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1
CHAPTER 1
HYBRID CAR
1.1 WHAT MAKE A HYBRID
Any vehicle is a hybrid when it combines two or more sources of power.
Many people have probably owned a hybrid vehicle at some point. For example, a
mo-ped (a motorized pedal bike) is a type of hybrid because it combines the power
of a petrol engine with the pedal power of its rider. Giant trucks and pulling trains
are diesel electric hybrid. Submarines are also hybrid vehicles. Some are nuclear-
electric and some are diesel-electric. Any vehicle that combines two or more
sources of power that can directly or indirectly provide propulsion power is a
hybrid. The gasoline electric hybrid car is just a cross between a gasoline-powered
car and an electric car.
1.2Types of Hybrid Car
1.2.1 Gasoline powered car
The figure shows a schematic representation of gasoline-powered car. It
consists of a fuel tank, an engine and transmission. The fuel from the fuel tank
helps the running of engine. The power produced is transmitted to the transmission
system and it turns the road wheels.
ENGINE TRANSMISSION FUEL TANK
Figure 1.2.1.1: Gasoline powered car
2
ENGINE
TRANSMISSION
Fig. 1.2.1.2: Gasoline car
1.2.2 PROBLEMS WITH GASOLINE POWERED CAR
To be useful for a car must meet certain requirements. A car should be able
to
a) Drive at least 300 miles between refueling.
b) Refueled quickly and easily.
c) Keep up other traffic on the road.
A gasoline car meets these requirements but produces a relative large
amount of pollution and generally gets poor mileage. On the other hand
the electric car produces almost no pollution but it can only go up to 100 miles
between charges and the problem is that it is very slow and Inconvenient to
recharge.
When driver for quick acceleration causes a car to be much less efficient
than they could be. Less powerful engine gets better mileage than an identical car
with a more power engine. The amusing thing is that most of what we require a car
to do uses a small percentage of its hybrid power when driving along the freeway
at 60 mph.
2.3 EVOLUTION OF HYBRID
The hybrid is a compromise. It attempts to significantly increase the mileage and
reduce the emissions of a gas-powered car while overcoming the shortcomings of
an electric car. It is wonder that while any one would build such a complicated
machine when most people are perfectly happy with their gasoline-powered car.
FUEL TANK
3
The reason is two told that is to reduce tail pipe emission and to improve the
mileage.
1.2.3 ELECTRIC POWERED CAR
The figure shows a typical electric car. It consists of a set of batteries,
electric motor and transmission. The batteries provide power to electric motor and
the motor turns the transmission.
MOTOR TRANSMISSION BATTERY
Fig. 1.2.3.1: Electric powered car
Fig. 1.2.3.2: Electric Powered Car
BATTERY
ELECTRIC
MOTOR
TRANSMISSION
4
CHAPTER 2
HYBRID STRUCTURE
2. HYBRID STRUCTURE
Two power sources found in a hybrid car in different ways. It has a fuel
tank, which supplies gasoline to the engine and set of batteries that supplies power
to an electric motor. Both the engine and the electric motor can turn the
transmission at the same time, and the transmission then turns the road wheels. The
different ways in which the power sources found in a hybrid car are describe as
follows.
i) Parallel Hybrid
ii) Series Hybrid
2.1 PARALLEL HYBRID CAR
The figure shows a typical parallel hybrid. You'll notice that the fuel tank
and gas engine connect to the transmission. The batteries and electric motor also
connect to the transmission independently. As a result, in a parallel hybrid, both
the electric motor and the gas engine can provide propulsion power.
BATTERY FUEL TANK
ENGINE
TRANSMISSION
ELECTRIC MOTOR
Fig.2.1.1: Parallel Hybrid Car
5
BATTERY
ELECTRIC
MOTOR
Fig.2.1.2: Parallel hybrid
2.2 SERIES HYBRID CAR
The figure shows a series hybrid. The gasoline engine turns a generator,
and the generator can either charge the batteries or power an electric motor that
drives the transmission. Thus, the gasoline engine never directly powers the
vehicle. Take a look at the diagram of the series hybrid, starting with the fuel tank,
and 4- cylinder engine, Generator, Battery, Electric motor and transmission.
GENERATOR
TRANSMISSION FUEL TANK
ELECTRIC BATTERY
MOTOR ENGIN Fig.2.2.1: Series Hybrid Car
FUEL
TANK ENGINE
TRANSMISSION
6
FUEL TANK ENGINE
GENERATOR
BATTERY
ELECTRIC MOTOR
TRANSMISSION
2.3 HYBRID COMPONENTS
Hybrid car contains the following parts.
i) Gasoline engine
The hybrid car has a gasoline engine much like the one you will find on
most cars. However, the engine on a hybrid is smaller and uses advanced
technologies to reduce emissions and increase efficiency.
ii) Fuel tank
The fuel tank in a hybrid is the energy storage device for the gasoline
engine. Gasoline has a much higher energy density than batteries do. For example,
it takes about 1,000 pounds of batteries to store as much energy as 7 pounds of
gasoline.
iii) Electric motor
The electric motor on a hybrid car is very sophisticated. Advanced
electronics allow it to act as a motor as well as a generator. For example, when it
needs to, it can draw energy from the batteries to accelerate the car. But acting as a
generator, it can slow the car down and return energy to the batteries.
iv) Generator
The generator is similar to an electric motor, but it acts only to produce
electrical power. It is used mostly on series hybrids.
v) Batteries
The batteries in a hybrid car are the energy storage device for the electric
motor. Unlike the gasoline in the fuel tank, which can only power the gasoline
engine, the electric motor on a hybrid car can put energy into the batteries as well
as draw energy from them
7
vi) Transmission
The transmission on a hybrid car performs the same basic function as the
transmission on a conventional car.
2.4 USES OF HYBRID CAR
California emissions standards announced how much of each type of
pollution a car is allowed to emit in California. The amount is usually specified in
gm/mile of carbon monoxide. The key thing here is that the amount of pollutions
allowed does not depend on the mileage a car gets. But a car that burns twice as
much fuel to go a mile will generate approximately twice as much pollution. The
pollution will have to be reduced by the emission control equipment on the car. So
decreasing the fuel consumption of the car is one of the surest ways to decrease
emission. Carbon dioxide is another type of pollution a car produce it cause global
warming. So the hybrid cars are used to reduce the air pollution. They produce
very less Amount of exhaust gases like carbon dioxide and carbon monoxide with
respect to the other vehicles. The pollutants in this type of gas are very less and are
in a controlled manner
2.5 HYBRID MILEAGE TIPS
You can get the best mileage from a hybrid car by using the same kind of
driving habits that give you better mileage in your gasoline-engine car:
i) Drive slower
The aerodynamic drag on the car increases dramatically the faster you
drive. For example, the drag force at 70 mph (113 kph) is about double that at 50
mph (81 kph). So, keeping your speed down can increase your mileage
significantly.
ii) Maintain a constant speed
Each time you speed up the car you use energy, some of which is wasted
when you slow the car down again. By maintaining a constant speed, you will
make the most efficient use of your fuel.
8
iii) Avoid abrupt stops
When you stop your car, the electric motor in the hybrid acts like a
generator and takes some of the energy out of the car while slowing it down. If you
give the electric motor more time to slow the vehicle, it can recover more of the
energy.
9
CHAPTER 3
SOLAR SYSTEM
3. SOLAR CELLS
A device which gets heated by the sun’s energy is called solar heating
device. All the solar heating devices are designed in such a way that they help in
collecting as much sunlight as possible. The solar heating devices such as solar
cooker, solar water h eater and solar cells have greatly helped in solving the energy
problem, its consumption and future energy demands of our country. Solar energy
also reduces our dependence on fossil fuel.
3.1SOLAR CELL
It is a device which converts solar energy directly into electricity. Since
solar energy is a light energy so we can say, “Solar cell is a device which converts
light energy into electrical energy.”
Solar cells are made by a semiconductors such as silicon and galium. Those
solar devices which convert the solar radiation into electricity are called Solar
Cells. Before we discuss the solar Cells, we should know the meaning of semi-
conductors.
3.2 SEMI-CONDUCTORS
i) Semi-conductors are those substances which have very low electrical
conductivity.
ii) They are neither bad conductors nor good conductors of electricity.
iii) They are not good conductors., but unlike an insulator, they allow Some
current to pass through them.
iv) Two common semi-conductor are (1) Silicon, (2) Galium.
3.3 PROCESS OF TRANSFORMATION OF SOLAR ENERGY INTO
ELECTRICAL ENERGY
Solar energy is transformed in the form of electromagnetic radiations of
different wavelength. These radiations comprise visible light and invisible light
10
(infra red) Solar cells can transform light energy into electrical energy which can
also be converted into mechanical energy.
The conductivity of solar cells, that is ability to conduct electricity of semi
conduct electricity of semi-conductors increases if certain impurities like Boron
and Arsenic are added to them. These can be explained from following Fig. 1.
Collection
Electric Current
Solar Raditions
Fig.3.3.1:Flow of Electrons
3.4 USE OF SEMI-CONDUCTORS IN SOLAR CELLS
Due to use of semi-conductors materials for making solar cells, efficiency
of solar cells has increased tremendously. The efficiency of Solar cells has
increased tremendously. The efficiency of solar cells, made from silicon, galium
and germanium is limited upto 10% to 15% that is they can convert about 10% to
15% of solar energy into electrical energy. Efficiency of modern solar cells mode
from selenium is upto 25% which is quite high.
3.5 CONSTRUCTION OF SOLAR CELLS
These days solar cells are usually made from semi-conductors like silicon,
galium and selenium. To make solar cells, wafer (think layer) of semi-conductor
materials is arranged in such a way that when the light falls on them, a potential
difference is produced between the two regions of wafer (See Fig. 2). When the
sunlight falls on wafer of selenium, it is converted into electricity due to emission
of electrons.
Solar Radiations Ctric Current
Borona Major impurity Flow of Electrons Arsenic Major impurity
Fig.3.5.1: Construction of solar cell
11
Potential difference produced by a single solar cell of 4 sq cm size is about 0.4
volts and generates current of 0 milli-amperes.
3.6 SOLAR CELL PANEL
A lot of electricity is required for working of various device such as
artificial satellites, water pumps, street lighting, etc. No single cell can provide
such energy. But by joining a large number of solar cells in a particular way, we
can obtain any amount of electrical energy at any desired voltage. A solar cell
panel contains large number of solar cells joined together in a definite pattern. The
solar panel converts solar energy into electricity during day. The energy so
produced is stored in condensers and is used during nights. A solar panel can
provide much more electric power than a single solar cell.
3.7 ADVANTAGES OF SOLAR CELL PANEL
i) Solar cell panel provide a large amount of electricity than a single cell. The
electricity provided by it is used to run electric motors and lift water from deep
wells.
ii) The electric power required for working of artificial satellites stationed in
outer space. Street lighting in remove areas and running of irrigation water pumps,
etc., is obtained with the help of a solar cell panel.
iii) Solar cell panel is very helpful to overcome energy crises in the modern
times. In Fig. 3 solar energy is being used for running a water pump for irrigation
with the help of solar cell panel.
iv) There is also a solar cell panel ‘S’ in which hundreds of solar cells are
joined together. The electricity produced by solar panel is stored in battery ‘B’.
This battery runs the electric motor M and finally motor M drives the pump P,
which pumps out the underground water.
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3.8 APPLICATIONS OF SOLAR CELLS
i)The uses of solar cells have been very effective in providing electric power to
remote inaccessible and isolated places.
ii)Solar cells are used for providing electricity in artificial satellites and space
probes depend mainly on the electricity generated by solar panels. In India, solar
cells are being used for street light, for running water pumps and for operating
radio and televisions sets in remote areas.
iii)Solar cells are used for providing electricity to light houses situated in the sea
and to off shore oil drilling rig platforms Solar cells are used for operating
electronic watches and calculators. Solar cells have gained a lot of importance in
the last few decade because they are being used increasingly for providing
electricity to artificial satellites and space probes, for providing electricity to
remote areas and for operating modern instruments, like electronic watches and
calculators.
iv)In India, efforts are being made to harness solar energy on a large scale to meet
its ever-increasing needs for energy.
v)The department of Non-conventional Energy Sources (DNS) of Government of
India and similar departments of the State level are making all efforts to popularise
use of solar cells of generating electricity. Solar cells and solar panels are available
to the public at highly subsidised rates. In order to harness solar energy on a
commercial scale, many solar energy parks are being established in India.
vi)The greatest advantage of solar energy cells is that they make use of ever-lasting
solar energy and their use does not produce any environmental pollution.
3.8 SOLAR PANEL
A solar cell or photovoltaic cell is a device that converts solar energy into
electricity by the photovoltaic effect. Photovoltaics is the field of technology and
research related to the application of solar cells as solar energy. Sometimes the
term solar cell is reserved for devices intended specifically to capture energy from
sunlight, while the term photovoltaic cell is used when the source is unspecified.
13
Assemblies of cells are used to make solar modules, which may in turn be linked
in photovoltaic arrays. Solar cells have many applications. Individual cells are used
for powering small devices such as electronic calculators. Photovoltaic arrays
generate a form of renewable electricity, particularly useful in situations where
electrical power from the grid is unavailable such as in remote area power systems,
Earth-orbiting satellites and space probes, remote radiotelephones and water
pumping applications. Photovoltaic electricity is also increasingly deployed in
grid-tied electrical systems.
Fig.3.8.1:Solar Panel
A solar cell, made from a monocrystalline sil
3.9 History
3.9.1 Timeline of solar cells
The term "photovoltaic" comes from the Greek φώς:phos meaning "light",
and "voltaic", meaning electrical, from the name of the Italian physicist Volta,
after whom the measurement unit volt is named. The term "photo-voltaic" has been
in use in English since 1849.
The photovoltaic effect was first recognized in 1839 by French physicist
Alexandre-Edmond Becquerel. However, it was not until 1883 that the first solar
cell was built, by Charles Fritts, who coated the semiconductor selenium with an
extremely thin layer of gold to form the junctions. The device was only around 1%
efficient. Russell Ohl patented the modern solar cell in 1946 (U.S. Patent
2,402,662 , "Light sensitive device"). Sven Ason Berglund had a prior patent
14
concerning methods of increasing the capacity of photosensitive cells. The modern
age of solar power technology arrived in 1954 when Bell Laboratories,
experimenting with semiconductors, accidentally found that silicon doped with
certain impurities was very sensitive to light.
This resulted in the production of the first practical solar cells with a sunlight
energy conversion efficiency of around 6 percent. Russia launched the first
artificial satellite in 1957, and the United States' first artificial satellite was
launched in 1958 using solar cells created by Peter Iles in an effort spearheaded by
Hoffman Electronics. The first spacecraft to use solar panels was the US satellite
Explorer 1 in January 1958. This milestone created interest in producing and
launching a geostationary communications satellite, in which solar energy would
provide a viable power supply. This was a crucial development which stimulated
funding from several governments into research for improved solar cells.
In 1970 the first highly effective GaAs heterostructure solar cells were created by
Zhores Alferov and his team in the USSR. Metal Organic Chemical Vapor
Deposition (MOCVD, or OMCVD) production equipment was not developed until
the early 1980's, limiting the ability of companies to manufacture the GaAs solar
cell. In the United States, the first 17% efficient air mass zero (AM0) single-
junction GaAs solar cells were manufactured in production quantities in 1988 by
Applied Solar Energy Corporation (ASEC). The "dual junction" cell was
accidentally produced in quantity by ASEC in 1989 as a result of the change from
GaAs on GaAs substrates to GaAs on Germanium (Ge) substrates. The accidental
doping of Ge with the GaAs buffer layer created higher open circuit voltages,
demonstrating the potential of using the Ge substrate as another cell. As GaAs
single-junction cells topped 19% AM0 production efficiency in 1993, ASEC
developed the first dual junction cells for spacecraft use in the United States, with
a starting efficiency of approximately 20%. These cells did not utilize the Ge as a
second cell, but used another GaAs-based cell with different doping. Eventually
GaAs dual junction cells reached production efficiencies of about 22%. Triple
Junction solar cells began with AM0 efficiencies of approximately 24% in 2000,
26% in 2002, 28% in 2005, and in 2007 have evolved to a 30% AM0 production
15
efficiency, currently in qualification. In 2007, two companies in the United States,
Emcore Photovoltaics and Spectrolab, produce 95% of the world's 28% efficient
solar cells.
3.10 Three generations of solar cells
The first generation photovoltaic cell consists of a large-area, single-
crystal, single layer p-n junction diode, capable of generating usable electrical
energy from light sources with the wavelengths of sunlight. These cells are
typically made using a diffusion process with silicon wafers. First-generation
photovoltaic cells (also known as silicon wafer-based solar cells) are the dominant
technology in the commercial production of solar cells, accounting for more than
86% of the terrestrial solar cell market.
The second generation of photovoltaic materials is based on the use of thin
epitaxial deposits of semiconductors on lattice-matched wafers. There are two
classes of epitaxial photovoltaics - space and terrestrial. Space cells typically have
higher AM0 efficiencies (28-30%) in production, but have a higher cost per watt.
Their "thin-film" cousins have been developed using lower-cost processes, but
have lower AM0 efficiencies (7-9%) in production and are questionable for space
applications. The advent of thin-film technology contributed to a prediction of
greatly reduced costs for thin film solar cells that has yet to be achieved. There are
currently (2007) a number of technologies/semiconductor materials under
investigation or in mass production. Examples include amorphous silicon,
polycrystalline silicon, micro-crystalline silicon, cadmium telluride, copper indium
selenide/sulfide. An advantage of thin-film technology theoretically results in
reduced mass so it allows fitting panels on light or flexible materials, even textiles.
The advent of thin GaAs-based films for space applications (so-called "thin cells")
with potential AM0 efficiencies of up to 37% are currently in the development
stage for high specific power applications. Second generation solar cells now
16
comprise a small segment of the terrestrial photovoltaic market, and approximately
90% of the space market.
Third-generation photovoltaics are proposed to be very different from the
previous semiconductor devices as they do not rely on a traditional p-n junction to
separate photogenerated charge carriers. For space applications quantum well
devices (quantum dots, quantum ropes, etc.) and devices incorporating carbon
nanotubes are being studied - with a potential for up to 45% AM0 production
efficiency. For terrestrial applications, these new devices include
photoelectrochemical cells, polymer solar cells, nanocrystal solar cells, Dye-
sensitized solar cells and are still in the research phase.
3.11 Applications and implementations
3.11.1 Photovoltaic array
Solar cells are often electrically connected and encapsulated as a module.
PV modules often have a sheet of glass on the front (sun up) side , allowing light
to pass while protecting the semiconductor wafers from the elements (rain, hail,
etc.). Solar cells are also usually connected in series in modules, creating an
additive voltage. Connecting cells in parallel will yield a higher current. Modules
are then interconnected, in series or parallel, or both, to create an array with the
desired peak DC voltage and current.
The power output of a solar array is measured in watts or kilowatts. In
order to calculate the typical energy needs of the application, a measurement in
watt-hours, kilowatt-hours or kilowatt-hours per day is often used. A common rule
of thumb is that average power is equal to 20% of peak power, so that each peak
kilowatt of solar array output power corresponds to energy production of 4.8 kW·h
per day.
To make practical use of the solar-generated energy, the electricity is most often
fed into the electricity grid using inverters (grid-connected PV systems); in stand
alone systems, batteries are used to store the energy that is not needed
immediately.
17
Fig.3.11.1.1:Laminated Cell
Polycrystalline PV cells laminated to backing material in a PV module
3.12 Theory
Fig.3.12.1: Solar Panel
i) Photons in sunlight hit the solar panel and are absorbed by semiconducting
materials, such as silicon.
ii) Electrons (negatively charged) are knocked loose from their atoms, allowing
them to flow through the material to produce electricity. The complementary
positive charges that are also created (like bubbles) are called holes and flow in the
direction opposite of the electrons in a silicon solar panel.
iii) An array of solar panels converts solar energy into a usable amount of direct
current (DC) electricity.
3.13 Optionally:
i) The DC current enters an inverter.
18
ii) The inverter turns DC electricity into 120 or 240-volt AC (alternating current)
electricity needed for home appliances.
iii) The AC power enters the utility panel in the house.
iv) The electricity is then distributed to appliances or lights in the
house.
v) The electricity that is not used will be re-routed and used in other
facilities.
3.14 Photogeneration of charge carriers
When a photon hits a piece of silicon, one of three things can happen:
i) The photon can pass straight through the silicon — this (generally) happens for
lower energy photons,
ii) The photon can reflect off the surface,
iii) The photon can be absorbed by the silicon, if the photon energy is higher than
the silicon band gap value. This generates an electron-hole pair and sometimes
heat, depending on the band structure.
When a photon is absorbed, its energy is given to an electron in the crystal
lattice. Usually this electron is in the valence band, and is tightly bound in covalent
bonds between neighboring atoms, and hence unable to move far. The energy
given to it by the photon "excites" it into the conduction band, where it is free to
move around within the semiconductor. The covalent bond that the electron was
previously a part of now has one fewer electron — this is known as a hole. The
presence of a missing covalent bond allows the bonded electrons of neighboring
atoms to move into the "hole," leaving another hole behind, and in this way a hole
can move through the lattice. Thus, it can be said that photons absorbed in the
semiconductor create mobile electron-hole pairs.
A photon need only have greater energy than that of the band gap in order
to excite an electron from the valence band into the conduction band. However, the
solar frequency spectrum approximates a black body spectrum at ~6000 K, and as
such, much of the solar radiation reaching the Earth is composed of photons with
energies greater than the band gap of silicon. These higher energy photons will be
19
absorbed by the solar cell, but the difference in energy between these photons and
the silicon band gap is converted into heat (via lattice vibrations — called
phonons) rather than into usable electrical energy.
3.15 Charge carrier separation
There are two main modes for charge carrier separation in a solar cell:
i) Drift of carriers, driven by an electrostatic field established across the device
ii) Diffusion of carriers from zones of high carrier concentration to zones of low
carrier concentration (following a gradient of electrochemical potential).
In the widely used p-n junction solar cells, the dominant mode of charge
carrier separation is by drift. However, in non-p-n-junction solar cells (typical of
the third generation of solar cell research such as dye and polymer thin-film solar
cells), a general electrostatic field has been confirmed to be absent, and the
dominant mode of separation is via charge carrier diffusion.
3.16 The p-n junction
3.16.1Semiconductor
The most commonly known solar cell is configured as a large-area p-n
junction made from silicon. As a simplification, one can imagine bringing a layer
of n-type silicon into direct contact with a layer of p-type silicon. In practice, p-n
junctions of silicon solar cells are not made in this way, but rather, by diffusing an
n-type dopant into one side of a p-type wafer (or vice versa).
If a piece of p-type silicon is placed in intimate contact with a piece of n-
type silicon, then a diffusion of electrons occurs from the region of high electron
concentration (the n-type side of the junction) into the region of low electron
concentration (p-type side of the junction). When the electrons diffuse across the
p-n junction, they recombine with holes on the p-type side. The diffusion of
carriers does not happen indefinitely however, because of an electric field which is
created by the imbalance of charge immediately on either side of the junction
which this diffusion creates. The electric field established across the p-n junction
creates a diode that promotes current to flow in only one direction across the
20
junction. Electrons may pass from the n-type side into the p-type side, and holes
may pass from the p-type side to the n-type side. This region where electrons have
diffused across the junction is called the depletion region because it no longer
contains any mobile charge carriers. It is also known as the "space charge region".
3.17 Connection to an external load
Ohmic metal-semiconductor contacts are made to both the n-type and p-
type sides of the solar cell, and the electrodes connected to an external load.
Electrons that are created on the n-type side, or have been "collected" by the
junction and swept onto the n-type side, may travel through the wire, power the
load, and continue through the wire until they reach the p-type semiconductor-
metal contact. Here, they recombine with a hole that was either created as an
electron-hole pair on the p-type side of the solar cell, or swept across the junction
from the n-type side after being created there.
3.18 Equivalent circuit of a solar cell
To understand the electronic behavior of a solar cell, it is useful to create a
model which is electrically equivalent, and is based on discrete electrical
components whose behavior is well known. An ideal solar cell may be modelled
by a current source in parallel with a diode; in practice no solar cell is ideal, so a
shunt resistance and a series resistance component are added to the model.[5] The
resulting equivalent circuit of a solar cell is shown on the left. Also shown, on the
right, is the schematic representation of a solar cell for use in circuit diagrams.
Fig.3.17.1: The equivalent circuit of a solar cell
21
Fig.3.17.2: The schematic symbol of a solar cell
3.18 Circuit Equations defining solar cell
The equations which describe the I-V characteristics of the cell are
1. I = I_L - I_o \left( e^{q \left( V + I \times R_s \right) / nkT} - 1 \right)
2. I_L = I_{L \left( T_1 \right)} \left( 1 + K_o \left(T - T_1 \right) \right)
3. I_{L \left( T_1 \right)} = G \times I_{sc \left( T_1 \right) }
4. K_o = \left( I_{ sc \left(T_2 \right) } - I_{sc \left( T_1 \right) } \right) / \left(
T_2 - T_1 \right)
5. I_o = I_{ o \left( T_1 \right) } \times {\left( T / T_1 \right)}^{3/n} * e^{ -q *
V_g / nk * \left( 1/T - 1/T_1 \right)}
6. I_{ o \left( T_1 \right) } = I_{ sc \left( T_1 \right) }/ \left( e^{ q*V_{ oc \left(
T_1 \right) } / nkT_1 } - 1 \right)
7. R_s = - { \left( \tfrac{dV}{dI} \right) }_{ V_{oc} } - 1 / X_v
8. X_v = I_{ o \left( T_1 \right) } \times q / nkT_1 \times e^{ q*V_{ oc \left(
T_1 \right) } / nkT_1 }
where
* k is Boltzman's constant
* q is charge on an electron
* Vg is band gap voltage
* n is diode quality factor
* Rs is series resistance of cell
* Rsh is shunt resistance
* T1 and T2 are reference temperature in Kelvin
* T is working temperature of cell in Kelvin
* Voc is open circuit voltage
22
* Isc is short circuit current
* Io is reverse saturation current of diode
* I is the current through Rs
* IL is current produced by the cell
in many sources IL is also known as Iph.
3.19 Solar cell efficiency factors
3.19.1 Energy conversion efficiency
A solar cell's energy conversion efficiency (η, "eta"), is the percentage of
power converted (from absorbed light to electrical energy) and collected, when a
solar cell is connected to an electrical circuit. This term is calculated using the ratio
of the maximum power point, Pm, divided by the input light irradiance (E, in
W/m²) under standard test conditions (STC) and the surface area of the solar cell
(Ac in m²).
\eta = \frac{P_{m}}{E \times A_c}
STC specifies a temperature of 25°C and an irradiance of 1000 W/m² with
an air mass 1.5 (AM1.5) spectrum. These correspond to the irradiance and
spectrum of sunlight incident on a clear day upon a sun-facing 37°-tilted surface
with the sun at an angle of 41.81° above the horizon.[6][7] This condition
approximately represents solar noon near the spring and autumn equinoxes in the
continental United States with surface of the cell aimed directly at the sun. Thus,
under these conditions a solar cell of 12% efficiency with a 100 cm2 (0.01 m2)
surface area can be expected to produce approximately 1.2 watts of power.
The losses of a solar cell may be broken down into reflectance losses,
thermodynamic efficiency, recombination losses and resistive electrical loss. The
overall efficiency is the product of each of these individual losses.
Due to the difficulty in measuring these parameters directly, other
parameters are measured instead: Thermodynamic Efficiency, Quantum
Efficiency, VOC ratio, and Fill Factor. Reflectance losses are a portion of the
Quantum Efficiency under "External Quantum Efficiency". Recombination losses
make up a portion of the Quantum Efficiency, VOC ratio, and Fill Factor.
23
Resistive losses are predominantly categorized under Fill Factor, but also make up
minor portions of the Quantum Efficiency, VOC ratio.
3.20 Thermodynamic Efficiency Limit
Solar cells operate as quantum energy conversion devices, and are therefore
subject to the "Thermodynamic Efficiency Limit". Photons with an energy below
the band gap of the absorber material cannot generate a hole-electron pair, and so
their energy is not converted to useful output and only generates heat if absorbed.
For photons with an energy above the band gap energy, only a fraction of the
energy above the band gap can be converted to useful output. When a photon of
greater energy is absorbed, the excess energy above the band gap is converted to
kinetic energy of the carrier combination. The excess kinetic energy is converted to
heat through phonon interactions as the kinetic energy of the carriers slows to
equilibrium velocity.
Solar cells with multiple band gap absorber materials are able to more
efficiently convert the solar spectrum. By using multiple band gaps, the solar
spectrum may be broken down into smaller bins where the thermodynamic
efficiency limit is higher for each bin.
3.21 Quantum efficiency
As described above, when a photon is absorbed by a solar cell it is
converted to an electron-hole pair. This electron-hole pair may then travel to the
surface of the solar cell and contribute to the current produced by the cell; such a
carrier is said to be collected. Alternatively, the carrier may give up its energy and
once again become bound to an atom within the solar cell without reaching the
surface; this is called recombination, and carriers that recombine do not contribute
to the production of electrical current.
Quantum efficiency refers to the percentage of photons that are converted
to electric current (i.e., collected carriers) when the cell is operated under short
circuit conditions. External quantum efficiency is the fraction of incident photons
24
that are converted to electrical current, while internal quantum efficiency is the
fraction of absorbed photons that are converted to electrical current.
Mathematically, internal quantum efficiency is related to external quantum
efficiency by the reflectance of the solar cell; given a perfect anti-reflection
coating, they are the same.
Quantum efficiency should not be confused with energy conversion
efficiency, as it does not convey information about the power collected from the
solar cell. Furthermore, quantum efficiency is most usefully expressed as a spectral
measurement (that is, as a function of photon wavelength or energy). Since some
wavelengths are absorbed more effectively than others in most semiconductors,
spectral measurements of quantum efficiency can yield information about which
parts of a particular solar cell design are most in need of improvement.
3.22 VOC ratio
Due to recombination, the open circuit voltage (VOC) of the cell will be
below the band gap voltage of the cell. Since the energy of the photons must be at
or above the band gap to generate a carrier pair, cell voltage below the band gap
voltage represents a loss. This loss is represented by the ratio of VOC divided by
VG
3.23 Maximum-power point
A solar cell may operate over a wide range of voltages (V) and currents (I).
By increasing the resistive load on an irradiated cell continuously from zero (a
short circuit) to a very high value (an open circuit) one can determine the
maximum-power point, the point that maximizes V×I; that is, the load for which
the cell can deliver maximum electrical power at that level of irradiation. (The
output power is zero in both the short circuit and open circuit extremes).
A high quality, monocrystalline silicon solar cell, at 25 °C cell temperature,
may produce 0.60 volts open-circuit (Voc). The cell temperature in full sunlight,
even with 25 °C air temperature, will probably be close to 45 °C, reducing the
open-circuit voltage to 0.55 volts per cell. The voltage drops modestly, with this
25
type of cell, until the short-circuit current is approached (Isc). Maximum power
(with 45 °C cell temperature) is typically produced with 75% to 80% of the open-
circuit voltage (0.43 volts in this case) and 90% of the short-circuit current. This
output can be up to 70% of the Voc x Isc product. The short-circuit current (Isc)
from a cell is nearly proportional to the illumination, while the open-circuit voltage
(Voc) may drop only 10% with a 80% drop in illumination. Lower-quality cells
have a more rapid drop in voltage with increasing current and could produce only
1/2 Voc at 1/2 Isc. The usable power output could thus drop from 70% of the Voc
x Isc product to 50% or even as little as 25%. Vendors who rate their solar cell
"power" only as Voc x Isc, without giving load curves, can be seriously distorting
their actual performance.
The maximum power point of a photovoltaic varies with incident
illumination. For systems large enough to justify the extra expense, a maximum
power point tracker tracks the instantaneous power by continually measuring the
voltage and current (and hence, power transfer), and uses this information to
dynamically adjust the load so the maximum power is always transferred,
regardless of the variation in lighting.
3.24 Fill factor
Another defining term in the overall behavior of a solar cell is the fill factor
(FF). This is the ratio of the maximum power point divided by the open circuit
voltage (Voc) and the short circuit current (Isc):
FF = \frac{P_{m}}{V_{oc} \times I_{sc}} = \frac{\eta \times A_c \times
E}{V_{oc} \times I_{sc}}
3.25 Comparison of energy conversion efficiencies
3.25.1 Photovoltaics
At this point, discussion of the different ways to calculate efficiency for
space cells and terrestrial cells is necessary to alleviate confusion. In space, where
there is no atmosphere, the spectrum of the sun is relatively unfiltered. However on
earth, with air filtering the incoming light, the solar spectrum changes. To account
26
for the spectral differences, a system was devised to calculate this filtering effect.
Simply, the filtering effect ranges from Air Mass 0 in space, to approximately Air
Mass 1.5 on earth. Multiplying the spectral differences by the quantum efficiency
of the solar cell in question will yield the efficiency of the device. For example, a
Silicon solar cell in space might have an efficiency of 14% at AM0, but have an
efficiency of 16% on earth at AM 1.5. Terrestrial efficiencies typically are greater
than space efficiencies.
Solar cell efficiencies vary from 6% for amorphous silicon-based solar
cells to 40.7% with multiple-junction research lab cells and 42.8% with multiple
dies assembled into a hybrid package.[9] Solar cell energy conversion efficiencies
for commercially available multicrystalline Si solar cells are around 14-19%[10].
The highest efficiency cells have not always been the most economical — for
example a 30% efficient multijunction cell based on exotic materials such as
gallium arsenide or indium selenide and produced in low volume might well cost
one hundred times as much as an 8% efficient amorphous silicon cell in mass
production, while only delivering about four times the electrical power.
However, there is a way to "boost" solar power. By increasing the light
intensity, typically photogenerated carriers are increased, resulting in increased
efficiency by up to 15%. These so-called "concentrator systems" have only begun
to become cost-competitive as a result of the development of high efficiency GaAs
cells. The increase in intensity is typically accomplished by using concentrating
optics. A typical concentrator system may use a light intensity 6-400 times the sun,
and increase the efficiency of a one sun GaAs cell from 31% at AM 1.5 to 35%.
A common method used to express economic costs of electricity-generating
systems is to calculate a price per delivered kilowatt-hour (kWh). The solar cell
efficiency in combination with the available irradiation has a major influence on
the costs, but generally speaking the overall system efficiency is important. Using
the commercially available solar cells (as of 2006) and system technology leads to
system efficiencies between 5 and 19%. As of 2005, photovoltaic electricity
generation costs ranged from ~0.60 US$/kWh (0.50 €/kWh) (central Europe) down
to ~0.30 US$/kWh (0.25 €/kWh) in regions of high solar irradiation. This
27
electricity is generally fed into the electrical grid on the customer's side of the
meter. The cost can be compared to prevailing retail electric pricing (as of 2005),
which varied from between 0.04 and 0.50 US$/kWh worldwide. (Note: in addition
to solar irradiance profiles, these costs/kwh calculations will vary depending on
assumptions for years of useful life of a system. Most c-Si panels are warrantied
for 25 years and should see 35+ years of useful life.)
The chart at the right illustrates the various commercial large-area module
energy conversion efficiencies and the best laboratory efficiencies obtained for
various materials and technologies.
Reported timeline of solar cell energy conversion efficiencies (from National
Renewable Energy Laboratory (USA)
Reported timeline of solar cell energy conversion efficiencies (from National
Renewable Energy Laboratory (USA)
3.26 Watts peak
Since solar cell output power depends on multiple factors, such as the sun's
incidence angle, for comparison purposes between different cells and panels, the
measure of watts peak (Wp) is used. It is the output power under these conditions
known as STC
i) Insolation (solar irradiance) 1000 W/m²
ii) solar reference spectrum AM (airmass) 1.5
iii) cell temperature 25°C
3.27 Solar cells and energy payback
In the 1990s, when silicon cells were twice as thick, efficiencies were 30%
lower than today and lifetimes were shorter, it may well have cost more energy to
make a cell than it could generate in a lifetime. In the meantime, the technology
has progressed significantly, and the energy payback time of a modern
photovoltaic module is typically from 1 to 4 years [13] depending on the type and
where it is used (see net energy gain). With a typical lifetime of 20 to 30 years, this
28
means that modern solar cells are net energy producers, i.e they generate much
more energy over their lifetime than the energy expended in producing them.
3.28 Light-absorbing materials
All solar cells require a light absorbing material contained within the cell
structure to absorb photons and generate electrons via the photovoltaic effect. The
materials used in solar cells tend to have the property of preferentially absorbing
the wavelengths of solar light that reach the earth surface; however, some solar
cells are optimized for light absorption beyond Earth's atmosphere as well. Light
absorbing materials can often be used in multiple physical configurations to take
advantage of different light absorption and charge separation mechanisms. Many
currently available solar cells are configured as bulk materials that are
subsequently cut into wafers and treated in a "top-down" method of synthesis
(silicon being the most prevalent bulk material). Other materials are configured as
thin-films (inorganic layers, organic dyes, and organic polymers) that are deposited
on supporting substrates, while a third group are configured as nanocrystals and
used as quantum dots (electron-confined nanoparticles) embedded in a supporting
matrix in a "bottom-up" approach. Silicon remains the only material that is well-
researched in both bulk and thin-film configurations. The following is a current list
of light absorbing materials, listed by configuration and substance-name:
3.29 Bulk
These bulk technologies are often referred to as wafer-based
manufacturing. In other words, in each of these approaches, self-supporting wafers
between 180 to 240 micrometers thick are processed and then soldered together to
form a solar cell module. A general description of silicon wafer processing is
provided in Manufacture and Devices.
3.30 Silicon
3.30.1 Polysilicon, Silicon, and list of silicon producers
29
Fig.3.30..1.1: Basic structure of a silicon based solar cell and its working mechanism.
By far, the most prevalent bulk material for solar cells is crystalline silicon
(abbreviated as a group as c-Si), also known as "solar grade silicon". Bulk silicon
is separated into multiple categories according to crystallinity and crystal size in
the resulting ingot, ribbon, or wafer.
i) monocrystalline silicon (c-Si): often made using the Czochralski process.
Single-crystal wafer cells tend to be expensive, and because they are cut from
cylindrical ingots, do not completely cover a square solar cell module without a
substantial waste of refined silicon. Hence most c-Si panels have uncovered gaps
at the corners of four cells.
ii) Poly- or multicrystalline silicon (poly-Si or mc-Si): made from cast square
ingots — large blocks of molten silicon carefully cooled and solidified. These cells
are less expensive to produce than single crystal cells but are less efficient.
iii) Ribbon silicon: formed by drawing flat thin films from molten silicon and
having a multicrystalline structure. These cells have lower efficiencies than poly-
30
Si, but save on production costs due to a great reduction in silicon waste, as this
approach does not require sawing from ingots.
3.31 Thin films
The various thin-film technologies currently being developed reduce the
amount (or mass) of light absorbing material required in creating a solar cell. This
can lead to reduced processing costs from that of bulk materials (in the case of
silicon thin films) but also tends to reduce energy conversion efficiency, although
many multi-layer thin films have efficiencies above those of bulk silicon wafers.
3.32 CdTe
Cadmium telluride is an efficient light-absorbing material for thin-film
solar cells. Compared to other thin-film materials, CdTe is easier to deposit and
more suitable for large-scale production. Despite much discussion of the toxicity
of CdTe-based solar cells, this is the only technology (apart from amorphous
silicon) that can be delivered on a large scale[citation needed]. The perception of
the toxicity of CdTe is based on the toxicity of elemental cadmium, a heavy metal
that is a cumulative poison. However it has been shown that the release of
cadmium to the atmosphere is lower with CdTe-based solar cells than with silicon
photovoltaics and other thin-film solar cell technologies.
3.33 Copper-Indium Selenide
The materials based on CuInSe2 that are of interest for photovoltaic
applications include several elements from groups I, III and VI in the periodic
31
table. These semiconductors are especially attractive for thin film solar cell
application because of their high optical absorption coefficients and versatile
optical and electrical characteristics which can in principle be manipulated and
tuned for a specific need in a given device. CIS is an abbreviation for general
chalcopyrite films of copper indium selenide (CuInSe2), CIGS mentioned below is
a variation of CIS. CIS films (no Ga) achieved greater than 14% efficiency.
However, manufacturing costs of CIS solar cells at present are high when
compared with amorphous silicon solar cells but continuing work is leading to
more cost-effective production processes. The first large-scale production of CIS
modules was started in 2006 in Germany by Wuerth Solar.
When gallium is substituted for some of the indium in CIS, the material is
sometimes called CIGS , or copper indium/gallium diselenide, a solid mixture of
the semiconductors CuInSe2 and CuGaSe2, often abbreviated by the chemical
formula CuInxGa(1-x)Se2. Unlike the conventional silicon based solar cell, which
can be modelled as a simple p-n junction (see under semiconductor), these cells are
best described by a more complex heterojunction model. The best efficiency of a
thin-film solar cell as of March 2008 was 19.9% with CIGS absorber layer. Higher
efficiencies (around 30%) can be obtained by using optics to concentrate the
incident light. The use of gallium increases the optical bandgap of the CIGS layer
as compared to pure CIS, thus increasing the open-circuit voltage. In another point
of view, gallium is added to replace as much indium as possible due to gallium's
relative availability to indium. Approximately 70% of indium currently produced
is used by the flat-screen monitor industry. Some investors in solar technology
worry that production of CIGS cells will be limited by the availability of indium.
Producing 2 GW of CIGS cells (roughly the amount of silicon cells produced in
2006) would use about 10% of the indium produced in 2004.[20] For comparison,
silicon solar cells used up 33% of the world's electronic grade silicon production in
2006. Nanosolar claims to waste only 5% of the indium it uses. As of 2006, the
best conversion efficiency for flexible CIGS cells on polyimide is 14.1% by Tiwari
et al, at the ETH, Switzerland. Conversion efficiency values on metallic flexible
32
foils were reported by AbuShama et al in the proceedings of the IEEE 4th World
Conference on Photovoltaic Energy Conversion 2006 in Hawaii, USA.
That being said, indium can easily be recycled from decommissioned PV
modules. The recycling program in Germany would be is an example that
highlights the regenerative industrial paradigm: "From cradle to cradle".
Selenium allows for better uniformity across the layer and so the number of
recombination sites in the film are reduced which benefits the quantum efficiency
and thus the conversion efficiency.
3.34 Gallium arsenide (GaAs) multijunction
3.34.1 Multi junction photovoltaic cell
High-efficiency cells have been developed for special applications such as
satellites and space exploration. These multijunction cells consist of multiple thin
films produced using molecular beam epitaxy. A triple-junction cell, for example,
may consist of the semiconductors: GaAs, Ge, and GaInP2. Each type of
semiconductor will have a characteristic band gap energy which, loosely speaking,
causes it to absorb light most efficiently at a certain color, or more precisely, to
absorb electromagnetic radiation over a portion of the spectrum. The
semiconductors are carefully chosen to absorb nearly all of the solar spectrum, thus
generating electricity from as much of the solar energy as possible.
GaAs multijunction devices are the most efficient solar cells to date,
reaching a record high of 40.7% efficiency under solar concentration and
laboratory conditions. These devices use 20 to 30 different semiconductors layered
in series.
This technology is currently being utilised in the Mars rover missions.
Solar arrays made with a material which contains gallium arsenide GaAs
and germanium Ge is seeing demand rapidly rise. In just the past 12 months
(12/2006 - 12/2007), the cost of 4N gallium metal has risen from about $350 per
kg to $680 per kg. Additionally, germanium metal prices have risen substantially
to $1000-$1200 per kg this year. Although some Chinese producers of these
materials may be able to offset some of the price increases with their lower labor
33
costs. Those materials include gallium (4N, 6N and 7N Ga), arsenic (4N, 6N and
7N) and germanium, pyrolitic boron nitride (pBN) crucibles for growing crystals,
and boron oxide, these products are critical to the entire substrate manufacturing
industry.
Triple-junction GaAs solar cells were also being used as the power source
of the Dutch four-time World Solar Challenge winners Nuna in 2005 and 2007.
3.35 Light-absorbing dyes
3.35.1Dye-sensitized solar cells
Typically a ruthenium metalorganic dye (Ru-centered) is used as a
monolayer of light-absorbing material. The dye-sensitized solar cell depends on a
mesoporous layer of nanoparticulate titanium dioxide to greatly amplify the
surface area (200-300 m²/g TiO2, as compared to approximately 10 m²/g of flat
single crystal). The photogenerated electrons from the light absorbing dye are
passed on to the n-type TiO2, and the holes are passed to an electrolyte on the
other side of the dye. The circuit is completed by a redox couple in the electrolyte,
which can be liquid or solid. This type of cell allows a more flexible use of
materials, and is typically manufactured by screen printing, with the potential for
lower processing costs than those used for bulk solar cells. However, the dyes in
these cells also suffer from degradation under heat and UV light, and the cell
casing is difficult to seal due to the solvents used in assembly. In spite of the
above, this is a popular emerging technology with some commercial impact
forecast within this decade.
3.36 Organic polymer solar cells
Organic solar cells and Polymer solar cells are built from thin films
(typically 100 nm) of organic semiconductors such as polymers and small-
molecule compounds like polyphenylene vinylene, copper phthalocyanine (a blue
or green organic pigment) and carbon fullerenes. Energy conversion efficiencies
achieved to date using conductive polymers are low at 6% efficiency[24] for the
34
best cells to date. However, these cells could be beneficial for some applications
where mechanical flexibility and disposability are important.
3.37 Silicon
Silicon thin-films are mainly deposited by chemical vapor deposition
(typically plasma-enhanced (PE-CVD)) from silane gas and hydrogen gas.
Depending on the deposition's parameters, this can yield:
i) Amorphous silicon (a-Si or a-Si:H)
ii) protocrystalline silicon or
iii) Nanocrystalline silicon (nc-Si or nc-Si:H).
These types of silicon present dangling and twisted bonds, which results in
deep defects (energy levels in the bandgap) as well as deformation of the valence
and conduction bands (band tails). The solar cells made from these materials tend
to have lower energy conversion efficiency than bulk silicon, but are also less
expensive to produce. The quantum efficiency of thin film solar cells is also lower
due to reduced number of collected charge carriers per incident photon.
Amorphous silicon has a higher bandgap (1.7 eV) than crystalline silicon
(c-Si) (1.1 eV), which means it absorbs the visible part of the solar spectrum more
strongly than the infrared portion of the spectrum. As nc-Si has about the same
bandgap as c-Si, the two material can be combined in thin layers, creating a
layered cell called a tandem cell. The top cell in a-Si absorbs the visible light and
leaves the infrared part of the spectrum for the bottom cell in nanocrystalline Si.
Recently, solutions to overcome the limitations of thin-film crystalline
silicon have been developed. Light trapping schemes where the incoming light is
obliquely coupled into the silicon and the light traverses the film several times
enhance the absorption of sunlight in the films. Thermal processing techniques
enhance the crystallinity of the silicon and pacify electronic defects.[citation
needed
A silicon thin film technology is being developed for building integrated
photovoltaics (BIPV) in the form of semi-transparent solar cells which can be
35
applied as window glazing. These cells function as window tinting while
generating electricity.
3.38 Nanocrystalline solar cells
These structures make use of some of the same thin-film light absorbing
materials but are overlain as an extremely thin absorber on a supporting matrix of
conductive polymer or mesoporous metal oxide having a very high surface area to
increase internal reflections (and hence increase the probability of light
absorption).
3.39 Concentrating photovoltaics (CPV)
Concentrating photovoltaic systems use a large area of lenses or mirrors to
focus sunlight on a small area of photovoltaic cells.[25] If these systems use single
or dual-axis tracking to improve performance, they may be referred to as Heliostat
Concentrator Photovoltaics (HCPV). The primary attraction of CPV systems is
their reduced usage of semiconducting material which is expensive and currently
in short supply. Additionally, increasing the concentration ratio improves the
performance of general photovoltaic materials[26]. Despite the advantages of CPV
technologies their application has been limited by the costs of focusing, tracking
and cooling equipment. On October 25, 2006, the Australian federal government
and the Victorian state government together with photovoltaic technology
company Solar Systems announced a project using this technology, Solar power
station in Victoria, planned to come online in 2008 and be completed by 2013.
This plant, at 154 MW, would be ten times larger than the largest current
photovoltaic plant in the world.
3.40 Silicon solar cell device manufacture
36
Fig.3.39.1:Solar powered scientific calculator
Because solar cells are semiconductor devices, they share many of the
same processing and manufacturing techniques as other semiconductor devices
such as computer and memory chips. However, the stringent requirements for
cleanliness and quality control of semiconductor fabrication are a little more
relaxed for solar cells. Most large-scale commercial solar cell factories today make
screen printed poly-crystalline silicon solar cells. Single crystalline wafers which
are used in the semiconductor industry can be made into excellent high efficiency
solar cells, but they are generally considered to be too expensive for large-scale
mass production.
Poly-crystalline silicon wafers are made by wire-sawing block-cast silicon
ingots into very thin (180 to 350 micrometer) slices or wafers. The wafers are
usually lightly p-type doped. To make a solar cell from the wafer, a surface
diffusion of n-type dopants is performed on the front side of the wafer. This forms
a p-n junction a few hundred nanometers below the surface.
37
Antireflection coatings, which increase the amount of light coupled into the
solar cell, are typically next applied. Over the past decade, silicon nitride has
gradually replaced titanium dioxide as the antireflection coating of choice because
of its excellent surface passivation qualities (i.e., it prevents carrier recombination
at the surface of the solar cell). It is typically applied in a layer several hundred
nanometers thick using plasma-enhanced chemical vapor deposition (PECVD).
Some solar cells have textured front surfaces that, like antireflection coatings,
serve to increase the amount of light coupled into the cell. Such surfaces can
usually only be formed on single-crystal silicon, though in recent years methods of
forming them on multicrystalline silicon have been developed.
The wafer then has a full area metal contact made on the back surface, and
a grid-like metal contact made up of fine "fingers" and larger "busbars" are screen-
printed onto the front surface using a silver paste. The rear contact is also formed
by screen-printing a metal paste, typically aluminium. Usually this contact covers
the entire rear side of the cell, though in some cell designs it is printed in a grid
pattern. The paste is then fired at several hundred degrees Celsius to form metal
electrodes in ohmic contact with the silicon. After the metal contacts are made, the
solar cells are interconnected in series (and/or parallel) by flat wires or metal
ribbons, and assembled into modules or "solar panels". Solar panels have a sheet of
tempered glass on the front, and a polymer encapsulation on the back. Tempered
glass cannot be used with amorphous silicon cells because of the high temperatures
during the deposition process.
3.40 Current research on materials and devices
3.40.1 Timeline of solar cells
There are currently many research groups active in the field of
photovoltaics in universities and research institutions around the world. This
research can be divided into three areas: making current technology solar cells
cheaper and/or more efficient to effectively compete with other energy sources;
38
developing new technologies based on new solar cell architectural designs; and
developing new materials to serve as light absorbers and charge carriers.
Silicon processing
One way of reducing the cost is to develop cheaper methods of obtaining
silicon that is sufficiently pure. Silicon is a very common element, but is normally
bound in silica, or silica sand. Processing silica (SiO2) to produce silicon is a very
high energy process - at current efficiencies, it takes over two years for a
conventional solar cell to generate as much energy as was used to make the silicon
it contains. More energy efficient methods of synthesis are not only beneficial to
the solar industry, but also to industries surrounding silicon technology as a whole.
The current industrial production of silicon is via the reaction between
carbon (charcoal) and silica at a temperature around 1700 degrees Celsius. In this
process, known as carbothermic reduction, each tonne of silicon (metallurgical
grade, about 98% pure) is produced with the emission of about 1.5 tonnes of
carbon dioxide.
Solid silica can be directly converted (reduced) to pure silicon by
electrolysis in a molten salt bath at a fairly mild temperature (800 to 900 degrees
Celsius). While this new process is in principle the same as the FFC Cambridge
Process which was first discovered in late 1996, the interesting laboratory finding
is that such electrolytic silicon is in the form of porous silicon which turns readily
into a fine powder, (with a particle size of a few micrometres), and may therefore
offer new opportunities for development of solar cell technologies.
Another approach is also to reduce the amount of silicon used and thus
cost, is by micromachining wafers into very thin, virtually transparent layers that
could be used as transparent architectural coverings. The technique involves taking
a silicon wafer, typically 1 to 2 mm thick, and making a multitude of parallel,
transverse slices across the wafer, creating a large number of slivers that have a
thickness of 50 micrometres and a width equal to the thickness of the original
wafer. These slices are rotated 90 degrees, so that the surfaces corresponding to the
faces of the original wafer become the edges of the slivers. The result is to convert,
39
for example, a 150 mm diameter, 2 mm-thick wafer having an exposed silicon
surface area of about 175 cm² per side into about 1000 slivers having dimensions
of 100 mm x 2 mm x 0.1 mm, yielding a total exposed silicon surface area of about
2000 cm² per side. As a result of this rotation, the electrical doping and contacts
that were on the face of the wafer are located the edges of the sliver, rather than the
front and rear as is the case with conventional wafer cells. This has the interesting
effect of making the cell sensitive from both the front and rear of the cell (a
property known as bifaciality). Using this technique, one silicon wafer is enough to
build a 140 watt panel, compared to about 60 wafers needed for conventional
modules of same power output.
3.42 Thin-film processing
Thin-film solar cells use less than 1% of the raw material (silicon or other
light absorbers) compared to wafer based solar cells, leading to a significant price
drop per kWh. There are many research groups around the world actively
researching different thin-film approaches and/or materials, however it remains to
be seen if these solutions can generate the same space-efficiency as traditional
silicon processing.
One particularly promising technology is crystalline silicon thin films on
glass substrates. This technology makes use of the advantages of crystalline silicon
as a solar cell material, with the cost savings of using a thin-film approach.
Another interesting aspect of thin-film solar cells is the possibility to deposit the
cells on all kind of materials, including flexible substrates (PET for example),
which opens a new dimension for new applications.
3.43 Polymer processing
The invention of conductive polymers (for which Alan Heeger, Alan G.
MacDiarmid and Hideki Shirakawa were awarded a Nobel prize) may lead to the
development of much cheaper cells that are based on inexpensive plastics.
However, all organic solar cells made to date suffer from degradation upon
exposure to UV light, and hence have lifetimes which are far too short to be viable.
40
The conjugated double bond systems in the polymers, which carry the charge, are
always susceptible to breaking up when radiated with shorter wavelengths.
Additionally, most conductive polymers, being highly unsaturated and reactive, are
highly sensitive to atmospheric moisture and oxidation, making commercial
applications difficult.
3.44 Nanoparticle processing
Experimental non-silicon solar panels can be made of quantum
heterostructures, eg. carbon nanotubes or quantum dots, embedded in conductive
polymers or mesoporous metal oxides. In addition, thin films of many of these
materials on conventional silicon solar cells can increase the optical coupling
efficiency into the silicon cell, thus boosting the overall efficiency. By varying the
size of the quantum dots, the cells can be tuned to absorb different wavelengths.
Although the research is still in its infancy, quantum dot-modified photovoltaics
may be able to achieve up to 42 percent energy conversion efficiency due to
multiple exciton generation(MEG).
3.45 Transparent conductors
Many new solar cells use transparent thin films that are also conductors of
electrical charge. The dominant conductive thin films used in research now are
transparent conductive oxides (abbreviated "TCO"), and include fluorine-doped tin
oxide (SnO2:F, or "FTO"), doped zinc oxide (e.g.: ZnO:Al), and indium tin oxide
(abbreviated "ITO"). These conductive films are also used in the LCD industry for
flat panel displays. The dual function of a TCO allows light to pass through a
substrate window to the active light absorbing material beneath, and also serves as
an ohmic contact to transport photogenerated charge carriers away from that light
absorbing material. The present TCO materials are effective for research, but
perhaps are not yet optimized for large-scale photovoltaic production. They require
very special deposition conditions at high vacuum, they can sometimes suffer from
poor mechanical strength, and most have poor transmittance in the infrared portion
41
of the spectrum (e.g.: ITO thin films can also be used as infrared filters in airplane
windows). These factors make large-scale manufacturing more costly.
A relatively new area has emerged using carbon nanotube networks as a
transparent conductor for organic solar cells. Nanotube networks are flexible and
can be deposited on surfaces a variety of ways. With some treatment, nanotube
films can be highly transparent in the infrared, possibly enabling efficient low
bandgap solar cells. Nanotube networks are p-type conductors, whereas traditional
transparent conductors are exclusively n-type. The availability of a p-type
transparent conductor could lead to new cell designs that simplify manufacturing
and improve efficiency.
3.46 Silicon wafer based solar cells
Despite the numerous attempts at making better solar cells by using new
and exotic materials, the reality is that the photovoltaics market is still dominated
by silicon wafer-based solar cells (first-generation solar cells). This means that
most solar cell manufacturers are equipped to produce these type of solar cells.
Therefore, a large body of research is currently being done all over the world to
create silicon wafer-based solar cells that can achieve higher conversion efficiency
without an exorbitant increase in production cost. The aim of the research is to
achieve the lowest cost per watt solar cell design that is suitable for commercial
production.
3.47 Manufacturers
Solar cells are manufactured primarily in Japan, Germany, USA, and
China, though numerous other nations have or are acquiring significant solar cell
production capacity. While technologies are constantly evolving toward higher
efficiencies, the most effective cells for low cost electrical production are not
necessarily those with the highest efficiency, but those with a balance between
low-cost production and efficiency high enough to minimize area-related balance
of systems cost. Those companies with large scale manufacturing technology for
coating inexpensive substrates may, in fact, ultimately be the lowest cost net
42
electricity producers, even with cell efficiencies that are lower than those of single-
crystal technologies.
43
CHAPTER 4
WIND ENERGY
4. INTRODUCTION
The Earth is unevenly heated by the sun, such that the poles receive less
energy from the sun than the equator; along with this, dry land heats up (and cools
down) more quickly than the seas do. The differential heating drives a
global atmospheric convection system reaching from the Earth's surface to
the stratosphere which acts as a virtual ceiling. Most of the energy stored in these
wind movements can be found at high altitudes where continuous wind speeds of
over 160 km/h (99 mph) occur. Eventually, the wind energy is converted through
friction into diffuse heat throughout the Earth's surface and the atmosphere. The
total amount of economically extractable power available from the wind is
considerably more than present human power use from all sources. The most
comprehensive study as of 2005 found the potential of wind power on land and
near-shore to be 72 TW, equivalent to 54,000 MToE (million tons of oil
equivalent) per year, or over five times the world's current energy use in all forms.
The potential takes into account only locations with mean annual wind speeds ≥
6.9 m/s at 80 m. The study assumes six 1.5 megawatt, 77 m diameter turbines per
square kilometer on roughly 13% of the total global land area (though that land
would also be available for other compatible uses such as farming).
Large-scale wind farms are connected to the electric power
transmission network; smaller facilities are used to provide electricity to isolated
locations. Utility companies increasingly buy back surplus electricity produced by
small domestic turbines. Wind energy, as an alternative to fossil fuels, is
plentiful, renewable, widely distributed, clean, and produces no greenhouse gas
emissions during operation. However, the construction of wind farms is not
universally welcomed because of their visual impact but any effects on the
environment are generally among the least problematic of any power source.
44
Wind energy is the one of the renewable means of electricity generation
that is part of the worldwide discussion on the future of energy generation and
use and consequent effect on the environment. We believe that there should be
wider participation in this important debate, including technocrats like us. There
are many research problems that could be attracted by physicists including e.g.
Material for turbine vanes(limit wind turbine size),semiconductor materials and
power electronics for electric power conditioning. In this report we discuss basic
technical and some economic aspects of wind energy and hope to provide a helpful
guide to some important issues and literature relevant to this technology.
45
4.1 WORKING PRINCIPLE
Wind energy is the kinetic energy of the air in motion. Total wind energy
flowing through an imaginary area A during the time t is:
E = A . v . t . ρ . ½ v2,
where v is the wind velocity and ρ is the air density. The formula presented is
structured in two parts: (A . v . t) is the volume of air passing through A, which is
considered perpendicular to the wind velocity; (ρ . ½ v2) is the kinetic energy of
the moving air per unit volume.
P = ½ * air density * Area Swept by Rotor * Wind Speed3
P = E / t = ½ . ρ .A . v3
i) Power in the wind is correlated 1:1 with area and is extremely sensitive to wind
speed (the cubic amplifies the power significantly)
ii) If the wind speed is twice as high, it contains 23 = 2 x 2 x 2 = 8 times as much
energy
iii) A site with 16 mph average wind speed will generate nearly 50% more
electricity and be more cost effective than one with 14 mph average wind
speed (16*16*16) / (14*14*14) = 1.4927
iv) Therefore, it “pay$” to hunt for good wind sites with better wind speeds
v) Wind speed increases with height above the ground
vi) Energy in the wind increases as height increases (theoretically)
V2/V1 = (H2/H1)1/7
46
Fig.4.1.1: Wind Energy Graph
4.2 WIND TURBINE:
A wind turbine is a device that converts kinetic energy from the wind
into mechanical energy. If the mechanical energy is used to produce electricity, the
device may be called a wind generator or wind charger. If the mechanical energy is
used to drive machinery, such as for grinding grain or pumping water, the device is
called a windmill or wind pump. Developed for over a millennium, today's wind
turbines are manufactured in a range of vertical and horizontal axis types. The
smallest turbines are used for applications such as battery charging or auxiliary
power on sailing boats; while large grid-connected arrays of turbines are becoming
an increasingly large source of commercial electric power.
Fig.4.2.1: Wind Turbine Schematic diagram
47
4.3 TYPES OF WIND TURBINE:-
a) Horizontal-axis
i) Horizontal-axis wind turbines (HAWT)
ii) Downwind turbine HAWT
iii) Upwind turbine HAWT
iv) Counter Rotating HAWT
v) Modern wind turbines
b) Vertical-axis
i) Vertical-axis wind turbines (or VAWT)
4.4 HORIZONTAL-AXIS WIND TURBINES (HAWT):
Horizontal-axis wind turbines (HAWT) have the main rotor shaft
and electrical generator at the top of a tower, and must be pointed into the wind.
Small turbines are pointed by a simple wind vane, while large turbines generally
use a wind sensor coupled with a servo motor. Most have a gearbox, which turns
the slow rotation of the blades into a quicker rotation that is more suitable to drive
an electrical generator. Since a tower produces turbulence behind it, the turbine is
usually positioned upwind of its supporting tower. Turbine blades are made stiff to
prevent the blades from being pushed into the tower by high winds. Additionally,
the blades are placed a considerable distance in front of the tower and are
sometimes tilted forward into the wind a small amount.
Fig.4.4.1: HAWT
48
DOWNWIND TURBINE UPWIND TURBINE
Fig.4.5.1: Turbine Type
a) Upwind turbines are used mostly.Because wind velocity increases at higher
altitudes, the backward force and torque on a horizontal axis wind turbine
(HAWT) blade peaks as it turns through the highest point in its circle. The
tower hinders the airflow at the lowest point in the circle, which produces a
local dip in force and torque. These two effects combine to produce a
cyclic twist on the main bearings of a HAWT. The combined twist is worst
in machines with an even number of blades, where one is straight up when
another is straight down. To improve reliability, teetering hubs are used
which allow the main shaft to rock through a few degrees, so that the main
bearings do not have to resist the torque peaks.
b) MODERN WIND TURBINES:- Turbines used in wind farms for
commercial production of electric power are usually three-bladed and
pointed into the wind by computer-controlled motors. These have high tip
speeds of over 320 kilometres per hour (200 mph), high efficiency, and low
torque ripple, which contribute to good reliability. The blades are usually
colored light gray to blend in with the clouds and range in length from 20
to 40 metres (66 to 130 ft) or more. The tubular steel towers range from 60
to 90 metres (200 to 300 ft) tall. The blades rotate at 10-22 revolutions per
minute. A gear box is commonly used for stepping up the speed of the
generator, although designs may also use direct drive of an annular
generator. Some models operate at constant speed, but more energy can be
49
collected by variable-speed turbines which use a solid-state power
converter to interface to the transmission system. All turbines are equipped
with protective features to avoid damage at high wind speeds,
by feathering the blades into the wind which ceases their rotation,
supplemented by brakes.
ii) COUNTER ROTATING TURBINES
Fig.4.5.2: Counter rotating Turbine
Counter rotating turbines can be used to increase the rotation speed of the
electrical generator. When the counter rotating turbines are on the same side of the
tower, the blades on the one in front are angled forwards slightly so as to never hit
the rear ones. They are either both geared to the same generator or, more often, one
is connected to the rotor and the other to the field windings. Counter rotating
turbines geared to the same generator have additional gearing losses. Counter
rotating turbines connected to the rotor and stator are mechanically simpler; but,
the field windings need slip rings which adds complexity, wastes some electricity
and wastes some mechanical power. As of 2005, no large practical counter-rotating
HAWTs are commercially sold. Counter rotating turbines can be on opposite sides
of the tower. In this case it is best that the one at the back be smaller than the one
at the front and set to stall at a higher wind speed. This way, at low wind speeds,
50
both turn and the generator taps the maximum proportion of the wind's power. At
intermediate speeds, the front turbine stalls; but, the rear one keeps turning, so the
wind generator has a smaller wind resistance and the tower can still support the
generator. At high wind speeds both turbines stall, the wind resistance is at a
minimum and the tower can still support the generator. This allows the generator
to function at a wider wind speed range than a single-turbine generator for a given
tower. To reduce sympathetic vibrations, the two turbines should turn at speeds
with few common factors, for example 7:3 speed ratio. Overall, this is a more
complicated design than the single-turbine wind generator, but it taps more of the
wind's energy at a wider range of wind speeds.
iii) VERTICAL-AXIS WIND TURBINES ( VAWT)
Vertical-axis wind turbines (VAWTs) have the main rotor shaft arranged
vertically. Key advantages of this arrangement are that the turbine does not need to
be pointed into the wind to be effective. This is an advantage on sites where the
wind direction is highly variable, for example when integrated into buildings. The
key disadvantages include the low rotational speed with the consequential
higher torque and hence higher cost of the drive train, the inherently lower power
coefficient, the 360 degree rotation of the aerofoil within the wind flow during
each cycle and hence the highly dynamic loading on the blade, the pulsating torque
generated by some rotor designs on the drive train, and the difficulty of modelling
the wind flow accurately and hence the challenges of analysing and designing the
rotor prior to fabricating a prototype.
With a vertical axis, the generator and gearbox can be placed near the ground,
hence avoiding the need of a tower and improving accessibility for maintenance.
Drawbacks of this configuration include (i) wind speeds are lower close to the
ground, so less wind energy is available for a given size turbine, and (ii) wind
shear is more severe close to the ground, so the rotor experiences higher loads. Air
flow near the ground and other objects can create turbulent flow, which can
introduce problems associated with vibration, such as noise and bearing wear
51
which may increase the maintenance or shorten the service life. However, when a
turbine is mounted on a rooftop, the building generally redirects wind over the roof
and this can double the wind speed at the turbine. If the height of the rooftop
mounted turbine tower is approximately 50% of the building height, this is near the
optimum for maximum wind energy and minimum wind turbulence. It should be
borne in mind that wind speeds within the built environment are generally much
lower than at exposed rural sites.
Fig.4.5.3:VAWT
4.5 TURBINE DESIGN AND CONSTRUCTION
Wind turbines are designed to exploit the wind energy that exists at a
location. Aerodynamic modeling is used to determine the optimum tower height,
control systems, number of blades and blade shape.
i) One of the best construction materials available (in 2001) is graphite-fibre in
epoxy. Graphite composites can be used to build turbines of sixty meters radius,
enough to tap a few megawatts of power. Smaller household turbines can be made
of lightweight fiberglass, aluminum, or sometimes laminated wood.
ii) Wood and canvas sails were originally used on early windmills. Unfortunately
they require much maintenance over their service life. Also, they have a relatively
52
high drag (low aerodynamic efficiency) for the force they capture. For these
reasons they were superseded with solid airfoils.
iii) Wind power intercepted by the turbine is proportional to the square of its
blade-length. The maximum blade-length of a turbine is limited by both the
strength and stiffness of its material.
iv) The wind blows faster at higher altitudes because of the drag of the surface (sea
or land) and the viscosity of the air. The variation in velocity with altitude, called
wind shear is most dramatic near the surface. Typically, the variation follows the
1/7th power law, which predicts that wind speed rises proportionally to the seventh
root of altitude. Doubling the altitude of a turbine, then, increases the expected
wind speeds by 10% and the expected power by 34%. Doubling the tower height
generally requires doubling the diameter as well, increasing the amount of material
by a factor of eight.
v) For HAWTs, tower heights approximately twice the blade length have been
found to balance material costs of the tower against better utilisation of the more
expensive active components.
vi) Although turbines can be built with any number of blades, there are many
constraints. There are a number of vibration modes that increase in peak intensity
as the number of blades decreases. Some of these vibrations, besides causing wear
on the machine, are also audible. Thus, noise and wear considerations point to
larger numbers of blades, generally at least 3.Many small scale wind turbines, such
as the Whisper 175, use 2 blades because such turbines are easy to construct as
they avoid the need for using a hub with linkages to individual blades, and the
blade(s) can be shipped easily in one long package. Three-bladed turbines, which
are much more efficient, and more quiet, require more complicated onsite
assembly
4.6 NEED OF ROTATIONAL CONTROL
The speed at which wind turbines rotate must be controlled for several reasons:
53
i) Maintenance:- because it is dangerous to have people working on a wind turbine
while it is active, it is sometimes necessary to bring a turbine to a full stop.
ii) Noise reduction:- As a rule of thumb, the noise from a wind turbine increases
with the fifth power of the relative wind speed (as seen from the moving tip of the
blades). In noise-sensitive environments (nearly all onshore installations), noise
limits the tip speed to approximately 60 m/s. High efficiency turbines may have tip
speed ratios of 5-6, which, for onshore turbines, limits high efficiency operation to
winds of just 10 m/s.
iii) Centripetal force reduction:- as the rotational speed increases, so does the
centripetal force working on the central hub or axis. When it exceeds safe limits
blades could snap off, and the turbine would fail dramatically.
iv) Compatible with wind speed and direction:-On a pitch controlled wind turbine
the turbine's electronic controller checks the power output of the turbine several
times per second. When the power output becomes too high, it sends an order to
the blade pitch mechanism which immediately pitches (turns) the rotor blades
slightly out of the wind. Conversely, the blades are turned back into the wind
whenever the wind drops again. (Passive) stall controlled wind turbines have the
rotor blades bolted onto the hub at a fixed angle. The geometry of the rotor blade
profile, however has been aerodynamically designed to ensure that the moment the
wind speed becomes too high, it creates turbulence on the side of the rotor blade
which is not facing the wind as shown in the picture on the previous page. This
stall prevents the lifting force of the rotor blade from acting on the rotor. An
increasing number of larger wind turbines (1 MW and up) are being developed
with an active stall power control mechanism. Technically the active stall
machines resemble pitch controlled machines, since they have pitchable blades. In
order to get a reasonably large torque (turning force) at low wind speeds, the
machines will usually be programmed to pitch their blades much like a pitch
controlled machine at low wind speeds. (Often they use only a few fixed steps
depending upon the wind speed).
54
v) Sensor:-It simply consists of a ball resting on a ring. The ball is connected to a
switch through a chain. If the turbine starts shaking, the ball will fall off the ring
and switch the turbine off. There are many other sensors in the nacelle, e.g.
electronic thermometers which check the oil temperature in the gearbox and the
temperature of the generator.
vi) Overspeed Protection :-It is essential that wind turbines stop automatically in
case of malfunction of a critical component.
vii) Aerodynamic Braking System: Tip Brakes The primary braking system for
most modern wind turbines is the aerodynamic braking system, which essentially
consists in turning the rotor blades about 90 degrees along their longitudinal axis
(in the case of a pitch controlled turbine or an active stall controlled turbine ), or in
turning the rotor blade tips 90 degrees (in the case of a stall controlled turbine ).
Aerodynamic braking systems are extremely safe.
vii) Mechanical Braking System :- The mechanical brake is used as a backup
system for the aerodynamic braking system.
4.7 DETERMINING FACTORS
Some of the factors determining the economics of the utility scale wind energy are:
i) Costs depend very much on the wind speed at that site since the power varies as
cube of the wind speed.
ii) Turbine design and construction: more than 60% of total costs are contributed
by the turbine costs.
iii) Rated capacity of the turbine: larger wind farms are known to be more
economical than small wind farms.
iv) Exact location and orientation of the turbine greatly affects the economics of
the wind energy.
55
v) Improvements of turbine design: for example, use of light weight material .
vi) Wind energy is a capital intensive source of energy.
vii) Assuming the same size project, the better the wind resource, the lower the
cost.
viii) Assuming the same wind speed of 8.08 m/s, a large wind farm is more
economical.
4.8 WIND PLANT CLASSIFICATION
a) OFFSHORE WIND POWER PLANT
Offshore wind power refers to the construction of wind farms in bodies of
water to generate electricity from wind. Better wind speeds are available offshore
compared to on land, so offshore wind power’s contribution in terms of electricity
supplied is higher. Offshore wind turbines are less obtrusive than turbines on land,
as their apparent size and noise is mitigated by distance. Because water has less
surface roughness than land (especially deeper water), the average wind speed is
usually considerably higher over open water. Capacity factors (utilisation rates) are
considerably higher than for onshore locations
ii) ADVANTAGES : higher wind speed results in low turbine height
iii) DISADVANTAGES: higher o&m costs due to harsh, corrosive environment
and tougher approach.
b) ON SHORE POWER PLANT
These are installed on hills/ridges. These turbine installations in hilly or
mountainous regions tend to be on ridgelines generally three kilometers or more
inland from the nearest shoreline. This is done to exploit the topographic
acceleration as the wind accelerates over a ridge. The additional wind speeds
56
gained in this way can increase the amount of energy produced because more wind
is going through the turbines. Great attention must be paid to the exact positions of
the turbines (a process known as micro-sitting) because a difference of 30 m can
sometimes mean a doubling in output.
i) ADVANTAGES: low o&m costs due to easy approach.
ii) DISADVANTAGES: relatively large turbine height
4.9 ADVANTAGES OF WIND POWER PLANT
a)Economic Advantages:
i) No delay in construction: wind turbines are easy to construct and does not
require long gestation periods.
ii) Low maintenance costs: maintenance costs are very small compared to
installation costs.
iii) Reliable and durable equipment: except for wind speeds greater than 30 mi/hr,
once installed, wind equipment last for more than 25 years.
iv) Farmers and ranchers earn additional income by leasing their land for wind
turbine.
v) Wind industry produces more jobs per unit energy produced than other forms
of energy.
vi) No hidden costs, which greatly reduces the environmental impacts
vii) Greater fuel diversity.
viii) Wind energy is big business turning over A$13 billion globally and
employing 100,000 people in 2003. By 2020, the industry is expected to employ
1.8 million people and be worth $A120 billion a year.
57
b) Additional advantages:
i) Clean,pollution free, renewable
ii) Price stability
iii) Energy independence from foreign fuel sources
iv) Can reduce green house effect if used in place of fossil fuel plants
4.10 TYPICAL CONCERNS AND THEIR REMEDIES
a) VISUAL IMPACT:
i) Off shore turbines
ii) Arrangement:
Aesthetic concerns may be addressed through the use of modern turbines --
tubular towers and sleek, minimalist features contribute to a more attractive
appearance. Avoiding construction of conspicuous roads and clearings, burying
transmission lines, and hiding buildings and structures behind ridges or vegetation
are also prudent steps. Finally, educating nearby communities prior to construction
about wind energy and its benefits can reduce opposition to visual effects
b) AVIAN CONCERNS
i) Suitable choice of site
ii) Using tubular towers instead of lattice tower.
For wind plants currently experiencing bird conflicts, the immediate task is to
find practical measures to reduce bird deaths and injuries. Mitigation proposals
include changing the color of wind turbine blades, using tubular towers with
diagonal stringers, eliminating places for birds to perch on the towers (especially
perches near uninsulated electricity transmission lines).
iii) Using radars
58
using radar to alert wind project operators to the passage of large flocks of birds.
Federal and state agencies and environmental organizations are collaborating on a
research program to address the bird issue.
b) NOISE
i) Varies as 5th
power of relative wind speed
ii) Streamlining of tower and nacelle
Streamlining (rounding or giving an aerodynamic shape to any protruding features
and to the nacelle itself) reduces any noise that is created by the wind passing the
turbine. Turbines also incorporate design features to reduce vibration and any
associated noise.
iii) Acoustic insulation of nacelle
The generator, gears, and other moving parts located in the turbine nacelle produce
mechanical noise. Soundproofing and mounting equipment on sound-dampening
buffer pads helps to deal with this issue.
iv) Specially designed gear box
Wind turbines use special gearboxes, in which the gear wheels are designed to flex
slightly and reduce mechanical noise. In addition, special sound-dampening buffer
pads separate the gearboxes from the nacelle frame to minimize the possibility that
any vibrations could become sound.
v) Use of upwind turbines
A wind turbine can be either "upwind" (that is, where the rotor faces into the
wind) or "downwind" (where the rotor faces away from the wind). A downwind
design offers some engineering advantages, but when a rotor blade passes the
"wind shadow" of the tower as the rotor revolves, it tends to produce an
"impulsive" or thumping sound that can be annoying. Today, almost all of the
commercial wind machines on the market are upwind designs, and the few that are
59
downwind have incorporated design features aimed at reducing impulsive noise
(for example, positioning the rotor so that it is further away from the tower).
1. Reducing angle of attack
2. Low tip speed ratios
c) CHANGES IN WIND PATTERNS
i) Reducing turbulence
d) INTERMITTENT
i) Coupling with hydro or solar energy
e) TV, MICROWAVE, RADAR INTERFERENCE
Switching from conducting material to non-conducting and composite material
4.11 POTENTIAL AND APPLICATION OF WIND ENERGY
4.11.1 WIND SECTOR IN INDIA:-
i) The development of wind power in India began in the 1990s, and has
significantly increased in the last few years. Although a relative newcomer to the
wind industry compared with Denmark or the US, domestic policy support
for wind power has led India to become the country with the fifth largest installed
wind power capacity in the world.
ii) As of December 2010 the installed capacity of wind power in India was
13,065.37 MW, mainly spread across Tamil Nadu (4132.72
MW), Maharashtra (1837.85MW), Karnataka (1184.45MW), Rajasthan (670.97
MW), Gujarat (1432.71 MW), Andhra Pradesh (122.45 MW), Madhya
Pradesh (187.69 MW), Kerala (23.00 MW), West Bengal (1.10 MW), other states
(3.20 MW) It is estimated that 6,000 MW of additional wind power capacity will
60
be installed in India by 2012. Wind power accounts for 6% of India's total installed
power capacity, and it generates 1.6% of the country's power.
4.12 INDIA’S LARGEST WIND PLANT:-
i) Commissioned by suzlon in dhule – satara,maharashtra.
ii) Asia’s largest with present capacity of 400 mw with 320turbines of 1.25mw .
iii) Soon its going to become world’s largest after commissioning 1000mw.
4.13 WORLD’S LARGEST WIND PLANTS:-
i) The Roscoe Wind Farm in Roscoe, Texas, owned and operated
by E.ON Climate & Renewables is the world's largest wind farm (as of October
2009) with 627 wind turbines and a total installed capacity of 781.5 MW.
4.14 CONCLUSION
Wind energy, as an alternative to fossil fuels, is plentiful, renewable, widely
distributed, clean, and produces no greenhouse gas emissions during operation,also
Wind energy has very good potential and it is the fastest growing energy source.
The future looks bright for wind energy because technology is becoming more
advanced and windmills are becoming more efficient. Also as a technocrat it is our
responsibility to make wind energy as a part of the worldwide discussion on the
future of energy generation and use and consequent effect on the environment.
61
CHAPTER 5
SHOCK ABSORBER SYSTEM
5. Shock Absorber System
The project here is all about Power-Generating Shock Absorber (PGSA).
The Power-Generating Shock Absorber (PGSA) converts kinetic energy into
electricity through the use of a Linear Motion Electromagnetic System (LMES).
There are at least two entities who have spent time/resources developing this
concept: Goldneretal and Oxenreider. An electromagnetic linear generator and
regenerative electromagnetic shock absorber is disclosed which converts variable
frequency, repetitive intermittent linear displacement motion to useful electrical
power. The innovative device provides for superposition of radial components of
the magnetic flux density from a plurality of adjacent magnets to produce a
maximum average radial magnetic flux density within a coil winding array. Due to
the vector superposition of the magnetic fields and magnetic flux from a plurality
of magnets, a nearly four-fold increase in magnetic flux density is achieved over
conventional electromagnetic generator designs with a potential sixteen-fold
increase in power generating capacity. As a regenerative shock absorber, the
disclosed device is capable of converting parasitic displacement motion and
vibrations encountered under normal urban driving conditions to a useful electrical
energy for powering vehicles and accessories or charging batteries in electric and
fossil fuel powered vehicles. The disclosed device is capable of high power
generation capacity and energy conversion efficiency with minimal weight penalty
for improved fuel efficiency.
5.1 Transformation of sound into electric energy
Sound energy is also a type of wave motion. We are heard by others when
we talk because of the sound energy we produce. It is due to the effect of the air
molecules vibrating when we talk. The vibrating molecules hit our eardrums,
62
which enable us to hear others talk. Sound energy may be converted into electrical
energy for transmission, and later the electrical energy can be converted back into
sound energy at the receiving end. An example of such transformations could be
seen in the microphone and the loudspeaker. Sound, like heat energy is easily
lost. The transformation of one form of energy into another may be accompanied
by losses in the form of sound and/or heat that are often not desirable.
5.2 COMPONENTS USED IN THIS HYBRID MODEL OF CAR
i) Stepper motor
ii) Induction braking coil
iii) Gears
iv) Resistances
vi) Capacitors
vii) Diode
5. 3 VOLTS MOTOR CYCLE BATTERY
We have used a motorcycle lead acid battery. This battery is of 6 volts. Power
of this battery is used for glowing tube light when the power supply is off.
Otherwise, the power supply keeps on charging the battery.
5.4 NEON LAMP
230 volts neon lamps are connected between 220 volts AC input and
transformer. This lamp is working as an indicator. It indicates whether the power is
on or off.
5.5 FLUORESCENT TUBE LIGHT
A fluorescent 20 watts tube is used as a source of light. The given circuit
operates it automatically.
63
Fig 5.5.1: Battery Types
5.7 CAPACITORS
It is an electronic component whose function is to accumulate charges and
then release it. To understand the
concept of capacitance,
consider a pair of metal plates which all are placed near to each other without
touching. If a battery is connected to these plates the positive pole to one and the
negative pole to the other, electrons from the battery will be attracted from the plate
connected to the positive terminal of the battery. If the battery is then disconnected,
one plate will be left with an excess of electrons, the other with a shortage, and a
potential or voltage difference will exists between them. These plates will be acting
as capacitors. Capacitors are of two types: - (1) fixed type like ceramic, polyester,
electrolytic capacitors-these names refer to the material they are made of aluminium
foil. (2) Variable type like gang condenser in radio or trimmer. In fixed type
capacitors, it has two leads and its value is written over its body and variable type
has three leads. Unit of measurement of a capacitor is farad denoted by the symbol
F. It is a very big unit of capacitance. Small unit capacitor are pico-farad denoted by
pf (Ipf=1/1000,000,000,000 f) Above all, in case of electrolytic capacitors, it's two
terminal are marked as (-) and (+) so check it while using capacitors in the circuit in
right direction. Mistake can destroy the capacitor or entire circuit in operational.
64
Fig 5.6.1: Types of Capacitors
5.7 crystal oscillators
Crystal oscillators are oscillators where the primary frequency determining
element is a quartz crystal. Because of the inherent characteristics of the quartz
crystal the crystal oscillator may be held to extreme accuracy of frequency
stability. Temperature compensation may be applied to crystal oscillators to
improve thermal stability of the crystal oscillator. Crystal oscillators are usually,
fixed frequency oscillators where stability and accuracy are the primary
considerations. For example it is almost impossible to design a stable and accurate
LC oscillator for the upper HF and higher frequencies without resorting to some
sort of crystal control. Hence the reason for crystal oscillators. The frequency of
older FT-243 crystals can be moved upward by crystal grinding. I won't be
discussing frequency synthesizers and direct digital synthesis (DDS) here. They
are particularly interesting topics to be covered later.
5.7.1 A practical example of a Crystal Oscillator
This is a typical example of the type of crystal oscillators which may be
used for say converters. Some points of interest on crystal oscillators in relation to
figure 1.
Figure 5.7.1.1: schematic of a crystal oscillator
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The transistor could be a general purpose type with an Ft of at least 150 Mhz for
HF use. A typical example would be a 2N2222A. The turns ratio on the tuned
circuit depicts an anticipated nominal load of 50 ohms. This allows theoretical 2K5
ohms on the collector. If it is followed by a buffer amplifier (highly recommended)
I would simply maintain the typical 7:1 turns ratio. I have included a formula for
determining L and C in the tuned circuits of crystal oscillators in case you have
forgotten earlier tutorials. Personally I would make L a reactance of around 250
ohms. In this case I'd make C a smaller trimmer in parallel with a standard fixed
value.
5.8 RESISTANCE
Resistance is the opposition of a material to the current. It is measured in
conductor is 100% efficient. To control the electron flow (current) in a predictable
manner, we use resistors. Electronic circuits use calibrated lumped resistance to
control the flow of current. Broadly speaking, resistor can be divided into two
groups viz. fixed & adjustable (variable) resistors. In fixed resistors, the value is
fixed & cannot be varied. In variable resistors, the resistance value can be varied
by an adjuster knob. It can be divided into (a) Carbon composition (b) Wire wound
(c) Special type. The most common type of resistors used in our projects is carbon
type. The resistance value is normally indicated by colour bands. Each resistance
has four colours, one of the band on either side will be gold or silver, this is called
fourth band and indicates the tolerance, others three band will give the value of
resistance (see table). For example if a resistor has the following marking on it say
red, violet, gold. Comparing these coloured rings with the colour code, its value is
27000 ohms or 27 kilo ohms and its tolerance is ±5%. Resistor comes in various
sizes (Power rating). The bigger, the size, the more power rating of 1/4 watts. The
four colour rings on its body tells us the value of resistor value as given below.
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5.9 Colour Code
Colour Code
Black 0
Brawn 1
Red 2
Orange 3
Yellow 4
Green 5
Blue 6
Violet 7
Grey 8
White 9
Fig 5.8.1: Type of Resistor
The first rings give the first digit. The second ring gives the second digit.
The third ring indicates the number of zeroes to be placed after the digits. The
fourth ring gives tolerance (gold ±5%, silver ± 10%, No colour ± 20%). Resistance
coils of different values are connected b/w the gaps. When the knob is rotated, the
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pointer also moves over the brass pieces. If a gap is skipped over, its resistance is
included in the circuit. If two gaps are skipped over, the resistances of both
together are included in the circuit and so on. A dial type of resistance box
contains many dials depending upon the range, which it has to cover. If a
resistance box has to read up to 10,000ohm, it will have three dials each having ten
gaps i.e. ten resistance coils each of resistance 10 ohm. The third dial will have ten
resistances each of 100 ohm. The dial type of resistance boxes is better because the
contact resistance in this case is small & constant.
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CHAPTER 6
PARKING SYSTEM
6. Easy Parallel Parking – Car Goes Sideways, Wheels Turn 90 degrees
It gives immense pleasure when a person rides a car and goes on a smooth
nice highway. On the other hand, the same person gets frustrated when he has
to make sharp manoeuvres to parallel park his car in closely packed parking spot
with front and rear car taking a portion of his parking space. I suggest a feature in
cars that may alleviate the pain. Let’s see the pain and the solution.
6.1 Parallel Parking
Parking on the streets, in apartment complexes and even some other
parking areas, requires one to Parallel Park a car. In Parallel parking of car, a
person needs to park his car lengthwise parallel to curb. There may be car ahead
and behind the parking spot where this car needs to be parked. The space provided
to park this car may be around 1.5 car length – 1 length for the car and 1/4 length
ahead of and 1/4 length behind the car.
6.2 Method
The recommended and easy way for parallel parking of a car is
i) Stop the car parallel to the car in front
ii) Turn steering wheel towards the curb and move the car backwards (reverse
gear)
iii) when car has entered around 80% length in the parking spot, turn steering
wheel away from the curb and continue moving
iv) Watch for the car in the back and the curb on side
v) Straighten the steering wheel and move the car to maintain equal distance
from car in front and back
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6.3 Drawback of parallel parking in conventional way
i) Parallel parking a car is a skill that one needs to learn and practice. One may not
perform parallel parking without first learning it. This is the reason, Department of
Motor Vehicles in USA
ii) Even with all the learning drivers do make some mistake while parallel parking
a car and the result is nicks on the bumper of car.
iii) Most of the time, parallel parking requires one vehicle to acquire 1.5 to 2 car
parking length so that the car can get in the parking spot and come out of it.
iv)Parallel parking requires a vehicle to move forward and then reverse causing it
to travel more distance than was really required.
a)More distance leads to more fuel consumption
b)More distance leads to more wear and tear of the car
c)More distance and parallel parking causes more time spent
d)Above point also causes more money spent to Parallel Park a car.
6.4 Available Solutions
i)Park Assist
ii)Parking sensors or vicinity sensors and rear view or back up camera
iii)Extreme parking skills
6.5 Your Venture
The available solutions, undoubtedly, seem fascinating but they are not
natural and some of the drawbacks still exist. My suggestion (your venture) may
supplement these solutions rather than replace it. Design axle and driving
mechanism of a car such that wheels can turn 90 degrees to park a car straight
away rather than making manoeuvres required for parallel parking. In other words.
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i) A car stops moving forward (or backward)
ii) All the wheels of the car turns 90 degrees
iii) The car moves to the left or right hand side i.e. the car moves sideways on
the instructions of driver.
Fig 6.5.1: Ways of Parking System
6.6Advantages
i) The mechanism is intuitive so a driver needs to spend very little time to
learn if such mechanism is implemented in a car.
ii) Cars can be parked very close to each other. This may save costly parking
space, lead to more parking fees to commercial parking spaces and
eventually passing on the benefit to drivers (reduction of parking fees).
iii) In congested apartment complexes where parking is limited, this
mechanism can be very helpful.
iv) This mechanism addresses all the drawbacks of parallel parking
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v) On a congested road, if the car in front breaks down and you have little
space to pass (overtake) that car then this feature may be very helpful.
6.6 Challenges
i) This solution is not in market for all this time cars/automobiles are there in the
world (200 to 400 years) implies this solution is extremely challenging.
ii) Axle and driving mechanism will need to be designed so that wheels can turn
90 degrees. There will be no precedence of such mechanism so one will need to
undergo all the Research and Design for this solution.
iii) The 2 wheel drive vehicles will need to be redesigned so that power can be
shifted to a front and rear wheel rather than to pair of front wheels or rear wheels.
Four wheel drive vehicles may have an advantage in this term.
iv) Sideways driving of car may be dangerous when passing a struck car in front.
Car’s signal mechanism will need to be designed to warn the driver coming from
behind that car is moving sideways.
v) The forward and reverse gears in current vehicles may not work well for
sideways movement. Sideways movement should be slower than forward and rear
movement.
vi) Mechanism will need to be made for a driver to stop the car completely before
he makes sideways movement.
vii) The cost of this mechanism once implemented may be high. One may
consider designing this solution for bigger and costlier car, which may derive
premium from wealthy customers, and the car’s bottom has more room to turn
wheels 90 degrees.
viii) The competition with Park Assist Technology and Backup Camera may be
steep even though these technologies can supplement our solution.
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CHAPTER 7
ANDROID SYSTEM
7. ANDROID SYSTEM
7.1 What is Robot?
Robot is a mechanical device that sometimes resembles a human and is
capable of performing a variety of often complex human tasks on command or by
being programmed in advance.
7.2Project is about
Here at this robot I have used a Bluetooth module to control the robot via 2
BO motors at 100RPM approx the robot is control by an android phone
application. Microcontroller used is AT89S52 form 8051 family to work in a serial
communication UART mode the communication is configured on 9800bps to
communicate it with the Bluetooth module. The Bluetooth module used is a HC-05
in smd package which works on a 3.3v and have a serial communication with any
device connected to it the communication speed can be configured on various
speed via AT Command. The BT module is a SPP supported profile so it can be
connected easily to any module or phone. In this profile the data can be sent and
receive to module. The BT module is connected to the RX pin of microcontroller.
The L293D is a motor driver IC to operate the motors in any direction required
dependent on the logic applied to the logic pins. A readymade compact size chassis
I have used to avoid the chassis assembly the chassis contains 2 decks the lower is
used for BO motors fitting the upper is used as a battery stack .on top the plate the
board is mounted by screw fitting.
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7.3 Block Diagram
Fig 7.3.1: Android System
Fig 7.3.2: Circuit Diagram for Bluetooth Module
BT
module
Motor 2
Motor 1 L293D
Motor
driver
AT89S52
Micro
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The reset circuit at pin 11 and KEY circuit at pin24 can be ignored to work
the module in communication mode.
Fig 7.3.3: Integrated Circuit
7.4 Motor driver circuit :
The pin 8 of IC should b connected to the 9v battery or 12v.
This pin8 is internally connected to the driver ckt inside the IC which helps the
motor to get the good supply which also helps the smooth functioning of motors
Fig.7.4.1: Motor Circuit
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5.5 The complete circuit:
The crystal circuit and the reset circuit is as common usually used in every
AT89S52 circuit. The leds connected to the motor indicates the flow of
current….the direction of the motor rotation.
Fig 7.4.2: Complete circuit of android system
7.5 The Bluetooth module:
The Bluetooth module used is a HC-05 based on SPP support
Features:
i)Wireless serial Bluetooth port.
ii) With free power adapter bottom board come with well power regulator. User
can connect 3.3 to 5VDC and connect TX and RX to your control IO (general 3.3
to 5V digital input output of MCU or IO is ok, or general TLL IO)
iii) Easy to connect this module with PC, just search and key "1234" pass code.
iv) With white SMD LED on the adapter board, can see the Bluetooth connection
status.
Step to connect:
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i) Connect the wiring, power up, while the device is not connected, the Bluetooth
module board has a white LED flashing
ii) At PC side, search Bluetooth device.
iii) Found name called "HC-05" device
iv) Connect it, and pass code is "1234"
v) While connection is ok, you can see the LED become always on
7.6 Usage:
i)Coupled Mode: Two modules will establish communication automatically when
powered.
ii)PC hosted mode: Pair the module with Bluetooth dongle directly as virtual
serial.
iii)Bluetooth protocol : Bluetooth Specification v2.0+EDR
iv)Frequency : 2.4GHz ISM band
v)Modulation : GFSK(Gaussian Frequency Shift Keying)
vi)Speed : Asynchronous: 2.1Mbps(Max) / 160 kbps, Synchronous:
1Mbps/1Mbps
vii)Security : Authentication and encryption
viii)Profiles : Bluetooth serial port
ix)CSR chip : Bluetooth v2.0
x)Wave band : 2.4GHz-2.8GHz, ISM Band
xi)Protocol : Bluetooth V2.0
xii)Voltage : 5V (3.6V-6V, NO more than 7V)
xiii)User defined Baud Rate : 4800, 9600, 19200, 38400, 57600, 115200,
230400,460800,921600 ,1382400.
7.7 Pin Definition:
i) PIO8 connects with LED cathodea with 470ohm series resistor in between. LED
NEGATIVE connects to ground. It is used to indicate the module state. After
powered on, flashing intervals differ in different states.
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ii) PIO9 is used to control LED indicating paring. It will be steady on when paring
is successful.
iii) PIO11, module state switching pin. HIGH -> response to AT command; LOW
or floating -> regular work status.
iv) With build-in reset circuit, reset is completed automatically after powered on.
v) Steps to set to MASTER:
vi) Set PIO11 HIGH with a 10K resistor in between.
vii) Power on, module comes into AT Command Response Status
Viii) Open HyperTerminal or other serial tool, set the baud rate 38400, 8 data bits,
1 stop bit, no parity bit, no Flow
7.8 At commands to check the Module:
i) Set PIO11 HIGH with a 10K resistor in between.
ii) Power on, module comes into AT Command Response Status
iii) Open HyperTerminal or other serial tool, set the baud rate 38400, 8 data bits, 1
stop bit, no parity bit, no Flow
iv) Now u r enter in AT commond mode.
Command AT
Response OK the module is connected successfully
Commond AT+ROLE?
Response +ROLE=0 the module is in slave mode
Commond AT+ROLE=1
Response OK the module is set in master mode
For more command refer data sheet ……;)
In slave mode the PC, Phone etc will search the module and pair with the module
In master mode the module will search for any PC, Phone etc
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CHAPTER 8
DC MOTORS
8.1 DC Motor
Faradays used oersteds discovered, that electricity could be used to produce
motion, to build the world first electric motor in 1821. Ten years later, using the
same logic in reverse, faraday was interested in getting the motion produced by
oersteds experiment to be continuous, rather then just a rotatory shift in position. In
his experiments, faraday thought in terms of magnetic lines of force. He visualized
how flux lines existing around a current carrying wire and a bar magnet. He was
then able to produce a device in which the different lines of force could interact a
produce continues rotation. The basic faradays motor uses a free-swinging wire
that circles around the end of a bar magnet. The bottom end of the wire is in a pool
of mercury. Which allows the wire to rotate while keeping a complete electric
circuit?
Fig.8.1.1: DC Motor
Although Faraday's motor was ingenious. It could not be used to do any practical
work. This is because its drive shaft was enclosed and it could only produce an
internal orbital motion. It could not transfer its mechanical energy to the outside
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for deriving an external load. However it did show how the magnetic fields of a
conductor and a magnet could be made to interact to produce continuous motion.
Faradays motor orbited its wire rotor must pass through the magnet’s lines of
force.
Fig.8.1.2: Magnetic Lines Of Forces
When a current is passes through the wire ,circular lines of force are
produced around the wire. Those flux lines go in a direction described by the left-
hand rule. The lines of force of the magnet go from the N pole to the S pole You
can see that on one side of the wire, the magnetic lines of force are going in the
opposite direction as a result the wire, s flux lines oppose the magnet’s flux line
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since flux lines takes the path of least resistance, more lines concentrate on the
other side of the wire conductor, the lines are bent and are very closely spaced. The
lines tend to straighten and be wider spaced. Because of this the denser, curve
field pushes the wire in the opposite direction. The direction in which the wire is
moved is determined by the right hand rule. If the current in the wire went in the
opposite direction. The direction of its flux lines would reverse, and the wire
would be pushed the other way.
8.2 Rules for motor action
The left hand rule shows the direction of the flux lines around a wire that is
carrying current. When the thumb points in the direction of the magnetic lines of
force. The right hand rule for motors shows the direction that a current carrying
wire will be moved in a magnetic field. When the forefinger is pointed in the
direction of the magnetic field lines, and the centre finger is pointed in the
direction of the current in the wire the thumb will point in the direction that the
wire will be moved.
FIG.8.2.1: TORQUE AND ROTATING MOTION
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In the basic action you just studied the wire only moves in a straight line and stops
moving once out of the field even though the current is still on. A practical motor
must develop a basic twisting force called torque loop. We can see how torque is
produced. If the loop is connected to a battery. Current flows in one direction
oneside of the loop, and in the opposite direction on the other. Therefore the
concentric direction on the two sides. If we mount the loop in a fixed magnetic
field and supply the current the flux lines of the field and both sides of the loop
will interact, causing the loop to act like a lever with a force pushing on its two
sides in opposite directions. The combined forces result in turning force, or torque
because the loop is arranged to pivot on its axis. In a motor the loop that moves in
the field is called an armature or rotor. The overall turning force on the armature
depends upon several factors including field strength armature current strength and
the physical construction of the armature especially the distance from the loop
sides to the axis lines. Because of the lever action the force on the sides are further
from the axis; thus large armature will produce greater torques.
Fig.8.2.2: Magnetic Poles
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Fig.8.2.3: Torque
In the practical motor the torque determines the energy available for doing useful
work. The greater the torque the greater the energy. If a motor does not develop
enough torque to pull its load it stalls.
8.3 Producing continuous rotation
The armature turns when torque is produced and torque is produced as long
as the fields of the magnet and armature interact. When the loop reaches a position
perpendicular to the field, the interaction of the magnetic field stops. This position
is known as the neutral plane. In the neutral plane, no torque is produced and the
rotation of the armature should stop; however inertia tends to keep a moving object
in the motion even after the prime moving force is removed and thus the armature
tends to rotate past the neutral plane. But when the armature continues o the sides
of the loop start to swing back in to the flux lines, and apply a force to push the
sides of the loop back and a torque is developed in the opposite direction. Instead
of a continuous rotation an oscillating motion is produced until the armature stops
in the neutral plane.
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Fig.8.3.1: Neutral Plane
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To get continuous rotation we must keep the armature turning in the same
direction as it passes through the neutral plane .We could do this by reversing
either the direction of the current flow through the armature at the instant the
armature goes through the neutral pole. Current reversals of this type are normally
the job of circuit switching devices. Since the switch would have to be
synchronized with the armature, it is more logical to build it into the armature then
in to the field. The practical switching device, which can change the direction of
current flow through an armature to maintain continuous rotation, is called a
commutator.
8.4 The commutator
For the single-loop armature, the commutator is simple. It is a conducting
ring that is split into two segment with each segment connected to an end of the
armature loop. Power for the armature from an external power source such as a
battery is brought to the commutator segments by means of brushes. The
arrangement is almost identical to that for the basic dc generator. The logic behind
the operation of the commutator is easy to see in the figures. You can see in figure
A that current flows into the side of the armature closest to the South Pole of the
field and out of the side closest to the North Pole. The interaction of the two fields
produces a torque in the direction indicated, and the armature rotates in that
direction.
No torque is produced but the armature continues to rotate past the neutral plane
due to inertia. Notice that at the neutral position the commutator disconnects from
the brushes sides of the loop reverse positions. But the switching action of the
commutator keeps the direction of current flow through the armature the same as it
was in the figure. A. Current still flows into the armature side that is now closest to
the South Pole. Since the magnet’s field direction remains the same throughout the
interaction of fields after commutation keeps the torque going in the original
direction; thus the same direction of rotation is maintained. As you can see in
figure D, Inertia again carries the armature past neutral to the position shown in the
fig. A while communication keeps the current flowing in the direction that
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continues to maintain rotation. In this way, the commutator keeps switching the
current through the loop, so that the field it produces always interacts with the pole
field to develop a continuous torque in the same direction.
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Fig.8.4.1:ABCD For Commutator
8.5 THE ELEMENTRY DC MOTOR
At this point, you have been introduced to the four principal parts that
make up the elementary D.C motor. These parts are the same as those you met in
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your study of the basic D.C generator .a magnetic field, a movable conductor, a
commutator and brushes. In practice, the magnetic field can be supplied by a
permanent magnet or by an electromagnet. For most discussions covering various
motor operating principles, we will assume that a permanent magnet is used at
other times when it is important for you to understand that the field of the motor is
develop electrically, we will show that an electromagnet is used. In either case, the
magnetic field itself consists of magnetic flux lines that form a closed magnetic
circuit. The flux lines leave the north pole of the magnet, extend across the air gap
between the poles of the magnet, enter the South Pole and then travel through the
magnet itself back to the north pole. The movable conductor, usually a loop, called
armature, therefore is in the magnetic field.
When D.C motor is supplied to the armature through the brushes and commutator,
magnetic flux is also build up around the armature. It is this armature flux that
interacts with the magnetic field in which the armature is suspended to develop the
torque that makes the motor operate.
Fig.8.5.1: Inertia
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Almost every mechanical movement that we see around us is accomplished
by an electric motor. Electric machines are a means of converting energy. Motors
take electrical energy and produce mechanical energy. Electric motors are used to
power hundreds of devices we use in everyday life. Motors come in various sizes.
Huge motors that can take loads of 1000’s of Horsepower are typically used in the
industry. Some examples of large motor applications include elevators, electric
trains, hoists, and heavy metal rolling mills. Examples of small motor applications
include motors used in automobiles, robots, hand power tools and food blenders.
Micro-machines are electric machines with parts the size of red blood cells, and
find many applications in medicine.
Electric motors are broadly classified into two different categories: DC (Direct
Current) and AC (Alternating Current). Within these categories are numerous
types, each offering unique abilities that suit them well for specific applications.
In most cases, regardless of type, electric motors consist of a stator (stationary
field) and a rotor (the rotating field or armature) and operate through the
interaction of magnetic flux and electric current to produce rotational speed and
torque. DC motors are distinguished by their ability to operate from direct current.
There are different kinds of D.C. motors, but they all work on the same principles.
In this chapter, we will study their basic principle of operation and their
characteristics. It’s important to understand motor characteristics so we can choose
the right one for our application requirement. The learning objectives for this
chapter are listed below.
8.6 Learning Objectives
Understand the basic principles of operation of a DC motor.
Understand the operation and basic characteristics of simple DC motors.
Compute electrical and mechanical quantities using the equivalent circuit.
Use motor nameplate data.
Study some applications of DC motors.
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8.7 Electromechanical Energy Conversion
An electromechanical energy conversion device is essentially a medium of
transfer between an input side and an output side. Three electrical machines (DC,
induction and synchronous) are used extensively for electromechanical energy
conversion. Electromechanical energy conversion occurs when there is a change in
magnetic flux linking a coil, associated with mechanical motion.
8.8 Electric Motor
The input is electrical energy (from the supply source), and the output is
mechanical energy (to the load).
ELECTRICAL Electromechanical Energy Mechanical
ENERGY Conversion Device Energy
SOURCE Motor Load
8.9 Electric Generator
The Input is mechanical energy (from the prime mover), and the output is
electrical energy.
Mechanical Energy Electromechanical Energy Electrical
Conversion Device Energy
Source Generator Load
8.10 Construction
DC motors consist of one set of coils, called armature winding, inside another set
of coils or a set of permanent magnets, called the stator. Applying a voltage to the
coils produces a torque in the armature, resulting in motion.
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8.11 Stator
The stator is the stationary outside part of a motor.
The stator of a permanent magnet dc motor is composed of two or more
permanent magnet pole pieces.
The magnetic field can alternatively be created by an electromagnet. In
this case, a DC coil (field winding) is wound around a magnetic material
that forms part of the stator.
8.12 Rotor
The rotor is the inner part which rotates.
The rotor is composed of windings (called armature windings) which are
connected to the external circuit through a mechanical commutator.
Both stator and rotor are made of ferromagnetic materials. The two are
separated by air-gap.
8.13 Winding
A winding is made up of series or parallel connection of coils.
Armature winding - The winding through which the voltage is applied or
induced.
Field winding - The winding through which a current is passed to produce
flux (for the electromagnet)
Windings are usually made of copper.
8.14 DC Motor Basic Principles
8.14.1 Energy Conversion
If electrical energy is supplied to a conductor lying perpendicular to a
magnetic field, the interaction of current flowing in the conductor and the magnetic
field will produce mechanical force (and therefore, mechanical energy).
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8.15 Value of Mechanical Force
There are two conditions which are necessary to produce a force on the
conductor. The conductor must be carrying current, and must be within a magnetic
field. When these two conditions exist, a force will be applied to the conductor,
which will attempt to move the conductor in a direction perpendicular to the
magnetic field. This is the basic theory by which all DC motors operate. The force
exerted upon the conductor can be expressed as follows.
F = B i l Newton
where B is the density of the magnetic field, l is the length of conductor, and i the
value of current flowing in the conductor. The direction of motion can be found
using Fleming’s Left Hand Rule.
Figure 8.15.1: Fleming’s Left Hand Rule
The first finger points in the direction of the magnetic field (first - field),
which goes from the North Pole to the South Pole. The second finger points in the
direction of the current in the wire (second - current). The thumb then points in the
direction the wire is thrust or pushed while in the magnetic field (thumb - torque or
thrust).
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8.16 Principle of operation
Consider a coil in a magnetic field of flux density B (figure 4). When the
two ends of the coil are connected across a DC voltage source, current I flow
through it. A force is exerted on the coil as a result of the interaction of magnetic
field and electric current. The force on the two sides of the coil is such that the coil
starts to move in the direction of force.
Figure 8.16.1: Torque production in a DC motor
In an actual DC motor, several such coils are wound on the rotor, all of
which experience force, resulting in rotation. The greater the current in the wire, or
the greater the magnetic field, the faster the wire moves because of the greater
force created. At the same time this torque is being produced, the conductors are
moving in a magnetic field. At different positions, the flux linked with it changes,
which causes an emf to be induced (e = d /dt) as shown in figure 5. This voltage
is in opposition to the voltage that causes current flow through the conductor and is
Referred to as a counter-voltage or back emf.
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Figure 8.16.2: Induced voltage in the armature winding of DC motor
The value of current flowing through the armature is dependent upon the
difference between the applied voltage and this counter-voltage. The current due to
this counter-voltage tends to oppose the very cause for its production according to
Lenz’s law. It results in the rotor slowing down. Eventually, the rotor slows just
enough so that the force created by the magnetic field (F = Bil) equals the load
force applied on the shaft. Then the system moves at constant velocity.
8.17 Torque Developed
The equation for torque developed in a DC motor can be derived as
follows. The force on one coil of wire F =i l x B Newton
Note that l and B are vector quantities since B = /A where A is the area of the
coil, Therefore the torque for a multi turn coil with an armature current of Ia
T = K *fie *Ia
Where fie is the flux/pole in weber, K is a constant depending on coil geometry,
and Ia is the current flowing in the armature winding.
Note: Torque T is a function of force and the distance, equation (2) lumps all the
constant parameters (eg. length, area and distance) in constant K.
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The mechanical power generated is the product of the machine torque and the
mechanical speed of rotation, wm
Or, Pm = wm T
= wm K fie Ia
It is interesting to note that the same DC machine can be used either as a
motor or as a generator, by reversing the terminal connections.
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CHAPTER 9
EFFICIENCY
9.1 HYBRID EFFICIENCY
Besides a smaller, more efficient engine, today's hybrids use many other tricks
to increase fuel efficiency. Some of those tricks will help any type of car get better
mileage, and some only apply to a hybrid.
9.2 Recover energy and store it in the battery
Whenever you step on the brake pedal in your car, you are removing energy
from the car. The faster a car is going, the more kinetic energy it has. The brakes
of a car remove this energy and dissipate it in the form of heat. A hybrid car can
capture some of this energy and store it in the battery to use later. It does this by
using "regenerative braking." That is, instead of just using the brakes to stop the
car, the electric motor that drives the hybrid can also slow the car. In this mode, the
electric motor acts as a generator and charges the batteries while the car is slowing
down.
9.3 Sometimes shut off the engine
A hybrid car does not need to rely on the gasoline engine all of the time
because it has an alternate power source -- the electric motor and batteries So the
hybrid car can sometimes turn off the gasoline engine.
9.4 It uses advanced aerodynamics
This is used to the air resistance. This helps the hybrid car to increase its
efficiency. A less power is required when the car is made in an aerodynamic
model.
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9.5 It uses small efficient engine
The engine in a hybrid weight only 56 kg and it is a small, 1 litre, three
cylinder that produces 67 HP and 5700 rpm. The additional power provide by the
small electric motor is able to accelerate the hybrid car from 0-60 mph in about 11
seconds.
9.6 ADVANTAGES OF HYBRID CAR
i) Decreasing fuel consumption while it turns reduces emission.
ii) Pollution is reduced considerably
iii) Improve mileage.
iv) Reduced tail pipe emission
v) Refueling problem reduced
vi) No need of more fuel consumption at the time of acceleration.
vii) Uses small engine and components their by reduces weight.
viii) Better performance.
9.7 WHAT’S AVAILABLE NOW
Three hybrid cars are now available in the United States – the Honda Civic
Hybrid, the Honda Insight and the Toyota Prius. We will be discussing the latter
two, and although both of these cars are hybrids, they are actually quite different in
character. The Honda Insight, which was introduced in early 2000 in the United
States, is designed to get the best possible mileage. The Insight is a small,
lightweight two-seater with a tiny, high-efficiency gas engine. The Toyota Prius,
which came out in Japan at the end of 1997, is designed to reduce emissions in
urban areas. It meets California's super ultra low emissions vehicle (SULEV)
standard. It is a four-door sedan that seats five, and the power train is capable of
accelerating the vehicle to speeds up to 15 mph (24 kph) on electric power alone.
The Prius was honoured as the 2004 North American Car of the Year.
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9.8 CONCLUSION
The hybrid car gives very good mileage and it produces less pollution. So in
the incoming years the hybrid car will become the most common vehicle in the
world. Hybrid car produces less pollution than gasoline powered car and good
mileage than electric car.
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