sources
include the fossil fuels (coal, oil and natural gas), hydroelectric power and nuclear power,
while the non-commercial sources include wood, animal waste and agricultural wastes. In
an industrialized country like, U.S.A., most of the energy requirements are met from
commercial sources, while in an industrially less developed country like India, the use of
commercial and non-commercial sources are about equal.

Commercial or Conventional Energy Sources:

Major Sources of energy include:
(1) Fossil fuels i.e. solid fuels (mainly coal including anthracite, bituminous, and brown
coals lignits and peals), liquid and gaseous fuels including petroleum and its derivatives
and natural gas
(2) Water power or energy stored in water.
(3) Energy of nuclear fission.

Minor Sources of energy include:

Minor sources of energy include sun, wind, tides in the sea, geothermal, ocean thermal
electric conversion, fuel cells, thermionic, thermoelectric generators.
• Wood was dominant source of energy in the pre-industrialization era. It gave way to
coal and coke.
• Use of coal reached a peak in the early part of the twentieth century.
• Wood is no more regarded as a conventional source.
• Oil get introduced at that time and has taken a substantial share from wood and coal
• Hydroelectricity has already grown to a stable level in most of the developed
countries. A brief account of the various important sources of energy and their future
possibilities is given below.

• Coal, oil, gas, uranium and hydro are commonly known as commercial or
conventional energy sources.
Solar Radiation and Its Measurement:
• In general, the energy produced and radiated by the sun, more specifically the term refers to
the sun’s energy that reaches the earth.
• Solar energy, received in the form of radiation, can be converted directly or indirectly into
other forms of energy, such as heat and electricity, which can be utilized by man.
The major drawbacks to the extensive application of solar energy are:
1. The intermittent and variable manner in which it arrives at the earth’s
surface and
2. The large area required to collect the energy at a useful rat
Experiments are underway to use this energy for power production, house heating, air-
conditioning, cooking and high temperature melting of metals.

• Energy is radiated by the sun as electromagnetic waves of which 99 per cent have wave
lengths in the range of 0.2 to 4.0 micrometers (1 micrometer = 10-6 meter).
• Solar energy reaching the top of the earth’s atmosphere consists of about 8% ultraviolet
radiation (short wave length, less than 0.39 micrometer), 46% visible light (0.39 to 0.78
micrometer), and 46% infrared radiation (long wave length more than 0.78 micrometer).
Solar Constant:
• The sun is a large sphere of very hot gases, the heat being generated by various kinds of fusion
reactions. Its diameter is 1.39 x 106 km while that of the earth is 1.27 x 104 km.
• The mean distance between the two is 1.50 x 108 km. Although the sun is large, it subtends an
angle of only 32 minutes at the earth’s surface. This is because it is also at a very large
distance.
• Thus the beam radiation received from the sun on the earth is almost parallel. The, brightness
of the sun varies from its centre to its edge. However for engineering calculations, it is
customary to assume that the brightness all over the solar disc is uniform.

The rate at which solar energy arrives at the top of the atmosphere is called the solar constant. It
is denoted by Isc.
• This is the amount of energy received in unit time on a unit area perpendicular to the sun’s
direction at the mean distance of the earth from the sun.
• Because of the suns distance and activity vary throughout the year, the rate of arrival of solar
radiation varies accordingly.
• The so called solar constant is thus an average from which the actual values vary upto about 3
percent most practical purposes.
• The National Aeronautics and Space Administration’s (NASA) standard value for the solar
constant, expressed in three common units, is as follows:
• 1.353 kw/ m2 or 1353 w/m2.
• 116.5 langleys (calories per sq. cm)/ hr, or 1165 kcal/m2/hr (1 langley being equal to 1 cal/cm2
of solar radiation received in one day).
• 429.2 Btu/sq. ft/ hr.
• The distance between the earth and the sun varies a little through the year. Because of this
variation, the extraterrestrial (out side the earth’s atmosphere) flux also varies. The earth is
closest to the sun in the summer and farthest away in the winter.
• This variation in distance produces a nearly sinusoidal variation in the intensity of solar
radiation I that reaches the earth. This can be calculated by the equation

where, n is the day of the year. As the distance between earth and sun varies a little through the
year, due to it extraterrestrial radiation also varies.
The percentage of radiation obtained up to certain wave length is obtained in table
Solar Radiation at the Earth’s Surface:
Solar radiation received at the surface of the earth is entirely different due to the various reasons.
Before studying this it is important to know the following terms:
1. Beam and Diffuse Solar Radiation
2. Sun at Zenith
3. Air mass (m)
4. Attenuation of Beam Radiation a) Absorption b) Scattering
1. Beam and Diffuse Solar Radiation:
1. Beam Radiation, Diffuse Solar Radiation and Total Radiation:
• The solar radiation that penetrates the earth’s atmosphere and reaches the surface differs in
both amount and character from the radiation at the top of the atmosphere. In the first place,
part of the radiation is reflected back into the space, especially by clouds. Further more, the
radiation entering the atmosphere is partly absorbed by molecules in the air. Oxygen and ozone
(O3) formed from oxygen, absorb nearly all the ultraviolet radiation, and water vapour and
carbon dioxide absorb some of the energy in the infrared range.
• In addition, part of the solar radiation is scattered (i.e., its direction has been changed) by
droplets in clouds by atmospheric molecules, and by dust particles.
• Solar radiation that has not been absorbed or scattered and reaches the ground directly from the
sun is called “direct radiation” or Beam radiation. It is the radiation which produces a shadow
when interrupted by an opaque object. Diffuse radiation is that solar radiation received from
the sun after its direction has been changed by reflection and scattering by the atmosphere.
• Because of the solar radiation is scattered in all directions in the atmosphere, diffuse radiation
comes to the earth from all parts of the sky.
• The total solar radiation received at any point on the earth’s surface is the sum of the direct and
diffuse radiation. This is referred to in a general sense as the insolation at that point. More
specifically, the insolation is defined as the total solar radiation energy received on a horizontal
surface of unit area (e.g., 1 sq. m) on the ground in unit time (e.g., 1day).
2. Sun at Zenith: Position of the sun directly over head.
3. Air mass (m):
• It is the path length of radiation through the atmosphere, considering the vertical path at sea
level as unity.

• The air mass m is the ratio of the path of the sun’s rays through the atmosphere to the length
of path when the sun is at the zenith.
 m = 1 when the sun is at zenith, i.e., directly over head.
m = 2 when zenith angle is 60° (0z, the angle subtended by the zenith and the line of
sight to the sun).
m = sec Ɵz when m > 3.
m = 0 just above the earth’s atmosphere.
4. Attenuation of Beam Radiation:
The variation in solar radiation reaching the earth than received at the outside of the atmosphere is
due to absorption and scattering in atmosphere.
a)Absorption: As solar radiation passes through the earth’s atmosphere the short-wave ultraviolet
rays are absorbed by the ozone in the atmosphere and the long wave infrared waves are absorbed
by the carbon dioxide and moisture in the atmosphere. This results in narrowing of the band
width. In fact most of the terrestrial solar energy (i.e., energy received by the earth) lies within the
range of 0.29 µm to 2.5 µm.
b) Scattering: As solar radiation passes through the earth’s atmosphere the components of the
atmosphere, such as water vapour and dust, scatter a portion of the radiation. A portion of this
scattered radiation always reaches the earths surface as diffuse radiation. Thus the radiation finally
received at the earths surface consists partly of beam radiation and partly of diffuse radiation.
Ozone absorbs mainly in the ultraviolet band. It absorbs almost completely the short wave
radiation below 0.29 µm, and its transmittance is almost unity above wavelengths of 0.35 µm.
Water vapour absorbs mainly in the infrared bands. At wavelength lower than 2.3 µm, the extra
terrestrial solar radiation by H2O and CO2 in atmosphere, is strong. Hence for terrestrial
application of solar energy, only wavelengths between 0.29 and 2.5 µm need be considered.
shows spectral distribution curves. Under favourable atmospheric conditions, the maximum
intensity observed at noon on an oriented surface at sea level is 1 kW/m2. At an altitude of 1000
metres, the value rises to about 1.05 kW/m2, and in higher mountains values slightly above 1.1
kW/m2 are obtained, compared with 1.353 kW/m2 (the solar constant) in outer space.
Solar Radiation Geometry:
Solar Radiation Measurement:
Measurements of solar radiation are important because of the increasing number of solar heating
and cooling applications, and the need for accurate solar irradiation data to predict performance.
Two basic types of instruments are employed for solar radiation measurement:
1. pyrheliometer a) Angstrom pyrheliometer b) Abbot silver disk pyrheliometer
2. pyranometer
3. sunshine Recorder

1. Pyrheliometer:
• A pyrheliometer is an instrument which measures beam radiation on a surface normal to the
sun’s rays.
• The sensor is a thermopile and its disc is located at the base of a tube whose axis
is aligned in the direction of the sun’s rays.
• Thus, diffuse radiation is blocked from the sensor surface.
• The pyrheliometer designed by Eppley Laboratories, USA, consists of bismuth silver
thermopile, with 15 temperature-compensated junctions connected in series.
• It is mounted at the end of a cylindrical tube, with a series of diaphragms (the aperture is
limited to an angle of 5.42°).
• The instrument is mounted on a motor-driven heliostat which is adjusted every week to
cover changes in the sun’s declination. The output of the pyreheliometer can either be recorded
on a strip chart recorder or integrated over a suitable time period.
a. Angstrom pyrheliometer :
 In this pyrheliometer, a thin blackened shaded manganin strip (Size 20 x 2 x 0.1 mm) is heated
electrically until it is at the same temperature as a similar strip which is exposed to solar
radiation.
 Under steady state conditions (both strips at identical temperature) the energy used for heating
is equal to the absorbed solar energy.
 The thermocouples on the back of each strip, connected in opposition through a sensitive
galvanometer (or other null detector), are used to test for the equality of temperature.
 The energy H of direct radiation is calculated by means of the formula,
HDN= Ki2
HDN = Direct radiation incident on area normal to sun rays
i = heating current in amperes
K = dimension and instrument constant
= R/Wα
R = resistance per unit length of the absorbing strip.
W = mean width of the absorbing strip.
α = absorbing coefficient
a. Angstrom pyrheliometer :

b. Abbot silver disk pyrheliometer:

 It consists essentially of a blackened silver disk positioned at the lower end of a tube with
diaphragms to limit the whole aperture to 5.7°.
 A mercury in glass thermometer is used to measure the temperature at the disk.
 A shutter made of three polished metal leaves is provided at the upper end of the tube to allow
solar radiation to fall on the disk at regular intervals and the corresponding changes in
temperature of the disk are measured.
 The thermometer stem is bent through 90° so that it lies along the tube to minimize its
exposure to the sun. The instrument must of course be celibrated against a primary standard,
but their stability has been found to be very good and they are widely used for calibrating
pyranometers.
2. Pyranometer:
 It is based on the principle as stated above that there is a difference between the temperature of
black surfaces (which absorb most solar radiation) and white surfaces
(which reflect most solar radiation).
 The detection of temperature difference is achieved by thermopile.
 It uses concentric silver rings 0.25 mm thick, appropriate coated black and white, with either
10 or 50 thermocouple junctions to detect temperature difference between coated rings.
 Later models use wedges arranged in a circular pattern, with alternate black and white
coatings.
 The disks or wedges are enclosed in a hemispherical glass cover.
3. Sunshine recorder:
 The duration of bright sunshine in a day is measured by means of a sunshine recorder.
 The sun’s rays are focussed by a glass-sphere to point on a card strip held in a groove in a
spherical bowl mounted concentrically with the sphere.
 Whenever there is a bright sunshine, the image formed is intense enough to burn a spot on the
card strip.
 Through the days the sun moves across the sky, the image moves along the strip. Thus a burnt
space whose length is proportional to the duration of sun shine is obtained on the strip.
Solar Radiation Data for India:
 India lies within the latitudes of 7° N and 37° N, with annual intensity of solar radiation
between 400 and 700 cal/cm2/day. Most parts of India receive 4–7 kWh/m2/day of solar
radiation with 250–300 sunny days in a year.
 The annual average daily global solar radiation in India (in kWh/m2/day) is shown in Figure
3.12. A similar map can also be drawn for average daily diffuse radiation.
 The highest annual radiation energy is received in the western Rajasthan while the northeastern
region receives the lowest annual radiation.

Annual solar radiation pattern:

 India is divided into five regions as shown in Figure 3.13, with changing solar radiation pattern
between January and December. It gives the annual average of global solar energy received on
a horizontal plane.
 The daily record of global radiation data is useful for industry as India lies
in the sunny regions of the world. Other countries having a rich solar flux belt are Saudi Arabia,
Central Australia and South Africa.
 Solar energy can be used through two routes. One is the thermal route for water heating,
cooking, drying, water purification and power generation. The other is photovoltaic route that
converts solar radiation into electricity which can be used for pumping water, communications
and power supply in unelectrified areas.
 The daily solar insolation values over selected cities in India with seasonal variations are
shown in Table 3.2. The peak values are measured from March to May, when the western
Rajasthan and Gujarat receive over 600 cal/cm2/day (25,100 kJ/m2/day). During monsoon and
winter months the daily solar radiation decreases to 400 cal/cm2/day (16,700 kJ/m2/day).
 The annual average daily diffuse radiation received over the whole country is about
175 cal/cm2/day (7300 kJ/m2/day). Maximum values observed are 300 cal/cm2/day during July
in Gujarat, while the minimum values between 75 and 100 cal/cm2/day during December are
observed over many locations.