RENEWABLE ENERGY SOURCES Module II

MODULE-II
SOLAR ENERGY

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Solar Energy: Solar energy has the greatest potential of all the sources of
renewable energy. This energy keeps the temperatures of earths above that in
colder space, causes current in the atmosphere and in the ocean, causes the
water cycle and generate photosynthesis in plants. Solar power on earth’s
surface is 1016 Watts. Total worldwide power demand 1013 Watts. Therefore
sun gives us 1000 times more power than we need, if we can use 5% of this
energy, it will be 50 times what world will be required.

Applications of solar energy:

1. Heating and cooling of residential building

2. Solar water heater

3. Solar drying of agricultural and animal products

4. Solar Cookers

5. Solar photovoltaic cell

6. Food refrigeration

Fundamentals: Solar energy, received in the form of radiation, can be

converted directly or indirectly into other forms of energy such as heat and
electricity. Sun is expected to radiate at an essentially constant rate for a few
billon years. Major drawbacks to extensive application of solar energy are:

1. The intermittent and variable manner in which it arrives at the earth’s

surface.
2. The large area required to collect the energy at a useful rate.

Energy is radiated by the sun as electromagnetic waves of which 99% have

wave lengths in the range of 0.2 to 4.0 micro meter (1 micro meter = 10-6 meter)
The solar spectrum has the following three basic levels:

(i) “Infrared band” with wavelengths too long for response by human eye
(frequency range: 4 × 1014 to 7.5 × 1010 Hz) wavelengths: between 0.75
micron and 1.95 microns (1 micro, mm = 10-6 m).
(ii) Visible band: Frequency range: 6 × 1016 to 7.69 × 1014 Hz; Wave
lengths: Between 0.39 micron and 0.75 micron.
(iii) Ultraviolet band: Frequency range: 6 × 1016 to 7.5 × 1010 Hz.
Wavelengths: Between 0.005 micron to 0.39 micron.

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Terms used in solar radiations:

Beam (or direct) radiation (Ib): Solar radiation received on the surface of earth
without change in directions is known as “beam or direct radiation”.
Diffuse radiation (Id): The solar radiation received from the sun after its
direction has been changed by reflection and scattering by atmosphere is
known as “diffuse radiation”.
Part of the radiation is reflected back into the space, especially by clouds.
Radiation entering the atmosphere is partly absorbed by molecules in the air.
Part of solar radiation is scattered by droplets in clouds, atmospheric
molecules and dust particles. Radiation that has not been absorbed or
scattered and reaches the ground directly from the sun is called direct
radiation. Diffuse radiation is that radiation received from sun after its
direction has been changed by reflection and scattering by atmosphere.

Solar Constant (Isc):

The “solar constant” (Isc) is the energy from the sun received on a unit area
perpendicular to solar rays at the mean distance from the sun (1.5 × 108 km)
outside the atmosphere.
Solar constant is characterised by the following:
(i) It is constant and not affected by daily, seasonal, atmospheric
condition, clarity of atmosphere etc.
(ii) It is on a unit area on imaginary spherical surface around earth’s
atmosphere for mean distance between the sun and the earth.

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(iii) It is on surface normal to sun’s rays. Sun rays are practically parallel
(beam radiation).
(iv) It has a measured value of “1353 W/m2”.

The value of solar constant remains constant throughout the year. However,
this value changes with location because earth-sun distance changes
seasonally with time. The extraterrestrial relation observed on different days is
known as apparent solar irradiance and can be calculated on any of the year
using the following relation:

I0 = Apparent extraterrestrial solar irradiance (W/m2),

n = Number of days of the year counting January 1 as the first day of the year,
and

Isc = Solar constant = 1353 W/m2.

Solar Radiation Geometry : The various angles which are useful for
conversion of beam radiation on the arbitrary surface are:

Altitude angle () or solar altitude: It is a vertical angle between the

projection of the sun rays on the horizontal plane and direction of the sunrays,
passing through the point.

Zenith angle (θz): It is a vertical angle between sun’s rays and a line
perpendicular to the horizontal plane through the point.

Solar azimuth angle (s): It is the polar angle (in degrees) along the horizontal
east or west of north. Or It is a horizontal angle measured from north to the
horizontal projection of the sun’s rays. This angle is positive when measured
west wise.

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Surface azimuth angle (): It is the angle of deviation of the normal to the
surface from the local meridian, the zero point being south, east positive and
west negative.

Slope or tilt angle (): It is the angle made by the plane surface with the
horizontal. It is taken to be positive for surfaces sloping towards the south and
negative for surfaces sloping towards north.

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MEASUREMENT OF SOLAR RADIATION:

It is important to measure solar radiation, owing to the increasing number of
solar heating and cooling applications, and the necessity for accurate solar
radiation data to predict performance.
The following three devices are used for measuring the solar radiations.
1. Pyranometer 2. Pyrheliometers 3. Sunshine recorders.

Pyranometer:
A pyranometer is a device used to measure the “total hemispherical solar
radiation”. The total solar radiation arriving at the outer edge of the
atmosphere is called the ‘solar constant’. The working principle of this
instrument is that sensitive surface is exposed to total (beam, diffuse and
reflected from the earth and surrounding) radiations.
Construction: It consists of a “black surface” which receives the beam as well
diffuse radiations which rises heat. A “glass dome” prevents the loss of
radiation received by the black surface. A “thermopile” is a temperature sensor,
and consists of a number of thermocouples connected in series to increase the
sensitivity. The “supporting stand” keeps the black surface in a proper position.
Working: When the pyranometer is exposed to sun, it starts receiving the
radiations. As a result, the surface temperature starts rising due to absorption
of the radiation. The increase in the temperature of the absorbing surface is
detected by the thermopile. The thermopile generates a thermo emf which is

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proportional to the radiations absorbed; this thermo emf is calibrated in terms

of the received radiations. This will measure the global solar radiations.

Pyrheliometer: (Angstrom electrical compensation)

A pyrheliometer is a device used to measure “beam or direct radiations”. It
collimates the radiation to determine the beam intensity as a function of
incident angle. This instrument uses a collimated detector for measuring solar
radiation from the sun and from a small portion of the sky around the sun at
normal incidence.

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This pyrheliometer has a rectangular aperture, two manganin-strip sensors

(20.0 mm × 2.0 mm ×0.02 mm) and several diaphragms to let only direct
sunlight reach the sensor. The sensor surface is painted optical black and has
uniform absorption characteristics for short-wave radiation. A copper-
constantan thermocouple is attached to the rear of each sensor strip, and the
thermocouple is connected to a galvanometer. The sensor strips also work as
electric resistors and generate heat when a current flows across them. When
solar irradiance is measured with this type of pyrheliometer, the small shutter
on the front face of the cylinder shields one sensor strip from sunlight, allowing
it to reach only the other sensor. A temperature difference is therefore
produced between the two sensor strips because one absorbs solar radiation
and the other does not, and a thermo electromotive force proportional to this
difference induces current flow through the galvanometer. Then, a current is
supplied to the cooler sensor strip (the one shaded from solar radiation) until
the pointer in the galvanometer indicates zero, at which point the temperature
raised by solar radiation is compensated by Joule heat. A value for direct solar
irradiance is obtained by converting the compensated current at this time.
Sunshine Recorder:
A sunshine recorder is a device used to measure the “hours of bright sunshine
in a day”.
Construction: It consist of a “glass-sphere” installed in a section of “spherical
metal bowl” having grooves for holding a recorder card strip” and the glass
sphere.

Working: The glass-sphere, which acts as a convex lens, focusses the sun’s
rays/beams to a point on the card strip held in a groove in the spherical bowl
mounted concentrically with the sphere. Whenever there is a bright sunshine,

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the image formed is intense enough to burn a spot on the card strip. Through
the day, the sun moves across the sky, the image moves along the strip. Thus a
burnt space whose length is proportional to the duration of sunshine is
obtained on the strip.
SOLAR THERMAL ENERGY
General Aspects: The solar thermal energy is a clean, cheap and abundantly
available renewable energy which has been used since ancient times. The sun
is a sustainable source of providing solar energy in the form of radiations,
visible light and infrared radiation. This solar energy is captured naturally by
different surfaces to produce thermal effect or to produce electricity by means
of photovoltaic or day lighting of the buildings. Solar energy can be converted
into ‘thermal energy’ by using solar collector. It can be converted into
‘electricity’ by using photovoltaic cell.
Flat Plate Collector: Solar collector is a device for collecting solar radiation
and transfer the energy to a fluid passing in contact with it. Utilization of solar
energy requires solar collectors.
1. Non Concentrating of flat plate type solar collector.
2. Concentrating (focusing) type solar collector.
The solar energy collector, with its associated absorber, is the essential
component of any system for conversion of solar radiation energy into more
usable form. In the non concentrating type, the collector area (the area
intercepts the solar radiation) is the same as the absorber area (the area
absorbing the radiation). In concentrating collectors the area intercepting the
solar radiation is greater than absorber area. In concentrating collectors much
higher temperatures can be obtained.
Principles (physical) of Conversion of Solar Energy into Heat:
When solar radiation from the sun, in the form of light (a shortwave radiation),
reaches earth, visible sunlight is absorbed on the ground and converted into
heat energy but nonvisible light is re-radiated by earth (a longwave radiation).
CO2 in atmosphere absorbs this light and radiates back a part of it to the
earth, which results in the increase in temperature. This whole process is
called Green-house effect. Hence, the Greenhouse effect brings about an
accumulation of energy of the ground. The name ‘Green-house effect’ related to
its first use in green houses, in which it is possible to grow exotic plants in cold
climates through better utilisation of the available light. Glass easily transmits
short-wave radiation, which means that it poses little interference to incoming
solar energy, but it is very poor transmitter of long –wave radiation.
Once the sun energy passed through the glass windows and has been absorbed
by some material inside, the heat will not be reradiated back outside. Glass
therefore acts as heat trap. Solar collectors almost have one or more glass

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covers (plastic and other transparent materials are often used instead of glass).

Flat Plate collectors may be divided into two main classifications based on the
type of heat transfer fluid used.
1. Liquid heating collectors are used for heating water.
2. Air or gas heating collectors are employed as solar air heater.

1. An absorber plate. It intercepts and absorbs solar radiation. This plate is

usually metallic (copper, aluminium or steel), although plastics have
been used in some low temperature applications. In most cases it is
coated with a material to enhance the absorption of solar radiation. The
coating may also be tailored to minimize the amount of infrared radiation
emitted. A heat transport fluid (usually air or water) is used to extract
the energy collected and passes over, under or through passages which
form an integral part of the plate.

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2. Transparent covers. These are one or more sheets of solar radiation

transmitting materials and are placed above the absorber plate. They
allow solar energy to reach the absorber plate while reducing convection,
conduction and re-radiation heat losses.
3. Insulation beneath the absorber plate. It minimizes and protects the
absorbing surface from heat losses.
4. Box-like structure. It contains the above components and keeps them in
position.
5. Tubes, fins, passages or channels are integral with the collector absorber
plate or connected to it, which carry the water, air or other fluid.
Liquid Collector: It is the plate and tube type collector, basically consists of a
flat surface with high absorptivity for solar radiation called absorbing surface.
It is a metal plate of copper, steel or aluminium material with tubing of copper
in thermal contact with the plates. Absorber plate made from metal sheet 1 to
2 mm in thickness, while the tubes are also of metal range in diameter from 1
to 1.5 cm. They are soldered, brazed or clamped to the bottom of the absorber
plate. Heat is transferred from the absorber plate to a point of use by
circulation of fluid across the solar heated surface. Thermal insulation of 5 to
10 cm thickness is usually placed behind the absorber plate to prevent the
heat losses from the rear surface. Insulation materials are wool, fiber glass. The
front covers are generally glass ,is transparent to in-coming solar radiation.
Glass covers act as convection shield to reduce losses from absorber plate.
Thickness of 3 and 4 mm are commonly used.

Air Collector: Air is used as the heat transport medium. Fins attached to the
plate to increase the contact surface. It eliminates corrosion and leakage
problems compared to liquid collector. Solar air heaters used for drying or
curing of agricultural products, space heating for comfort, curing of industrial
product such as plastics.

ENERGY STORAGE SYSTEM

In order to take care of intermittency of solar energy availability and to fill up
the gap between power demand and supply of the power plant, there is need for
energy storage system. An energy storage system stores the collected amount of
energy in excess of requirement of the demand and supplies this energy when
the demand exceeds the supply energy.
SOLAR THERMAL ENERGY STORAGE:
Thermal energy storage is the storage of energy by heating, melting or
vaporisation of material and the energy becomes available as heat.
Thermal energy storage is of two types:

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1. Sensible heat storage: The storage by causing a material to rise in

temperature
2. Latent heat storage: In this system of heat storage, heat is stored in a
material when it melts and extracted from the material when it freezes.

SOLAR POND:
Solar pond, also called solar ‘salt pond’, is an artificially designed pond, filled
with salty water, maintaining a definite concentration gradient. It combines
solar energy radiation and sensible heat storage, and as such, it is utilized for
collecting and storing solar energy. A solar pond reduces the convective and
evaporative heat losses by reversing the temperature gradient with the help of
non-uniform vertical concentration of salts.

The vertical configuration of “salt gradient solar pond” normally consist of the
following three zones:
1. “Surface (homogeneous) convective zone (SCZ)”. It is adjacent to the surface
and serves as a buffer zone between environmental fluctuations at the surface
and conductive heat transport from the layer below. It is about 10 to 20 cm
thick with a low uniform concentration at nearly the ambient air temperature.
2. “Lower connective zone (LCG)”. It is at the bottom of the pond, and this is
the layer with highest salt concentration, where high temperatures are built
up.

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3. “Concentration/Intermediate gradient zone (CGZ)”. This zone keeps the two

convective zones (SCG and LCG) apart and gives the solar pond its unique
thermal performance. It provides excellent insulation for the storage layer,
while transmitting the solar radiation. To maintain a solar pond in this non-
equilibrium stationary state it is necessary to replace the amount of salt that is
transported by molecular diffusion from the LCG to SCZ. This means that salt
must be added to the LCG, and fresh water to the SCG whilst brine is removed.
The brine can be recycled, divided into water and salt (by solar distillation) and
returned to the pond.
The major heat loss occurs from the surface of the solar pond. This heat loss
can be prevented by spreading a plastic grid over the pond’s surface to prevent
disturbance by the wind. Disturbed water tends to lose heat transfer faster
than when calm. Due to the excessively high salt concentration of the LCZ, a
plastic liner or impermeable soil must be used to prevent infiltration into the
nearby ground water or soil.
SOLAR POND ELECTRIC POWER PLANT:

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A low temperature thermal electric power production scheme using solar pond
is shown schematically in Fig. The energy obtained from a solar pond is used to
drive a Rankine cycle heat engine. Hot water from the bottom level of the pond
is pumped to the evaporator where the organic working fluid is vapourized. The
vapour then flows under high pressure to the turbine where it expands and
work thus obtained runs an electric generator producing electricity. The
exhaust vapour is then condensed in a condenser and the liquid is pumped
back to the evaporator and the cycle is repeated.
SOLAR DISTILLATION (SOLAR STILL):
Solar still is a device which is used to convert saline water into pure water by
using solar energy. Soft drinking water, an essential requirement for
supporting life, is scarce in arid, semi-arid and coastal areas. Saline water, at
such places, is available in underground or in the ocean. This water can be
distilled utilising abundant solar radiation available in that area, by solar
still(s).
The simplest ‘solar still’, generally known as, ‘basin type solar still’ is shown in
Fig.

Construction:It is a shallow basin having blackened surface called basin liner.

The filler supplies the saline water to the basin and an overflow pipe allows the
excess water to flow out from the basin. The top of the basin is covered with a
sloping airtight transparent cover that encloses the space above the basin. This
cover is made of glass or plastic and slope is provided towards a collection
trough.

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Working: Solar radiation passes through the transparent cover and is absorbed
and converted into heat by the black surface of the still. The saline water is
then heated and the water vapours condense over the cool interior surface of
the transparent cover. The condensate flows down the sloping roof and gets
collected in troughs installed at the outer frame of the solar still. The distilled
water is then transferred into a storage tank.
“Desalination output” increases with the rise in ambient temperature and is
independent of the salt content in raw feed water. The “solar still performance”
is expressed as the quantity of water produced by each unit of basin area per
day. Solar still installations may provide about 15 to 50 litres/day/10 m2.

SOLAR PHOTOVOLTAIC (SPV) SYSTEMS

Semiconductors : ‘‘Semiconductors’’ are solid materials, either non-metallic
elements or compounds, which allow electrons to pass through them so that
they conduct electricity in much the same way as a metal.
Atomic Structure: To understand how semiconductors work, it is necessary to
study briefly the structure of matter. All atoms are made of electrons, protons
and neutrons. Most solid materials are classed, as conductors, semiconductors
or insulators. To be conductor, the substance must contain some mobile
electrons—one that can move freely between the atoms. These free electrons
come only from the valence (outer) orbit of the atom. Physical force associated
with the valence electrons bind adjacent atoms together. The inner electrons
below the valence level do not normally enter into the conduction process.
Intrinsic Semiconductor: A pure semiconductor is called “intrinsic
semiconductor“. Here no free electrons are available since all the co-valent
bonds are complete. A pure semiconductor, therefore, behaves as an insulator.
It exhibits a peculiar behaviour even at room temperature or with rise in
temperature. The resistance of a semiconductor decreases with increase in
temperature.

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When an electric field is applied to an intrinsic semiconductor at a temperature

greater than 0°K, conduction electrons move to the anode and the holes (when
an electron is liberated into the conduction band a positively charged hole is
created in valence band) move to cathode. Hence semiconductor current
consists of movement of electrons in opposite direction.
Extrinsic Semiconductor: In a pure semiconductor, which behaves like an
insulator under ordinary conditions, if small amount of certain metallic
impurity is added, it attains current conducting properties.
The impure semiconductor is then called “impurity semiconductor“ or
“extrinsic semiconductor“. The process of adding impurity (extremely in small
amounts about 1 part in 108 ) to a semiconductor to make it extrinsic
(impurity) semiconductor is called Doping.
Generally following doping agents are used:
(i) Pentavalent atom having five valence electrons (arsenic, antimony,
phosphorus) called donor atoms.
(ii) Trivalent atoms having three valence electrons (gallium aluminium, boron)
called acceptor atoms.
With the addition of suitable impurities to semiconductor, two type of
semiconductors are:
(i) N-type semiconductor.
(ii) P-type semiconductor.
N-type semiconductor: If the impurity atom has one valence electron more
than the semiconductor atom which it has substituted, this extra electron will
be loosely bound to the atom. Such an impurity into a semiconductor is called
“donor impurity” (or donor).

P-type semiconductor: P-type extrinsic semiconductor can be produced if the

impurity atom has one valence electrons less than the semiconductor atom.
This impurity atom cannot fill all the interatomic bonds, and the free bond can

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accept an electron from the neighbouring bond; leaving behind a vacancy of

hole. Such an impurity is called an “acceptor impurity” (or acceptor). In this
type of semiconductor, conduction is by means of holes in the valence band.
Accordingly, holes form the majority carriers whereas electrons constitute
minority carriers.

Photovoltaic Effect: When a solar cell (p-n junction) is illuminated, electron-

hole pairs are generated and the electric current I is obtained. Solar
Photovoltaic Cells: The “photovoltaic or solar cell“ is a samiconductor device.
The ‘photovoltaic effect‘ was first observed in 1839 by Becquerel who found
that, when light was directed on to one side of an electrochemical cell, a voltage
was created. The development of selenium and cuprous oxide photovoltaic cells
led to many applications, including photographic exposure meters. In the late
1950s, silicon solar cells were made with a conversion efficiency high enough
for power generators.
Silicon photovoltaic cell (Single crystal solar cell): The main feature of a
silicon photovoltaic cell is a thin wafer of high purity silicon crystal, doped with
a minute quantity of boron.

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The ‘phosphorous’ in the silicon causes an excess of conduction-band

electrons, and the ‘boron’ causes an excess of valence electron vacancies, or
holes, which act like positive charges. At the “junction“ between the two types
of silicon, conduction electrons from the negative (n) region diffuse into the
positive (p) region and combine with holes, thus cancelling their charges. The
opposite action also occurs, with holes from the p-region crossing into the n-
region and combining electrons.
When light falls on the active-surface, photons with energy exceeding a certain
critical level known as band or energy gap interact with the valence electrons
and elevate them to the conduction band. This activity also leaves holes, so
that the photons are said to generate ‘electron-hole pairs. The electrons move
throughout the crystal, and the less mobile holes also move by valence-electron
substitution from atom to atom. The electrons are accelerated towards the
negative contact and the holes towards the positive. A potential difference is
established across the cell, and this will drive a current through an external
load.
Classification of Solar Cells: Solar cells can be classified on the basis of:
(i) Cell size; (ii) Thickness of active material; (iii) Type of junction
structure; (iv) Type of active material.
1. Cell size. The size of the silicon solar cells can be divided into four groups;
(i) Round single crystalline having 100 mm diameter; (ii) Square single
crystalline having area of 100 cm2, (iii) 1000 mm × 1000 mm square
multicrystalline, and (iv) 125 mm × 125 mm square multicrystalline.
2. Thickness of active material. Such solar cells are of two types:
(i) Bulk material cell,and (ii) Thin film cell.
Bulk material single crystal and multicrystalline cells are most successful for
terrestrial applications.
Thin film cells are not commercially successful.
3. Type of junction structure. These cells are classified as:
(i) p-n homojunction cell, (ii) p-n hetrojunction cell, (iii) p-n multijunction cell,
and (iv) Metal semiconductor Schottky junction.
4. Type of active material. Such cells are classified as:
(i) Single crystal silicon cell, (ii) Multicrystalline silicon cell, (iii) Amorphous
silicon cell, (iv) Gallium arsenide cell, (v) Copper indium diselenide cell, (vi)
Cadmium telluride cell, and (vii) Organic P-V cell.
Silicon Cell Modules: ‘Solar cells‘ are electrically connected in series and
parallel to give suitable voltages and currents for a particular application.
A number of cells are generally encapsulated into a module, which is the
building block of a photovoltaic system. A typical module measures about 1000

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mm × 300 mm × 50 mm and contains 36 cells, which produce 30-35 W at 12 V

in bright sunshine. Several modules combine to form a ‘photovoltaic array’.
Photovoltaic (PV) Systems
Solar photovoltaic systems refer to a wide variety of solar electricity systems.
Such a system uses solar array made of silicon to convert sunlight into
electricity. Components other than PV array are collectively known as ‘balance
of system (BOS)’ which includes storage batteries, an electronic charge
controller and an inverter.
These systems are of the following two types:
1. Stand-alone power systems. In such a system, the photovoltaic array is the
principal or only source of energy. Energy is stored, often in batteries, for
periods when there is insufficient solar radiation. There may also be a back-up
power supply such as an engine-generator set.
2. Grid connected power systems. In this type of system, load is connected to
both a photovoltaic power system and an electricity grid. In periods when there
is sufficient solar radiation, the array powers the load, otherwise grid is used.
In some cases, any surplus electricity produced by the array (i.e. when the load
output exceeds the load) is fed back into the grid.
Stand-alone power systems
A photovoltaic system can be designed to meet any electrical load. The
principal components of stand-alone photovoltaic systems are shown in Figure.

Photovoltaic array: It consists of the required number of modules

interconnected in series and parallel to give the desired system voltage and
current.
Storage battery: The battery supplies energy to the load during periods of little
or no solar irradiance and stores energy from the array during periods of high
irradiance. This enables the systems to meet momentary peak power demands
and to maintain stable voltage to the load.

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Power conditioner: Because the voltage output of the photovoltaic array varies
with insolation and temperature, systems with battery storage require voltage
regulator to prevent excessive overcharging of the battery. Further controls are
used, as required, to prevent discharge or to ensure that the array is operating
at its maximum power point.
To determine the capacity and size of a photovoltaic system, it is necessary to
select an optimum combination of battery capacity and array size for a
particular location. Methodologies for sizing systems are relatively well-
developed and employ an hour-by-hour computer model of the system under
consideration. The annual energy output from the system can then be
calculated for a range of array sizes and battery capacities to select an
optimum combination, i.e., the one with the lowest cost.
Advantages and limitations of photovoltaic systems are as follows:
Advantages:
1. Systems are durable.
2. No operational cost.
3. Low maintenance.
4. More flexibility available.
5. Systems are eco-friendly.
6. Highly reliable.
7. Long effective life.
8. Absence of moving parts.
9. Can function unattended for long periods.
10. High power to weight ratio.
Limitations:
1. Weather dependent.
2. Low efficiency
3. High installation cost.
The applications of photovoltaic systems are:
1. Solar street lighting system.
2. Home lighting systems.
3. Water pumping systems (for micro irrigation and drinking water supply)
4. Solar vehicles.
5. Radio beacons for ship navigation at ports.
6. Community radio and television sets.
7. Cathodic protection of oil pipelines.
8. Railway signalling equipment.
9. Weather monitoring.
10. Battery charging

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