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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.
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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

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