Solar photovoltaic (PV) systems convert solar energy directly into
electrical energy.
The basic conversion device used is known as a solar photovoltaic
cell or a solar cell.
A solar cell is the most expensive component in a solar PV system
(about 60% of the total system cost) though its cost is falling slowly.
Commercial photocells may have efficiencies in the range of 10-
20% and can produce electrical energy of 1-2 kWh per sq. m per
day in ordinary sunshine.
MAJOR ADVANTAGES OF SOLAR PV SYSTEMS
(i)It converts solar energy directly into electrical energy without going
through the thermal-mechanical link. It has no moving parts.
(ⅱ) Solar PV systems are reliable, modular, durable and generally
maintenance-free.
(iii)These systems are quiet, compatible with almost all environments,
respond instantaneously to solar radiation and have an expected lifespan
of 20 years or more.
(iv) A solar PV system can be located at the place of use and hence no or
minimum distribution network is required, as it is universally available
Disadvantages
(i) At present, the costs of solar cells are high, making them economically
uncompetitive with other conventional power sources.
(ii) The efficiency of solar cells is low. As solar radiation density is also
low, a large area of solar cell modules are required to generate sufficient
useful power.
(iii)As solar energy is intermittent, some kind of electrical energy storage is
required, which makes the whole system more expensive.
PHOTOVOLTAIC EFFECT
Photovoltaic effect is a process in which two dissimilar materials in close contact
produce an electrical charge when struck by light or any other radiant energy.
When light strikes crystals such as
silicon or germanium (p-n junction)
in which electrons are usually not
free to move from n-region to p-
region due to the potential barrier,
the light provides the energy
(e.m.f.) needed to free some
electrons from the bound condition
depending on the absorption of
solar energy (Figure 3.23).
• Free electrons cross the junction between two dissimilar
crystals more easily in one direction than in the other,
giving one side of the junction a negative charge, and this
results in a negative voltage with respect to the other side,
as in the case of a battery in which one electrode has
negative voltage with respect to the other.
• The photovoltaic effect can continue to provide voltage
and current as long as light falls on the junction of two
materials.
• A solar cell is essentially a p-n junction with a large
surface area.
• The n-type material is kept thin to allow light to pass
through it and strike the p-n junction.
• The light travels in packets of energy called photons.
• The generation of electric current takes place inside the
depletion zone of the p-n junction.
• The depletion zone as explained previously is the area
around the p-n junction where the electrons from the n-
region diffuse into the holes of the p-region.
• When a photon of light is absorbed by one of these atoms in
n-region of silicon, it will dislodge an electron from any
atom, thereby creating a free electron and hole pair.
• The free electron and hole pair has sufficient energy to jump
out of the depletion zone.
• If a wire is connected from the cathode at n-type silicon to an
anode of p-type silicon, electrons flow through the wire.
• The electron is attracted to the positive charge of p-type
material and travels through the external load (bulb or
resistance), thereby creating a flow of electric current.
• The hole created by the dislodged electron is attracted to the
negative charge of the n-type material and travels to back
electrical contact.
• As the electron reaches the p-type silicon from the back
electrical contact, it combines with the hole, thereby restoring
the electrical neutrality (Figure 3.23).
SOLAR CELL, MODULE, PANEL AND ARRAY
Solar Cell
The solar cell consists of
(i) p-type silicon material layer,
(ii) n-type silicon material layer,
(iii) front metallic grid and
(iv) opaque back metal contact
as shown in Figure 3.24
Figure 3.24 Construction of a solar cell. (a) Side view of the solar cell. (b) Top view of
the solar cell.
• The bulk material consists of p-type silicon
having thickness about 100-350 mm.
• A thin layer of n-type silicon having
thickness of about 2 mm is diffused on this
bulk material, providing p-n junction.
• A metallic grid at top with n-type material
and an opaque back metal contact at the
bottom of p-type material are provided
which also act as negative and positive
terminals.
Figure 3.24 Construction of a solar cell. (a) Side view of the solar cell. (b) Top view of
the solar cell.
Solar PV Module
• A Single solar cell cannot be used as such as it has
(1) A very small output
(2) No protection against dust moisture , mechanical impacts and atmospheric
harsh condition
Figure 3.24 Construction of a solar cell. (a) Side view of the solar cell. (b) Top view of
the solar cell.
Solar PV Module
Suitable voltage and adequate power can be obtained by suitably interconnecting
a number of solar cells. This assembly of solar cell is called solar module. Solar
cells are provided with transparent cover and these are hermetically sealed for
assembly into solar module.
It is necessary that all solar
cells should match as closely
as possible with each other
so that peak power of the
module is the algebraic sum
of the peak power of
individual solar cells. A
typical module is shown in
Figure 3.25.
The construction of a solar PV panel.
• Solar PV panel consists of a number of solar PV modules
connected in series and parallel to obtain the power of desired
voltage and current. When modules are connected in series, it is
desirable that each module should produce maximum power at the
same current.
Figure 3.26 A solar PV panel of four modules.
• When solar PV modules are connected in parallel, it is desirable
that each module should produce maximum power at the same
voltage. A frame is used to mount several modules to form a solar
PV panel as shown in Figure 3.26.
Figure 3.26 A solar PV panel of four modules.
In the panel, bypass diodes are
installed across each module so that
any defective module can be bypassed
by the output of remaining modules.
The blocking diodes are connected in
series with each series string of
modules which enable the output of
the remaining series strings should not
be absorbed by the failed string. A
typical panel with the series and the Figure 3.27 A typical panel
parallel connections is shown in with the series and the
Figure 3.27. parallel connection.
Solar PV Array
A PV array consists of a number of solar panels which are installed in an array
field. The solar panels may be installed as stationary facing the sun or installed
with some tracking mechanism. The installation should ensure that no panel
should cast shadow on any of the neighboring panels and those panels can be
easily maintained.
TYPES OF SOLAR CELL
The solar cells depending on the type of material
used can be classified as
(1) single crystal silicon solar cell,
(2) polycrystalline and amorphous silicon cell,
(3) cadmium sulphide-cadmium telluride cell,
(4) copper indium diselenide cell
(5) gallium arsenide cell.
Single crystal silicon
• It is produced from silicon dioxide which is reduced to silica
with 1% impurities.
• It is first purified to polycrystalline form and then further
converted into the single crystal state.
• The conversion process into single crystal state is very
expensive.
• The single crystal p-type silicon is obtained in the form of a
long cylindrical block (diameter of about 6-15 cm). The
block is sawed using diamond cutter to obtain a number of
silicon slices or wafers having thickness of about 300 μm.
• The p-type silicon wafers are then exposed to phosphorous
vapour (doping material) in a furnace so that phosphorous
can diffuse into the silicon wafer for a short depth, thereby
forming n-silicon region over the p-silicon bulk material.
• The efficiency of single crystal silicon is about 22%. It is
most efficient and robust. It has two main drawbacks:
(i)it needs high energy to produce and hence is costly
and
(ii) it requires high intensity of radiation to produce
solar electricity.
Polycrystalline and amorphous silicon
• The cells made of these materials are cost-effective but these
have lower efficiency compared to a single crystal silicon cell.
• The process to produce polycrystalline silicon cells is similar
to that of single crystal silicon except that the costly step of
converting polycrystalline state to the single crystal is not
required.
• The polycrystalline silicon is directly melted, doped with
phosphorous and cooked to the desired shape and size.
• This helps in economy of materials and energy consumption
for the production of cells.
• Amorphous silicon cells are produced using thin film
technology.
• These cells are a cheaper alternative to single crystal or
multicrystallinc cells.
• The main drawbacks are that they have low efficiency (4-8%)
and they degrade easily when used in outdoor applications.
• These cells are useful for indoor lights, pocket calculator,
electronic watches and electronic instruments.
Cadmium sulphide cadmium telluride cells
• These cells are also produced using thin film technology.
• The cells require very less material.
• In thin film technology, the semiconductor (cadmium
telluride) is vapourised and its film (10 μm) is deposited
on a thin layer (12 μm) of cadmium sulphide.
• A barrier layer of copper sulphide is then deposited on
top of the CdS-CdTe cell.
• The cell consists of n-type CdS and p-type CdTe.
• The cell has efficiency of 10% and it has no deterioration
during outside applications.
Copper indium diselenide
• It is a thin film polycrystalline cell made from copper
indium diselenide.
• It has an efficiency of about 14%.
• Its properties remain stable.
• It has an easier manufacturing process.
Gallium arsenide
• The cell has thin film of n-type and p-type gallium
arsenide (GaAs) grown on a suitable substrate.
• The efficiency of the cell is about 20%, but it has high
cost of production.
• The cell has high performance in extraterrestrial
applications.
TYPES OF SOLAR PV SYSTEM Central Power Station System
• This type of solar power station is
similar to other conventional power
stations which are required to feed
generated power into some
national grid.
• This type of solar power stations
are designed to meet high peak
daytime load only and these have
large generation capacity in
megawatt (up to 6 MW).
Figure 3.30 Classification • Only few such power stations have
of solar PV systems. been installed worldwide as the
capital cost of these plants is high.
Stand-Alone System
• Solar PV power station is planned and located at the load centre.
• Its complete electricity generation is meant to meet the electrical load of any
remote area, village or installation.
• Energy storage is
essential to meet the
requirement during
non-sunshine hours.
• A typical stand-
alone solar PV
system is shown in
Figure 3.31.
Figure 3.31 A schematic layout of a stand-alone solar PV system.
• The maximum power point tracker (MPPT) senses the voltage and
current outputs from the solar array and then suitably adjusts the
operating point to obtain maximum power output from the solar array as
possible from the climatic conditions.
• The solar electric output in direct current is converted into alternating current
and it is fed into the load.
• The excess power is
preferably stored by
charging the dumped in
the electric heaters.
• When the sun radiation is
unavailable, the batteries
supply the electricity
through the converter.
Figure 3.31 A schematic layout of a stand-alone solar PV system.
stand-alone pv system block diagram
Grid Interactive Solar PV System
• In grid interactive solar PV system, the system first meets the requirement
of house, village or installation and then all excess power is fed to an
electric grid during sunshine hours (Figure 3.32).
Figure 3.32 Grid interactive solar PV system.
Grid Interactive Solar PV System
• This arrangement helps in preventing any dumping of electricity
as required in the stand-alone solar PV system.
• The second advantage of this system is that during absence of
insufficient sunshine, the supply of electricity is maintained from
the electric grid, thereby eliminating any need of battery.
• This system is very popular in the United Kingdom, where two-
way electric meters provided to record
• (i) the electricity generated and supplied by rooftop PV
system of various houses to the electric grid system during
non-peak sunshine hours and
• (ii) electricity supplied to the houses from the electric grid
during non-sunshine hours. The difference of two is paid to
consumers or vice versa.
Small Consumer Systems
These systems are designed to meet the power requirement of low energy
devices which are generally used for indoor applications, such calculators,
watches and electric devices.
Hybrid Solar PV System
• The hybrid solar PV system is designed to provide electric power by some
other means besides solar electricity.
• It is difficult and uneconomical to provide all of the power from only solar
PV system.
• It may be more economical to meet the power requirement by some other
means, such as windmills, fuel cells and diesel or petrol generators.
• The best hybrid solar PV system is the one in which no amount of solar PV
generated power is wasted.
Hybrid Solar PV System
Advantages and Disadvantages of PV System
Advantages are as follows: Disadvantages are as follows:
(i) It directly converts solar energy to
electric power without any use of (i)It has high cost of
moving parts. installation.
(ii) It is more reliable, durable and (ii) It has low efficiency.
maintenance free. (iii) It requires a large area
(iii) It works without any noise. for installation to produce
(iv) It is non-polluting. sufficient power.
(v) It has long lifespan. (iv) Its output is intermittent,
(vi) It can be located near the point thereby requiring some
of load and requires no distribution means to store energy to use
system. during non-sunshine hours.
Figure.3.33 The p-n junction forward biased.
SOLAR CELL CHARACTERISTICS
In case the p-n junction is forward biased,
electrons from p-region start moving
towards the positive terminal of the battery,
there by reducing the potential barrier at the
junction (Figure 3.33). This facilitates the
flow of current through the p-n junction.
In case the junction is reversed biased, the
potential barrier at the junction increases,
which further reduces the possibility of any
flow of current through the junction. The
Current Voltage (I vs V) characteristic of a
p-n junction is shown in Figure 3.34.
Figure 3.34 Current Voltage characteristic of p-n junction
when forward and backward biased.
As the voltage (V) increases, the current (I) in the junction also increases.
However, there is a very small reverse saturation current (I0) instead of
zero current when reversed voltage is applied. The flow of current can be
given by diode current equation (Schottky equation)
where I0 is the reverse saturation current and VT is the voltage equivalent of
temperature and it is given by
Here,
k is the Boltzmann constant,
T is the temperature in kelvin and
q is the charge of an electron.
The current-voltage characteristics of a p-n junction (solar cell)
gets modified due to photon or solar generated current (Isc)
flowing through the p-n junction as this (Isc) is added with the
reverse leakage current (I0). The diode current equation is now
modified as
V= 0; that is junction is short circuited.
where Vsc is the short circuit voltage. The above relation
shows that when junction is radiated with suns ray and it is
short-circuited at its terminals, there is a finite current called
short circuit current (Isc) that flows through the external
circuit made with the short circuiting of the junction
terminals.
The magnitude of Isc depends upon solar radiation.
Figure 3.35 shows current- voltage characteristics at different isolations which
include
(i) dark,
(ii) lesser amount of radiation, and
(iii) larger amount of radiation.
Hence, p-n junction can be considered an energy source or e.m.f having open
circuit voltage as Voc and short circuit current as Isc.
Figure 3.35 Current voltage characteristics at different isolation levels.
In case we use standard convention in which current flowing out from a
positive terminal of any energy source is always taken as positive and
apply the same convention on a solar cell, the current and voltage
characteristic can be redrawn with suitable modification as shown in
Figure 3.36, and mathematically the current voltage relationship can be
written as follows:
FILL FACTOR
Another term defining the overall behavior of a solar cell is the fill factor (FF).
• It is a measure of squareness of the I-V characteristics of the solar cell and is
defined as
FF = Maximum output power/(open-circuit voltage x short-circuit
current)
• It is the available power at the maximum power point (Pmax) divided by the
product of open circuit voltage (Voc) and short-circuit current (Is) as
FF = Pmax (Voc * Isc) = (Vmp * Imp)/(Voc × Isc)
where Vmp and Imp are the voltage and current at the maximum
power point.
• The above equation can be redefined as,
FF = (hxSxE)/(Voc x Isc)
EFFICIENCY OF SOLAR CELLS
Energy conversion efficiency (h) is defined as the ratio of power
output of cell (in watts) at its maximum power point (PMAX)
and the product of input light power (E, in W/m2 ) and the
surface area of the solar cell (S in m2 ) under standard
conditions
η= maximum output power/(irradiance × area) = PMAX/(E × S)
Factors Limiting the Efficiency of the Cell
1. Wavelength of solar spectrum: Cell response to only a portion of wavelength
available in the solar spectrum. Photon with wavelength >1.1 μm does not have
sufficient energy to create electron–hole pair in silicon cell.
2. Temperature: Normal operating temperature of silicon cells can reach 60°C in
peak sunlight and these temperature decreases the efficiency of the cells.
Therefore, it is important to provide heat sinks of the best quality available.
Gallium arsenide cells are capable of operating at high temperature where
focused energy can be used.
3. Mounting of the cells: It should be to a heat sink (usually an aluminium plate)
either heat conductive but electrically insulated. This will reduce operating
temperatures and make the cell more efficient. In case free water source is
available, heat sinks can be water cooled.
Factors Limiting the Efficiency of the Cell
4. Arrangement and maintenance of solar cell: The negative side of the
cells usually faces the sun and has antireflection coatings. These coatings
should be protected from dust, bird dropping, by a clear plastic or glass
cover. Accumulated dust on the cover will reduce the output power by
about 10%.
5. Position of the cell: The cell or panel should be positioned either facing
south in the north of equator or facing north in the south of equator for
maximum power output and fixed panel applications. The angle off the
ground should be equal to the latitude of the place for year around average or
can be changed monthly to face the sun at noon for more efficiency.
