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ABSTRACT
In this paper, a detailed dynamic model and simulation of a solar cell/wind
turbine/fuel cell hybrid power system is Developed using a novel topology to
complement each other and to alleviate the effects of environmental variations.
Comparing with the nuclear energy and thermal power, the renewable energy is
inexhaustible and has non-pollution Characteristics. The solar energy, wind power,
hydraulic power and tide energy are natural resources of the interest to generate
electrical sources. As the wind turbine output power varies with the wind speed and
the solar cell output power varies with both the ambient temperature and radiation, a
FC system with an UC bank can be integrated to ensure that the system performs
under all conditions. Excess wind and solar energies when available are converted to
hydrogen using electrolysis for later use in the fuel cell. In this paper Dynamic
modeling of various components of this isolated system is presented. Transient
responses of the system to step changes in the load, ambient temperature, radiation,
and wind speed in a number of possible situations are studied. The recent commercial
availability of small PEMFC units has created many new opportunities to design
hybrid energy systems for remote applications with energy storage in hydrogen form.
Here Ultra-capacitors are used in power applications requiring short duration peakpower.The voltage variation at the output is found to be within the acceptable range.
The output fluctuations of the wind turbine varying with wind speed and the solar cell
varying with both environmental temperature and sun radiation are reduced using a
fuel cell. Therefore, this system can tolerate the rapid changes in load and
environmental conditions, and suppress the effects of these fluctuations on the
equipment side voltage. The proposed system can be used for off-grid power
generation in non interconnected areas or remote isolated communities.
Modeling and simulations are conducted using MATLAB/Simulink software
packages to verify the effectiveness of the proposed system. The results show that the
proposed hybrid power system can tolerate the rapid changes in natural conditions and
suppress the effects of these fluctuations on the voltage within the acceptable range.
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CHAPTER-1
INTRODUCTION
Comparing with the nuclear energy and thermal power, the renewable energy
is inexhaustible and has non-pollution characteristics. The solar energy, wind power,
hydraulic power and tide energy are natural resources of the interest to generate
electrical sources. Extensive and generalized usage of renewable energy is very
popular to reduce the pollutions we have cause on earth. The wind and solar energy
are welcome substitution for many other energy resources because it is natural,
inexhaustible resource of sunlight to generate electricity. The main disadvantage of
wind turbines is that naturally variable wind speed causes voltage and power
fluctuation problems at the load side. This problem can be solved by using appropriatepower converters and control strategies. Another significant problem is to store the
energy generated by wind turbines for future usage when no wind is available but the
user demand exists.
The solar cell depends on the weather factors, mainly the irradiation and the cell
temperature. Therefore, the weather factors such as the irradiation and the temperature
are utilized for the estimation of the maximum power in this paper. After many
technological advances, proton exchange membrane fuel cell technology has now
reached the test and demonstration phase. The recent commercial availability of small
PEMFC units has created many new opportunities to design hybrid energy systems
for remote applications with energy storage in hydrogen form. By using an
electrolyzer, hydrogen conversion allows both storage and transportation of large
amounts of power at much higher energy densities. Furthermore, coupling a wind
turbine, a solar cell, fuel cells and electrolyzers is efficacious to improve environment
pollution because of by using natural energy.
In this paper, a detailed dynamic model and simulation of a solar cell/wind
turbine/fuel cell hybrid power system is developed using a novel topology to
complement each other and to alleviate the effects of environmental variations.
Modeling and simulations are conducted using MATLAB/Simulink software
packages to verify the effectiveness of the proposed system. The results show that the
proposed hybrid power system can tolerate the rapid changes in natural conditions and
suppress the effects of these fluctuations on the voltage within the acceptable range.
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1.1 MATLAB INTRODUCTION
MATLAB (Matrix Laboratory)
MATLAB is developed by The Math Works, Inc.
MATLAB is a high-level technical computing language and interactive
environment for algorithm development, data visualization, data analysis, and
numeric computation.
MATLAB can be install on Unix, Windows
MATLAB is a high-level language and interactive environment that enables
you to perform computationally intensive tasks faster than with traditional
programming languages such as C, C++, and FORTRAN.
History of MATLAB
Fortran subroutines for solving linear (LINPACK) and eigenvalue (EISPACK)
problems
Developed primarily by Cleve Moler in the 1970s
Later, when teaching courses in mathematics, Moler wanted his students to be
able to use LINPACK and EISPACK without requiring knowledge of Fortran MATLAB developed as an interactive system to access LINPACK and
EISPACK
MATLAB gained popularity primarily through word of mouth because it was
not officially distributed
In the 1980s, MATLAB was rewritten in C with more functionality (such as
plotting routines)
The Math works, Inc. was created in 1984
The Math works is now responsible for development, sale, and support for
MATLAB
The Math works is located in Natick
The Math works is an employer that hires co-ops through our co-op program
Strengths of MATLAB
MATLAB is relatively easy to learn.
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MATLAB code is optimized to be relatively quick when performing matrix
operations.
MATLAB may behave like a calculator or as a programming language.
MATLAB is interpreted, errors are easier to fix.
Although primarily procedural, MATLAB does have some object-oriented
elements.
Other Features
2-D and 3-D graphics functions for visualizing data
Tools for building custom graphical user interfaces
Functions for integrating MATLAB based algorithms with external
applications and languages, such as C, C++, Fortran, Java, COM, and
Microsoft Excel
Weakness of MATLAB
MATLAB is NOT a general purpose programming language.
MATLAB is an interpreted language (making it for the most part slower than
a compiled language such as C++).
MATLAB is designed for scientific computation and is not suitable for some
things (such as parsing text).
Components of MATLAB interface
Workspace
Current Directory
Command History
Command Window
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1.2 SIMULINK
It is a commercial tool for modeling, simulating and analyzing multidomain systems.
Its primary interface is a graphical block diagramming tool and a customizable set of
block libraries. Simulink is widely used in control theory and digital signal
processing for multidomain simulation and Model-based design
Generally there are three ways to open Simulink
By using start in Matlab
By typing Simulink in Command prompt
By clicking Simulink icon in toolbar
1.3 HYBRID POWER SYSTEMS
Electrical energy requirements for many remote applications are too large to
allow the cost-effective use of stand-alone or autonomous PV systems. In these cases,
it may prove more feasible to combine several different types of power sources to
form what is known as a "hybrid" system. To date, PV has been effectively combined
with other types of power generators such as wind, hydro, thermoelectric, petroleum-
fueled and even hydrogen. The selection process for hybrid power source types at a
given site can include a combination of many factors including site topography,
seasonal availability of energy sources, cost of source implementation, cost of energy
storage and delivery, total site energy requirements, etc.
Hybrid power systems use local renewable resource to provide power.
Village hybrid power systems can range in size from small household systems
(100Wh/day) to ones supplying a whole area (10s MWh/day).
They combine many technologies to provide reliable power that is tailored to
the local resources and community.
Potential components include: PV, wind, micro-hydro, river-run hydro,
biomass, batteries and conventional generators.
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1.4 RENEWABLE ENERGY
Renewable energy sources also called non-conventional energy are sources that are
continuously replenished by natural processes. For example, solar energy, wind
energy, bio-energy - bio-fuels grown sustain ably, hydropower etc., are some of the
examples of renewable energy sources.
A renewable energy system converts the energy found in sunlight, wind, falling-
water, sea-waves, geothermal heat, or biomass into a form, we can use such as heat or
electricity. Most of the renewable energy comes either directly or indirectly from sun
and wind and can never be exhausted, and therefore they are called renewable.
However, most of the world's energy sources are derived from conventional sources
fossil fuels such as coal, oil, and natural gases. These fuels are often termed non-renewable energy sources. Although, the available quantity of these fuels are
extremely large, they are nevertheless finite and so will in principle run out at some
time in the future.Due to industrializations and population growth our economy and
technologies today largely depend upon natural resources, which are not replaceable.
Approximately 90% of our energy consumption comes from fossil fuels. The another
advantage using renewable resources is that they are distributed over a wide
geographical area, ensuring that developing regions have access to electricity
generation at a stable cost for the long-term future.Renewable energy sources are
essentially flows of energy, whereas the fossil and nuclear fuels are, in essence, stocks
of energy.
Various forms of renewable energy
Solar energy
Wind energy
Bio mass energy
Geothermal energy
Tidal energy
Fuel cell
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Renewable energy is an alternative to fossil fuels and nuclear power, and was
commonly called alternative energy in the 1970s and 1980s.
Scientists have advanced a plan to power 100% of the world's energy with wind,
hydroelectric, and solar power by the year 2030, recommending renewable energy
subsidies and a price on carbon reflecting its cost for flood and related expenses
Difference between Renewable and Non-Renewable Sources.
RENEWABLE NON-RENEWABLE
1. Renewable energy is energy
which comes from natural resources
such as sunlight, wind, rain, tides
and geothermal heat.
1. The energy produced from
fossil fuels is non renewable
energy (coal, oil.)
2. The energy can be producedagain and again (continuous).
2. A non-renewable resource is
a natural resource which cannot
be produced, grown, generated,
or used on a scale which
can sustain its consumption rate.
3. The advantage of renewableenergy sources is that they are
ready, cheap, and they are difficult
easy to use.
3.The disadvantage of non-renewable energy sources is
costly to extract.
4. There is no expiration, it iscontinuous.
4.They are finite and willexpire sometime in the future.
5.No pollution, energy is clean. 5. Lot of pollution, energy isfilled with carbon elements.
6. Initially taking energy cost isvery high.
6.Energy cost is low
7.Energy produced is lowEx: wind,tidal,solar
7.Energy produced is highEx: coal, oil,(fossil fuels)
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CHAPTER-2
DYNAMIC SYSTEM MODELS
2.1 SOLAR CELL
The photovoltaic effect was first reported by Edmund Bequerel in 1839 when
he observed that the action of light on a silver coated platinum electrode immersed in
electrolyte produced an electric current. Forty years later the first solid state
photovoltaic devices were constructed by workers investigating the recently
discovered photoconductivity of selenium. In 1876 William Adams and Richard Day
found that a photocurrent could be produced in a sample of selenium when contacted
by two heated platinum contacts. The photovoltaic action of the selenium deferred
from its photoconductive action in that a current was produced spontaneously by theaction of light. No external power supply was needed. In this early photovoltaic
device, a rectifying junction had been formed between the semiconductor and the
metal contact. In 1894, Charles Fritts prepared what was probably the first large area
solar cell by pressing a layer of selenium between gold and another metal. In the
following years photovoltaic effects were observed in copper{copper oxide thin film
structures, in lead sulphide and thallium sulphide. These early cells were thin film
Schottky barrier devices, where a semitransparent layer of metal deposited on top of
the semiconductor provided both the asymmetric electronic junction, which is
necessary for photovoltaic action, and access to the junction for the incident light. The
photovoltaic effect of structures like this was related to the existence of a barrier to
current own at one of the semiconductor {metal interfaces (i.e., rectifying action) by
Goldman and Brodsky in 1914. Later, during the 1930s, the theory of
metal{semiconductor barrier layers was developed by Walter Schottky, Neville Mott
and others.
However, it was not the photovoltaic properties of materials like selenium
which excited researchers, but the photoconductivity. The fact that the current
produced was proportional to the intensity of the incident light, and related to the
wavelength in a definite way meant that photoconductive materials were ideal for
photographic light meters. The photovoltaic effect in barrier structures was an added
benefit, meaning that the light meter could operate without a power supply. It was not
until the 1950s, with the development of good quality silicon wafers for applications
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in the new solid state electronics, that potentially useful quantities of power were
produced by photovoltaic devices in crystalline silicon.
In the 1950s, the development of silicon electronics followed the discovery of
a way to manufacture pn junctions in silicon. Naturally n type silicon wafers
developed a p type skin when exposed to the gas boron trichloride. Part of the skin
could be etched away to give access to the n type layer beneath. These p{n junction
structures produced much better rectifying action than Schottky barriers, and better
photovoltaic behaviour. The first silicon solar cell was reported by Chapin, Fuller and
Pearson in 1954 and converted sunlight with an efficiency of 6%, six times higher
than the best previous attempt. That was to rise significantly over the following years
and decades but, at an estimated production cost of some $200 per Watt, these cells
were not seriously considered for power generation for several decades. Nevertheless,
the early silicon solar cell did introduce the possibility of power generation in remote
locations where fuel could not easily be delivered. The obvious application was to
satellites where the requirement of reliability and low weight made the cost of the
cells unimportant and during the 1950s and 60s, silicon solar cells were widely
developed for applications in space.
Also in 1954, a cadmium sulphide p{n junction was produced with an
efficiency of 6%, and in the following years studies of p{n junction photovoltaic
devices in gallium arsenide, indium phosphide and cadmium telluride were stimulated
by theoretical work indicating that these materials would over a higher efficiency.
However, silicon remained and remains the foremost photovoltaic material,
benefitting from the advances of silicon technology for the microelectronics industry.
Short histories of the solar cell are given elsewhere [Shive, 1959; Wolf, 1972; Green,
1990].
In the 1970s the crisis in energy supply experienced by the oil-dependent
western world led to a sudden growth of interest in alternative sources of energy, and
funding for research and development in those areas. Photovoltaics was a subject of
intense interest during this period, and a range of strategies for producing photovoltaic
devices and materials more cheaply and for improving device efficiency were
explored. Routes to lower cost included photoelectrochemical junctions, and
alternative materials such as polycrystalline silicon, amorphous silicon, other `thin
film' materials and organic conductors. Strategies for higher efficiency includedtandem and other multiple band gap designs. Although none of these led to
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widespread commercial development, our understanding of the science of
photovoltaics is mainly rooted in this period.
During the 1990s, interest in photovoltaics expanded, along with growing
awareness of the need to secure sources of electricity alternative to fossil fuels. The
trend coincides with the widespread deregulation of the electricity markets and
growing recognition of the viability of decentralized power. During this period, the
economics of photovoltaics improved primarily through economies of scale. In the
late 1990s the photovoltaic production expanded at a rate of 15{25% per annum,
driving a reduction in cost. Photovoltaics first became competitive in contexts where
conventional electricity supply is most expensive, for instance, for remote low power
applications such as navigation, telecommunications, and rural electrification and for
enhancement of supply in grid-connected loads at peak use [Anderson,2001]. As
prices fall, new markets are opened up. An important example is building integrated
photovoltaic applications, where the cost of the photovoltaic system is onset by the
savings in building materials.
There are several types of solar cells. However, more than 90 % of the solar
cells currently made worldwide consist of wafer-based silicon cells. They are either
cut from a single crystal rod or from a block composed of many crystals and are
correspondingly called mono-crystalline or multi-crystalline silicon solar cells.
Wafer-based silicon solar cells are approximately 200 m thick. Another important
family of solar cells is based on thin-films, which are approximately 1-2 m thick and
therefore require significantly less active, semiconducting material. Thin-film solar
cells can be manufactured at lower cost in large production quantities; hence their
market share will likely increase in the future. However, they indicate lower
efficiencies than wafer-based silicon solar cells, which mean that more exposure
surface and material for the installation is required for a similar performance.
A number of solar cells electrically connected to each other and mounted in a
single support structure or frame is called a photovoltaic module. Modules are
designed to supply electricity at a certain voltage, such as a common 12 volt system.
The current produced is directly dependent on the intensity of light reaching the
module. Several modules can be wired together to form an array. Photovoltaic
modules and arrays produce direct-current electricity. They can be connected in both
series and parallel electrical arrangements to produce any required voltage and currentcombination.
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A solar cell (also called photovoltaic cell) is asolid statedevice that converts
the energy ofsunlightdirectly into electricityby thephotovoltaic effect. Assemblies of
cells are used to make solar modules, also known as solar panels. The energy generated
from these solar modules, referred to assolar power, is an example ofsolar energy.
The term "photovoltaic" comes from the Greek (phs) meaning "light", and
"voltaic", meaning electric, from the name of theItalianphysicistVolta, after whom a
unit of electro-motive force, thevolt, is named.
The solar cell works in three steps:
1. Photons in sunlight hit the solar panel and are absorbed by semiconducting
materials, such as silicon.
2. Electrons (negatively charged) are knocked loose from their atoms, allowing
them to flow through the material to produce electricity. Due to the special
composition of solar cells, the electrons are only allowed to move in a single
direction.
3. An array of solar cells converts solar energy into a usable amount of direct
current (DC) electricity.
Solar panels use light energy (photons) from the sun to generate electricity
through the photovoltaic effect. The structural (load carrying) member of a module
can either be the top layer (superstrate) or the back layer (substrate). The majority of
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modules use wafer-based crystalline silicon cells or thin-film cells based on cadmium
telluride or silicon. Crystalline silicon is a commonly used semiconductor.
ELECTRICAL CONNECTION OF THE CELLS
The electrical output of a single cell is dependent on the design of the device
and the Semi-conductor material(s) chosen, but is usually insufficient for most
applications. In order to provide the appropriate quantity of electrical power, a
number of cells must be electrically connected. There are two basic connection
methods: series connection, in which the top contact of each cell is connected to the
back contact of the next cell in the sequence, and parallel connection, in which all the
top contacts are connected together, as are all the bottom contacts. In both cases, this
results in just two electrical connection points for the group of cells.
Series connection
Figure shows the series connection of three individual cells as an example and the
resultant group of connected cells is commonly referred to as a series string. The
current output of the string is equivalent to the current of a single cell, but the voltage
output is increased, being an addition of the voltages from all the cells in the string
(i.e. in this case, the voltage output is equal to 3Vcell).
Fig. Series connection of cells, with resulting currentvoltage characteristic.
It is important to have well matched cells in the series string, particularly with
respect to current. If one cell produces a significantly lower current than the other
cells (under the same illumination conditions), then the string will operate at that
lower current level and the remaining cells will not be operating at their maximum
power points.
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Parallel connection
Figure shows the parallel connection of three individual cells as an example. In
this case, the current from the cell group is equivalent to the addition of the current
from each cell (in this case, 3 Icell), but the voltage remains equivalent to that of a
single cell.
As before, it is important to have the cells well matched in order to gain
maximum output, but this time the voltage is the important parameter since all cells
must be at the same operating voltage. If the voltage at the maximum power point is
substantially different for one of the cells, then this will force all the cells to operate
off their maximum power point, with the poorer cell being pushed towards its open-
circuit voltage value and the better cells to voltages below the maximum power point
voltage. In all cases, the power level will be reduced below the optimum.
Fig. Parallel connection of cells, with resulting currentvoltage characteristic.
THE PHOTOVOLTAIC ARRAY
A PV array consists of a number of PV modules, mounted in the same plane
and electrically connected to give the required electrical output for the application.
The PV array can be of any size from a few hundred watts to hundreds of kilowatts,
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although the larger systems are often divided into several electrically independent sub
arrays each feeding into their own power conditioning system.
Advantages of Solar cell:
This system of energy conversion is noise less and cheap. Maintenance cost is low.
They have long life.
Pollution free.
Highly reliable.
Disadvantages of Solar cell:
Large area is required to collect the solar energy.
Direction of rays changes continuously.
Energy is not uniform during cloudy weather and not available during nights.
Energy storage is essential.
High initial cost.
Low efficiency.
Applications:
Water pumping in agriculture.
For low-power portable electronics, like calculators or small fans.
Industrial applications.
Developing remote areas.
2.2 WIND TURBINES
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.Kinetic energy from the wind is used to turn the generator
inside the wind turbine to produce electricity. There are several factors that contribute
to the efficiency of the wind turbine in extracting the power from the wind.Wind
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turbine works on the basis of Bernoullis principle. The power in the wind is
proportional to:
The area of windmill being swept by the wind.
The cube of the wind speed.
The air density - which varies with altitude.
Wind Turbine
Wind Turbine types:
There are two types of wind turbine in relation to their rotor settings. They are:
Horizontal-axis rotors, and
Vertical-axis rotor
Horizontal axis wind turbine:
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.
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Horizontal axis wind turbine
Vertical axis wind turbine:
Vertical-axis wind turbines (or 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. With a vertical axis, the generator and gearbox can be placed near the
ground, so the tower doesn't need to support it, and it is more accessible for
maintenance.
Vertical axis wind turbine
In this report, only the horizontal-axis wind turbine will be discussed since the
modeling of the wind driven electric generator is assumed to have the horizontal-axis
rotor.
The horizontal-axis wind turbine is designed so that the blades rotate in front of the
tower with respect to the wind direction i.e. the axis of rotation are parallel to the
wind direction. These are generally referred to as upwind rotors. Another type of
horizontal axis wind turbine is called downwind rotors which has blades rotating in
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back of the tower. Nowadays, only the upwind rotors are used in large-scale power
generation and in this report, the term horizontal-axis wind turbine refers to the
upwind rotor arrangement.
The main components of a wind turbine for electricity generation are the rotor, the
transmission system, the generator, and the yaw and control system. The following
figures show the general layout of a typical horizontal-axis wind turbine, different
parts of the typical grid-connected wind turbine, and cross-section view of a nacelle
of a wind turbine.
(a) (b)
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(c)
Figs: (a) Main Components of Horizontal-axis Wind Turbine
(b) Cross-section of a Typical Grid-connected Wind Turbine
(c) Cross-section of a Nacelle in A Grid-connected Wind Turbine
Main Components of a wind turbine
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The main components of a wind turbine can be classified as
Tower
Rotor System
Generator
Yaw
Control System and transmission system
Tower:
It is the most expensive element of the wind turbine system. The lattice or tubular
types of towers are constructed with steel or concrete. Cheaper and smaller towers
may be supported by guy wires. The major components such as rotor brake, gearbox,
electrical switch boxes, controller, and generator are fixed on to or inside nacelle,
which can rotate or yaw according to wind direction, are mounted on the tower. The
tower should be designed to withstand gravity and wind loads. The tower has to be
supported on a strong foundation in the ground. The design should consider the
resonant frequencies of the tower do not coincide with induced frequencies from the
rotor and methods to damp out if any. If the natural frequency of the tower lies above
the blade passing frequency, it is called stifftower and if below is called softtower.
Rotor:
The aerodynamic forces acting on a wind turbine rotor is explained by aerofoil theory.
When the aerofoil moves in a flow, a pressure distribution is established around the
symmetric aerofoil shown in Fig (a).
Zones of low and high pressure an aerofoil section in an air stream
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Forces acting on the rotor blade
A reference line from which measurements are made on an aerofoil section is referred
to as chord line and the length is known as chord. The angle, which an aerofoil makes
with the direction of airflow measured against the chord line is called the angle of
attack . The generation of lift forceL on an aerofoil placed at an angle of attack
to an oncoming flow is a consequence of the distortion of the streamlines of the fluid
passing above and below the aerofoil. When a blade is subjected to unperturbed wind
flow, the pressure decreases towards the center of curvature of a streamline. The
consequence is the reduction of pressure (suction) on the upper surface of the aerofoil
compared to ambient pressure, while on the lower side the pressure is positive or
greater. The pressure difference results in lift force responsible for rotation of the
blades. The drag force D is the component that is in line with the direction of
oncoming flow is shown in above Figure
These forces are both proportional to the energy in the wind. To attain a high
efficiency of rotor in wind turbine design is for the blade to have a relatively high lift-
to-drag ratio. This ratio can be varied along the length of the blade to optimize the
turbines energy output at various wind speeds. The lift force, drag force or both
extract the energy from wind. For aerofoil to be aerodynamically efficient, the liftforce can be 30 times greater than the drag force.
Cambered or asymmetrical aerofoils have curved chord lines. The chord line is now
defined as the straight line joining the ends of the camber line and is measured
from this chord line. Cambered aerofoil is preferred to symmetrical aerofoil because
they have higher lift/drag ratio for positive angles of attack. It is observed that the lift
at zero angle of attack is no longer zero and that the zero lift occurs at a small
negative angle of attack of approximately 4 o. The center of pressure, which is at the
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chord position on symmetrical aerofoil has at the chord position on cambered
aerofoil and moves towards the trailing edge with increasing angle of attack.
Arching or cambering a flat plate will cause it to induce higher lift force for a given
angle of attack and blades with a cambered plate profile work well, under the
conditions experienced by high solidity, multi bladed wind turbines. For low solidity
turbines, the use of aerofoil section is more effective.
The characteristics of an aerofoil, the angle of attack, the magnitude of the relative
wind speed are the prime parameters responsible for the lift and drag forces. These
forces acting on the blades of a wind turbine rotor are transformed into a rotational
torque and axial thrust force. The useful work is produced by the torque where as the
thrust will overturn the turbine. This axial thrust should be resisted by the tower and
foundations.
Rotor speed:
Low speed and high-speed propeller are the two types of rotors. A large design tip
speed ratio would require a long, slender blade having high aspect ratio. A low
design tip speed would require a short, flat blade. The low speed rotor runs with high
torque and the high-speed rotor runs with low torque. The wind energy converters of
the same size have essentially the same power output, as the power output depends on
rotor area. The low speed rotor has curved metal plates. The number of blades,
weight, and difficulty of balancing the blades makes the rotors to be typically small.
They get self-started because of their aerodynamic characteristics. The propeller type
rotor comprises of a few narrow blades with more sophisticated airfoil section. When
not working, the blades are completely stalled and the rotor cannot be self-started.
Therefore, propeller type rotors should be started either by changing the blade pitch or
by turning the rotor with the aid of an external power source (such as generator used
as a motor to turn the rotor). Rotor is allowed to run at variable speed or constrained
to operate at a constant speed. When operated at variable speed, the tip speed ratio
remains constant and aerodynamic efficiency is increased.
Rotor alignment:
The alignment of turbine blades with the direction of wind is made by upwind ordownwind rotors. Upwind rotors face the wind in front of the vertical tower and have
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the advantage of somewhat avoiding the wind shade effect from the presence of the
tower. Upwind rotors need a yaw mechanism to keep the rotor axis aligned with the
direction of the wind. Downwind rotors are placed on the lee side of the tower. A
great disadvantage in this design is the fluctuations in the wind power due to the rotor
passing through the wind shade of the tower which gives rise to more fatigue loads.
Downwind rotors can be built without a yaw mechanism, if the rotor and nacelle can
be designed in such a way that the nacelle will follow the wind passively. This may
however include gyroscopic loads and hamper the possibility of unwinding the cables
when the rotor has been yawing passively in the same direction for a long time,
thereby causing the power cables to twist. Upwind rotors need to be rather inflexible
to keep the rotor blades clear of the tower, downwind rotors can be made more
flexible. The latter implies possible savings with respect to weight and may
contribute to reducing the loads on the tower. The vast majority of wind turbines in
operation today have upwind rotors.
Number of rotor blades:
The three bladed rotors are the most common in modern aero generators.
Compared to three bladed concepts, the two and one bladed concepts have the
advantage of representing a possible saving in relation to cost and weight of the rotor.
However, the use of fewer rotor blades implies that a higher rotational speed or a
larger chord is needed to yield the same energy output as a three bladed turbine of a
similar size. The use of one or two blades will also result in more fluctuating loads
because of the variation of the inertia, depending on the blades being in horizontal or
vertical position and on the variation of wind speed when the blade is pointing upward
or downward.
Therefore, the two and one bladed concepts usually have so-called teetering
hubs, implying that they have the rotor hinged to the main shaft. This design allows
the rotor to teeter in order to eliminate some of the unbalanced loads. One bladed
wind turbines are less widespread than twobladed turbines. This is because they in
addition to a higher rotational speed, more noise and visual intrusion problems, need a
counter weight to balance the rotor blade.
Generator:
Electricity is an excellent energy vector to transmit the high quality mechanical powerof a wind turbine. Generator is usually 95% efficient and transmission losses should
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be less than 10%. The frequency and voltage of transmission need not be
standardized, since the end use requirements vary. There are already many designs of
wind/ electricity systems including a wide range of generators. The distinctive
features of wind/electricity generating systems are:
Wind turbine efficiency is greatest if rotational frequency varies to maintain
constant tip speed ratio, yet electricity generation is most efficient at constant
or near constant frequency.
Mechanical control of turbine to maintain constant frequency increases
complexity and expense. An alternative method, usually cheaper and more
efficient is to vary the electrical load on the turbine to control the rotational
frequency.
The optimum rotational frequency of a turbine in a particular wind speed
decreases with increase in radius in order to maintain constant tip speed ratio.
Thus, only small turbines of less than 2 m radius can be coupled directly to
generators. Larger machines require a gearbox to increase the generator drive
frequency.
Gearboxes are relatively expensive and heavy. They require maintenance and
can be noisy. To overcome this problem, generators with a large number ofpoles are being manufactured to operate at lower frequency.
The turbine can be coupled with the generator to provide an indirect drive
through a mechanical accumulator (weight lifted by hydraulic pressure) or
chemical storage (battery). Thus, generator control is independent of turbine
operation.
Wind Turbine Generator System (WTGS):
A wind turbine generator system (WTGS) transforms the energy present in the
blowing wind into electrical energy. As wind is highly variable resource that cannot
be stored, operation of a WTGS must be done according to this feature. The general
scheme of a WTGS is shown in Figure.
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General scheme of a WTGS where three types of energy states are presented
wind, mechanical, and electrical
Wind energy is transformed into mechanical energy by a wind turbine that has several
blades. It usually includes a gearbox that matches the turbine low speed to the higherspeed of the generator. Some turbines include a blade pitch angle control for
controlling the amount of power to be transformed. Wind speed is measured with an
anemometer. The electrical generator transforms mechanical energy into electrical
energy. Commercially available wind generators installed at present are squirrel cage
induction generator, doubly fed induction generator, wound field synchronous
generator (WFSG), and permanent magnet synchronous generator (PMSG). Based on
rotational speed, in general, the wind turbine generator systems can be split into two
types.
Fixed speed WTGS
Variable speed WTGS
Schematic diagram of a fixed speed WTGS
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Schematic diagram of VSWT-DFIG
The generators used with wind machines are
Synchronous AC generator
Induction AC generator
Variable speed generator
Synchronous AC generator:
The Synchronous speed will be in the range of 1500 rpm4 pole, 1000 rpm6 pole
or 750 rpm, - 8 pole for connection to a 50 Hz net work. The ingress of moisture is to
be avoided by providing suitable protection of the generator. Air borne noise is
reduced by using liquid cooling in some wind turbines. An increase of the damping in
the wind turbine drive train at the expense of losses in the rotor can be obtained by
high slip at rated power output. Synchronous generators run at a fixed or synchronous
speed, sN . We have pfNs 120 , where p the number of poles is, f is the
electrical frequency and sN is the speed in rpm.
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Induction AC generator:
They are identical to conventional industrial induction motors and are used on
constant speed wind turbines. The torque is applied to or removed from the shaft if
the rotor speed is above or below synchronous. The power flow direction in wires is
the factor to be considered to differentiate between a synchronous generator and
induction motor. Some design modifications are to be incorporated for induction
generators considering the different operating regime of wind turbines and the need
for high efficiency at part load, etc.
Variable speed generator:
Electrical variable speed operation can be approached as:
All the output power of the wind turbine may be passed through the
frequency converters to give a broad range of variable speed operation.
A restricted speed range may be achieved by converting only a fraction of
the output power.
Yaw system:
It turns the nacelle according to the actuator engaging on a gear ring at the top of the
tower. Yaw control is the arrangement in which the entire rotor is rotated horizontally
or yawed out of the wind. During normal operation of the system, the wind direction
should be perpendicular to the swept area of the rotor. The yaw drive is controlled by
a slow closed- loop control system. The yaw drive is operated by a wind vane, which
is usually mounted on the top of the nacelle sensing the relative wind direction, and
the wind turbine controller. In some designs, the nacelle is yawed to attain reduction
in power during high winds. In extremity, the turbine can be stopped with nacelle
turned such that the rotor axis is at right angles to the wind direction. One of the more
difficult parts of a wind turbine designs is the yaw system, though it is apparently
simple. Especially in turbulent wind conditions, the prediction of yaw loads is
uncertain.
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Control systems:
A wind turbine power plant operates in a range of two characteristic wind speed
values referred to as Cut in wind speed inu and Cut out wind speed outu . The turbine
starts to produce power at Cut in wind speed usually between 4 and 5 m/s. Below this
speed, the turbine does not generate power. The turbine is stopped at Cut out wind
speed usually at 25 m/s to reduce load and prevent damage to blades. They are
designed to yield maximum power at wind speeds that lies usually between 12 and 15
m/s. It would not be economical to design turbines at strong winds, as they are too
rare. However, in case of stronger winds, it is necessary to waste part of the excess
energy to avoid damage on the wind turbine. Thus, the wind turbine needs some sort
of automatic control for the protection and operation of wind turbine. The functionalcapabilities of the control system are required for:
i Controlling the automatic startup
ii Altering the blade pitch mechanism
iii Shutting down when needed in the normal and abnormal condition
iv Obtaining information on the status of operation, wind speed, direction
and power production for monitoring purpose
As can be seen in figure, the nacelle consists of several components. They are the
generator, yaw motor, gearbox, tower, yaw ring, main bearings, main shaft, hub,
blade, clutch, brake, blade and spinner. Other equipment that is not shown in the
figure might include the anemometer, the controller inside the nacelle, the sensors and
so on. The generator is responsible for the conversion of mechanical to electrical
energy.
Yaw motor is used power the yaw drive to turn the nacelle to the direction of the
wind. The gearbox is used to connect the low-speed shaft (main shaft in the figure) to
the high-speed shaft which drives the generator rotor. The brake is used to stop the
main shaft from over speeding. The blades are used to extract the kinetic power from
the wind to mechanical power i.e. lifting and rotating the blades. The tower is made
from tubular steel or steel lattice and it is usually very high in order to expose the
rotor blades to higher wind speed.
Anemometer: Measures the wind speed and transmits wind speed data to thecontroller.
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Blades: Most turbines have either two or three blades. Wind blowing over the blades
causes the blades to "lift" and rotate.
Brake: A disc brake which can be applied mechanically, electrically, or hydraulically
to stop the rotor in emergencies.
Controller: The controller starts up the machine at wind speeds of about 8 to 16
miles per hour (mph) and shuts off the machine at about 65 mph. Turbines cannot
operate at wind speeds above about 65 mph because their generators could overheat.
Gear box: Gears connect the low-speed shaft to the high-speed shaft and increase the
rotational speeds from about 30 to 60 rotations per minute (rpm) to about 1200 to
1500 rpm, the rotational speed required by most generators to produce electricity. The
gear box is a costly (and heavy) part of the wind turbine and engineers are exploring
"direct-drive" generators that operate at lower rotational speeds and don't need gear
boxes.
Generator: Usually an off-the-shelf induction generator that produces 60-cycle AC
electricity.
High-speed shaft: Drives the generator.
Low-speed shaft: The rotor turns the low-speed shaft at about 30 to 60 rpm.
Nacelle: The rotor attaches to the nacelle, which sits atop the tower and includes the
gear box, low- and high-speed shafts, generator, controller, and brake. A cover
protects the components inside the nacelle. Some nacelles are large enough for a
technician to stand inside while working.
Pitch: Blades are turned, or pitched, out of the wind to keep the rotor from turning in
winds that are too high or too low to produce electricity.
Rotor: The blades and the hub together are called the rotor.
Tower: Towers are made from tubular steel (shown here) or steel lattice. Because
wind speed increases with height, taller towers enable turbines to capture more energy
and generate more electricity.
Wind direction: This is an "upwind" turbine, so-called because it operates facing into
the wind. Other turbines are designed to run "downwind", facing away from the wind.
Wind vane: Measures wind direction and communicates with the yaw drive to orient
the turbine properly with respect to the wind.
Yaw drive: Upwind turbines face into the wind; the yaw drive is used to keep the
rotor facing into the wind as the wind direction changes. Downwind turbines don'trequire a yaw drive; the wind blows the rotor downwind.
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Yaw motor: Powers the yaw drive.
TamilNadu, Andhra and Gujarat are considered suitable for wind power generation.
The location of wind turbines is a very important factor, which influences the
performance of the machine. The Wind power potential of the country is estimated as
20,000 MW and India now ranks FOURTH in the world. Wind mills are operated at
wind speed normally not less than 3 mph. To avoid turbulence from one turbine
affecting the wind flow at others it is located at 5-15 times blades diameter. Wind
turbines will not work in winds below 13 km an hour.
Advantages of Wind turbine:
Improving price competitiveness
Modular installation
Rapid construction
Complementary generation
Improved system reliability and
Non-polluting.
Disadvantages of wind turbine:
These are noisy
Construction can be very expensive and costly
Applications:
Used as coolant
Used in water pumping
2.3 FUEL CELL
A fuel cell is an electrochemical cell that converts a source fuel into an
electrical current. It generates electricity inside a cell through reactions between a fuel
and an oxidant, triggered in the presence of an electrolyte. The reactants flow into the
cell, and the reaction products flow out of it, while the electrolyte remains within it.
Fuel cells can operate continuously as long as the necessary reactant and oxidant
flows are maintained.
Fuel cells are different from conventional electrochemical cell batteries in that
they consume reactant from an external source, which must be replenished a
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thermodynamically open system. By contrast, batteries store electrical energy
chemically and hence represent a thermodynamically closed system.
Many combinations of fuels and oxidants are possible. A hydrogen fuel cell
uses hydrogen as its fuel and oxygen (usually from air) as its oxidant. Other fuels
include hydrocarbons and alcohols. Other oxidants include chlorine and chlorine
dioxide.
Fuel cells come in many varieties; however, they all work in the same general
manner. They are made up of three segments which are sandwiched together: the
anode, the electrolyte, and the cathode. Two chemical reactions occur at the interfaces
of the three different segments. The net result of the two reactions is that fuel is
consumed, water or carbon dioxide is created, and an electrical current is created,
which can be used to power electrical devices, normally referred to as the load.
Fuel cell
At the anode a catalyst oxidizes the fuel, usually hydrogen, turning the fuel
into a positively charged ion and a negatively charged electron. The electrolyte is a
substance specifically designed so ions can pass through it, but the electrons cannot.
The freed electrons travel through a wire creating the electrical current. The ions
travel through the electrolyte to the cathode. Once reaching the cathode, the ions are
reunited with the electrons and the two react with a third chemical, usually oxygen, to
create water or carbon dioxide.
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DESIGN FEATURES IN A FUEL CELL ARE:
The electrolyte substance. The electrolyte substance usually defines the type
of fuel cell.
The fuel that is used. The most common fuel is hydrogen.
The anode catalyst, which breaks down the fuel into electrons and ions. The
anode catalyst is usually made up of very fine platinum powder.
The cathode catalyst, which turns the ions into the waste chemicals like water
or carbon dioxide. The cathode catalyst is often made up ofnickel.
A typical fuel cell produces a voltage from 0.6 V to 0.7 V at full rated load. Voltage
decreases as current increases, due to several factors:
Activation loss
Ohmic loss (voltage drop due to resistance of the cell components and
interconnects)
Mass transport loss (depletion of reactants at catalyst sites under high loads,
causing rapid loss of voltage).
To deliver the desired amount of energy, the fuel cells can be combined in
series and parallel circuits, where series yields higher voltage, and parallel allows a
higher current to be supplied. Such a design is called a fuel cell stack. The cell surface
area can be increased, to allow stronger current from each cell.
Types of fuel cells:
Proton exchange Fuel cell
High temperature Fuel cell
Molten Carbonate Fuel cell
Proton exchange fuel cell:
There are different fuel cell technologies that have been successfully used.
Among others, the polymer electrolyte (PE) fuel cell, also named proton exchange
membrane (PEM) fuel cell, can be considered a good alternative for the use aboard of
electric Vehicles in which simplicity, high specific power and rapid start-up at
different temperatures have a significative importance.
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Structure of a PEM fuel cell
(a) Bipolar plate; (b) Gas flow channel; (c) Electrode layer; (d) Catalyst layer and
(e) polymer layer.
A PEM fuel cell is constituted by a stack with a central membrane able to
conduct protons. The external layers work as two electrodes. The set of layers is
pressed by two conductive plates containing some channels in which the reactants
flow. A basic diagram showing the structure of the cell is shown in Fig. The main
elements inside the cell are: conductor plates, electrodes and membrane. The
electrodes are composed by a gas diffusion layer and a catalyst layer. Both layers
have a porous, partially hydrophobic, structure. Air is fed to the cathodic layer, and
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hydrogen is fed to the anodic one. The central membrane works as a electrolyte that
performs both the functions of transferring H+ from the anode to the cathode and
reactant separation. The electrochemical reactions involved are summarized below,
H2 2H+ + 2e (1)
2H+ +1/2O2 + 2eH2O (2)
H2 +1/2O2H2O (3)
Eq. (1) describes the chemical reaction at the anode. The electrons are transferred to
the platinum layer and protons to the central membrane. Eq. (2) shows what happens
at the cathode. The oxygen reacts with the protons coming from the membrane and
with the electrons fed by the catalyst. The result is water. Finally, eq. (3) shows the
overall reaction.
In the archetypal hydrogenoxygen proton exchange membrane fuel cell
(PEMFC) design, a proton-conducting polymer membrane, (the electrolyte), separates
the anode and cathode sides. This was called a "solid polymer electrolyte fuel cell"
(SPEFC) in the early 1970s, before the proton exchange mechanism was well-
understood. (Notice that "polymer electrolyte membrane" and "proton exchange
mechanism" result in the same acronym.)
On the anode side, hydrogen diffuses to the anode catalyst where it later
dissociates into protons and electrons. These protons often react with oxidants causing
them to become what is commonly referred to as multi-facilitated proton membranes.
The protons are conducted through the membrane to the cathode, but the electrons are
forced to travel in an external circuit (supplying power) because the membrane is
electrically insulating. On the cathode catalyst, oxygen molecules react with the
electrons (which have traveled through the external circuit) and protons to form water.
The materials used in fuel cells differ by type. In a typical membrane electrode
assembly (MEA), the electrodebipolar plates are usually made of metal, nickel or
carbon nanotubes, and are coated with a catalyst (like platinum, nano iron powders or
palladium) for higher efficiency. Carbon paper separates them from the electrolyte.
The electrolyte could be ceramic or a membrane.
http://en.wikipedia.org/wiki/Proton_exchange_membrane_fuel_cellhttp://en.wikipedia.org/wiki/Electrolytehttp://en.wikipedia.org/wiki/Anodehttp://en.wikipedia.org/wiki/Cathodehttp://en.wikipedia.org/wiki/Acronymhttp://en.wikipedia.org/wiki/Moleculehttp://en.wikipedia.org/wiki/Membrane_electrode_assemblyhttp://en.wikipedia.org/wiki/Membrane_electrode_assemblyhttp://en.wikipedia.org/wiki/Bipolarhttp://en.wikipedia.org/wiki/Bipolarhttp://en.wikipedia.org/wiki/Metalhttp://en.wikipedia.org/wiki/Nickelhttp://en.wikipedia.org/wiki/Carbon_nanotubehttp://en.wikipedia.org/wiki/Catalysthttp://en.wikipedia.org/wiki/Platinumhttp://en.wikipedia.org/wiki/Nano_iron_powderhttp://en.wikipedia.org/wiki/Palladiumhttp://en.wikipedia.org/wiki/Carbon_paperhttp://en.wikipedia.org/wiki/Ceramichttp://en.wikipedia.org/wiki/Artificial_membranehttp://en.wikipedia.org/wiki/Artificial_membranehttp://en.wikipedia.org/wiki/Ceramichttp://en.wikipedia.org/wiki/Carbon_paperhttp://en.wikipedia.org/wiki/Palladiumhttp://en.wikipedia.org/wiki/Nano_iron_powderhttp://en.wikipedia.org/wiki/Platinumhttp://en.wikipedia.org/wiki/Catalysthttp://en.wikipedia.org/wiki/Carbon_nanotubehttp://en.wikipedia.org/wiki/Nickelhttp://en.wikipedia.org/wiki/Metalhttp://en.wikipedia.org/wiki/Bipolarhttp://en.wikipedia.org/wiki/Membrane_electrode_assemblyhttp://en.wikipedia.org/wiki/Membrane_electrode_assemblyhttp://en.wikipedia.org/wiki/Moleculehttp://en.wikipedia.org/wiki/Acronymhttp://en.wikipedia.org/wiki/Cathodehttp://en.wikipedia.org/wiki/Anodehttp://en.wikipedia.org/wiki/Electrolytehttp://en.wikipedia.org/wiki/Proton_exchange_membrane_fuel_cell -
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Proton exchange membrane fuel cell design issues:
Many companies are working on techniques to reduce cost in a variety of ways
including reducing the amount of platinum needed in each individual cell. Ballard
Power Systems have experiments with a catalyst enhanced with carbon silkwhich
allows a 30% reduction (1 mg/cm to 0.7 mg/cm) in platinum usage without
reduction in performance. Monash University, Melbourne uses PEDOT as a
cathode.
The production costs of the PEM (proton exchange membrane). The Nafion
membrane currently costs $566/m. In 2005 Ballard Power Systems announced
that its fuel cells will use Sholapur, a porous polyethylene film patented by DSM.
Water and air management (in PEMFCs). In this type of fuel cell, the membrane
must be hydrated, requiring water to be evaporated at precisely the same rate that
it is produced. If water is evaporated too quickly, the membrane dries, resistance
across it increases, and eventually it will crack, creating a gas "short circuit"
where hydrogen and oxygen combine directly, generating heat that will damage
the fuel cell. If the water is evaporated too slowly, the electrodes will flood,
preventing the reactants from reaching the catalyst and stopping the reaction.
Temperature management. The same temperature must be maintained throughout
the cell in order to prevent destruction of the cell through thermal loading. This is
particularly challenging as the 2H2 + O2 -> 2H2O reaction is highly exothermic,
so a large quantity of heat is generated within the fuel cell.
Durability, service life, and special requirements for some type of cells. Stationary
fuel cell applications typically require more than 40,000 hours of reliable
operation at a temperature of -35 C to 40 C (-31 F to 104 F), while automotive
fuel cells require a 5,000 hour lifespan (the equivalent of 150,000 miles) under
extreme temperatures. Current service life is 7,300 hours under cycling
conditions. Automotive engines must also be able to start reliably at -30 C (-22
F) and have a high power to volume ratio (typically 2.5 kW per liter).
Limited carbon monoxide tolerance of the cathode.
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High temperature fuel cell:
Solid oxide fuel cell:
A solid oxide fuel cell (SOFC) is extremely advantageous because of a
possibility of using a wide variety of fuel. Unlike most other fuel cells which only use
hydrogen, SOFCs can run on hydrogen, butane, methanol, and other petroleum products.
The different fuels each have their own chemistry.
For methanol fuel cells, on the anode side, a catalyst breaks methanol and water
down to form carbon dioxide, hydrogen ions, and free electrons. The hydrogen ions move
across the electrolyte to the cathode side, where they react with oxygen to create water. A
load connected externally between the anode and cathode completes the electrical circuit.
Below are the chemical equations for the reaction:
Anode Reaction: CH3OH + H2O CO2 + 6H+ + 6e-
Cathode Reaction: 3/2 O2 + 6H+ + 6e- 3H2O
Overall Reaction: CH3OH + 3/2 O2 CO2 + 2H2O + electrical energy
At the anode SOFCs can use nickel or other catalysts to break apart the methanol
and create hydrogen ions and CO2. A solid called yttrium stabilized zirconia (YSZ) is
used as the electrolyte. Like all fuel cell electrolytes YSZ is conductive to ions, allowing
them to pass from the anode to cathode, but is non-conductive to electrons. YSZ is a
durable solid and is advantageous in large industrial systems. Although YSZ is a goodion conductor, it only works at very high temperatures.
The standard operating temperature is about 950oC. Running the fuel cell at such
a high temperature easily breaks down the methane and oxygen into ions. A major
disadvantage of the SOFC, as a result of the high heat, is that it places considerable
constraints on the materials which can be used for interconnections. Another
disadvantage of running the cell at such a high temperature is that other unwanted
reactions may occur inside the fuel cell. It is common for carbon dust, graphite, to build
up on the anode, preventing the fuel from reaching the catalyst. Much research is
currently being done to find alternatives to YSZ that will carry ions at a lower
temperature.
Solid oxide fuel cells (SOFCs) offer a clean, low-pollution technology to
electrochemically generate electricity at high efficiencies; since their efficiencies are not
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limited the way conventional heat engine's is. These fuel cells provide many advantages
over traditional energy conversion systems including high efficiency, reliability,
modularity, fuel adaptability, and very low levels of polluting emissions. Quiet,
vibration-free operation of SOFCs also eliminates noise usually associated with
conventional power generation systems.
Up until about six years ago, SOFCs were being developed for operation
primarily in the temperature range of 900 to 1000oC (1692 to 1832oF); in addition to the
capability of internally reforming hydrocarbon fuels (for example, natural gas), such high
temperature SOFCs provide high quality exhaust heat for cogeneration, and when
pressurized, can be integrated with a gas turbine to further increase the overall efficiency
of the power system. However, reduction of the SOFC operating temperature by 200oC
(392oF) or more allows use of a broader set of materials, is less demanding on the seals
and the balance-of-plant components, simplifies thermal management, aids in faster start
up and cool down, and results in less degradation of cell and stack components. Because
of these advantages, activity in the development of SOFCs capable of operating in the
temperature range of 650 to 800oC (1202 to 1472oF) has increased dramatically in the last
few years. However, at lower temperatures, electrolyte conductivity and electrode
kinetics decrease significantly; to overcome these drawbacks, alternative cell materials
and designs are being extensively investigated.
Structure of Solid Oxide Fuel cell
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Figure 1: Operating principle of a solid state fuel cell
An SOFC essentially consists of two porous electrodes separated by a dense,
oxide ion conducting electrolyte. The operating principle of such a cell is illustrated in
Figure 1. Oxygen supplied at the cathode (air electrode) reacts with incoming electrons
from the external circuit to form oxide ions, which migrate to the anode (fuel electrode)
through the oxide ion conducting electrolyte. At the anode, oxide ions combine with
hydrogen (and/or carbon monoxide) in the fuel to form water (and/or carbon dioxide),
liberating electrons. Electrons (electricity) flow from the anode through the external
circuit to the cathode.
The materials for the cell components are selected based on suitable electrical
conducting properties required of these components to perform their intended cell
functions; adequate chemical and structural stability at high temperatures encountered
during cell operation as well as during cell fabrication; minimal reactivity and inter
diffusion among different components; and matching thermal expansion among different
components.
Molten-Carbonate fuel cell:
Molten-carbonate fuel cells (MCFCs) are high-temperature fuel cells, that operate
at temperatures of 600C and above.
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Molten carbonate fuel cells (MCFCs) are currently being developed for natural
gas and coal-based power plants for electrical utility, industrial, and military applications.
MCFCs are high-temperature fuel cells that use an electrolyte composed of a molten
carbonate salt mixture suspended in a porous, chemically inert ceramic matrix of beta-
alumina solid electrolyte (BASE). Since they operate at extremely high temperatures of
650C (roughly 1,200F) and above, non-precious metals can be used as catalysts at the
anode and cathode, reducing costs.
Structure of Molten Carbonate Fuel Cell
Improved efficiency is another reason MCFCs offer significant cost reductions
over phosphoric acid fuel cells (PAFCs). Molten carbonate fuel cells can reach
efficiencies approaching 60 percent, considerably higher than the 37-42 percent
efficiencies of a phosphoric acid fuel cell plant. When the waste heat is captured and
used, overall fuel efficiencies can be as high as 85 percent.
Unlike alkaline, phosphoric acid, and polymer electrolyte membrane fuel cells,
MCFCs don't require an external reformer to convert more energy-dense fuels to
hydrogen. Due to the high temperatures at which MCFCs operate, these fuels are
converted to hydrogen within the fuel cell itself by a process called internal reforming,
which also reduces cost.
The primary disadvantage of current MCFC technology is durability. The high
temperatures at which these cells operate and the corrosive electrolyte used accelerate
component breakdown and corrosion, decreasing cell life. Scientists are currently
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exploring corrosion-resistant materials for components as well as fuel cell designs that
increase cell life without decreasing performance.
Fuel cell efficiency:The efficiency of a fuel cell is dependent on the amount of power drawn from it.
Drawing more power means drawing more current, which increases the losses in the fuel
cell. As a general rule, the more power (current) drawn, the lower the efficiency. Most
losses manifest themselves as a voltage drop in the cell, so the efficiency of a cell is
almost proportional to its voltage. For this reason, it is common to show graphs of
voltage versus current (so-called polarization curves) for fuel cells. A typical cell running
at 0.7 V has an efficiency of about 50%, meaning that 50% of the energy content of the
hydrogen is converted into electrical energy; the remaining 50% will be converted into
heat. (Depending on the fuel cell system design, some fuel might leave the system
unreacted, constituting an additional loss.)
For a hydrogen cell operating at standard conditions with no reactant leaks, the
efficiency is equal to the cell voltage divided by 1.48 V, based on the enthalpy, or heating
value, of the reaction. For the same cell, the second law efficiency is equal to cell voltage
divided by 1.23 V. (This voltage varies with fuel used, and quality and temperature of the
cell.) The difference between these numbers represents the difference between the
reaction's enthalpy and Gibbs free energy. This difference always appears as heat, along
with any losses in electrical conversion efficiency.
Fuel cells do not operate on a thermal cycle. As such, they are not constrained, as
combustion engines are, in the same way by thermodynamic limits, such as Carnot cycle
efficiency. At times this is misrepresented by saying that fuel cells are exempt from the
laws of thermodynamics, because most people think of thermodynamics in terms of
combustion processes (enthalpy of formation). The laws of thermodynamics also hold for
chemical processes (Gibbs free energy) like fuel cells, but the maximum theoretical
efficiency is higher (83% efficient at 298K in the case of hydrogen/oxygen reaction) than
the Otto cycle thermal efficiency (60% for compression ratio of 10 and specific heat ratio
of 1.4).
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Comparing limits imposed by thermodynamics is not a good predictor of
practically achievable efficiencies. Also, if propulsion is the goal, electrical output of the
fuel cell has to still be converted into mechanical power with another efficiency drop. In
reference to the exemption claim, the correct claim is that the "limitations imposed by the
second law of thermodynamics on the operation of fuel cells are much less severe than
the limitations imposed on conventional energy conversion systems". Consequently, they
can have very high efficiencies in converting chemical energy to electrical energy,
especially when they are operated at low power density, and using pure hydrogen and
oxygen as reactants.
It should be underlined that fuel cell (especially high temperature) can be used as
a heat source in conventional heat engine (gas turbine system). In this case the ultra high
efficiency is predicted (above 70%).
In practice:
For a fuel cell operating on air, losses due to the air supply system must also be
taken into account. This refers to the pressurization of the air and dehumidifying it. This
reduces the efficiency significantly and brings it near to that of a compression ignition
engine. Furthermore, fuel cell efficiency decreases as load increases.
The tank-to-wheel efficiency of a fuel cell vehicle is greater than 45% at low
loads and shows average values of about 36% when a driving cycle like the NEDC (New
European Driving Cycle) is used as test procedure. The comparable NEDC value for a
Diesel vehicle is 22%. In 2008 Honda released a fuel cell electric vehicle (the Honda
FCX Clarity) with fuel stack claiming a 60% tank-to-wheel efficiency.
It is also important to take losses due to fuel production, transportation, and
storage into account. Fuel cell vehicles running on compressed hydrogen may have a
power-plant-to-wheel efficiency of 22% if the hydrogen is stored as high-pressure gas,
and 17% if it is stored as liquid hydrogen. In addition to the production losses, over 70%
of US electricity used for hydrogen production comes from thermal power, which only
has an efficiency of 33% to 48%, resulting in a net increase in carbon dioxide production
by using hydrogen in vehicles.
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Fuel cells cannot store energy like a battery, but in some applications, such as
stand-alone power plants based on discontinuous sources such as solar or wind power,
they are combined with electrolyzes and storage systems to form an energy storage
system. The overall efficiency (electricity to hydrogen and back to electricity) of such
plants (known as round-trip efficiency) is between 30 and 50%, depending on conditions.
While a much cheaper lead-acid battery might return about 90%, the electrolyze/fuel cell
system can store indefinite quantities of hydrogen, and is therefore better suited for long-
term storage.
Solid-oxide fuel cells produce exothermic heat from the recombination of the
oxygen and hydrogen. The ceramic can run as hot as 800 degrees Celsius. This heat can
be captured and used to heat water in a micro combined heat and power (m-CHP)
application. When the heat is captured, total efficiency can reach 80-90% at the unit, but
does not consider production and distribution losses. CHP units are being developed
today for the European home market.
Stationary fuel cell applications (or stationary fuel cell power systems) are
stationary that are either connected to the electric grid (distributed generation) to provide
supplemental power and as emergency power system for critical areas, or installed as a
grid-independent generator for on-site service.
Codes and standards Stationary fuel cell applications is a classification in FCHydrogen codes and standards and fuel cell codes and standards. The other main
standards are Porta