Renewable Energy Resources (RES)

R20 Regulations Open elective

III Year Semester 1-B. Tech. (CSE/AIML/IoT)

UNIT–I
Solar Energy: Introduction - Renewable Sources - prospects, Solar radiation at the Earth
Surface - Equivalent circuit of a Photovoltaic (PV) Cell - I-V & P-V Characteristics - Solar
Energy Collectors: Flat plate Collectors, concentrating collectors - Solar Energy storage
systems and Applications: Solar Pond - Solar water heating - Solar Green house.
Energy from Sun
 Energy from the sun is called solar energy.
 Sun’s energy comes from nuclear fusion reaction
that takes place deep in the sun.
 Hydrogen nucleus fuse into helium nucleus.
 The energy from these reactions flow out from
the sun and escape into space.
 Solar energy is some times called radiant energy.
 These are different kinds of radiant energy
emitted by sun.
 The most important are light infrared Rays, Ultra
violet rays, and X- Rays.
Sun-Earth Relationship
 The sun is a large sphere of very hot gases.
 It’s diameter is 1.39x106KM, While that of the
earth is 1.27x104 KM.
 The mean distance between the two is
1.5x108KM.
 The beam radiation received from the sun on
 the earth is reflected in to space, another 15% is
absorbed by the earth atmosphere and the rest
is absorbed by the earth’s surface.
 This absorbed radiation consists of light and
infrared radiation with out which the earth
would be barren.
 All life on the earth depends on solar energy.
 Green plants make food by means of
photosynthesis.
 Light is essential from in this process to take place.
 This light usually comes from sun.
 Animals get their food from plants or by eating other animals that feed on plants.
 Plants and animals also need some heat to stay alive. Thus plants are store houses of solar energy.
Solar Constant (ISC)
 The sun is a large sphere of very hot gases,
the heat being generated by various kinds
of fusion reactions.
 Its diameter is 1.39x106 KM While that
 of the earth is 1.27x104 KM.
 The mean distance between the two is
1.50x108 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 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 center to its edge.
 However for engineering calculations, it is customary to assume that the brightness all over the solar disc
in uniform.
 As viewed from the earth, the radiation coming from the sun appears to be essentially equivalent to that
coming from a back surface at 5762ok.
Solar Constant (ISC)
 “The rate at which solar energy arrives at the top of the atmosphere is called solar
constant”.
 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 sun’s distance and activity vary through out the year, the rate of arrival of
solar constant is thus an average from which the actual values vary up to 3 percent in
either direction.
Solar Constant (ISC)

Solar constant is characterised by the following:

 It is constant and not affected by daily, seasonal, atmospheric condition, clarity of
atmosphere etc.
 It is on a unit area on imaginary spherical surface around earth’s atmosphere for mean
distance between the sun and the earth.
 It is on surface normal to sun’s rays. Sun rays are practically parallel (beam radiation).
 It has a measured value of “1353 W/m2”.
 Isc in terms of kJ/m2. hour = (1353 × 3600)/1000 = 4870.8 kJ/m2 hour.
Solar Radiation at the Earth’s Surface

 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 in to 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 atmosphere molecules, and
by dust particles.
Depletion of Solar Radiation
Absorption
 Selective absorption of various wavelengths
occurs by different molecules.
 The absorbed radiation increases the energy of
the absorbing molecules, thus raising their
temperatures.
Scattering
 Scattering by dust particles, and air molecules (or gaseous particles of different sizes) involves
redistribution of incident energy.
 A part of scattered radiation is lost (reflected back) to space while remaining is directed downwards to the
earth’s surface from different directions as diffuse radiation.
 Diffuse radiation: Solar radiation scattered by aerosols, dust and molecules is known as diffuse radiation. It
does not have a unique direction.
 Beam radiation: Solar radiation propagating in a straight line and received at the earth surface without
change of direction, i.e., in line with sun is called beam or direct radiation.
 Global radiation: The sum of beam and diffuse radiation is referred to as total or global radiation.
 The path length of solar beam through the atmosphere is accounted for in the term ‘Air Mass’, which is
defined as the ratio of the path length through the atmosphere, which the solar beam actually traverses
up to the ground to the vertical path length (which is minimum) through the atmosphere.
SOLAR ENERGY TERMS AND DEFINITIONS

Solar radiation is the energy radiated by the sun.

— The radiated energy received on earth surface is called Solar irradiation.

— Solar radiation received on a flat horizontal surface on earth is called Solar insolation.

The solar radiation is of the following two types:

1. Extraterrestrial solar radiation:

 The intensity of sun’s radiation outside the earth’s atmosphere is called “extraterrestrial” and has no
diffuse components.
 Extraterrestrial radiation is the measure of solar radiation that would be received in the absence of
atmosphere.

2. Terrestrial solar radiation:

 The radiation received on the earth surface is called “terrestrial radiation” and is nearly 70% of
extraterrestrial radiation.
Clarity Index and Concentration Ratio

Clarity Index:
 The ratio of radiation received on earth’s horizontal surface over a given period to radiation on equal
surface area beyond earth’s atmosphere in direction perpendicular to the beam is called “Clarity index”.
 It depends upon the clarity of atmosphere for passage of solar beam radiation.
 Clarity index can be between 0.1 to 0.7.

Concentration ratio:
 It is the ratio of solar power per unit area of the concentrator surface (kW/m2) to power per unit area on
the line focus or point focus (kW/m2).
Earth-Sun angles and their relationships
In order to understand how to collect energy from the sun, one must first be able to predict the location of the
sun relative to the collection device.
Hour Angle (Ꙍ )
The hour angle is the angular distance between the meridian of the observer and the meridian whose plane
contains the sun. An expression to calculate the hour angle from solar time is,
Ꙍ = 15 × (ts − 12); (in degrees)
Where, ts is the solar time in hours.

Hour angle (w) can be calculated simply as follows:

Since the earth makes one revolution on its axis in 24 h, then 15 minutes will be equal to 15/60 = 1/4 min
Therefore, Ꙍ = 1/4 × tm; (in degrees)
Where, tm is the time in minutes after local solar noon.
w will be +ve if solar time is after solar
noon. However, w will be −ve if solar
time is before solar noon as shown in
Figure.
Latitude angle (ɸ):

The ‘latitude of a place’ is the angle subtended by

the radial line joining the place to the centre of the
earth, with the projection of the line on the
equatorial plane.

The latitude is taken as positive for any location towards the ‘northern hemisphere’ and negative towards
the ‘southern hemisphere’ i.e., the latitude(s) at equator is 00 while at north and south poles are + 90° and –
90° respectively.
Pyranometer
 A precision pyranometer is designed to
respond to radiation of all wavelengths
and hence measures accurately the
total power in the incident spectrum.
 It contains a thermopile whose sensitive
surface consists of circular, blackened,
hot junctions, exposed to the sun and
cold junctions are completely shaded.
 The temperature difference between
the hot and cold junctions is the
function of radiation falling on the
sensitive surface.
 The sensing element is covered by two concentric hemispherical glass domes to shield it from wind and rain.
 This also reduces the convection currents.
 A radiation shield surrounding the outer dome and coplanar with the sensing element, prevents direct solar
radiation from heating the base of the instrument.
Pyrheliometer
 The normal incidence pyranometer, shown in Fig.
uses a long collimator tube to collect beam
radiation whose field of view is limited to a solid
angle of 5.5° (generally) by appropriate
diaphragms inside the tube.
 The inside of the tube is blackened to absorb any
radiation incident at angles outside the collection
solid angle.
 At the base of the tube a wire wound thermopile
having a sensitivity of approximately
8 mV/W/m2 and an output impedance of
approximately 200 Ω is provided.
 The tube is sealed with dry air to eliminate
absorption of beam radiation within the tube by
water vapor.
 A tracker is needed if continuous readings are
desired.
Sunshine Recorder
 This instrument measures the duration in hours, of
bright sunshine during the course of the day.
 It essentially consists of glass sphere (about 10 cm
in diameter) mounted on its axis parallel to that of
earth, within a spherical section (bowl) as shown in
Fig.
 The bowl and glass sphere is arranged in such a way that sun’s
rays are focused sharply at a spot on a card held in a groove in
the bowl.
 The card is prepared from special paper bearing a time scale.
 As the sun moves, the focused bright sunshine burns a path
along this paper.
 The length of the trace thus obtained on the paper is the
measure of the duration of the bright sunshine.
 Three overlapping pairs of grooves are provided in the spherical
segment to take care of the different seasons of the year.
Boltzmann constant = 1.380649 × 10-23 m2 kg s-2 K-1
Solar Energy storage systems and Applications:

Solar Pond - Solar water heating - Solar Green house.

Assignment 1 (Submit within a week)

1) Explain about Local Solar Time (LST) or Local Apparent Time (LAT).
2) List the advantages and limitations of renewable energy sources.
3) Explain different angles that are used in solar radiation geometry.
4) Draw and explain the equivalent circuit of a PV cell along with I-V, P-V characteristics.
5) Compare conventional energy sources with renewable energy sources.
6) Give the advantages and disadvantages of concentrating collectors over flat plate
collectors and explain.
7) Explain about the term ‘Fill Factor’ and its importance as a performance parameter for a
solar cell.
8) Explain about solar energy storage systems and applications.