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SIMULATION MODEL FOR GRID CONNECTED FUEL CELL

SYSTEMS

A PROJECT REPORT

Submitted by

K.PALANI VEL (92103105029)

K.SYED ABDUL RAHMAN (92103105034)

S.RAJAGANAPATHY (92103105036)

S.RAJARAMAN (92103105038)

in partial fulfillment for the award of the degree

Of

BACHELOR OF ENGINEERING in

ELECTRICAL AND ELECTRONICS ENGINEERING

SYED AMMAL ENGINEERING COLLEGE, RAMANATHAPURAM.

ANNA UNIVERSITY:: CHENNAI 600 025 MAY 2007

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ANNA UNIVERSITY:: CHENNAI 600 025

BONAFIDE CERTIFICATE

Certified that this project report “SIMULATION MODEL FOR GRID

CONNECTED FUEL CELL SYSTEMS” is the bonafide work of

“K.PALANI VEL, K.SYED ABDUL RAHMAN, S.RAJAGANAPATHY,

S.RAJARAMAN” who carried out the project work under my supervision.

SIGNATURE SIGNATURE K. PANDIARAJAN, M.E., M.I.S.T.E., J. BASTIN SOLAI NAZARAN, M.E., HEAD OF THE DEPARTMENT SUPERVISOR Asst. Professor Asst. Professor Department of EEE, Department of EEE, Syed Ammal Engg.College, Syed Ammal Engg.College, Ramanathapuram-623502. Ramanathapuram-623502. INTERNAL EXAMINER EXTERNAL EXAMINER

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Abstract:

The research activity aimed to analyze and model fuel cell systems for stationary

applications. In particular, PEM and SOFC models have been developed and set up in

the Matlab-Simulink. The steady state and dynamical operation of PEM and SOFC

technologies have been analyzed, and their main characteristics have been compared.

The model is applied to a distributed utility that uses fuel cells to investigate the

nature and magnitude of their interaction. The validity of the analysis is verified when

the model is used to predict the response of the system.

The response curves indicate the load-following characteristics of the model and

the predicted changes in the analytical parameters predicated by the analysis. The

developed model, being simple, could provide a useful tool for the planning of

distributed generation.

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ACKNOWLEDGEMENT

We acknowledge our indebtedness and sincere thanks to our college

MANAGEMENT who are constantly stimulating us to do innovative actions and

taking care about our carrier development.

It is of immense pleasure to all of us to express our sincere gratitude to our

beloved principal Prof. MOHAMED SHAHABUDDIN, B.E., M.Tech., F.I.E.,

M.I.S.T.E., who helped us in all ways to complete This project successfully.

We would like to thank our vice principal Mr.M.PERIYASAMY M.E.,

Asst.Prof., Department of Electronics and Communication Engineering, who gave

moral support to do this project.

We wish to express our gratitude to our beloved head of the department

Mr. K. PANDIARAJAN, M.E., M.I.S.T.E., Electrical and Electronics Engineering,

who gave valuable suggestion and helped us in successful of this project.

We renter our heartful and sincere thanks to our respective guide,

Mr. J. BASTIN SOLAI NAZARAN, ME., Department of Electrical and Electronics

Engineering, for his guidance and encouragement in completion of the project.

We convey our heartful gratitude to our loving parents for their support and

continuous encouragement and also the staff members and friends who directly or

indirectly support for completing this project successfully.

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CONTENTS

CHAPTER TITLE PAGE NO

ABSTRACT iii

ACKNOWLEDGEMENT iv

LIST OF FIGURES vii

LIST OF TABLES viii

LIST OF SYMBOLS ix

1. FUEL CELL TECHNOLOGY OVERVIEW 1

1.1 INTRODUCTION 1

1.2 UNIT CELLS 2

1.3 FC STACKING 4

1.3.1 PLANAR BIPOLAR 4

1.3.2 STACK TUBLAR 5

1.4 FC SYSTEM 5

1.4.1 FUEL PREPERATION 6

1.4.2 AIR SUPPLY 6

1.4.3 THERMAL MANAGEMENT 6

1.4.4 WATER MANAGEMENT 6

1.4.5 ELECTRIC POWER CONDITIONING 7

1.5 FC TYPES 8

1.6 ADVANTAGES 9

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2 . PEMFC 10

2.1 INTRODUCTION 10

2.2 CELL COMPONENT 11

2.2.1 MEMBRANE 12

2.2.2 POROUS BACKING LAYER 13

2.2.3 WATER MANAGEMENT 14

2.3 PEMFC SYSTEM 15

2.3.1 DIRECT HYDROGEN PEMFC 15

2.3.2 REFORMER BASED PEMFC 16

2.3.3 DIRECT METHANOL PEMFC 17

2.4 PEMFC APPLICATION 17

2.4.1 TRANSPORTATION 17

2.4.2 STATIONARY 18

3 . SOFC 19

3.1 CELL COMPONENTS 21

3.1.1 ELECTROLYTE MATERIAL 22

3.1.2 ANODE MATERIAL 22

3.1.3 CATHODE MATERIAL 22

3.1.4 INTERCONNECT MATERIAL 22

3.1.5 SEAL MATERIAL 23

3.2 CELL AND STACK DESIGN 24

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4. FUEL PROCESSING 25

5. POWER CONDITIONING 26

6. MATLAB/SIMULINK 28

MATLAB 28

MATLAB TOOLBOXES 28

SIMULINK 28

PEM FUEL CELL MODELLING 29

6.4.1 FUEL CELL MODELLING 29

6.4.2 REFORMER MODELLING 30

LINE DATA 32

FC OUTPUT 33

7. SIMULATION OF SOFC 34

PSAT 34

SOFC SCHEME 37

SIMULATION MODEL 40

7.3.1 POWER FLOW REPORT 42

7.3.2 LOAD PROFILE 44

7.3.3 TIME DOMAIN ANALYSIS 45

8. CONCLUSION 46

9. REFERENCES 47

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LIST OF FIGURES

FIGURE NO TITLE PAGE NO

1.1 INDIVIDUAL FC 2

1.2 FC STACK 4

1.3 FC POWER PLANT 6

2.1 INTERSECTIONAL VIEW OF PEMFC 11

2.2 PEMFC UNIT CELL 11

2.3 COMPONENTS OF PEMFC 13

2.4 DHPEMFC 14

2.5 REFORMER BASED PEMFC 16

2.6 HONDA FCX 17

2.7 5KW PEMFC 17

2.8 MOTOROLA FC 18

3.1 250KW TUBULAR SOFC 23

3.2 PLANAR SOFC 24

3.3 COMPONENTS OF SOFC 24

4.1 FUEL PROCESSING 25

5.1 POWER CONDITIONING 26

5.2 FC POWER SYSTEM 27

6.1 FC MODEL 28

6.2 REFORMER MODEL 30

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6.3 FC SYSTEM 31

6.4 FC VOLTAGE 33

6.5 OXYGEN FLOW 33

6.6 HYDROGEN FLOW 34

7.1 SYNOPTIC OF PSAT 36

7.2 SOFC SIMULINK 37

7.3 SOFC-GRID 39

7.4 VOLTAGE CONTROL 39

7.5 SOFC PSAT MODEL 40

7.6 VOLTAGE PROFILE 44

7.7 PHASE PROFILE 44

7.8 REAL POWER PROFILE 44

7.9 REACTIVE POWER PROFILE 44

7.10 BUS VOLTAGE 45

7.11 REAL POWER 45

7.12 REACTIVE POWER 45

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LIST OF TABLES

TABLE NO CONTENT PAGE NO

1.1 COMPARISION OF FC TYPES 8

7.1 SOFC DATA FORMAT 38

7.2 OBSERVATION 41

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1. FUEL CELL - A TECHNOLOGY OVERVIEW

1.1 INTRODUCTION

Fuel cells are electrochemical devices that convert chemical energy in

fuels into electrical energy directly, promising power generation with high

efficiency and low environmental impact.

Most fuel cell power systems comprise a number of components:

Unit cells, in which the electrochemical reactions take place.

Stacks, in which individual cells are modularly

combined by electrically connecting the cells to form

units with the desired output capacity.

Balance of plant which comprises components that provide

feed stream conditioning (including a fuel processor if

needed), thermal management, and electric power conditioning

among other ancillary and interface functions.

1.2 Unit Cells

Unit cells form the core of a fuel cell. These devices convert the chemical

energy contained in a fuel electrochemically into electrical energy. The basic physical

structure, or building block, of a fuel cell consists of an electrolyte layer in contact with

an anode and a cathode on either side. A schematic representation of a unit cell with the

reactant/product gases and the ion conduction flow directions through the cell is shown

in Figure 1-1.

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Fig 1.1 Schematic of an Individual Fuel Cell

In a typical fuel cell, fuel is fed continuously to the anode (negative electrode)

and an oxidant (often oxygen from air) is fed continuously to the cathode (positive

electrode). The electrochemical reactions take place at the electrodes to produce an

electric current through the electrolyte, while driving a complementary electric

current that performs work on the load. Although a fuel cell is similar to a typical

battery in many ways, it differs in several respects. The battery is an energy storage

device in which all the energy available is stored within the battery itself (at least

the reluctant). The battery will cease to produce electrical energy when the chemical

reactants are consumed (i.e., discharged). A fuel cell, on the other hand, is an

energy conversion device to which fuel and oxidant are supplied continuously. In

principle, the fuel cell produces power for as long as fuel is supplied.

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1.3 Fuel Cell Stacking

For most practical fuel cell applications, unit cells must be combined in a

modular fashion into a cell stack to achieve the voltage and power output level

required for the application. Generally, the stacking involves connecting multiple

unit cells in series via electrically conductive interconnects. Different stacking

arrangements have been developed, which are described below.

1.3.1 Planar-Bipolar Stacking

The most common fuel cell stack design is the so-called planar-bipolar

arrangement (Figure 1-2 depicts a PAFC). Individual unit cells are electrically

connected with interconnects.

Because of the configuration of a flat plate cell, the interconnect becomes a

separator plate with two functions:

1) To provide an electrical series connection between adjacent cells,

specifically for flat plate cells, and

2) To provide a gas barrier that separates the fuel and oxidant of

adjacent cells.

In many planar-bipolar designs, the interconnect also includes channels that

distribute the gas flow over the cells. The planar-bipolar design is electrically

simple and leads to short electronic current paths (which helps to minimize cell

resistance).

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Fig.1.2 Expanded View of a Basic Fuel Cell Unit in a Fuel Cell Stack

1.3.2 Stacks with Tubular Cells

Especially for high-temperature fuel cells, stacks with tubular cells have

been developed. Tubular cells have significant advantages in sealing and in the

structural integrity of the cells. However, they represent a special geometric

challenge to the stack designer when it comes to achieving high power density and

short current paths. In one of the earliest tubular designs the current is conducted

tangentially around the tube. Interconnects between the tubes are used to form

rectangular arrays of tubes. Alternatively, the current can be conducted along the

axis of the tube, in which case interconnection is done at the end of the tubes. To

minimize the length of electronic conduction paths for individual cells, sequential

series connected cells are being developed. The cell arrays can be connected in

series or in parallel.

1.4 Fuel Cell Systems

In addition to the stack, practical fuel cell systems require several other sub-

systems and components; the so-called balance of plant (BoP). Together with the

stack, the BoP forms the fuel cell system. The precise arrangement of the BoP

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depends heavily on the fuel cell type, the fuel choice, and the application. In

addition, specific operating conditions and requirements of individual cell and stack

designs determine the characteristics of the BoP. Still, most fuel cell systems

contain:

1.4.1 Fuel preparation:

Except when pure fuels (such as pure hydrogen) are used, some fuel

preparation is required, usually involving the removal of impurities and thermal

conditioning. In addition, many fuel cells that use fuels other than pure hydrogen

require some fuel processing, such as reforming, in which the fuel is reacted with

some oxidant (usually steam or air) to form a hydrogen-rich anode feed mixture.

1.4.2 Air supply:

In most practical fuel cell systems, this includes air compressors or blowers

as well as air filters.

1.4.3 Thermal management:

All fuel cell systems require careful management of the fuel cell stack

temperature.

1.4.4 Water management:

Water is needed in some parts of the fuel cell, while overall water is a

reaction product. To avoid having to feed water in addition to fuel, and to ensure

smooth operation, water management systems are required in most fuel cell

systems.

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1.4.5 Electric power conditioning equipment:

Since fuel cell stacks provide a variable DC voltage output that is typically

not directly usable for the load, electric power conditioning is typically required.

Figure 1-3 shows a simple rendition of a fuel cell power plant. Beginning

with fuel processing, a conventional fuel (natural gas, other gaseous hydrocarbons,

methanol, naphtha, or coal) is cleaned, and then converted into a gas containing

hydrogen. Energy conversion occurs when dc electricity is generated by means of

individual fuel cells combined in stacks or bundles. A varying number of cells or

stacks can be matched to a particular power application. Finally, power

conditioning converts the electric power from dc into regulated dc or ac for

consumer use.

Fig.1.3 Fuel Cell Power Plant Major Processes

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1.5 Fuel Cell Types

A variety of fuel cells are in different stages of development. The most

common classification of fuel cells is by the type of electrolyte used in the cells and

includes

1) Polymer electrolyte fuel cell (PEFC),

2) Alkaline fuel cell (AFC),

3) Phosphoric acid fuel cell (PAFC),

4) Molten carbonate fuel cell (MCFC), and

5) Solid oxide fuel cell (SOFC).

Broadly, the choice of electrolyte dictates the operating temperature

range of the fuel cell. The operating temperature also plays an important role in

dictating the degree of fuel processing required. In low-temperature fuel cells, all the

fuel must be converted to hydrogen prior to entering the fuel cell. In addition, the

anode catalyst in low temperature fuel cells (mainly platinum) is strongly poisoned

by CO. In high temperature fuel cells, CO and even CH4 can be internally converted

to hydrogen or even directly oxidized electrochemically. Table 1-1 provides an

overview of the key characteristics of the main fuel cell types.

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Table 1.1 comparisons between several fuel cells

1.6 Advantages:

Direct energy conversion (no combustion)

High efficiency (35 to 60 percent)

No moving parts in the energy converter

Quiet

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Demonstrated high availability of lower temperature units

Siting ability

Fuel flexibility

Demonstrated endurance/reliability of lower temperature units

Good performance at off-design load operation

Modular installations to match load and increase reliability

Remote/unattended operation

Size flexibility

Rapid load following capability

Here in particular we are concentrating on PEMFC and SOFC because of some

attractive features.

2. Proton exchange membrane fuel cell (PEMFC)

2.1 Introduction:

PEMFC is otherwise called as polymer electrolyte membrane fuel cell. PEMFC are

able to efficiently generate high power densities, thereby making the technology

potentially attractive for certain mobile and portable applications.

PEM fuel cells operate at low temperatures (less than 100 degrees Celsius), making

them temperature-compatible with many of today's automotive systems and also

allowing faster startups. However, due to a relatively small temperature gradient to

the ambient atmosphere, the produced waste heat is low-grade and requires large

heat exchangers.

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PEMFC are particularly suitable for operation on pure hydrogen, fuel processors

have been developed that will allow the use of conventional fuels such as natural

gas or gasoline.

2.2 Cell Components

Typical cell components within a PEMFC stack include:

The ion exchange membrane

An electrically conductive porous backing layer

An electro-catalyst (the electrodes) at the interface between the backing

layer and the membrane

Cell interconnects and flow plates that deliver the fuel and oxidant to

reactive sites via flow channels and electrically connect the cells.

PEMFC stacks are almost universally of the planar bipolar type. Typically, the

electrodes are cast as thin films that are either transferred to the membrane or

applied directly to the membrane. Alternatively, the catalyst-electrode layer may be

deposited onto the backing layer, and then bonded to the membrane.

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Fig.2.1 Inter sectional view of PEM fuel cell

Fig.2.2 PEMFC unit cell structure

2.2.1 Membrane

This is an organic-based cation exchange membrane. In most of the PEMFCs

membrane is made up of perfluorosulfonic acid polymer. The function of the ion

exchange membrane is to provide a conductive path, while at the same time

separating the reactant gases. The material is an electrical insulator. As a result, ion

conduction takes place via ionic groups within the polymer structure. Ion transport

at such sites is highly dependent on the bound and free water associated with those

sites.

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2.2.2 Porous Backing Layer

The polymer membrane is sandwiched between two sheets of porous backing

media (also referred to as gas diffusion layers or current collectors). The functions

of the backing layer are to:

Act as a gas diffuser,

Provide mechanical support,

Provide an electrical pathway for electrons, and

Channel product water away from the electrodes.

The backing layer is typically carbon-based, and may be in cloth form, a non-

woven pressed carbon fiber configuration, or simply a felt-like material. The layer

incorporates a hydrophobic material, such as polytetrafluoroethylene. The function

of polytetrafluoroethylene is to prevent water from “pooling” within the pore

volume of the backing layer so that gases freely contact the catalyst sites.

Furthermore, it facilitates product water removal on the cathode as it creates a non-

wetting surface within the passages of the backing material.

Electrode-Catalyst Layer

In intimate contact with the membrane and the backing layer is the catalyst layer.

This catalyst layer, integral with its binder, forms the electrode. The catalyst and

binder electrode structure is applied either to the membrane or to the backing layer.

The catalyst is platinum-based for both the anode and cathode. To promote

hydrogen oxidation, the anode uses either pure platinum metal catalyst or, as is

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common in most modern PEMFC catalysts, a supported platinum catalyst, typically

on carbon or graphite for pure hydrogen feed streams. For other fuels, such as

reformate (containing H2, CO2, CO, and N2), the desired catalyst is an alloy of

platinum containing ruthenium. Oxygen reduction at the cathode may use either the

platinum metal or the supported catalyst.

2.2.3 Water and Thermal Management

Due to operation at less than 100 °C and atmospheric pressure, water is produced as

a liquid. A critical requirement is to maintain high water content in the electrolyte

to ensure high ionic conductivity. Maintaining high water content is particularly

critical when operating at high current densities (approximately 1 A/cm2) because

mass transport issues associated with water formation and distribution limit cell

output. The ionic conductivity of the electrolyte is higher when the membrane is

fully saturated: this impacts the overall efficiency of the fuel cell. Without adequate

water management, an imbalance will occur between water production and water

removal from the cell.

Fig.2.3 Overview of subsystems and components for PEMFC system

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2.3 PEMFC Systems

PEFC stacks require tight control of fuel and air feed quality, humidity level, and

temperature for sustained high-performance operation. To provide this, PEFC

stacks must be incorporated in a sophisticated system. Naturally, the architecture of

these systems depends strongly on whether they are fueled by hydrogen or by a

hydrocarbon fuel.

2.3.1 Direct Hydrogen PEMFC Systems

Direct hydrogen PEFC systems require extensive thermal and water management to

ensure that the PEFC stack operates under the desired design conditions (Figure 3-

10). Key components are

Heat exchangers,

Humidifiers, and

Condensers.

Fig shows Direct Hydrogen PEMFC System,

Fig.2.4 Direct Hydrogen PEMFC System

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2.3.2 Reformer-Based PEMFC Systems

Reformer-based PEMFC systems avoid the complexities and compromises of

hydrogen storage, but instead the system must be designed to handle hydrocarbon

fuels (similar considerations apply for alcohol fuels). This requires four major

additional unit operations (Figure 3-11), collectively referred to as fuel processing:

• Fuel preheat and vaporization:

Necessary to prepare the fuel to meet the reformer’s feed requirements. Often, this

unit operation is physically integrated with the reformer.

• Reformer:

This unit chemically converts hydrocarbon or alcohol to synthesis gas (a mixture of

hydrogen and carbon monoxide). The two most practical oxidants are steam and air.

If air is used, the reformer is referred to as a partial oxidation (POX) reformer; if

steam is used, a steam reformer (SR), and if a mix of air and steam is used, an auto

thermal reformer (ATR). The choice of reformer type depends on a number of

factors. Typically, POX reformers are smaller, cheaper, respond faster, and are

suitable for a wide range of fuels. Steam reformers enable higher system efficiency.

ATRs and catalytic POX reformers (CPOX) share some of the advantages of each

type:

• Water Gas Shift Reactor (WGSR):

The WGSR reacts carbon monoxide with water vapor to form hydrogen and carbon

dioxide. This reactor is critical in PEFC systems (as well as PAFC), since the stack

is unable to convert carbon monoxide.

• Reformate purification:

This is necessary because the PEFC stacks are sensitive to even trace

concentrations of contaminants. Especially CO and sulfur are problematic species,

and must be reduced to levels of around 10 and 1 ppm or less, respectively. Sulfur

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removal is, in actuality, done upstream in the process (just before or just after the

reformer), but CO removal must be done just prior to stack entry.

Fig shows Reformer-Based PEFMC System,

Fig.2.5 Schematic of Major Unit Operations Typical of Reformer-Based

PEFMC Systems.

2.3.3 Direct Methanol Fuel Cell Systems

Specially optimized PEMFCs can be fed with methanol (or fuels with similar

chemical structure), creating a so-called direct methanol fuel cell (DMFC).

Conceptually, this could lead to a very simple system with a fuel that has a

relatively high energy density and is a liquid under ambient conditions.

2.4 PEFC Applications

2.4.1 Transportation Applications

PEMFC powers cars and light trucks. PEMFC is the only type of fuel cell

considered for prime motive power in on-road vehicles (as opposed to APU power,

for which SOFC is also being developed). PEMFC systems fueled by hydrogen,

methanol, and gasoline have been integrated into light duty vehicles by at least

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twelve different carmakers. Early prototypes of fuel cell vehicles (Honda and

Toyota) have been released to controlled customer groups in Japan and the U.S. fig

shows PEMFC powered car designed by HONDA.

Fig.2.6 HONDA FCX

2.4.2 Stationary Applications

Several developers are also developing PEMFC systems for stationary applications.

These efforts are aimed at very small-scale distributed generation (~1 to 10 kW

AC). The vast majority of systems are designed for operation on natural gas or

propane. Hundreds of demonstration units have been sited in programs in the U.S.,

Europe, and Japan. Considerable progress has been made in system integration and

in achieving stand-alone operation.

Fig.2.7 5 KW fuel cell systems designed by Northwest power systems.

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Fig. Shows 5 KW fuel cell system designed by Northwest power systems.

The fuel cell can also be used in portable applications such as cellular telephones

and handheld computers.

Fig.2.8 Motorola is developing fuel cell powered cellular phones that would

run on methanol

3.solid oxide fuel cell (SOFC)

Solid oxide fuel cells (SOFCs) have an electrolyte that is a solid, non-

porous metal oxide, usually 32OY stabilized ZrO2.

Where ionic conduction by oxygen ions takes place.

SOFCs operate at extremely high temperatures - of the order of 700 to

1000 degrees Celsius.

As a result, they can tolerate relatively impure fuels, such as those

obtained from the gasification of coal.

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Waste heat is high-grade, allowing smaller heat exchangers and the

possibility for co-generation to produce additional power.

The reformer system for SOFCs is less complex than PEM reformers.

This is because SOFC can use carbon monoxide along with hydrogen as

fuel.

In addition, SOFCs show a high tolerance to fuel impurities such as

natural gas.

SOFCs do not need precious metal catalysts. The relatively simple design

(because of the solid electrolyte and fuel versatility), combined with the

significant time required to reach the operating temperature and to respond to

changes in electricity demand, make SOFCs suitable for large to very large

stationary power applications.

The cell is constructed with two porous electrodes that sandwich an electrolyte. Air

flows along the cathode. When an oxygen molecule contacts the cathode/electrolyte

interface, it acquires electrons from the cathode. The oxygen ions diffuse into the

electrolyte material and migrate to the other side of the cell where they contact the

anode. The oxygen ions encounter the fuel at the anode/electrolyte interface and

react catalytically, giving off water, carbon dioxide, heat, and electrons. The

electrons transport through the external circuit, providing electrical energy.

Solid oxide fuel cells (SOFC) allow conversion of a wide range of fuels, including

various hydrocarbon fuels. The relatively high operating temperature allows for

highly efficient conversion to power, internal reforming, and high quality by-

product heat for cogeneration or for use in a bottoming cycle. Indeed, both simple-

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cycle and hybrid SOFC systems have demonstrated among the highest efficiencies

of any power generation system, combined with minimal air pollutant emissions

and low greenhouse gas emissions. These capabilities have made SOFC an

attractive emerging technology for stationary power generation in the 2 kW to

100 MW capacity ranges.

3.1 Cell Components

The major components of an individual SOFC cell include the electrolyte, the

cathode, and the anode. Fuel cell stacks contain an electrical interconnect, which

links individual cells together in series or parallel. The electrolyte is made from a

ceramic such as yttria-stabilized zirconia (YSZ) and functions as a conductor of

oxide ions. Oxygen atoms are reduced into oxide ions on the porous cathode

surface by electrons, and then flow through the ceramic electrolyte to the fuel rich

porous anode where the oxide ions react with fuel (hydrogen), giving up electrons.

The interconnect serves to conduct the electrons through an external circuit.

3.1.1 Electrolyte Materials

As indicated by their name, SOFCs use solid oxide ceramics, typically perovskites,

as the electrolyte. Currently, yttrium stabilized zirconia (3, 8, or 10 percent yttria,

abbreviated to YSZ) is the most commonly used electrolyte for SOFC. YSZ

provides high conductivity at temperatures above 700 °C. In a fuel cell operating

with a current density of 250 mA/cm2 at 1000 °C and an electrolyte of 200-µm

thicknesses, the resistance loss in the electrolyte would be 50 mV. However, for

mechanical reasons it is desirable to operate the SOFC at lower temperatures. To

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operate at 800 °C, the electrolyte thickness would have to be reduced by about an

order of magnitude to maintain a similar ohmic loss in the electrolyte.

3.1.2 Anode Materials

Although a wide range of materials has been considered as anode materials for

SOFC most developers today use a cermets of nickel and YSZ. Early on in the

development of SOFC, precious metals such as platinum and gold were used, as

well as pure transition metals such as nickel and iron.

3.1.3 Cathode Materials

Most cathode materials used in SOFC today are lanthanum-based perovskite

materials (structure ABO 3 ). During early development, platinum and other noble

metals, and even magnetite, were used as cathode materials for SOFC. They are no

longer pursued actively because of chemical and physical instability,

incompatibility with most electrolytes, and, in the case of platinum, cost. Currently,

most cathodes are based on doped lanthanum manganites. In high temperature

SOFC (operating temperature ~1000 °C), strontium-doped LaMnO3 (LSM) is used.

3.1.4 Interconnect Materials

Broadly, interconnect materials for SOFC fall into two categories:

Conductive ceramic (perovskite) materials for operation at high

temperature (900 to 1000 °C), and

Metallic alloys for lower temperature operation.

The ceramic interconnects used in higher temperature SOFCs are primarily doped

lanthanum and yttrium chromites (dopants typically include Mg, Sr, Ca, Ca/Co).

These perovskites are unique in that they exhibit high electronic conductivity and

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resist reduction under exposure to syngas at high temperatures. Electronic

conductivity of these materials increases with temperature (making them unsuitable

for use at low temperatures).

Lower operating temperatures would allow the use of ferritic steels, that could

reduce the materials cost, and ferritic steels are typically easier to process with low-

cost processing techniques.

3.1.5 Seal Materials

The challenges of sealing the oxidant from fuel in planar SOFC stacks is

significant, hence a sub-section is devoted to potential seal materials here. The

function of SOFC seals includes:

Prevent mixing of fuel and oxidant,

In some configurations, prevent mixing of reactants with the ambient

environment,

In some configurations, provide mechanical bonding of components,

In some designs, provide electrical insulation between stack components.

Seal materials must be chemically and physically stable at operating conditions. In

some applications (e.g. in on-road vehicles), the seal must also be able to withstand

acceleration forces associated with vibration and shock. Finally, seal materials must

be low in cost and amenable to low-cost stack manufacturing methods.

Glass-ceramic seals------------------Bonded Seals

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Mica and hybrid mica seals--------Compressive Seals

3.2 Cell and Stack Designs

Two types of cell designs are being pursued for SOFC:

Tubular cells and

Planar cells.

Fig.3.1 Siemens Westinghouse 250 kW Tubular SOFC Installation

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Fig.3.2 Planar SOFC: Delphi (30 cells x 106 cm 2 , 3.5 liter, 13 kg)

Fig.3.3 Overview of subsystems and components for SOFC

4. Fuel Processing

Fuel processing is defined as the conversion of a commercially available gas,

liquid, or solid fuel to a fuel gas reformate suitable for the fuel cell anode reaction.

Fuel processing encompasses the cleaning and removal of harmful species in the

fuel, the conversion of the fuel to the fuel gas reformate, and downstream

processing to alter the fuel gas reformate according to specific fuel cell

requirements. Examples of these processes are:

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Fuel Cleaning –

Removal of sulfur, halides, and ammonia to prevent fuel processor and

fuel cell catalyst degradation.

Fuel Conversion –

Converting a fuel (primarily hydrocarbons) to a hydrogen-rich gas

reformate.

Reformate Gas Alteration –

Converting carbon monoxide (CO) and water (H2O) in the fuel gas

reformate to hydrogen (H2) and carbon dioxide (CO2) via the water-gas shift

reaction; selective oxidation to reduce CO to a few ppm, or removal of water by

condensing to increase the H2 concentration.

Figure depicts the Processing steps needed for a low temperature cell. Most fuel

processors make use of the chemical and heat energy left in the fuel cell effluent to

provide heat for fuel processing thus enhancing system efficiency.

Fig.4.1 Representative Fuel Processing Steps & Temperatures

36

5.Power Conditioning

Power conditioning is an enabling technology that is necessary to convert DC

electrical power generated by a fuel cell into usable AC power for stationary loads,

automotive applications, and interfaces with electric utilities. The purpose of this

section is to explore power-conditioning approaches for the following applications:

Fuel cell power conversion to supply a dedicated load,

Fuel cell power conversion to supply backup power (UPS) to a load

connected to a local utility,

Fuel cell power conversion to supply a load operating in parallel with the

local utility (utility interactive),

Fuel cell power conversion to connect directly to the local utility,

Power conversion for automotive fuel cell applications,

Power conversion architectures for a fuel cell turbine hybrid interfaced to

local utility,

Fig.5.1 Block diagram of the power-conditioning unit with line frequency

transformer

37

Fig.5.2 Block diagram of a fuel cell power system

6.MATLAB/SIMULINK

6.1 MATLAB:

38

MATLAB is developed by Math Works Inc., which is a software package for high

performance numerical computation and visualization. MATLAB actually means

Matrix Laboratory. MATLAB is powerful in matrix or vector programming; it is

also a brilliant tool in working with matrix for numerical and engineering

applications .It has the ability to be programmed to solve several tasks at one time

using the idea of matrix .In MATLAB, there are toolboxes of special collections of

functions and scripts. Script is a program without input and output, which is

actually a collection of MATLAB statement in one file. Function block accepts

variable inputs and allows variable outputs.

6.2 MATLAB Toolboxes:

There are some optional toolboxes written for special applications such as

Simulink, Signal Processing, Control Systems Design, System Identification,

Statistics, Neural Networks, Fuzzy Logic, Symbolic Computations, and others. In

this project, the Simulink toolbox was used to design the hybrid power system.

6.3 SIMULINK:

Simulink has a wide selection of dynamic systems for modeling, analyzing, and

simulating. It also offers a graphical user interface for creating block diagram

models. A system is configured in terms of block diagram representation from a

library of standard components. In the middle of a simulation, algorithms and

parameters can still be changed to get intuitive results, thus providing the user with

a Ready access-learning tool for simulating many of the operational problems found

in the real world. It also provides immediate access to the mathematical, graphical,

and programming capabilities of MATLAB. The model can then be analyzed either

through Menu commands or from the MATLAB command line. Results from the

analysis can then be passed to the MATLAB workspace for further work.

39

6.4 PEM Fuel cell modeling

The fuel cell modeling consist of

1. Fuel cell model

2. Reformer model

6.4.1 Fuel cell model

This model is based on simulating the relationship between output voltage and

partial pressure of hydrogen, oxygen, and water. A detailed model of the PEM fuel

cell is shown in Fig. 1.

Fig.6.1 Fuel cell model

40

6.4.2 Reformer model:

Here a simple model for the reformer generates hydrogen through reforming

methane. The model is a second-order transfer function. The mathematical form of

the model can be written as follows:

)1(1)( 21

221

2

ssCV

qq

methanol

H

Fig.6.2 Reformer model

To control hydrogen flow according to output power from the fuel cell, a

feedback from the stack current is considered. A proportional integral (PI)

controller is used to control the flow rate of methane in the reformer. Oxygen flow

is determined using the hydrogen-oxygen flow ratio OrH _ . The reformer and the

reformer controller are illustrated in Fig. 2, whereU is the Fuel utilization factor,

3K is the PI gain, and 3T is the PI time constant.

According to the basic electrochemical relationship between the hydrogen

flow and the FC system current, the flow rate of reacted hydrogen is given by

)2(222 IfcK

FNoIfcq r

rH

41

Assuming constant temperature and oxygen concentration FC output voltage may

be expressed

)3(ohmicactcell EV

Where

)ln( fcact cIB

fcohmic IRint

The amount of hydrogen available from the reformer can be used to control the

methane flow rate by using a PI controller can be expressed as

)4(23

11 21

in

Hfco

methnol qFU

INK

sTKKq

Fig.6.3 Fuel cell system model

42

6.5 Line data

B=0.04777;

C=0.0136;

CV=2;

F=96484600;

ToH2=3.37;

KH2=0.0000422;

rH_O=1.168;

Kr=0.00000022802;

Qmethref=0.000015;

Eo=0.6;

No=88;

ToO2=6.74;

KO2=0.0000211;

K1=0.25;

To1=2;To2=To1; To3=To1;

Rint=0.00303;

T=343;

R=8314.47;

U=0.8;

ToH2O=18.418;

KH2O=0.000007716;

43

6.6 Fuel cell output:

Fig.6.4 Fuel cell dc voltage Fig.6.5 oxygen flow

Fig.6.6 Hydrogen flow

44

7.SIMULATION OF SOLID OXIDE FUEL CELL (SOFC) SYSTEM

In our project, we are done the simulation of SOFC in Power System Analysis

Toolbox (PSAT) platform.

7.1 PSAT

PSAT is an open code MATLAB based toolbox for electric power

system analysis and control.

PSAT can handle a wide variety of Power Systems: from small-scale

educational networks to medium size realistic power systems.

PSAT is also GNU Octave compatible in it’s command line version.

Being PSAT and open code software it is suitable for research since it

allows to modify the existing models/routines and/or to include new

models/routines.

The GUIs and Simulink library make it easy to use, thus, it’s adequate for

educational purposes such as teaching and self-study; besides being free!

PSAT makes a full use of MATLAB vectorized computations and sparse

matrix functions, this gives an optimal performance.

45

The synoptic scheme of PSAT is depicted in Fig. Observe that PSAT kernel is the

power flow algorithm, which also takes care of the state variable initialization.

Once the power flow has been solved, the user can perform further static and/or

dynamic analyses. These are as follows.

1) Continuation Power Flow (CPF).

2) Optimal Power Flow (OPF).

3) Small-signal stability analysis.

4) Time-domain simulations.

Besides mathematical algorithms and models, PSAT includes a variety of

additional tools, as follows.

1) User-friendly graphical user interfaces.

2) Simulink library for one-line network diagrams.

3) Data file conversion to and from other formats.

4) User defined model editor and installer.

5) Command line usage.

PSAT can run on GNU/Octave, which is a free MATLAB clone.

46

Fig 7.1 Synoptic scheme of PSAT.

47

7.2 Solid Oxide Fuel Cell scheme.

Fig.7.2 SOFC simulink scheme

48

Solid Oxide Fuel Cell Data Format

Table7.1

The fuel cell scheme, which is based on the following equations (state variables):

49

Where R is the gas constant (R = 8.314 [J/(mol K)]), F is the Faraday constant (F =

96487 [C/mol]), T the absolute gas temperature, and To be a “small” time constant

that does not affects the fuel cell dynamics. The fuel cell current Ik can be subjected

to a constant power control:

Fig.7.3 Solid Oxide Fuel Cell connection with the AC grid.

Fig. 7.4 AC voltage control for the Solid Oxide Fuel Cell.

50

7.3 SIMULATION MODEL Fig.7.5 Shows the simulation model for SOFC system. The designed model

includes

SOFC

Slack bus

Double circuit line, and

Shunt admittance.

Fig.7.5 simulation model for SOFC system in PSAT

The SOFC and slack bus are connected to a generation bus .the load bus is

connected with the generation bus via double circuit line. Shunt admittance is

connected to the load bus.

The sofc can able to supply the real power consumed by the load. In order to

supply the reactive power consumed by real world load such as air conditioner

a slack bus is connected to the generating bus.

51

Table 7.2 shows, the observations corresponding to various values of G and B - of

the shunt admittance.

G

B

VOLTAGE

TOTAL POWER

GENRATION

TOTAL SHUNT

BUS1 BUS2 P Q P Q

0.6

0

1

0.99661

0.59773

0.01589

0.59594

0

0.8

0

1

0.99528

0.79563

0.02971

0.79246

0

1.0

0

1

0.99384

0.99267

0.0474

0.98773

0

1.0

0.2

1

1.0038

1.0128

-0.1511

1.0075

0.2015

1.0

0.4

1

1.0139

1.0339

-0.3535

1.0279

0.4111

1.0

0.6

1

1.0242

1.0561

-0.5600

1.049

0.6293

Table 7.2

All are per unit values.

52

7.3.1 POWER FLOW REPORT Here we are presenting power flow report for

Conductance (G) =0.8p.u;

Susceptance (B)=0.0p.u

P S A T 1.3.4

Author: Federico Milano, (c) 2002-2005 E-mail: fmilano@thunderbox.uwaterloo.ca

Website: http://thunderbox.uwaterloo.ca/~fmilano

File: C:\MATLAB6p5\psat\tests\dsofcnew1.mdl

Date: 18-Apr-2007 08:57:41

NETWORK STATISTICS Buses: 2

Lines: 2

Generators: 1

Loads: 0

SOLUTION STATISTICS

Number of Iterations: 3 Maximum P mismatch [p.u.] 0

Maximum Q mismatch [p.u.] 0

Power rate [MVA] 100

POWER FLOW RESULTS Bus V phase P gen Q gen P load Q load

[P.u.] [Rad] [P.u.] [P.u.] [P.u.] [P.u.]

Bus1 1 0 0.79563 0.02971 0 0

Bus2 0.99528 -0.03983 0 0 0 0

53

STATE VECTOR Ik_Sofc_1 0.23835

Vk_Sofc_1 0.88786

PH2_Sofc_1 0.0001

PH20_Sofc_1 0.00169

PO2_Sofc_1 0.0001

QH2_Sofc_1 0

M_Sofc_1 1.0061

LINE FLOWS From Bus To Bus Line P Flow Q Flow P Loss Q Loss

[p.u.] [p.u.] [p.u.] [p.u.]

Bus1 Bus2 1 0.39781 0.01485 0.00158 0.01485

Bus1 Bus2 2 0.39781 0.01485 0.00158 0.01485

LINE FLOWS From Bus To Bus Line P Flow Q Flow P Loss Q Loss

[p.u.] [p.u.] [p.u.] [p.u.]

Bus2 Bus1 1 -0.39623 0 0.00158 0.01485

Bus2 Bus1 2 -0.39623 0 0.00158 0.01485

GLOBAL SUMMARY REPORT

TOTAL GENERATION REAL POWER [p.u.] 0.79563

REACTIVE POWER [p.u.] 0.02971

TOTAL LOAD REAL POWER [p.u.] 0

REACTIVE POWER [p.u.] 0

54

TOTAL SHUNT REAL POWER [p.u.] 0.79246

REACTIVE POWER (IND) [p.u.] 0

REACTIVE POWER (CAP) [p.u.] 0

TOTAL LOSSES REAL POWER [p.u.] 0.00317

REACTIVE POWER [p.u.] 0.02971

7.3.2 LOAD PROFILE

Fig.7.6Voltage magnitude profile Fig.7.7 Voltage phase profile

Fig.7.8Real power profile Fig.7.9 Reactive power profile

55

7.3.3 TIME DOMAIN ANALYSIS

Fig.7.10 Bus voltages

Fig.7.11 Real power

Fig.7.12 Reactive power

56

8.CONCLUSION

The realized models can be used as a tool for the design optimization of fuel

cells, cell stacks and fuel cell power systems. If the models are implemented into an

integrated framework, the synergies among multiple technologies may also be

predicted. Using the models described in our project, various simulations have been

performed to find the optimal operation and control strategies of a PEM fuel cell

plant and of a SOFC system in order to analyse the systems behaviour. Numerical

results are provided.

The model can be used in future applications for the analysis of hybrid power

systems. A hybrid power system consists of a combination of two or more power

generation technologies to do the best use of their operating characteristics and to

obtain higher efficiencies than those obtainable from a single power source. The

resulting system exhibits a synergism in which the combination of different

technologies has far greater efficiency than the one that could be provided by each

system operating alone.

For instance, combining a fuel cell with a gas turbine increases the overall

cycle efficiency while reducing per kilowatt emissions. In addition, we decided to

combine fuel cells with wind power and solar power generation for back–up power

generation and energy storage. Getting higher efficiencies combined with low

emissions, hybrid systems are likely to be the choice for the next generation of

advanced power generation systems.

57

9.REFERENCES

1. Fuel cell hand book By EG&G Technical Services, Inc. U.S. Department of

Energy Office of Fossil Energy National Energy Technology Laboratory

Morgantown, West Virginia.

2. Federico Milano-Documentation for PSAT version 2.0.0, March 8, 2007 PSAT

Power System Analysis Toolbox.

3. M. Uzunoglu, and M.S.Alam Dynamic Modeling, Design, and Simulation of

a Combined PEM Fuel Cell and Ultracapacitor System for Stand-Alone

Residential Applications, IEEE TRANSACTIONS ON ENERGY

CONVERSION, VOL. 21, NO. 3, SEPTEMBER 2006.

4. M. Y. El-Sharkh, A. Rahman, M. S. Alam, P. C. Byrne, A. A. Sakla, and T.

Thomas, “A dynamic model for a stand-alone PEM fuel cell power plant for

residential applications,” J. Power Sources, vol. 138, no. 1–2, pp. 199–204,

Nov. 2004.

5. C. J. Hatziadoniu, A. A. Lobo, F. Pourboghrat, and M. Daneshdoost, “A

simplified dynamic model of grid-connected fuel-cell generators,” IEEE

Trans. Power Deliv., vol. 17, no. 2, pp. 467–473, Apr. 2002.

6. Honda Fuel Cell Power FCX (Dec. 2004). [Online]. Available:

http://world.honda.com/FuelCell/FCX/FCXPK.pdf,Press Information, 2004.

58

7. Chiara Boccaletti, Gerardo Duni, Gianluca Fabbri, Ezio Santini Chiara

Boccaletti, Gerardo Duni, Gianluca Fabbri, Ezio Santini -Simulation Models

of Fuel Cell Systems

8. A fuel cell priemer-IAEI-nov/dec2001

9. Residential fuel cells: hope or hype? - Home power#72 –aug/sep1999

10. J.J baschuk and xianguo li -Modeling of PEMFC and stacks-dept. of mech.

engg university of waterloo. Canada

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