Unit 2

Solar Energy

Solar Radiation and its measurements, Solar Thermal Energy Conversion from
plate Solar Collectors, Concentrating Collectors and its Types, Efficiency and
performance of collectors. Direct Solar Electricity Conversion from
Photovoltaic, types of solar cells and its application of battery charger, domestic
lighting, street lighting, and water pumping, power generation schemes. Recent
Advances in PV Applications: Building Integrated PV, Grid Connected PV
Systems.

Introduction

SOLAR CONSTANT

• The sun, being at a very large distance from the earth, solar rays subtend an angle of
only 32 minutes on earth, as shown in Figure.

• Energy flux received from the sun before entering the earth’s atmosphere, is a
constant quantity.

• The solar constant, Isc, is the energy from the sun received on a unit area
perpendicular to the solar rays at the mean distance from the sun outside the
atmosphere. Based on the experimental measurements, the standard value of the solar
constant is 1367 W/m 2 or 1.958 langley per minute (1 langley/min is the unit,
equivalent to 1 cal/cm 2 /min).

• In terms of other units, Isc = 432 Btu/ft2 /h or 4.921 MJ/m2 /h.

SPECTRAL DISTRIBUTION OF EXTRATERRESTRIAL RADIATION

• Extraterrestrial radiation is the measure of solar radiation that would be received in

the absence of atmosphere. A typical spectral distribution of extraterrestrial radiation
is shown in Figure

• The curve rises sharply with the wavelength and reaches the maximum value of 2074
W/m 2 /mm at a wavelength of 0.48 mm. It then decreases asymptotically to zero,
showing that 99% of the sun’s radiation is obtained up to a wavelength of 4 mm.
TERRESTRIAL SOLAR RADIATION

• Solar radiations pass through the earth’s atmosphere and are subjected to scattering
and atmospheric absorption.

• A part of scattered radiation is reflected back into space.

• Short wave ultraviolet rays are absorbed by ozone and long wave infrared rays are
absorbed by CO2 and water vapours.

• Scattering is due to air molecules, dust particles and water droplets that cause
attenuation of radiation.

• Minimum attenuation takes place in a clear sky when the earth’s surface receives
maximum radiation.

The terms pertaining to solar radiation are now defined as below:

• Beam radiation (Ib ): Solar radiation received on the earth’s surface without change
in direction, is called beam or direct radiation.

• Diffuse radiation (Id): The radiation received on a terrestrial surface (scattered by

aerosols and dust) from all parts of the sky dome, is known as diffuse radiation.

• Total radiation (IT): The sum of beam and diffuse radiations (Ib + Id) is referred to
as total radiation. When measured at a location on the earth’s surface, it is called solar
insolation at the place. When measured on a horizontal surface, it is called global
radiation (Ig ).

• Sun at zenith: It is the position of the sun directly overhead.

• Air mass (AM): It is the ratio of the path length of beam radiation through the
atmosphere, to the path length if the sun were at zenith. At sea level AM = 1, when
the sun is at zenith or directly overhead; AM = 2 when the angle subtended by zenith
and line of sight of the sun is 60°; AM = 0 just above the earth’s atmosphere.

• At zenith angle θz , the air mass is calculated as

• Irradiance (W/m2 ): The rate of incident energy per unit area of a surface is termed
irradiance.

• Albedo: The earth reflects back nearly 30% of the total solar radiant energy to the
space by reflection from clouds, by scattering and by reflection at the earth’s surface.
This is called the albedo of the earth’s atmosphere system

SOLAR RADIATION GEOMETRY

• Solar radiation varies in intensity at different locations on the earth, which revolves
elliptically around the sun.

• For the calculation of solar radiation, the position of a point P on the earth’s surface
with regard to sun’s rays can be located, if the latitude Φ, the hour angle ω for the
point and the sun’s declination δ are known.

• These basic angles for a location P on the northern hemisphere are shown in Figure

• Latitude (Φ): The latitude Φ 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.

Conventionally, the latitude for northern hemisphere is measured positive.

• Declination (δ): Declination δ is the angle subtended by a line joining the centres of
the earth and the sun with its projection on the earth’s equatorial plane. Declination
occurs as the axis of the earth is inclined to the plane of its orbit at an angle 66½°

where n is the total number of days counted from first January till the date of
calculation.

• Hour angle ( w): Hour angle w is the angle through which the earth must rotate to
bring the meridian of the point directly under the sun. It is the angular measure of
time at the rate of 15° per hour. Hour angle is measured from noon, based on local
apparent time being positive in the afternoon and negative in the forenoon.

• Altitude angle (α): It is a vertical angle between the direction of the sun’s rays
(passing through the point) and its projection on the horizontal plane
 SUNRISE, SUNSET AND DAY LENGTH

• The times of sunrise and sunset and the duration of the day-length depend upon the
latitude of the location and the month in the year.

• At sunrise and sunset, the sunlight is parallel to the ground surface with a zenith angle
of 90°.

• The hour angle pertaining to sunrise or sunset (ws )

Local apparent time (LAT)

• The time used for calculating the hour angle w is the ‘local apparent time’ which is
not the same as the ‘local clock time’.

• It can be obtained from the local time observed on a clock by applying two
corrections.

• The first correction arises due to the difference between the longitude of a location
and the meridian on which the standard time is determined.

• This correction has a magnitude of 4 minute for each degree difference in longitude.

The other correction is known as the ‘equation of time correction’ which is required due to
the fact that the earth’s orbit and the rate of rotation are subject to certain fluctuations.
SOLAR RADIATION MEASUREMENTS
 • The solar radiation data bank is required for many purposes, e.g. solar energy
appliances, hydrology and weather forecast.

Two types

• Pyranometer-The pyranometer measures global or diffuse radiation on a horizontal

surface. It covers total hemispherical solar radiation with a view angle of 2Π
steradians.

• Pyrheliometer- The Pyrheliometer is an instrument which measures beam radiation on

a surface normal to the sun’s rays.

Pyranometer
• The pyranometer measures global or diffuse radiation on a horizontal surface.

• The pyranometer designed by the Eppley laboratories, USA, operates on the principle
of thermopile.
• It consists of a black surface which heats up when exposed to solar radiation.

• Its temperature rises until the rate of heat gain from solar radiation equals the heat loss
by conduction, convection and radiation.

• On the black surface the hot junctions of a thermopile are attached, while the cold
junctions are placed in a position such that they do not receive the radiation.

• An electrical output voltage (0 to 10 mV range) generated by the temperature

difference between the black and the white surfaces indicates the intensity of solar
radiation.

• The output can be obtained on a strip chart or on a digital printout over a period of
time. This is a measure of global radiation.

• The pyranometer can also measure diffuse sky radiation by providing a shading ring
or disc to shade the direct sun rays.

• The shading ring is provided with an arrangement such that its plane is parallel to the
plane of the sun’s path across the sky.

• Consequently, it shades the thermopile element at all times from direct sunshine and
the pyranometer measures only the diffuse radiation obtained from the sky.

• A continuous record can be obtained either on an electronic chart or on an integrated

digital printout system.

• As the shading ring blocks a certain amount of diffuse sky radiation besides direct
radiation, a correction factor is applied to the measured value.

•
Sunshine recorder
• The duration in hours of bright sunshine in a day is measured by a sunshine recorder.

• It consists of a glass sphere installed in a section of spherical metal bowl, having

grooves for holding a recorder card strip.

• The glass sphere is adjusted to focus sun rays to a point on the card strip.
 • On a bright sunshine day, the focused image burns a trace on the card.

• Through the day the sun moves across the sky, the image moves along the strip. The
length of the image is a direct measure of the duration of bright sunshine.

SOLAR THERMAL ENERGY COLLECTORS

A solar thermal energy collector is an equipment in which solar energy is collected by
absorbing radiation in an absorber and then transferring to a fluid

There are two types of collectors:

Flat-plate solar collector: It has no optical concentrator. Here, the collector area and the
absorber area are numerically the same, the efficiency is low, and temperatures of the
working fluid can be raised only up to 100°C.

Concentrating-type solar collector: Here the area receiving the solar radiation is several
times greater than the absorber area and the efficiency is high. Mirrors and lenses are used to
concentrate the sun’s rays on the absorber, and the fluid temperature can be raised up to
osition. 500°C.For better performance, the collector is mounted on a tracking equipment to
face the sun always

FLAT-PLATE COLLECTOR

It consists of five major parts as mentioned below:

(i) A metallic flat absorber plate of high thermal conductivity made of copper, steel,
or aluminium, and having black surface. The thickness of the metal sheet ranges from
0.5 mm to 1 mm.

(ii) Tubes or channels are soldered to the absorber plate. Water flowing through these
tubes takes away the heat from the absorber plate. The diameter of tubes is around
1.25 cm,while that of the header pipe which leads water in and out of the collector and
distributes it to absorber tubes, is 2.5 cm.
(iii) A transparent toughened glass sheet of 5 mm thickness is provided as the cover plate. It
reduces convection losses through a stagnant air layer between the absorber plate and the
glass. Radiation losses are also reduced as the spectral transmissivity of glass is such that it is
transparent to short wave radiation and nearly opaque to long wave thermal radiation emitted
by interior collector walls and absorbing plate.

iv) Fibre glass insulation of thickness 2.5 cm to 8 cm is provided at the bottom and on the
sides in order to minimize heat loss.

(v) A container encloses the whole assembly in a box made of metallic sheet or fibre glass

 Since the heat transfer fluid is liquid, so, this type of flat-plate collector is also known
as liquid flat-plate collector.

 The commercially available collectors have a face area of 2 m2

 The whole assembly is fixed on a supporting structure that is installed in a tilted

position at a suitable angle facing south in the northern hemisphere.

 For the whole year, the optimum tilt angle of collectors is equal to the latitude of its
location.

 During winter, the tilt angle is kept 10 – 15° more than the latitude of the location
while in summer it should be 10 – 15° less than the latitude.

There are many parameters that affect the performance of a flat-plate collector.

 However, four important parameters are discussed below:

1. Heat Transport System

Heat from the absorber plate is removed by continuous flow of a heat transport medium.
When water is used, it flows through metal tubes that are welded to the absorber plate for
effective heat

transfer. Cold water enters the bottom header, flows upwards and gets warmed by the
absorber. The hot water then flows out through the top header.

When air is used as the heat transfer fluid, an air stream flows at the rear side of the collector
plate as shown in Figure. Fins welded to the plate increase the contact surface area. The rear
side of air passages is insulated with mineral wool. Solar air heaters are utilised for drying
agricultural products, space heating and seasoning of timber.
2. Selective Surfaces

 Absorber plate surfaces which provide high absorptivity for incoming solar radiation
and low emissivity for outgoing radiation are termed selective surfaces.

 Solar radiation lies in short wavelength band up to 4 µm while the absorber plate
emits long wave radiation with a maximum at 8.3µm. Thus, a selective surface needs
to have a high absorptivity for wavelengths shorter than 4 µm and a low emissivity for
wavelengths longer than 4 µm.

 No natural surface is available which possesses selective radiation characteristics.

 A selective surface is composed of a thin black metallic oxide coated on a bright

metal base. Black coating is sufficiently thick to be a good solar radiation absorber.

Bright metal base possesses low emissivity for thermal radiation. A successful selective
surface can be developed with a black chrome (Cr–Cr 2O 3 ) coating.

 It is a metal dielectric Cr2O3 layer over a Cr particle/ Cr2O3 composite prepared by

electroplating on a steel base.

 An effective selective surface has solar absorptivity of about 0.95 and thermal
emissivity close to 0.1.

 A selective surface of black chrome is durable with no degradation in performance

even in humid atmosphere and operating temperature of 300°C.

 Selective surfaces are important for low concentration solar equipment operating at
high temperatures.

 For high concentration devices the major requirement is high absorptance rather than
low emittance.

3.Number of Covers
  To minimize convection and radiation loss, a solar collector is provided with a
transparent glass sheet over the absorber plate. Solar radiation incident on glass sheet
passes through the glass cover.

 Glass sheet also absorbs heat radiation emitted by the hot absorber plate.

 Thus, the glass sheet cover reduces the heat loss coefficient to 10 W/m 2 .K.

 Experiments show that with two glass covers, the heat loss coefficient further reduces
to 4 W/m 2 .K.

4.Spacing

 The spacing between the absorber plate and the cover or between two covers also
influences the performance of a flat-plate collector.

 The operating performance varies with the spacing as well as with tilt and service
conditions and hence there is no way to specify the exact optimum spacing.

 However, researchers have suggested a spacing of 4 cm to 8 cm for improved

performance.

 It is also observed that a large spacing reduces the collector area requirements.

SOLAR CONCENTRATING COLLECTORS

• In Flat-plate collectors with heat transport medium as water or air, the area of glass
cover and that of absorber plate are practically the same.

• Thus, solar radiation intensity is uniformly distributed over the glass cover and the
absorber, keeping the temperature rise of the solar device up to 100°C.

• If solar radiation falling over a large surface is concentrated to a smaller area of the
absorber plate or receiver, the temperature can be enhanced up to 500°C.

• Concentration is achieved by an optical system either from the reflecting mirrors or

from the refracting lenses.

• These concentrators are used in medium temperature or high temperature energy

conversion cycles.

• An optical system of mirrors or lenses projects the radiation on to an absorber of

smaller area. This process compensates the reflection or absorption losses in mirrors
or lenses and losses on account of geometrical imperfections in the optical system.

• A term called ‘optical efficiency’ takes care of all such losses. For higher collection
efficiency, concentrating collectors are supported by a tracking arrangement that
tracks the sun all the time, so that beam radiation is on to the absorber surface.
• As collectors provide a high degree of concentration, a continuous adjustment of
collector orientation is required.

(i) ‘Concentrator’ is for the optical subsystem that projects solar radiation on to the
absorber. The term ‘receiver’ shall be used to represent the sub-system that includes
the absorber, its cover and accessories.

(ii) ‘Aperture’ (W) is the opening of the concentrator through which solar radiation
passes.

(iii) ‘Acceptance angle’ (2θa) is the angle across which beam radiation may deviate
from the normal to the aperture plane and then reach the absorber.

(iv) ‘Concentration ratio’ (CR) is the ratio of the effective area of the aperture to the
surface area of the absorber. The value of CR may change from unity (for flat-plate
collectors) to a thousand (for parabolic dish collectors). The CR is used to classify
collectors by their operating temperature range.

TYPES OF CONCENTRATING COLLECTORS

1.Plane receiver with plane collectors

• It is a simple concentrating collector, having up to four adjustable reflectors all

around, with a single collector as shown in Figure. The CR varies from 1 to 4 and the
non-imaging operating temperature can go up to 140°C.

2.Compound parabolic collector with plane receiver

• Reflectors are curved segments that are parts of two parabolas Figure .

• The CR varies from 3 to 10.

• For a CR of 10, the acceptance angle is 11.5° and tracking adjustment is required after
a few days to ensure collection of 8 hours a day.
3.Cylindrical parabolic collector

• The reflector is in the form of trough with a parabolic cross section in which the
image is formed on the focus of the parabola along a line as shown in Figure.

The basic parts are:

(i) an absorber tube with a selective coating located at the focal axis through which the liquid
to be heated flows,

(ii) a parabolic concentrator,

(iii) a concentric transparent cover.

• The aperture area ranges from 1 m2 to 6 m2 , where the length is more than the
aperture width.

• The CR range is from 10 to 30.

4.Collector with a fixed circular concentrator and a moving receiver

• The fixed circular concentrator consists of an array of long, narrow, flat mirror strips
fixed over a cylindrical surface as shown in Figure .
 • The mirror strips create a narrow line image that follows a circular path as the sun
moves across the sky.

• The CR varies from 10 to 100.

5.Fresnel lens collector

• Fresnel lens refraction type focusing collector is made of an acrylic plastic sheet, flat
on one side, with fine longitudinal grooves on the other as shown in Figure.

• The angles of grooves are designed to bring radiation to a line focus.

• The CR ranges between 10 and 80 with temperature varying between 150°C and
400°C.

6.Paraboloid dish collector

• To achieve high CRs and temperature, it is required to build a point-focusing

collector.

• A paraboloid dish collector is of point-focusing type as the receiver is placed at the

focus of the paraboloid reflector

• As a typical case, a dish of 6 m in diameter is constructed from 200 curved mirror

segments forming a paraboloidal surface. The absorber has a cavity shape made of
zirconium–copper alloy, with a selective coating of black chrome.
 • The CR ranges from 100 to a few thousands with maximum temperature up to
2000°C.

• For this, two-axis tracking is required so that the sun may remain in line with the
focus and vertex of the paraboloid.

7.Central receiver with heliostat

• To collect large amounts of heat energy at one point, the ‘Central Receiver Concept’
is followed.

• Solar radiation is reflected from a field of heliostats (an array of mirrors) to a centrally
located receiver on a tower

• Heliostats follow the sun to harness maximum solar heat.

• Water flowing through the receiver absorbs heat to produce steam which operates a
Rankine cycle turbo generator to generate electrical energy.

• With a central receiver optical system, a large number of small mirrors are installed,
each steerable to have an image at the absorber on the central receiver.

• A curvature is provided to the mirrors so as to focus the sunlight in addition to

directing it to the tower.

•
 Performance Analysis of a Collector

• The performance of solar collector can be improved by enhancing the useful energy
gain from incident solar radiation with minimum losses.
• Thermal losses have three components, namely
 the conductive loss,
 the convective loss
 the radiative loss.
• Conductive loss is reduced by providing insulation on the rear and sides of the
absorber plate.
• Convective loss is minimized by keeping an air gap of about 2 cm between the cover
and the plate.
• Radiative losses from the absorber plate are lowered by applying a spectrally selective
absorber coating.
• During normal steady-state operation, useful heat delivered by a solar collector is
equal to the heat gained by the liquid flowing through the tubes welded on to the
underside of the absorber plate minus the losses.

• The energy balance of the absorber can thus be represented by a mathematical

equation, i.e.
• Instantaneous collector efficiency and stagnation temperature which are required to
indicate the performances of the collector and also for comparing the designs of
different collectors.

• The instantaneous collector efficiency is defined as the ratio of useful heat gain to
radiation falling on the collector. It is expressed by

Depending on the given data, the collector aperture area Aa

or the collector gross area Ac is used in place of Ap in the above equation.

• The collector aperature area Aa is the net opening in the top cover through which
solar radiation passes into the collector.

• It is nearly 15% greater than the absorber plate area.

• The collector gross area is the top cover area including the frame and Aa is about 20%
higher than Ap.

• In case the flow of liquid through the collector is stopped, the useful heat gain and the
efficiency become zero.

• At this stage, the absorber plate attains a temperature so that

ApS = QL.

• This is the maximum temperature that the absorber plate can attain, and is called the
stagnation temperature.

• This data helps in selecting an appropriate material for manufacturing the collector
Since water heating through solar energy occurs comparatively at a slow pace, the
time base chosen is an hour.

• Accordingly, Qu, useful heat gain in one hour becomes kJ/h and IT the energy falling
on the collector face in one hour becomes kJ/m2. H

Solar photovoltaic (SPV) technology converts sunlight into DC electricity

without any moving parts and utilised for lighting, water pumping computers and
telecommunications etc.

• Stand alone SPV power plants in rural areas provide power for electrification.
• SPV roof top power plants are used for diesel saving in remote areas and tail end of
grid in rural areas.

• Photovoltaic power generation is a method of producing electricity using solar cells.

• A solar cell converts solar optical energy directly into electrical energy.

• A solar cell is essentially a semiconductor device fabricated in a manner which

generates a voltage when solar radiation falls on it.

• In semiconductors, atoms carry four electrons in the outer valence shell, some of
which can be displaced to move freely in the materials if extra energy is supplied.

• Then, a semiconductor attains the property to conduct the current. This is the basic
principle on which the solar cell works and generates power.

PHOTOVOLTAIC EFFECT

When a solar cell ( p-n junction) is illuminated, electron–hole pairs are generated and
the electric current obtained I is the difference between the solar light generated
current IL and the diode dark current Ij, i.e.,

Efficiency of Solar Cells

 The significant points of the curve are short-circuit current Isc and open circuit voltage
Voc .
 Maximum useful power of the cell is represented by the rectangle with the largest
area.
 When the cell yields maximum power, the current and voltage are represented by the
symbols Im and Vm respectively.
 Leakage across the cell increases with temperature which reduces voltage and
maximum power.
 Cell quality is maximum when the value of ‘fill factor’ approaches unity where the
Fill Factor (FF) is expressed as
 Maximum efficiency of a solar cell is defined as the ratio of maximum electric power
output to the incident solar radiation.

SOLAR PHOTOVOLTAIC SYSTEM (SPS)

• A PV module produces dc power.

• To operate electrical appliances used in households, inverters are used to convert dc

power into 220 V, 50 Hz, ac power.

• Components other than PV module are collectively known as Balance of System

(BOS) which includes storage batteries, an electronic charge controller, and an
inverter.

• Storage batteries with charge regulators are provided for back-up power supply during
periods of cloudy day and during nights.

• Batteries are charged during the day and supply power to loads as detailed in Figure.

• The capacity of a battery is expressed in ampere-hours (Ah) and each cell of the lead-
acid type battery is of 2 volts.

• Batteries are installed with a microprocessor-based charge regulator to monitor the

voltage and temperature and to regulate the input and the output currents to avoid
overcharging and excessive discharge, respectively.

• An inverter is provided for converting dc power from battery or PV array to ac power.

• It needs to have an automatic switch-off in case the output voltage from the array is
too low or too high.

• The inverter is also protected against overloading and short circuit.

 APPLICATION OF PV SYSTEMS

• Solar PV power systems may be categorized into four classes—standalone, PV

hybrid, grid connected and solar power satellite.

• The standalone systems are self-sufficient, unreachable by state grid but have a
battery system for continuous supply.

• A PV hybrid system is installed with a back-up system of diesel generator. Such

system are used in remote military installations, BSF border outposts, health centres,
and tourist bungalows.

• In grid-connected systems, a major part of the load during the day is supplied from the
PV array, and then from the grid when the sunlight is not sufficient.

Types of solar cell:

Solar cells are fabricated from semiconductor materials prepared in three physical
states–

 single multicrystal,

 many small crystals (polycrystalline)

 amorphous (noncrystalline)

1. Single Crystal Silicon

• Silicon solar cells are commonly used for both terrestrial and space applications.

• The basic raw material is sand (SiO2) from which silica (Si) is extracted and purified
repeatedly to obtain the metallurgical grade silicon.

• It contains about 1% impurities and further processed to convert it to a purer

semiconductor grade silicon.

• It is then finally converted into a single crystal ingot.

• A single crystal ingot is a long cylindrical block of about 6 cm to 15 cm in diameter.

• Crystalline cells basically require 300 µm to 400 µm of absorber material; the ingot is
sliced in wafers of 300 µm thickness as shown in Figure.

• These wafers are the starting material for a series of process steps such as surface
preparation, dopants diffusion, anti-reflection coating, contact grid on the surface and
base contact on the upper surface and on the lower one.
• Solar cells are fixed on a board and connected in series and parallel combinations to
provide the required voltage and power to form a PV module

• To protect the cells from damage a module is sealed between a plate of toughened
glass and layers of Ethyl Vinyl Acetate (EVA).

• A terminal box is attached to the back of a module where the two ends of the solar
string are soldered to the terminals.

• When the PV module is in use, terminals are connected directly to the load.
Single PV modules of capacities ranging from 10 Wp (peak watt) to 120 Wp can
provide power for different loads.
• Several panels of modules constitute an array, which is rated according to peak
wattage it delivers at noon on a clear day.

• For higher outputs an ‘array field’ is created.

• The size of an individual cell varies from 10 cm 2 to 100 cm 2 and a module contains
about 20 cells to 40 cells.

• A standard module constituting 30 cells, each of 7.5 cm diameter, can provide

electrical parameters of 12 volts, 1.2 ampere, and 18 watt peak power.

• To reduce cost, methods have been developed to produce a ribbon of single crystal
silicon from the molten pure silicon.

• The ribbon can be cut with minimum wastage into required sizes and processed
directly to make solar cells.

2.Polycrystalline Silicon Cells

• The production cost of a single crystal silicon cell is quite high compared to the
polycrystalline silicon cell.

• Polysilicon can be obtained in thin ribbons drawn from molten silicon bath and cooled
very slowly to obtain large size crystallites.

• Cells are made with care so that the grain boundaries cause no major interference with
the flow of electrons and grains are larger in size than the thickness of the cell.

 The polycrystalline silicon solar cell can be fabricated in three designs, namely p-n
junction cells, Metal Insulator Semiconductor (MIS) cells, and conducting oxide-
insulator semiconductor cells.

 For a p-n junction solar cell, a polycrystalline silicon film is deposited by chemical
vapour deposition on substrates like glass, graphite, metallurgical grade silicon and
metal.

 An MIS cell can be developed by inserting a thin insulting layer of SiO2 between the
metal and the semiconductor.
 A developed cell with chromium metal base with SiO2 insulation over it, the p-type
crystalline silicon can give efficiency up to 12% at AM-1 condition with cell
dimension of 0.2 cm 2

Bifacial crystalline cell over multicrystalline substrate

• A bifacial cell structure on a multicrystalline substrate is shown in Figure.

• With a double-sided cathode configuration, photo currents can be collected from the
nearest side of the cell.

• The p-n junctions and electrodes are formed on both sides of a cell to collect the
generated currents from both sides.

• The rear cathode acts as a current booster for the front cathode due to front sunlight
and vice-versa.

• The spectral response [Figure(b)] of the bifacial cell is the summation of the
independent front and rear cells.

• The spectral response improves in the long wavelength region due to effective
collection of photo-currents by the rear cathode.

• The conversion efficiency of a bifacial cell developed by Hitachi Japan is reported to

be up to 19%.

3.Amorphous Silicon Cells

• Amorphous silicon is pure silicon with no crystal properties.

• It is highly light absorbent and requires only 1 µm to 2 µm of material to absorb

photons of the incident light.

• Thin amorphous layers can be deposited on cheap substrates like steel, glass and
plastic.
• Hydrogenated amorphous silicon (a-Si : H) is a suitable material for thin film solar
cells, mainly due to its high photo-conductivity, high optical absorption of visible
light with optical band gap of 1.55 eV.

• Thin films of nearly 0.7 µm can produce solar cells comparatively at low cost.

• Amorphous silicon cells can be fabricated in four structures: (i) metal, insulator–
semiconductor (MIS), (ii) p-i-n devices, (iii) hetrojunction, and (iv) Schottky barriers.

• The p-i-n junction, a-Si solar cells are beneficial for commercial production due to
their good performance.

• A common type of p-i-n junction, a-Si solar cell, consists of a deposited layer of
boron doped a-Si : H(200 Å) and above it, is a deposited layer of n-doped a-Si : H (80
Å).

• Then, a 70 Å thick layer of Indium Tin Oxide (ITO) is deposited over the n-type layer
which serves in two ways, i.e., conducting electrode and anti reflective coating.

• In a single junction (a-Si : H) solar cell, a part of solar radiation with less energy than
band gap remains unutilized and wasted as heat, causing low cell efficiency.

This drawback is solved by adopting a ‘tandem structure’ that involves stacked

junctions where semiconductors having different energy gaps are created on top of
each other with decreasing band gap in the direction of light path.

• Hitachi of Japan has developed tandem thin film solar cells consisting of three
amorphous layers having different band gaps as shown in Figure .

• The top layer is of transparent conducting oxide and the first two cells are the
standard a-Si : H cells serving as the intrinsic layer, and the third (last) layer is an
alloy of silicon, germanium and hydrogen (a-Si Ge : H).

• In this structure, the a-Si : H cells utilise the blue–green end of the spectrum,

• while a-SiGe : H cell utilises the red part of the spectrum.

• The spectral response of a tandem cell is shown in Figure which shows the solar
spectrum performance of each cell and the summation of tandem cells.
• The spectral response is improved in long wavelength zones by the material provided
with narrow band gap characteristic controlled by Ge contents.

• This three-layered tandem cell with band gaps of 2.0, 1.7 and 1.45 eV respectively
can attain theoretical efficiency up to 24%
Standalone PV Systems

1.Solar street light

• Solar street light as shown in Figure describes a standalone PV power generating

device.

• It comprises a compact fluorescent lamp, two 35 watt solar modules, and an 80 Ah

tubular cell battery.

2.Home lighting system

• Home lighting systems are the most popular solar PV units, typically designed to
work with two light points and one TV point.

• When necessary, a small dc fan can also be run from this system.

GRID INTERACTIVE SOLAR PV POWER SYSTEM

 A grid-connected photovoltaic power system is connected with the state electric grid.

 The system operates to supplement the grid power during the daytime when a
substantial quantum of solar energy is extracted from the sunlight.

 During night the grid power alone feeds the load.

 This system also supplies emergency power during any short period of grid failure as
shown in Figure.

This system requires additional equipment to control voltage, frequency and

waveform so as to conform to conditions for feeding the power into the grid