SIM UNIVERSITY

ENG499

CAPSTONE ELECTRONICS PROJECT

FINAL REPORT

24th October 2008

By

V.K.Mohamed Buharie

Student No: K0604667 Low loss DC-DC converter for fuel cell application

Project No: CP0801/E11

STATEMENT

ASSISTANCE RECEIVED FROM EMPLOYER OR OTHER SOURCE This form is to be completed by all students and to be bound in the front of the final report.

BME499 ENG499 MTD499 ICT499 CAPSTONE PROJECT COURSE

Tick one box

The work described in this report is not associated in any way with my full or part-time employment.

The work described in this report is associated with my employer as described below.

1. Association of project to full or part-time employment.

2. Statement of assistance (provision of laboratory facilities, technical support, library services etc.) Include here any facilities provided by your tutor.

Name: V.K.Mohamed Buharie

Personal Identifier K0604667

Signed Date 1/10/2008

This project is not related in any way to my full or part-time employment.

Used UniSIM library extensively to read through journals and reference books. However, there was no provision of lab facilities or equipment provided by my tutor.

Acknowledgments First and foremost, my heartily profound thanks, gratitude and appreciation to my advisor Assoc Prof A.I Maswood for his encouragement, help and kind support. His invaluable technical advice, suggestions, discussions and guidance were a real support to complete this dissertation. Furthermore, I would like to express my deepest gratitude to my company’s departmental manager, Mr C C Lee, in rendering his moral support and showing patience in allowing me to complete the project on time. Finally, very special thanks to my family members for their patience, understanding and encouragement during the different phase of my project work.

Abstract In this project, proton exchange membrane (PEM) fuel cell is being investigated as an alternative power source for various applications like hybrid electric vehicles propulsion and power supplies. A novel circuit model for a PEM fuel cell is used to design and analyze the static characteristics though simulations using software namely, PSIM demo version circuit simulator. Software is used to emulate and understand the key characteristics and performance parameters of a fuel cell through circuit simulations. Due to the unavailability of a commercial fuel cell module to carry out experiments and to comprehend its actual performance behavior, this project takes the view of developing and modeling an electrical equivalent circuit of a fuel cell. The electrical circuit model is segregated into three stages. For the first stage, the PSIM-based model uses bipolar junction transistors (BJTs), diode, resistors (R), capacitors (C) and Inductor (L) elements available in the PSIM software library with some modifications. The model includes the phenomena like activation polarization, ohmic polarization, and mass transport effect present in a PEM fuel cell. The circuit fuel cell model presented should display simulated results for polarization curve (Voltage-Current) in close resemblance to the experimental results of a commercial fuel cell. In theory, this project is intended to boost the low DC voltage using a DC-DC boost converter. Hence, it was appropriate a suitable topology was chosen to design and simulate a 25W DC-DC boost converter. This represents the second stage of the model. As for the 3rd stage, a converter control circuit based on a voltage divider, comparator and a PWM generator is developed to control the duty cycle of the semiconductor device so as to extract maximum power and reduce voltage loss from the fuel cells. This paper mostly looks at the design derivation and implementation of an appropriate electrical equivalent fuel cell circuit model, coupled with simulations under various resistive loads. Comparisons are drawn between the actual behavior of the fuel cells and the simulated results. Finally, this paper discusses the development stages and the results obtained in simulating the circuit model, before putting forward some suggestions for future use and then drawing a conclusion.

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TABLE OF CONTENTS

ABSTRACT i

LIST OF FIGURES iv LIST OF TABLES vi ACRONYMS AND MEANINGS vii

1. INTRODUCTION 11

1.1 Background 11 1.2 Project Objectives 13 1.3 Proposed Approach and Methodology 14 1.4 Scope of Project 16 1.4.1 Project tasks 16 1.4.2 Assumptions and Limitations of Project 17 1.4.3 Project Management 18

2. FUEL CELL BACKGROUND 20

2.1 Fuel Cells Overview 20 2.2 Classification of Fuel Cells 21 2.2.1 Proton exchange membrane fuel cells (PEMFC) 23

3. PEM PSIM FUEL CELL MODEL 25

3.1 Polarization Characteristics of a PEM Fuel Cell 25 3.1.1 Activation polarization 28 3.1.2 Ohmic polarization 29 3.1.3 Concentration polarization (mass transportation losses) 29 3.2 Analogy of Equivalent Circuit Model of a PEM Fuel Cell 30 3.2.1 Proposed Equivalent Circuit Model of a Fuel Cell -1 31 3.2.2 Proposed Equivalent Circuit Model of a Fuel Cell -2 33 3.3 DC-DC Power Converters 36 3.3.1 DC-DC Converter Circuit Topologies 37 3.3.2 Analogy of DC-DC boost Converter 39 3.4 DC-DC control circuit using Pulse Width Modulation (PWM) 42 3.5 My Direction for PEMFC Circuit Model 44

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4. DESIGN CONSIDERATIONS FOR PEM FUEL CELLS 45

4.1 Design Considerations for 1 Stage of PEM Fuel Cellsst 45 4.2 Design Considerations for 2 Stage of DC-DC boost converter 48 nd

4.3 Design Considerations for 3 Stage rd converter control circuit 51

5. RESULTS AND FINDINGS 53

5.1 PEMFC circuit model: 1 Stage simulation and resultsst 53 5.2 PEMFC circuit model: 2 Stage simulation and results 57 nd

5.3 PEMFC circuit model: 3 Stage simulation and results 62 rd

6. DISCUSSION 67

6.1 Overall Design and Simulated Results 67

7. CONCLUSION 70

8. CRITICAL REVIEW AND REFLECTIONS 71 9. SUGGESTIONS FOR FUTURE WORK 73 References 74 Appendixes

Appendix A Appendix B Appendix C

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LIST OF FIGURES Figure Page 1-1 Block Diagram of a Fuel Cell……………………………………………………..12

1-2 Percentage of time spent on various project tasks………………………………...18

1-3 ‘DC-DC boost converter for fuel cell application’ Project Gantt Chart…………..19

2-1 Basic PEM fuel cell mechanism………………………………………………….20

2-2 Proton Exchange Membrane Fuel Cell…………………………………………...24

3-1 V-I characteristics of a single PEM fuel cell……………………………………..27

3-2 Terminal voltage drop of a single PEM fuel cell………………………………....27

3-3 Proposed equivalent circuit model of the PEM fuel cell – 1……………………..31

3-4 Proposed equivalent circuit model of the PEM fuel cell – 2……………………..33

3-5 General block diagram of converter module……………………………………..36

3-6 Basic DC-DC converters topology and their dc conversion ratio M(D) = Vin

V….39

3-7 Boost Converter Circuit…………………………………………………………..39

3-8 Two states of a boost converter, depending on the state of switch, S………….....40

3-9 Voltage and Current waveforms (Boost Converter)……………………………...41

3-10 Analogue PWM generator using a comparator…………………………………...42

3-11 PWM generation using a sawtooth waveform……………………………………43

4-1 1st stage design for PEM fuel cell module………………………………………..45

4-2 Calculation of R1 and R2 from illustrative V-I Curve…………………………...46

4-3 2nd stage design for PEM fuel cell module……………………………………….48

4-4 Duty cycle and frequency of square wave voltage source……………………….49

4-5 3rd stage design for PEM fuel cell module……………………………………….51

5-1 Equivalent circuit of the PEM fuel cell module – 1st stage……………………....53

5-2 Steady state output voltage under various resistive loads………………………..55

5-3 Polarization Curve obtained from the simulated results…………………………56

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Figure Page 5-4 Equivalent circuit of the PEM fuel cell module with DC booster circuit………..57

5-5 Plot of DC-DC booster curve for duty cycle = 0.85 and duty cycle = 0.83……...59

5-6 Output of the fuel cell and booster converter combined at reduced load………...60

5-7 Output of the fuel cell and booster converter combined at peak load……….........61

5-8 Equivalent circuit of the PEM fuel cell module – 3rd stage……………………....62

5-9 Controller circuit output waveform and control signal for RL = 50Ω……………64

5-10 Controller circuit output waveform and control signal for RL = 200Ω…………..65

5-11 Plot of controller curve based on simulated results from Table 5-3……………...66

6-1 Schematic diagram of the entire PEM fuel cell model……………………………67

6-2 Sketch of simulated V-I curve…………………………………………………….68

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LIST OF TABLES

Table Page 2-1 Properties of Main types of Fuel Cells…………………………………………...22

2-2 Summary of chemical reactions in different types of fuel cells………………......23

5-1 Simulated results of Voltage Vs Current for the 1st stage……………………......54

5-2 Simulated results of Booster Output Voltage Vs Load Resistance………………58

5-3 Simulated results for the converter control circuit……………………………….63

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ACRONYMS AND MEANINGS Acronyms

FC Fuel Cells

PEMFC Proton exchange membrane fuel cells

AFC Alkaline Fuel Cell

PAFC Phosphoric Acid Fuel Cell

MCFC Molten Carbonate Fuel Cell

SOFC Solid Oxide Fuel Cell

DC Direct Current

AC Alternating Current

IEC International Electrotechnical Commission

RF Ripple Factor

PI Proportional Integral

PSIM Simulation package specifically designed for power electronics

BJT Bipolar Junction Transistors

MOSFET Metal–oxide–semiconductor field-effect transistor

IGBT Insulated Gate Bipolar Transistor

PWM Pulse-width modulation

IEEE Institute of Electrical and Electronics Engineers

k Boltzmann’s Constant : 1.3807 X 10-23 JK-1 = 8.6174 X 10-5 eVK-1

T Absolute Temperature (Kelvins) at 300K (27oC)

q Electron Charge : 1.60218 X 10-19 C

1 Atm pressure 1.013 X 105 Pa

R Gas constant : 8.3145 JK -1 mol -1

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CHAPTER 1

INTRODUCTION

1.1 Background

Relative environmental reasons such as high emission of carbon monoxide and pollution

from the use of fossil fuels have been creating along the years, a steadily increasing

demand for new conversion technology and system of non-pollutants energy generation.

Within this context, fuel cells have been showing up as a highly promising alternative to

the present fossil fuel system Fuel cells demonstrate high efficiency, zero emission or

pollution to the environment, modularity , superior reliability and durability in

automotive, electronics and space applications. With an ever increasing oil price, currently

at US$80 per barrel, fuel cells offer low cost substitute to generate electricity.

The proton exchange membrane fuel cell (PEMFC) has been considered as a promising

kind of fuel cell during the last 10 years because of its low working temperature,

compactness, ease and safe operational modes. There is a need to model the PEMFC for

optimizing its performance and also for developing fuel cell power converters for various

applications. Previously, almost all the models proposed for the PEMFC consist of

mathematical equations and are not of much use in power converter/system simulation and

analysis [1.1]. Other Models of PEMFC use Matlab-Simulink and PSPICE software, but

they are still mathematical in nature. The models include several chemical phenomena such

as physical variables like pressure and hydrogen input present in the fuel cell and hence are

complex in computation.

This paper presents a novel circuit model for a PEMFC using solely the PSIM software

simulation tool, which is simple and at the same time includes all the important

characteristics of a fuel cell stack. The equations governing the different polarization

effects in a fuel cell are related to the equations of the electrical circuit elements making

up the model. Simulation was done to determine whether the fuel cells agree with the

polarization curve (V-I)

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Since the fuel cell circuit model produces DC voltage and at thermodynamic equilibrium

(zero current) has a typical cell voltage of 1.23V, it has to be boosted for most

applications. Its success depends on the correct power converter (buck, boost, buck-boost)

required to boost up the low voltage to the required 25V architecture. In this project, the

typical fuel cell power conditioner employs switch-mode DC-DC boost converters.

Important variables for the design of the converters are (a) variation of fuel cell terminal

voltage from no-load to full load; (b) range of current where the output voltage stays

constant [1.2].

In view of this, modeling the electrical equivalent circuit for a fuel cell after the power

conditioner stage could be daunting because maximum power must be achieved while

keeping losses to a minimum. An efficient fuel cell prototype model is one that reduces

the undesirable effects caused by electromagnetic interference and switching losses. This

would be achievable by adopting a methodology of using a small number of switching

components such as Insulated Gate Bipolar Transistor (IGBT) or MOSFET to reduce

power loss. Hence, modeling the power converters after the fuel cell stage represents the

next stage of the circuit design in this project. The construction of the fuel cell system is

shown in figure 1-1.

. Vin

Fuel Cell Stack

Vstage1

Power Conditioners

(DC-DC boost) Vstage2

2nd Stage 1st Stage

Figure 1-1: Block diagram of a Fuel Cell Understanding the fuel cell basics, characteristics, behavior, performance parameters and

finally designing, simulating the circuit model in compliance with IEC standard using

PSIM software makes this project important and worthwhile.

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1.2 Project Objectives

The utilization of fuel cells for the distributed power generation for hybrid electric

vehicles propulsion requires the development and modeling of an electrical equivalent

circuit of a fuel cell using software namely, PSIM circuit simulation. The key

characteristics and performance parameters of a fuel cell are analyzed through circuit

simulation.

Secondly, a topology is chosen to calculate, design and simulate a 25W DC-DC boost

converter with the least numbers of switching components so as to minimize switching

losses. This is done to reduce voltage loss and to minimize power loss from the fuel cells.

This stage of design is based on a DC/DC boost converter to enhance the 5V available

from a fuel cell to a usable 25V using only one MOSFET switch. The output voltage must

comply with the International Electrotechnical Commission (IEC) standard of less than

5% ripple factor (RF), and should be able to supply a continuous current of 1A, at 25V

with a maximum allowable +/- 5% deviation.

Thirdly, a converter control circuit is developed so as to control and stabilize the output

voltage from the fuel cells. It would be done using a voltage divider, comparator and a

PWM generator. The PWM generator would adjust the MOSFET conduction period. All

the components are connected in PSIM circuit simulator by combining the different stages

of design. Simulation is carried out to determine whether the fuel cells agree with the

polarization curve (V-I). Next, analysis is done and then the converter prototype is fine

tuned to meet the IEC standard on voltage and current distortions/harmonics. The results

are then tabulated and compared with the experimental results of a commercial fuel cell

stack

Finally, if time permits, do a comparative study and analysis on existing fuel cell based

converter.

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1.3 Proposed Approach and Methodology

Fuel cell technology is a comprehensive and yet a relatively new subject field which I had

to undertake in order to fulfill the objectives of this project. Although I had prior

knowledge and have worked with PSPICE/MATlab software emulator, there is a need to

understand and work with PSIM software tool to model, design and simulate the electrical

equivalent circuit for a fuel cell and a DC-DC boost converter with control circuitry to

extract maximum power. The project design is split into 3 stages of development.

To achieve the objective and to execute the first stage of development , firstly I need to

research and comprehend the key underlying principles of the electro-chemical properties

of fuel cells and their behavior under normal working conditions. Understanding the

performance parameters, such as voltage, current, temperature of a fuel cell and then

relating into an electrical equivalent circuit model is a key criterion.

Although there are numerous types of fuel cells available in the commercial market, my

study will greatly deal and focus on Proton Exchange Membrane (PEM) fuel cell behavior

and characteristics. The transient response of a typical fuel cell to voltage and current

(V-I) characteristics based on the polarization curve must be understood in order to

achieve maximum power. The source of this research primarily comes from Internet

websites, IEEE research journals and fuel cells articles/magazines. Websites such as

http://en.wikipedia.org/wiki/Fuel_cell

http://www.answers.com/topic/fuel-cell

http://www.fuelcelltoday.com/

http://www.howstuffworks.com/fuel-cell.htm/printable

http://www.hokuscientific.com/fcbasics.htm

is one of the many avenues that provide useful information for this project. After gaining

some insights on the key performance parameters of fuel cells, I need to model the

electrical equivalent circuit in PSIM software tool. The criteria and targets set for this

project are shown in detail in Appendix A.

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The PSIM software demo version 7.1 used for design and simulation purposes in this

project is downloaded from http://www.powersimtech.com/ .Hours of self practice in

PSIM software provided me the fundamental knowledge in dealing with circuit design,

simulation and analysis. Ultimately, getting the desired output voltage (1.0 to 5V) with

minimal RF (+/- 5%) from the fuel cell circuit design would be the prime target.

After completing the first stage of development in designing the fuel cell circuitry,

information was sourced on methods to boost the low DC voltage from the fuel cells to a

higher rating. , Preferably, the next step was to boost up the voltage to 25V with a

deviation of +/- 5% and to supply a continuous current of 1A using the DC-DC boost

converter circuit. This circuit is a continuation of the fuel cell circuitry. Here the emphasis

is placed on choosing a suitable topology to design a DC-DC boost converter (chopper).

Existing topology such as buck, boost and buck-boost converters are studied as references.

This completes the 2nd stage of development.

To minimize the fuel cell voltage loss caused by switching components such as

transistors, linear switches and etc, the DC-DC boost converter design would only consist

of one MOSFET switch due to their high switching speed.. Websites like

http://en.wikipedia.org/wiki/Boost_converter

http://www.discovercircuits.com/

http://www.powerdesigners.com/InfoWeb/design_center/articles/DC-DC/converter.shtm

provide general circuit designs on DC-DC boost converters.

The average desired output voltage is controlled by varying the conduction time of the

MOSFET. This closed loop control is essential to keep the output voltage constant and to

extract maximum power from the fuel cells. This concludes the final development stage.

Henceforth, my main approach is to re-define, modify and choose a suitable circuit design

to realize my project objectives. IEEE Journals, research books and magazines are some

of the information wealth which I had to delve into in order to complete this project.

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1.4 Scope of Project

This project will require proper planning and execution from the start till the end. There

are four main types of activity to be undertaken. They are

Preparing, brainstorming and planning.

Exploring and researching on PEM fuel cells and their key performance

characteristics.

Designing, executing and simulating the circuit model in PSIM software emulator.

Writing the technical report, reflecting and reviewing it constantly.

Most preparation and planning would be done at the start of the project term followed by

collating and execution at around mid-year. Once the circuit model is finalized, designing

it in PSIM and experimenting with the simulated results would be next logical step.

Finally reflection, fine adjustment to the circuit design and reviewing are done towards the

end of project semester. However, setting my own criteria, redefining my targets,

identifying my weak areas for improvements and constantly reviewing my progress so as

to judge how well I am doing are activities which are done throughout the project. For this

project, I have segregated the tasks and workflow as shown below.

1.4.1 Project Tasks

Task 1 - Research on fuel cell basics and their operating behavior.

Task 2 – Examine on different types of fuels with particular emphasis on PEM fuel cells.

Task 3 – Download the PSIM software tool and have hands-on practice session with it.

Task 4 – Study the key performance characteristics for PEM fuel cells.

Task 5 – Finish writing up the initial report.

Task 6 - Relate the characteristics into a simple electrical equivalent circuit model.

Task 7 – Design and execute the circuit model in PSIM.

Task 8 – Simulate the model in PSIM in compliance with the polarization curve (V-I).

Task 9 – Explore on DC-DC boost converter circuit and converter control circuit.

Task 10 – Design the second and third stages of the circuit model and simulate it

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Task 11 – Document the experimental results obtained, fine tune the circuitry in

compliance with IEC standards. Re-verify results based on fuel cells behavior.

Task 12 – Write up the drafted final year report. Re-edit them when necessary and include

the technical aspects of the circuit design.

Task 13 - Complete the final report and submit them.

Task 14 – Prepare an A1 size poster to explain the project summary and prepare for oral

presentation. Detail explanation on this is shown in Appendix B.

1.4.2 Assumptions and Limitations of Project

Software version: PSIM software emulator used in this project is version 7.1.2 Since it is

a demo version, it possesses certain limitations. For the simulation control, the maximum

time step allowed is about 0.001 seconds. Further reduction in the time step would

truncate the valuable graphical data. Furthermore, in order to generate the input square

wave for the MOSFET, the frequency of the conduction period has to be kept in hertz

range rather than kilohertz range so that the full graphical representation of the output can

be retrieved. Henceforth, the mathematically computed values for frequency, inductors or

capacitors may not always tally with the actual values when simulating the circuit model.

Fuel cell stacks: To accurately depict the performance characteristics, a lower voltage

(1 to 5V) fuel cell (with fewer cells stacked in series) becomes the optimal configuration

to design the fuel cells under 25W. Stacking many cells in series add to the complexity of

the system. Since this project is aimed at deriving and simulating the basic polarization

curve, such complex circuitry is therefore not essential. In addition, due to the available

lower output voltage, coupled with no-load to full-load variation of the fuel cell terminal

voltage, a DC-DC boost converter was therefore an ideal choice for such systems.

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1.4.3 Project Management

The project is embarked on by one part-time student. In order to efficiently and effectively

complete the project, it is sensible to assess the available resources and then dedicate the

workload systematically over the whole year. It is also prudent to get the necessary

research materials and sequentially execute the circuit design in the early stages to achieve

the desired results within the specified time frame.

Since the project is solely software based circuit implementation, greater share of planning

and project time went into this. In addition, a lot of research time is spent in getting a good

electrical equivalent fuel cell model as a head-start so that the 2nd and 3rd stage circuit

design can be consequentially introduced without any further adjustments in the later

stages of the project term. Figure 1-2 (pie-chart) showcases the percentage of time spent

on different elements of project tasks. Likewise, a good technical report is essential to

summarize the entire project. The Gantt chart in Figure 1-3 shows the planning and

scheduling for this project.

9%

18%17%

9%

13%

4%13%

17%

Brainstorming

Planning/Research

PEMFC Circuitmodel and designFirst stagesimulationDC-DC converter

Coverter controlcircuitCombined circuitsimulationTechnical Report

Figure 1-2: Percentage of time spent on various project tasks

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Figure 1-3: ‘DC-DC boost converter for fuel cell application’ Project Gantt Chart

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CHAPTER 2

FUEL CELL BACKGROUND

2.1 Fuel Cells Overview

Fuel cells are devices that utilize an electrochemical process to convert a fuel into

electrical energy. Fuel cells are intrinsically more efficient than most other energy-

conversion devices. It uses hydrogen as a fuel to produce electrons, protons, heat and

water. Fuel cell technology is based upon the simple combustion reaction given below.

2H2 + O2 →2H2O + heat + electricity

Every fuel cell has two electrodes, one positive and one negative, called respectively, the

cathode and anode. The reactions that produce electricity take place at the electrodes.

Hydrogen fuel is supplied to the anode (negative terminal) of the fuel cell while oxygen is

supplied to the cathode (positive terminal) of the fuel cell. Through a chemical reaction, the

hydrogen is split into electrons (e-) and protons (H+) [2.1]. The oxygen is consumed with the

protons and electrons and the by- products, liquid water (H2O), is produced with residual

heat in the surface of the catalytic particles. The electrons, which cannot pass through the

membrane, flow from the anode to the cathode via the external electric circuit, delivering

power to the electric circuit in the process. The process is shown in Figure 2-1.

Figure 2-1: Basic PEM fuel cell mechanism

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Unlike a battery which stores energy, a fuel cell converts fuel into electrical energy

without the use of stored materials within its structure. Fuel cells can operate virtually

continuously as long as the necessary flows are maintained. Fuel cell system is capable of

generating electricity using hydrogen extracted from a variety of sources like natural gas,

ethanol, methanol, coal, and even gasoline. Capable of operating on multiple fuels, fuel

cell systems can be deployed to operate in parallel with the grid, as independent energy

sources, or to complement solar and wind generating systems. With little or no pollution,

and greater flexibility in installation and operation, they will offer commercially viable

alternatives to existing power supplies [2.2]. The main drawback of fuel cells is their

extremely high cost. Their production cost has to be significantly reduced to become

commercially comparable with the conventional power plants.

Fuel cells are very useful as power sources for a wide variety of applications. They

include, base load power plants, electric and hybrid vehicles, Auxiliary power and off-grid

power supply.

2.2 Classification of Fuel Cells

The classification of the fuel cells is generally done on the basis of electrolyte being used.

The operating characteristics, constituent materials and fabrication techniques of Fuel

cells are significantly different. They are broadly classified into five main types: Alkaline

FC (AFC), Proton Exchange Membrane FC (PEMFC), Phosphoric Acid FC (PAFC),

Molten Carbonate FC (MCFC) and Solid Oxide FC (SOFC). In spite of their different

materials and operating temperatures of each type, Fuel Cells have the same basic

principles of operation. Due to the differences in materials and operating characteristics,

each type is suited for specific applications. An overview of the major design is illustrated

in Table 2-1. It summarizes the operating temperatures, utilized charge carrier and

catalyst, system efficiency level and electrolyte materials used [2.3].

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Table 2-1: Properties of the main types of fuel cells

The principle of operation of fuel cells is based on the reaction of hydrogen gas (H2),

which is supplied at the anode, and oxygen gas (O2), which is supplied at the cathode.

The process is attributed to the movement of charged particles towards regions of lower

electrochemical energy. It is essential to separate electrons from protons and to regulate

the movement of electrons via an external path to generate electricity.

Actual reaction occurs in two steps: the oxidation reaction at the anode and the reduction

reaction at the cathode. The oxidation reaction is the dissociation of hydrogen atoms into

protons and electrons. The reduction reaction occurs when the oxygen atoms dissociate

and bond with the protons coming through membrane and the electrons from the external

circuit forming water. The reactions of Alkaline (AFC), Proton Exchange Membrane

(PEMFC), Phosphoric Acid (PAFC), Molten Carbonate (MCFC) and Solid Oxide (SOFC)

types are summarized in Table 2-2.

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Table 2-2: Summary of chemical reactions in different types of fuel cells

2.2.1 Proton exchange membrane fuel cells (PEMFC)

Polymer electrolyte membrane (PEM) fuel cells - also called proton exchange membrane

fuel cells - deliver high power density and offer the advantages of low weight and volume.

The proton exchange membrane fuel cell (PEMFC) has been considered as a promising kind

of fuel cell during the last decade because of its low working temperature, compactness,

ease and safe operational modes

The Proton Exchange Membrane (PEM) fuel cell uses a thin, permeable polymeric

membrane as the electrolyte. The membrane is very small and light and in order to

catalyze the reaction, platinum electrodes are used on either side of the membrane. They

need only hydrogen, oxygen from the air, and water to operate and do not require

corrosive fluids like some fuel cells. They are typically fueled with pure hydrogen

supplied from storage tanks or onboard reformers as shown in figure 2-2.

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Compared to other electrolytes (which require temperatures up to 1,000°C to operate

effectively) PEMFCs operate at very low temperatures of about 80°C allowing rapid

start-up. The efficiency of a PEM unit usually reaches between 40 to 60 per cent and

the output of the system can be varied to meet shifting demand patterns. Typical

electric power is up to 250 kW. As a result of these characteristics, PEM units tend to be

the best candidates for hybrid cars, buildings and smaller stationary applications.

As the electrolyte is a solid rather than a liquid, the sealing of the anode and cathode

gases is far easier and this in turn makes the unit cheaper to manufacture than some

other types of fuel cell. Furthermore, the solid electrolyte can lead to a longer cell

and stack life [2.4].

My report analyses and focuses a great deal on the operating characteristics of a PEM fuel

cells due to its ease of use. It is being investigated as an alternative source of power for

automotive applications like hybrid vehicles. It is difficult to set up a high wattage PEM fuel

cell prototype in the lab environment. Henceforth, a method to understand and emulate the

fuel cell electrical characteristics based on its polarization curve must be studied as

reference. This methodology is then used to develop fuel cell power converters to optimize

its performance. Chapter 3 explores in greater detail about the fuel cell characteristics.

Figure 2-2: Proton Exchange Membrane Fuel Cell

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CHAPTER 3

PEM PSIM FUEL CELL MODEL

3.1 Polarization Characteristics of a PEM Fuel Cell

Low temperature fuel cells like PEM fuel cell require noble metal electro-catalysts to

achieve practical reaction rates at the anode and cathode, and hydrogen is the only

acceptable fuel. The ideal performance of a fuel cell is defined by its Nernst potential. The

stack output voltage at no load conditions can be calculated as follows [3.1].

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O2H20

H2T0

Pln2F

RT

P

Pln

2F

RTVV

(3-1)

: Ideal standard potential

T

R

F

VT

: Faraday’s constant = 96485.35°C mol -1

: Avogadro’s gas constant = 8.3145 JK-1mol -1

: Absolute Temperature

The Nernst equation provides a relationship between the ideal standard potential (VT) and

the ideal equilibrium potential (V0) at other temperatures (T) and partial pressures of

reactants and products (Px). The ideal cell potential at a given temperature can be

increased by operating at higher reactant pressures, and improvements in a fuel cell have

in fact been observed at higher pressures. The ideal standard potential of an H2 / O2 fuel

cell is 1.229V with liquid water product

The electrolyte resistance is a particularly important measure of single fuel cell (or fuel

cell stack) electrical performance since it quantifies internal cell losses. The variation of

the individual cell voltage is found from the maximum cell voltage (or EMF) and the

various voltages drop (losses). Multiple factors contribute to the irreversible losses

(voltage drop) in an actual fuel cell that cause the cell voltage to be less than its ideal

potential.

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The losses, which are also called polarization, originate primarily from three sources: (a)

activation polarization, (b) ohmic polarization, and (c) concentration (mass transport)

polarization. Each of these is associated with a voltage drop and is dominant in a

particular region of current density (low, medium, or high).

The V–I characteristic (polarization curve) of a typical single cell operating at room

temperature and normal air pressure is shown in figure 3-1. It shows the different regions

and the corresponding polarization effects. The theoretical EMF of a cell at zero current

and 80°C and 1 atmosphere gas pressure is V0 = 1.16V. As the current density increases,

the actual voltage at the electrical terminal drops. The terminal voltage (V) can be

calculated by:

n)RR(RIVV O2H2Ω0 (3-2)

where:

I, n : the stack current and the number of cells in series respectively

22 ,, OH RRR : Voltage drop due to internal resistance, anode and cathode reaction

The effect of resistive voltage drop as well as the cathode and anode over-potentials is

illustrated in figure 3-2 with respect to the equation (3-2).

The efficiency of a fuel cell is dependent on the amount of power drawn from it. Drawing

more power means drawing more current, this increases the losses in the fuel cell. As a

general rule, the more power (current) had 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. A typical cell running at 0.7 V has an efficiency of about 60%,

meaning that 60% of the energy content of the hydrogen is converted into electrical

energy; the remaining 40% will be converted into heat [3.2]

The fuel cell efficiency is given by

00 V

V

IV

IVη

(3-3)

and is simply the ratio of the terminal voltage (V) to the theoretical EMF (V0) for the cell.

26

Figure 3-1: V-I characteristics of a single PEM fuel cell

Figure 3-2: Terminal Voltage drop of a single PEM fuel cell

27

3.1.1 Activation polarization The activation polarization loss (dominant at low current density) is present due to initial

dramatic voltage losses in low temperature fuel cells. At this point, electronic barriers

have to overcome prior to current and ion flow. Activation losses show some increase as

current increases. These losses are basically representative of a loss of overall voltage at

the expense of forcing the reaction to completion, which is forcing the hydrogen to split

into electrons and protons, and for the protons to travel though the electrolyte, and then

combine with the oxygen and returning electrons. This loss is often termed over potential,

and is essentially the voltage difference between the two terminals. Through

experimentation, Tafel equation relates the rate of an electrochemical reaction to the over

potential [3.3]. On a single electrode the Tafel equation can be stated as

V = A ln

0i

i (3-4)

Where A is the constant, V is the over-voltage, i is the current density and i0 is the current

density at which voltage begins to drop.

For determining the values of A in the experimental Tafel equation, work has been done

to show what a theoretical value would be. The following equation has been simplified to

apply specifically to a hydrogen FC.

A = F2α

RT (3-5)

In this equation, R is the ideal gas constant, T is the temperature in Kelvin and F is the

Faraday’s constant. The value α is known as the charge transfer coefficient and its value

differs from one material to another. For typically used materials, the value is in a very

narrow range; it is always about α = 0.5 for the electrode, and it ranges from about α =0.1

to α = 0.5 for the cathode. These minor variations make experimenting with different

material to dramatically change the voltage predicted by the Tafel prediction [3.4].

28

3.1.2 Ohmic polarization Ohmic losses occur because of resistance to the flow of ions in the electrolyte and

resistance to flow of electrons through the electrode materials. The dominant ohmic

losses, through the electrolyte, are reduced by decreasing the electrode separation and

enhancing the ionic conductivity of the electrolyte. The loss (voltage drop) in the fuel cell

is approximately linear in this region. Since both the electrolyte and fuel cell electrodes

obey Ohm’s law, the ohmic losses can be expressed by the equation

Vohmic-loss = IR (3-6)

where I is the current density and R is the total cell resistance which includes electronic,

ionic and contact resistance [3.5].

3.1.3 Concentration polarization (mass transportation losses) As a reactant is consumed at the electrode by electrochemical reaction, there is a loss of

potential due to the inability of the surrounding material to maintain the initial

concentration of the bulk fluid. That is, a concentration gradient is formed. At practical

current densities, slow transport of reactants/products to/from the electrochemical reaction

site is a major contributor to concentration polarization.

The concentration polarization relates to the change in the concentration of the reactants at

the surface of the electrodes as the fuel (hydrogen) is used. If the hydrogen is being used

at a very vigorous rate at the anode then the partial pressure of the hydrogen drops, thus

slowing the reaction rate. This is also the same case that occurs at the cathode with

oxygen. The loss becomes significant at higher currents when the fuel and oxidant are

used at higher rates and the concentration in the gas channel is at a minimum. In general,

the mass transportation (transfer) losses are given by

V = Vs

’ - mIm (3-7) mIne

29

where Im = I−I1, Vs’ and I1 are the coordinates of the point where the V–I characteristic

graph starts to deviate from being linear (start of mass transport action), m and n are the

mass transfer parameters. While I1 is the limiting current at which the fuel is used up at a

rate equal to its maximum supply rate,

Understanding the polarization curve is paramount in designing the PEMFC equivalent

circuit model. The designed model has to accurately depict the different operating regions

of the V-I curve by taking into account the 3 source of losses as mentioned above.

3.2 Analogy of Equivalent Circuit Model of a PEM Fuel Cell

The starting point to analyze the behavior of the fuel cell stack is to obtain an electrical

equivalent circuit model. This model has to provide an accurate steady state response

under thermal equilibrium condition of 300K (room temperature). The model should

include all the important characteristics of a fuel cell stack. The equations governing the

different polarization effects in a fuel cell should preferably be related to the equations of

the electrical circuit elements making up the model in PSIM software emulator.

Over the years, many equivalent Proton Exchange Membrane Fuel Cell Stack (PEMFCS)

circuit models have been proposed which vary on complexity and accuracy. However,

these modeling methods require the impact of variables, such as temperature, pressure and

gas constituents, to assess the fuel cell performance [3.6]. These design parameters, are

not easy to obtain due to proprietary nature of the technology. Furthermore, most of the

models include complex chemical equations, which are not easy to solve, and are not

suitable for observing the electrical phenomena occurring when the fuel cell interacts with

its power conditioning unit (PCU).

In the upcoming section, we shall look at 2 existing models which were proposed in the

past to closely resemble and predict the electrical performance, voltage regulation and

ripple current of a fuel cell.

30

3.2.1 Proposed Equivalent Circuit Model of a Fuel Cell -1

Figure 3-3 shows the proposed equivalent circuit model of a PEMFC module.

Figure 3-3: Proposed equivalent circuit model of the PEM fuel cell -1

The performance of the fuel cell was evaluated using a simplified FC equivalent circuit

model using first-order discrete passive network elements. Even though Fuel Cells are

highly non-linear systems, a linear electrical equivalent circuit represents approximately

the FCs electrical behavior when operated with small signal at a quiescent operating point.

In Fig. 3-3, the ideal voltage represented by DC corresponds to Nerds equation, which has

a theoretical predicted value of 1.23V per cell.

Several effects cause a voltage drop or irreversibility, namely, activation losses (Tafel

equation), fuel crossover and internal currents, ohmic losses, and mass transport. The

voltage drops are represented by resistors Ra and Rohmic. A general equation 3-8 provides a

conceptual description of all the effects combined.

Vout = DC - ∆Vohmic – (∆Va + ∆Vt) (3-8)

31

The second term ∆Vohmic represents the voltage drop due to the electrodes resistance

(electrons) and membrane conduction properties (properties). It follows the ohmic

polarization characterization of the V-I curve.

The third term ∆Va corresponds to the activation of the anode and cathode. Finally ∆Vt is

due to the change of the concentration of the reactants in the region of the electrodes

(mass transport loss). In addition, an important element known as “charge double layer”

closely relates to the electrical dynamic behavior of the fuel cell. An equivalent capacitor

C was found suitable to represent this phenomenon [3.7]

Due to the diffusion effects and the reaction between the electrons (in the electrodes) and

the ions (in the electrolyte), there is a charge double-layer around the cathode in a fuel

cell. The layer of charge on or near the electrode interface behaves like an electrical

capacitor. The result is that if the current suddenly changes, the operating voltage takes

some time to arrive at its final equilibrium value. Thus, it is quite reasonable to use a

capacitor to model the capacitance effect resulting from the charge double layer [3.8].

32

3.2.2 Proposed Equivalent Circuit Model of a Fuel Cell -2

Figure 3-4 shows the second proposed circuit model of a commercial PEMFC module.

The complete model is developed by modeling the different operating regions using PSIM

simulation library

IB2

Figure 3-4: Proposed equivalent circuit model of the PEM fuel cell -2

1. Activation Polarization: A diode, D1 is used to model the activation polarization of

the fuel cell. In a semiconductor diode, the depletion region provides a potential barrier to

inhibit the migration of carriers across the junction, which is similar to the electrode

kinetics of the fuel cell. Similarity can be seen by comparing equation 3-4 and the diode

equation 3-9.

VD = nVTln( IsI D ) , VT =

q

kT (3-9)

where VD is the voltage across the diode and ID is the current through the diode, n is an

empirical constant between 1 and 2. IS is the reverse saturation current, k is Boltzmann’s

constant, T is the absolute temperature, q is the electronic charge. Thus it is reasonable to

use a diode to model the activation polarization in a fuel cell.

33

In the simulation package, P-SPICE, the diode model can be adjusted to match the V-I

characteristics of a fuel cell stack by choosing suitable values for the following

parameters: Is (saturation current), Rs (parasitic resistance) and N (emission coefficient).

Previous work has shown that Is = 0.02A, N =80 do a good job of modeling the activation

polarization.

2. Ohmic Polarization: A resistor is used to model the ohmic polarization. The “parasitic

resistance” (Rs) in the diode is used in the model instead of using a separate resistor.

3. Concentration polarization: To model the concentration or mass transport over

voltage, a “current-limiting circuit” is used. The current-limiting circuit is composed

of two BJTs Q1 and Q2 and the current-sensing resistor R2. When the current through R2

exceeds a set limit, Q2 starts conducting and reduces the base voltage of Q1. As a result,

the emitter voltage of Q1 (output voltage) will decrease at an exponential rate. Transistors

Q1 and Q2 are assumed to be identical with current gain β and base-emitter voltage VBE

The following equations can be written:

IB1 = β1

II0

2C (3-10)

IC2 ≈ ICSTV0I2R

e (3-11)

2B2C20BEC2B1SO )RII(IVIIRVV (3-12)

Substituting (3-10) and (3-11) in (3-12) and simplifying, we get

TV0I2RVV eCS2120 I

111

1

RRV

RRI BESO (3-13)

34

Assuming β to be large, equation (3-13) can be approximated as

TV0I2R

IRVIRVV eCS1BEo2so (3-14)

Equation 3-14 is of the same type as that of the mass transportation loss described in

equation 3-7. In this model, the design of the fuel system uses a diode and a pair of BJT

transistors to describe the static conditions [3.9].The values of C and LS are selected to

match the transient response of the fuel cell. Capacitor C functions as a ‘charge-double

layer’ and Inductor LS is used to bring the current to a steady state. To get a range of

voltage vs current readings, an array of load resistance, RL values are defined

Generally, the two equivalent circuit models presented are simple in design and

theoretically represent the different operating regions of a fuel cell. More simulation needs

to be done to verify this theory. In my design analogy, careful consideration on the choice

of components has to be made to model a circuit that provides an unregulated steady DC

output voltage of between 3 to 5V. Only then, would I be able to design a DC-DC boost

converter to step up the voltage to the required usable 25 V level.

Following this, the next section looks at some basic topology adopted previously to design

a DC-DC boost converter.

35

3.3 DC-DC Power Converters

DC-DC converter also known as chopper or switching regulator is a device that accepts a

DC input and produces a controlled DC output voltage with a desired voltage level. The

converter produces output voltage V, having a magnitude (and possibly polarity) that

differs from the input voltage. The general block diagram of power converter is shown in

figure 3-5.

Converter Circuit

Figure 3-5: General block diagram of converter module

High efficiency is invariably required, since cooling of inefficient power converters is

difficult and expensive. The ideal dc-dc converter exhibits 100% efficiency; in practice,

efficiencies of 70% to 95% are typically obtained. This is achieved using switched-mode,

or chopper circuits whose elements dissipate negligible power. A switching regulator is a

circuit that uses a power switch, an inductor, and a diode to transfer energy from input to

output.

Switching regulators offer three main advantages compared to linear regulators. Firstly,

switching efficiency can be much better than linear. Secondly, because less energy is lost

in the transfer, smaller components and less thermal management are required. Thirdly,

the energy stored by an inductor in a switching regulator can be transformed to output

voltages that can be greater than the input [3.10].

36

Usually a converter contains at least two semiconductor switches (a diode and a transistor

or power MOSFET). It must have least number of switching components so as to

minimize switching losses. Typical switching frequencies lie in the range 1 kHz to 1

MHz, depending on the speed of the semiconductor devices.

Filters made of inductor and capacitor combinations are often added to a converter’s

output to improve performance. Pulse-width modulation (PWM) allows control and

regulation of the total output voltage. It is designed to interface with the DC converter

circuits in order to fine-tune their output voltages.

To an extent, the inductor and capacitor are ideal; the filter removes the switching

harmonics without dissipation of power. Thus, the converter produces a dc output voltage,

whose magnitude is controllable via the duty cycle D, using circuit elements (transistors

or MOSFET) that (ideally) do not dissipate power.

The conversion ratio M(D) as shown in equation 3-15 is defined as the ratio of the dc

output voltage V to the dc input voltage, Vin under steady-state conditions:

M(D) = inV

V (3-15)

In the next sub-section, we shall look at some of the commonly DC-DC converter

topologies which either increase or decrease the output voltage.

3.3.1 DC-DC Converter Circuit topologies

A large number of dc-dc converter circuits are known that can increase or decrease the

magnitude of the dc voltage and/or invert its polarity [3-11]. Figure 3-6 illustrates several

commonly used dc-dc converter circuits, along with their respective conversion ratios. In

each example, the switch is realized using a power MOSFET and diode; however, other

semiconductor switches such as BJTs, or thyristors can be substituted if desired.

37

The first converter is the buck converter, which reduces the dc voltage and has conversion

ratio M(D) = D. In a similar topology known as the boost converter, the positions of the

switch and inductor are interchanged. This converter produces an output voltage V that is

greater in magnitude than the input voltage Vg. Its conversion ratio is M(D) = 1/(1 – D).

In the buck-boost converter, the switch alternately connects the inductor across the power

input and output voltages. This converter inverts the polarity of the voltage, and can either

increase or decrease the voltage magnitude. The conversion ratio is M(D) = - D/(1 – D).

The Cuk converter contains inductors in series with the converter input and output ports.

The switch network alternately connects a capacitor to the input and output inductors. The

conversion ratio M(D) is identical to that of the buck-boost converter. Hence, this

converter also inverts the voltage polarity, while either increasing or decreasing the

voltage magnitude.

V = D*Vin

V = )1( D

Vin

Diode

L

RL C

MOSFET

38

V = )1( D

D

V = )1( D

D

Fig 3-6: Basic DC-DC converters topology and their dc conversion ratio M(D) = Vin

V

3.3.2 Analogy of DC-DC boost Converter

The schematic in figure 3-7 shows the basic boost converter. This circuit is used when a

higher output voltage than input is required.

Figure 3-7: Boost Converter Circuit

39

The key principle that drives the boost converter is the tendency of an inductor to resist

changes in current. When being charged it acts as a load and absorbs energy (somewhat

like a resistor), when being discharged, it acts as an energy source (somewhat like a

battery). The voltage it produces during the discharge phase is related to the rate of change

of current, and not to the original charging voltage, thus allowing different input and

output voltages.

The basic principle of a Boost converter consists in 2 distinct states (see figure 3-8)

When switch S is closed, diode is reversed. Thus output is isolated. The input

supplies energy to the inductor. Voltage across the inductor is equal to Vd.

When switch is opened, the output stage receives energy from the input as well as

from the inductor. Hence output is large. Output voltage is approximately equal to

(Vd + VL)

VX

VX

ON State

OFF State

Fig 3-8: Two states of a boost converter, depending on the state of the switch, S

40

MOSFET switch is used due to their high switching speed. In addition, the desired output

voltage is controlled by varying the duty cycle (conduction time), D of the MOSFET.

Duty cycle, D is equal to the fraction of time the MOSFET is switched on/off and hence

takes the range of 0≤D≤1. Ton represents the switch ON time and Toff represents the

switch OFF time.

0

In figure 3-8, when the MOSFET switch is ON, Vx =Vd, and in the OFF state the inductor

current flows through the diode giving Vx =Vo. For this analysis it is assumed that the

inductor current is always remains flowing (continuous conduction). The voltage across

the inductor is shown in figure 3-9 and the average must be zero for the average current to

remain in steady state [3-12].

(3-16) )( TVVTV offOdONd

This can be rearranged as

)1(

1

DTT

VV

offd

O

(3-17)

Inductor Voltage

Vd

Vd – Vo

Figure 3-9: Voltage and current waveforms (Boost Converter)

Output voltage is maintained constant by virtue of small Capacitance, C. Boost converter

circuits would predominantly be used in my design process as the second stage circuit

model.

41

3.4 DC-DC Control Circuit using Pulse Width Modulation (PWM)

Pulse width modulation (PWM) of a signal or power source involves the modulation of its

duty cycle, to control the conduction period of semiconductor switches such as MOSFETs

or BJTs. The discrete on/off states of the modulation are used to control the state of the

switches which correspondingly control the voltage across the load. High frequency PWM

control systems are easily realizable with semiconductor switches.

The PWM signals can be generated in a number of ways. There are several methods:

1. Analogue method 2. Digital method 3. Discrete IC 4. Onboard microcontroller

However we shall look at the PWM generation using the analogue method. The simplest

way to generate a PWM signal requires only a sawtooth or a triangle waveform (easily

generated using a simple oscillator) and a comparator. When the value of the reference

signal (triangular wave generator) is more than the controlled waveform (DC level), the

PWM signal is in the high state, otherwise it is in the low state [3.12]. The higher the DC

level, the PWM ON pulses would be smaller. The block diagram of the analogue PWM

generator using a comparator is shown in figure 3.10

DC level

Vref

Comparator

Figure 3-10: Analogue PWM generator using a comparator

42

Likewise, PWM can be controlled using a sawtooth waveform as the input signal to the

comparator as shown in figure 3-11. Due to the difference between the Vo (desired) signal

and Vo (actual) from the DC-DC converter circuit, the Vcontrol signal can be adjusted.

Changing the level of the Vcontrol signal, would allow the conduction period of the

semiconductor switch to vary.

Figure 3-11: PWM generation using a sawtooth waveform

The principle behind generating the PWM generation is to control the duty cycle or the

conduction period of the MOSFET switch used in the DC-DC boost converter. By

controlling the duty cycle, the output voltage is stabilized at 25V regardless of drastic load

resistance change. Hence output voltage after the control circuit is dependent on the duty

cycle, D by virtue of the formula shown in equation (3-17).

43

3.5 My Direction for PEMFC Circuit Model

Based on the studies that were conducted, it is imperative that an effective electrical

equivalent circuit model must accurately depict the polarization curve of the PEM Fuel

cells, since voltage and current are the only two parameters that are being monitored.

Although other circuit models are considered as references, most of them involve dealing

with complex circuitry and equations. The need to monitor gas pressures and temperatures

of other circuit models in order to assess the fuel cell performance makes the design

process rather complicated. Henceforth, after careful analysis, the fuel cell model shown

in figure 3-4 would provide an ideal platform to represent the different polarization

effects.

The low DC voltage from the first stage has to be boosted to usable 25V architecture with

minimum number of switching components so as to reduce high frequency switching

losses and electromagnetic interference. Therefore a DC-DC boost converter circuit as

shown in figure 3-6 was used as the second stage of the model. Since a variable load, RL

is used in my design, the chopper voltage would vary as current drawn would change fo

different resistive loads. Hence, there is a need to specify a range of resistance values with

a tolerance range of +/- 35% to keep the voltage values close to 25V (+/-5% ripple factor).

r

In the final stage, closed loop control is formed between the DC-DC converter circuit and

the control circuit. The output voltage from the DC booster module is fed back around to

control the chopper PWM circuitry. The feedback loop includes a reference voltage,

voltage divider, comparator and a PWM controller to adjust the MOSFET conduction

period in order to stabilize the output voltage at around 25.0V when there is a drastic

change in load resistance, RL values.

44

CHAPTER 4

DESIGN CONSIDERATIONS FOR PEM FUEL CELLS

4.1 Design Considerations for 1st Stage of PEM Fuel Cells

Figure 4-1: 1st Stage design for PEM Fuel Cell Model

From design analysis, R1 and R2 decide the shape of the static characteristics in the

concentration polarization region of the fuel cell. In particular R2 decides the current I ,

which corresponds to the starting point on the concentration polarization region of the V-I

curve as shown in figure 4-2. Resistance R1 decides the rate of change of voltage in the

concentration region for output currents beyond I . The V-I curve is approximated by

straight lines drawn to get points A (I ) and B (I ). The values of R1 and R2 are calculated

as follows [4.1].

1

1

1 2

45

Figure 4-2: Calculation of R1 and R2 from illustrative V-I curve

In the fuel cell model, R2 is a series resistance, which actually decides the current limiting

action. Hence R2 is generally very small. The resistance R1 decides the regenerative

action in the two transistor circuit. The values of the two resistors are obtained from the

experimental data. For commercial PEM fuel cell modules, it is not possible to reach point

B by conducting load test. Over-current protection circuit shuts down operation slightly

beyond A. However, the characteristics can be extrapolated to get point B and the current

I . Based on previous experimental results, the following empirical equation relating I

and R1 is found

2 2

12 R

baI (4-1)

The constants for the PEM module tested are obtained as: a= 5 and b = 8.5

46

For a given value of R1 chosen from equation (4-1), the empirical relationship between I

and R2 is obtained as

1

21 R

cI (4-2)

For the module tested, the constant c is calculated as 5. The input voltage for the fuel cell is kept fixed at 5V. Capacitor, C and Inductor Ls are

used for charging the voltage and bringing the current to a steady state. They are kept

slightly small so that the rate of rise to steady state voltage is fast. To compute the values

of V-I, load resistance values, RL is varied from a small value of 0.1 ohms to 150 ohms,

while keeping resistance R1 and R2 values constant. The different voltage and current

values under various loads are tabulated in order to obtain the polarization curve.

47

4.2 Design Considerations for 2nd Stage of DC-DC boost converter

DC-DC Boost Converter

First Stage Design

Figure 4-3: 2nd Stage design for PEM Fuel Cell Model

A typical circuit consisting of fuel cell module and a boost converter is shown in figure

4-3. The power converter used here is a DC-DC boost converter that converts the DC

voltage output of a fuel cell to a different DC voltage level, often providing a regulated

output. The desired parameters required for the design of the 25V boost converter with

(+/- 5% ripple factor) are

V = 25V out

V = 5V S

Output Voltage ripple = Vout

Vout = Less than 5%

= 0.05

48

The ideal duty cycle for the boost converter;

Duty Cycle, D = Vout

Vs1

= 25

51

= 0.8 However to control the output voltage level at around 25V for various loading conditions,

the duty cycle is approximated from 0.80 to 0.85.The frequency of the square wave

voltage source is taken to be as Fs = 100Khz, which is greater than the audible hearing

range of 20Khz. This is shown in figure 4-4 where there are 10 cycles in 0.1 milliseconds.

The total simulation time step is taken to be as 1.0 seconds.

Period, T Ton= 0.9

Figure 4-4: Duty cycle and frequency of square wave voltage source

49

To compute the minimum value of L1 and C2;

Fs

RDDL2

)1( 2

min1

(4-3)

H8

)100000(2

50)8.01(8.0 2

To provide a margin and to ensure continuous current, let inductor L1 = 10μH

The switching frequency Fs is selected somewhat arbitrarily, and other combination will

also give continuous current [4.2]

(4-4)

F

Vout

VoutRFs

DC

2.3

)05.0)(100000(50

8.0

)(2min

A small capacitor C2 value reduces the ripple factor of the output voltage since the

average of the DC component is tabulated. To test the model, the boost converter output is

loaded with a load resistance, RL. In the computation shown above, RL is taken to be as

50Ω. For the design analysis and simulation, the load resistance is allowed to vary from 1

ohm to 100 ohms. When a constant output voltage with +/- 5% ripple factor is obtained,

the load resistance value with a disparity of +/- 35% is tagged as the most efficient area of

operation for the converter circuit.

The variation of the load resistance and the corresponding variation of the fuel cell voltage

are recorded for 2 different duty cycles as a comparison. Ultimately, a stable DC voltage

of 25V, taking into consideration +/-5% RF must be boosted from the 5V obtained in the

first stage.

50

4.3 Design Considerations for 3rd Stage converter control circuit

First Stage Design

Control Circuit

Figure 4-5: 3rd Stage design for PEM Fuel Cell Model

The circuit consisting of the DC-DC boost converter and the 3 stage control circuit is

shown in figure 4-5. The output voltage of the model is fed into the inverting terminal of

the comparator using voltage-divider rule through resistance R3 and R4. This voltage is

compared with a reference voltage, Vref at the non-inverting input.

rd

The output of a

comparator is high (value= 1) when the non-inverting input is higher than the inverting

input. When the positive input is lower, the output is zero. If the two input are equal, the

output is undefined and it will keep the previous value.

The Reference voltage, Vref is a triangular wave with operating frequency, fs = 100Khz.

From the derivation of the boost converter, it can be seen that changing the duty cycle

controls the steady state output with respect to the input voltage. This is a key concept

governing all inductor based switching circuits. Hence Vref is set to operate at 25V peak-

peak so as to control the voltage at that value.

51

Voltage fed into the inverting input terminal of the comparator by Voltage divider rule;

V- = VoutRR

R*

34

4

(4-5)

There is a need for V- to be as small as possible so that when compared with reference

voltage, Vref, output pulse width (PWM) is wider. It would be preferable if PWM cycle is

maintained with a duty cycle of 0.80 to 0.85 as mentioned in the previous section so that

the output voltage level remains at around 25.0V.

Preferably V- has to be in the range of 4 to 5V, so that PWM pulses would have a duty

cycle of

Min Duty Cycle = 1 - 25

5= 0.80

Max Duty Cycle = 1 - 25

4= 0.84

Calculation of R3 and R4 values;

Keep R3 fixed at 10kΩ

Assume V- = 4V and Vout = 25V

Then Minimum Value of R4 using formula (4-5)

= 4)104(25

4RkR = 1.9kΩ

Assume V- = 5V and Vout = 25V

Then Maximum Value of R4 using formula (4-5)

= 4)104(25

5RkR = 2.5kΩ

Hence R4 can range from 1.9kΩ to 2.5kΩ for efficient operation of the 3rd stage control

circuit.

52

CHAPTER 5

RESULTS AND FINDINGS

5.1 PEMFC circuit model: 1st Stage simulation and results

This chapter contains a detailed explanation on the simulation results obtained. The

simulation has been done using PSIM software emulator and has some approximations

that have been assumed for the resistor R1, R2 values. Figure 5-1 depicts the PEM fuel

cell model in the first stage. Simulation has been done for load resistance from 0.1Ω to

150Ω. The results are shown in Table 5.1. The components used in the circuit are;

Vs = 5V

R1= 0.29Ω

Ls = 10mH

R2 = 1Ω

C = 10mF

Figure 5-1: Equivalent circuit of the PEM fuel cell module – 1st stage

53

Load Resistance, RL (Ω)

Output Voltage, Vout (V) Output Current, Iout (A)

0.10 1.54 15.39

0.15 2.00 13.34

0.20 2.35 11.76

0.25 2.63 10.53

0.30 2.86 9.52

0.40 3.20 8.00

0.50 3.45 6.99

0.70 3.78 5.41

0.80 3.90 4.88

0.90 4.00 4.45

1.00 4.08 4.08

1.20 4.21 3.51

1.40 4.31 3.08

1.60 4.38 2.74

1.80 4.45 2.47

2.00 4.49 2.25

2.50 4.59 1.84

3.00 4.65 1.55

3.50 4.70 1.34

4.00 4.73 1.18

4.50 4.76 1.06

5.00 4.79 0.96

6.00 4.82 0.80

7.00 4.84 0.69

8.00 4.86 0.61

9.00 4.88 0.54

10.00 4.89 0.49

20.00 4.95 0.25

30.00 4.96 0.17

50.00 4.98 0.10

80.00 4.99 0.06

100.00 4.99 0.05

150.00 5.00 0.03

Table 5-1: Simulated Results of Voltage Vs Current for the 1st stage

54

Based on the simulated results, voltage decreases from 5.0V at high load of 150Ω to

approximately 1.5V at low load of 0.1Ω. It takes approximately 0.3 seconds for the circuit

model to reach its steady state voltage for different loads as shown in figure 5-2. Voltage

is stable at 5.0V from a load resistance value of 10Ω to 150Ω as shown in table 5-1.

Vout = 3.5V Vout = 1.5V

RL = 0.1Ω RL = 0.5Ω

Vout = 5.0V Vout = 4.5V

RL = 2.0Ω RL = 100.0Ω

Figure 5-2: Steady state output voltage under various resistive loads

55

From table 5-1, current is at its maximum at 15.4A when terminal voltage and resistive load is low. Likewise, current is at its minimum

when terminal voltage is at 5.0V. Although voltage and current have inverse relationship, they operate differently in the 3 regions as

indicated by the polarization curve shown in figure 5-3.

0

1

2

3

4

5

6

15.3

9

13.3

4

11.7

6

10.5

3

9.52

8.00

6.99

5.41

4.88

4.45

4.08

3.51

3.08

2.74

2.47

2.25

1.84

1.55

1.34

1.18

1.06

0.96

0.80

0.69

0.61

0.54

0.49

0.25

0.17

0.10

0.06

0.05

0.03

0.00

Current (A)

Voltage Vs Current

Vs = 5.0V

Activation Polarization

RL = 5 to 150 Ω

Ohmic Polarization

RL = 0.7 to 5 Ω

Concentration Polarization

RL = 0.10 to 0.7 Ω

Figure 5-3: Polarization Curve obtained from the simulated results

56

5.2 PEMFC circuit model: 2nd Stage simulation and results

The PSIM schematic of the fuel cell fed by the boost converter is shown in figure 5-4. The

boost converter and the fuel cell module ideally should have a regulated output voltage of

25.0V. Changing the load resistance drastically would cause current flow to differ

tremendously, hence terminal voltage would not be properly regulated. Hence there is a

need to specify the load resistance values, RL where voltage is stable and is within the 5%

RF.

Figure 5-4: Equivalent circuit of the PEM fuel cell module with DC booster circuit

Table 5-2 highlights the change in the load resistance and the corresponding output of the

boost converter when the duty cycle of the MOSFET is at 0.85. Likewise another

simulation is done when the duty cycle is set at 0.83. Comparative studies are made to

determine the efficient area of operation for the booster circuit, since higher loads have

not been considered, as usually the fuel cell is not operated under those circumstances.

57

MOSFET Duty cycle =0.85 MOSFET Duty Cycle = 0.83Load

Resistance,

RL (Ω)

Vin(V) Vout (V) Iout (A) Power

(W) Vout (V) Iout (A)

Power

(W)

1.00 3.02 3.02 9.12 3.34 3.34 11.16

2.00 5.60 2.80 15.68 6.00 3.02 18.12

3.00 7.67 2.56 19.64 8.20 2.73 22.39

4.00 9.50 2.40 22.80 9.97 2.50 24.93

5.00 11.07 2.21 24.46 11.50 2.30 26.45

6.00 12.46 2.07 25.79 12.80 2.13 27.26

7.00 13.68 1.95 26.68 14.00 2.00 28.00

8.00 14.76 1.84 27.16 14.90 1.86 27.71

9.00 15.70 1.73 27.16 15.75 1.75 27.56

10.00 16.60 1.60 26.56 16.50 1.65 27.23

15.00 19.90 1.32 26.27 19.40 1.30 25.22

20.00 22.20 1.11 24.64 21.13 1.04 21.98

25.00 23.50 0.94 22.09 22.50 0.90 20.25

30.00 24.87 0.83 20.64 23.50 0.78 18.33

35.00 25.70 0.74 19.02 24.00 0.70 16.80

40.00 26.50 0.66 17.49 24.50 0.62 15.19

45.00 27.00 0.60 16.20 25.00 0.55 13.75

50.00 27.60 0.55 15.18 25.00 0.50 12.50

55.00 27.80 0.51 14.18 25.00 0.45 11.25

60.00 27.90 0.47 13.11 25.08 0.42 10.53

80.00 28.10 0.35 9.84 26.90 0.34 9.15

90.00 29.10 0.32 9.31 27.70 0.31 8.59

100.00

5V

29.70 0.30 8.91 28.40 0.28 7.95

Table 5-2: Simulated Results of Booster Output Voltage Vs Load Resistance

As shown on the table 5-2, the effective area of operation by the booster circuit is

indicated by the boxed up area (red) for different duty cycle. This is illustrated in the plot

shown in figure 5-5. For this area, the output voltage is quite stable when range of current

value is from 0.42 to 0.78A giving a power output of 10 to 18W. Since voltage is the only

parameter monitored at this stage, current and power do not have much influence on the

operating resistive load range. Hence, a continuous current of 1A was not achieved. Since

duty cycle of 0.83 closely depicts the 25.0V required, it is preferred for the 2nd stage

design.

58

5 % Ripple factor

Effective Area of Operation for Duty

Cycle = 0.83

Effective Area of Operation for Duty

Cycle = 0.85

Figure 5-5: Plot of DC-DC booster Curve for Duty Cycle = 0.85 and Duty Cycle = 0.83

59

Henceforth, based on the simulated results and graphical representation shown in figure 5-5

for the MOSFET duty cycle of 0.83, load resistance varies from 30Ω to 65Ω in the effective

operation area when booster output voltage is in the range of 23.5V to 26.5V (+/- 5% RF

based on 25.0V).

Through experimental simulation, it is demonstrated that changing the load resistance

drastically would cause the terminal voltage and load current to change likewise as shown in

table 5-2. Henceforth, resistive load is fixed at 45Ω and is allowed to deviate by as much as

35% for efficient functionality. Figure 5-6 and figure 5-7 are provided to further illustrate the

regulated output voltage at reduced load of 10Ω and at peak load of 60Ω so as to emphasize

the importance of the load resistance in controlling the boost converter output.

Reduced Load, RL = 10Ω

Avg DC Voltage = 16.5V

Avg DC Current = 1.65A

Figure 5-6: Output of the fuel cell and boost converter combined at reduced load

60

Peak Load, RL = 60Ω

Avg DC Voltage = 25.2V

Avg DC Current = 0.42

Figure 5-7: Output of the fuel cell and boost converter combined at peak load

From this 2nd stage experiment and results, it can be concluded that the MOSFET duty cycle

has a greater influence in controlling the output voltage and the range of load resistance

values defines the effective area of operation. However, there is a need to stabilize the output

voltage at the required value of 25.0V whenever there is a great variation in load resistance.

For example, whenever resistance is in the range of RL<30Ω and RL>70Ω, terminal voltage,

Vout changes drastically and is uncontrollable. It is at this point that a closed loop feedback

control between the output voltage and the MOSFET conduction period be formed to

manipulate the duty cycle and hence control the output voltage. This is exactly what the 3rd

stage converter controller circuit model intended to do.

61

5.3 PEMFC circuit model: 3rd Stage simulation and results

The PSIM schematic of the converter controller circuit is shown in figure 5-8. Theoretically,

the duty cycle D of the output waveform is inversely proportional to the control voltage,

Vcontrol. When the control voltage is high, duty cycle is small and vice versa. If this control

system is well designed, then the duty cycle is automatically adjusted such that the converter

output voltage follows the reference voltage, Vsignal and is essentially independent of

variations in load resistance, RL.

Figure 5-8: Equivalent circuit of the PEM fuel cell module – 3rd stage As seen in the previous section for the DC-DC boost converter, output voltage was stable for

an effective area of operation of 30 to 70Ω. For the controller circuit, simulation was done

for a load resistance value of 1.0 to 200Ω to find out the effectiveness of the controller design

in manipulating the duty cycle and hence contain the output voltage to the required 25.0V.

The simulated results are shown in table 5-3.

62

Load Resistance, RL (Ω) Vout (V) Iout (A) Power (W)

1.00 0.71 0.50 0.36

2.00 1.63 0.81 1.32

5.00 8.10 1.62 13.12

8.00 14.00 1.75 24.50

10.00 16.00 1.60 25.60

20.00 20.80 1.04 21.63

30.00 22.40 0.75 16.80

40.00 24.20 0.60 14.52

50.00 24.20 0.50 12.10

60.00 24.50 0.41 10.05

70.00 24.80 0.36 8.93

80.00 24.80 0.31 7.69

90.00 24.80 0.28 6.94

100.00 25.07 0.25 6.27

120.00 25.00 0.21 5.25

140.00 24.90 0.18 4.48

150.00 24.90 0.17 4.23

160.00 24.90 0.16 3.98

170.00 25.00 0.15 3.75

180.00 25.00 0.14 3.50

190.00 25.00 0.13 3.25

200.00 25.00 0.13 3.25

210.00 28.30 0.13 3.68

Table 5-3: Simulated Results for the Converter Control Circuit

As shown in table 5-3, output voltage was maintained at 25.0V with the inclusion of 5%

ripple factor for load resistance value of 40 to 200Ω. When load resistance varies, the

average DC output value remains static for different PWM signals (Vcomparator) based on the

control signal from the voltage divider. This is illustrated in figure 5-9 and figure 5-10.The

focus here is only to stabilize the voltage and not the current. Therefore, current varies from a

small value of 0.13 to 0.60A for a stable output voltage. To establish the relationship, only

the RL values and voltage are boxed up (red) and the plot drawn in figure 5-11. Henceforth,

if current were to stabilize at 1A, a current sensing circuit is required.

63

RL = 50 ohms

Figure 5-9: Controller circuit output waveform and control signal for RL=50Ω

64

RL = 200 ohms

Figure 5-10: Controller circuit output waveform and control signal for RL=200Ω

65

5 % Ripple factor

Stable output voltage region from the Controller Circuit

Figure 5-11: Plot of controller circuit curve based on simulated results from Table 5-3

In conclusion, the controller circuit is effective in stabilizing the voltage above 40 ohms, whereas at lower resistance values, more control is

necessary. From 50 to 200 ohms, the control voltage (Vcontrol) is about 3.6 to 3.9V based on figure 5-10. Therefore resistance R4 = 1.8KΩ

should be ideal as per the calculation shown in section 4-3. Hence, the 3rd stage has met its intended aim of adjusting the MOSFET

conduction period and restricting the voltage from any further drift regardless of load resistance changes. This is only applicable at resistive

loads greater than 40Ω.

66

CHAPTER 6

DISCUSSION

6.1 Overall Design and Simulated Results

The structure of the PEM equivalent fuel cell circuit model is divided into 3 stages, with

each stage having specific characteristics and serving the required purpose. The first stage

is designed to depict the polarization curve (V-I) based on the electrical characteristics of

the fuel cell. As for the second stage, it was supposed to boost the low voltage of 5.0V

obtained from the first stage to the required 25V architecture. Since voltage is only

constant for certain range of resistive loads at the DC-DC booster stage, a 3rd stage or the

converter control stage, is necessary to stabilize the output voltage to 25.0V. The entire

circuit model of the PEM fuel cell is shown in figure 6-1 and each individual stage is

illustrated in Appendix C.

Figure 6-1: Schematic diagram of the entire PEM fuel cell model

67

For the first stage simulated results, the static characteristics of the model follows the

polarization curve accurately as shown in figure 5-3 when compared with the behavior of a

commercial fuel cell stack. With reference to the design considerations in chapter 4 and

according to equation (4-1) and (4-2), resistance R1 and R2 values are chosen after several

tries and are verified using the V-I curve. Hence a sketch of the simulated curve with

current values inserted is re-produced in figure 6-2.

5.0

5.41 20 Figure 6-2: Sketch of Simulated V-I Curve Based on the results obtained, I1 = 5.41A and I2 = 20.0A. Hence from equation (4-1)

1

5.850.20

R

Therefore, R1 = 0.32Ω. From equation (4-2), resistance R2 value is calculated to be

92.022

541.5

RR

The experimental values chosen for resistance R1 and R2 are very much close to the

calculated values and hence the first stage is operating as desired.

68

For the second stage of DC-DC boost converter, only one MOSFET switch is used to

reduce high frequency electromagnetic interference and switching loss. The switch is

operated at 100 kHz, which is way above the audible hearing frequency. The output

voltage curve obtained in figure 5-5 was able to boost up the voltage level to 25.0V

(subjected to 5% ripple factor) from 5.0V for duty cycle, D = 0.83. Taking into

consideration some noise introduced by transistor switching in the first stage, and voltage

fluctuations due to varying current drawn by the resistive loads, voltage was only stable for

loads from 30 to 65Ω. This stage was only able to partially fulfill its requirement as output

voltage swing was on the high side for low loads. As such, switching loss is not totally

annihilated at smaller loads even though a small capacitor C2 value is chosen.

In the 3rd stage, Vcontrol voltage is kept around 4V by careful choice of resistance R4 value

with reference to equation (4-5) so that MOSFET PWM cycles from the Vcomparator have a

duty cycle, D = 0.84. Maintaining this duty cycle is essential so that the output voltage is

controllable at 25.0V regardless of load resistance change. Simulated results from figure

5-11 conclude that voltage is indeed controlled at higher resistance of 40 to 200Ω.

However at lower resistance value, the system failed to restrict the output voltage swing.

Hence greater voltage loss was evident at this area. But, variation in load for a stable

voltage is definitely greater than the effective operation mode of the DC-DC converter and

hence the third stage has fulfilled its functionality of providing better control

The only drawback of the entire system is that current was not stabilized for the defined

range of load resistance values whereas voltage was controlled at 25.0V. The main reason

was that a current sensing circuit was not incorporated in the control circuit and as such

current was not maintained at 1.0A as preferred. This eventually caused the final output

power to vary from 3 to 14.5W because current loss was not minimized. Hence maximum

power extraction pegged at 25W was not possible and therefore it was not viable to

compare the results with a commercial fuel cell stack.

In general, the overall design was kept simple without too many components from the

PSIM library. Mathematical computations in deriving the components value was not

overly complex, hence circuit simulation and results were properly done and tabulated.

69

CHAPTER 7

CONCLUSION

This report sets down some preliminary work useful for developing an electrical equivalent

PEM fuel cell model for portable applications. Other items that were studied were design

methods of boosting the low DC output voltage and a feedback control scheme using

PWM to regulate the voltage to a stable level.

The fuel cell model is required to analyze the electrical characteristics, especially voltage

and current under normal operating conditions. It is designed as an alternative in

understanding the fuel cell behavior since high wattage commercial fuel cell stack could

not be implemented in this project. The model is simple and uses a diode and a pair of

transistors for static conditions and a capacitor and inductor for dynamic conditions. The

model is validated by comparing the simulated results with the polarization curve. The

effect of the load resistance in the model were studied too. From this study, a model to

increase the DC voltage was developed.

The performance of the fuel cell feeding a boost converter was also discussed. It had been

shown through experiment that voltage regulation at 25.0V was only possible for certain

range of load values. It was also observed that output current varies when the boost

converter had regulated output voltage. Current loss was not controlled, hence a 25W boost

converter was not achieved at this stage. Therefore a feedback control system was required

to stabilize the output voltage.

The general approach towards the design of a voltage control system as the 3 stage was

also shown. The main procedure followed was

rd

(1) Determine the control signal voltage so as to regulate the duty cycle of the MOSFET.

(2) Determine the resistance values of the voltage divider.

(3) Generate a triangular reference signal and compare it with the control voltage using a

comparator.

All simulated results were verified so that the general project objectives would be met.

70

CHAPTER 8

CRITICAL REVIEW AND REFLECTIONS

The nature of this project involved a great deal of planning and research work in getting

the equivalent PEM fuel cell model from the onset. This model is the pivotal point in

which the rest of the circuit structure is based upon. Good literature research, background

reading and hands-on activities in PSIM software is essential to undertake the numerous

tasks projected during the initial brainstorming sessions. Selecting key information for this

project as well as communicating the ideas clearly and adeptly is equally vital too. Proper

information gathering was one of my strong traits and I felt I did well in that department.

However, as a sole part-time student embarking on a project theme which is relatively new

to me but have greater potential in the alternative energy source market, there is bound to

be certain amount of weakness on my part that needs further development and

improvement.

Firstly, time management needs to be strictly adhered through the means of a realistic gantt

chart so that the project objectives can be fulfilled. Initially, I underestimated the amount

of time that need to be spent in understanding the required key performance parameters of

the fuel cell such as voltage and current. Henceforth valuable time was wasted in

researching other models that deals with complex equations and variable parameters such

as temperature and gas pressure. Fortunately, these problems occurred at the start of the

project and hence I was to rectify the situation, reorganize my criteria and reschedule my

time appropriately. This was one of the most important lessons I have learnt in project

management.

Secondly, learning to work independently and a regular feedback session with the tutor via

the electronic mail and regular meetings was critical in gaining valuable insights of my

performance and progress barometer. In this way, continuous improvement was achieved

for my circuit design. During the course of my circuit simulation, I have overlooked the

notion that my design methodology was mostly accurate enough to yield the expected

71

outcome. However, it was during one of the feedback sessions that I came to realize that

certain portions of the schematic were incorrect and hence modification was necessary.

Taking the tutor’s comments positively and responding in the right direction was

imperative in my decision making process. This helped me make a slow and steady

progress in my project report.

During the course of simulation process, it was easy to get frustrated at times when

experimental results did not produce the desired outcome since there was insufficient

knowledge in implementing the particular circuit design. The simulated data initially

amassed at that juncture was not very helpful either. I found this aspect to be fairly

daunting due to the fact that I was looking for patterns in the data which would allow me to

make predictions but did not know what was relevant and what could be discarded. As

such, I had to revise my 3rd stage objectives in changing the control circuit to another

design in order to work around the drawback. Rather than sticking with a predicament, I

had to search for alternatives and methodology to deal with the problem successfully. This

failure to detect the problem early was unavoidable as I did not anticipate the level of

complexity involved for the 3rd stage design.

However, there were some successes in my project design. After resolving the early stage

design problem, I needed to strengthen my commitment level in completing the project on

time. Hence, I ensured sufficient time was available to remedy any minor changes in my

technical report. Starting the report early allowed me to proofread and soften the impact of

any last minute work. The quality of research materials which I have gathered over the

entire year allowed me to effectively execute the entire circuit model. From this point, the

project ran more smoothly because the background foundation knowledge was very useful.

Running the circuit model well was the byproduct of a good simulation setup in PSIM

software emulator.

In conclusion, having a clear definite goal and objectives in achieving the intended target

while ensuring there is proper planning, time management, assessment and adaptability to

existing situations is essential for good project management.

72

CHAPTER 9

SUGGESTIONS FOR FUTURE WORK

This project has laid the groundwork for further improvement in PEM fuel cells circuit

design. It leaves a lot of possibilities to be explored, some of which are enlisted here;

1. Study the possibility of other type of fuel cell based power converters such as

half-bridge DC-DC converters or Cuk converter and then do a comparative

study on the current model which was proposed. Implement the system which

provides better voltage regulation and reduces voltage loss at low resistive

loads.

2. The next step would be to develop an additional feedback scheme in controlling

the output current to the required value so that maximum power can be

extracted from the fuel cells when operating at a fixed voltage.

3. Finally, work with a full version of PSIM software emulator so that individual

components’ parameters can be fine-tuned for better control of the system.

73

REFERENCES

[1.1] Chris Rayment, Scott Sherwin, “Introduction to Fuel cell technology”, in: University of Notre Dame, 2003, pp. 49-51.

[1.2] S.Yerramalla, A.Davari, A.Feliachi, “Dynamic modeling and analysis of polymer

electrolyte fuel cell”, in: Proceedings of the IEEE Power Engineering Society Summer Meeting, Vol 1, 2002, pp.82-86.

[2.1] National Energy Technology laboratory, Fuel Cell Handbook, sixth edition, 2002,

pp 2-9. [2.2] Ahmed Mohamed Refaat Azmy, “Simulation and Management of Distributed

Generating Units using Intelligent Techniques”, 2005, pp.16-21. [2.3] Jaiganesh Balasubramanian, “Investigation of Fuel cell Models and Auxiliary

Power Unit Configuration, pp. 15-20. [2.4] Philip T.Krein, Robert S. Balog, “High frequency Link Inverter for fuel cells based

on multiple-carrier PWM”, in: IEEE Transactions on Power Electronics, Vol 19, No 5, 2004, pp. 1279-1281.

[3.1] D.Chu, R.Jiang, C.Walker, “Analysis of PEM fuel cell stacks using an empirical

voltage-current equation”, in: J.Appl Electrochemical. 30, 2000, pp. 365-370. [3.2] Dachuan Yu, S.Yuvarajan, “Electronic circuit model for proton exchange

membrane fuel cells”, in: Journal of Power Sources 142, 2004, pp.238-241 [3.3] Chris Rayment, Scott Sherwin, ”Introduction to Fuel cell technology”, in:

University of Notre Dame, 2003, pp. 33-38. [3.4] Website, http://en.wikipedia.org/wiki/Tafel_equation [3.5] D.Yu, S.Yuvarajan, “A Novel Circuit Model for PEM Fuel cells”, in: Applied

Power Electronics Conference and Exposition, 2004, 19 Annual IEEE, Volume 1, 2004, pp.362-366.

th

[3.6] L.Palma, M Harfman Todorovic, “Design Considerations for a fuel cell powered

DC-DC Converter for portable applications”, in: Power Electronics & Fuel cell Power Systems laboratory, IEEE 2006, pp. 1263-1265.

[3.7] J.W. Jung, M.Dai, A.Keyhani, “Modeling and Control of Fuel Cell Based Z-Source

Converter for Distributed Generation Systems”, in: IEEE transactions on Energy Conversion, Oct 2004, pp. 17-20.

74

[3.8] Yu Jin Song, “Analysis and design of high frequency link power conversion

systems for Fuel Cell Power Conditioning”, 2004 [3.9] As mentioned in [3.2]. [3.10] Robert W. Erickson, “DC-DC Power Converters”, in: Wiley Encyclopedia of

Electrical and Electronic Engineering, pp.1-5. [3.11] Website, http://www.powerdesigners.com/InfoWeb/design_center/articles/DC-

DC/converter.shtm [3.12] Website, http://en.wikipedia.org/wiki/Pulse_width_modulation [4.1] As mentioned in [3.2]. [4.2] Jaiganesh Balasubramanian, “Investigation of Fuel cell Models and Auxiliary

Power Unit Configuration, pp. 43-44.

75

Appendixes

76

Appendix A

77

- To learn PSIM software and have hands-on activities with it. - Do research on PEM fuel cells and circuit model by April - Finish reading and designing on second stage DC-DC converter by July. - Start writing technical report in Sep and finish control circuit by end Sep 2008

- So as to manage the project well since it is done by one person only. - Better knowledge in understanding fuel cell behavior. - Circuit simulation and results would be done without any glitch

: 12/03/2008 : 12/03/2008

- Confident about organization skills and getting relevant information. - Need to communicate and have regular feedback session with tutor. - Learn to manage time well. - Learn PSIM thoroughly because have little experience in it

- Do a comprehensive research on the electro-chemical properties of fuel cells. - Study 2 to3 equivalent circuit models - Present to the tutor and ask for feedback. - Write the initial report.

- Improve on time management, as well as reading up on research journals. - Understand the key parameters affecting the PEM fuel cells - Be disciplined to finish up with the intended work

Appendix B

: 06/10/2008

- Practice before the actual presentation. - Look at samples from the web on how to deliver effective presentation that attracts audience attention.

- Keep presentation short and simple - Use 6 to 8 slides for presentation - Avoid technical terms and present only necessary items. - Use illustrations to explain.

- Good verbal communication and able to handle situations well. - Can answer questions posed by examiner - Tend to be long winded at times, need to reduce all the technical jargon - Need to practice on keeping to the time schedule

- Visual aids in power-point - Elaborate key points and stick to timing - Clear and precise presentation and maintain eye contact

78

Appendix C

1 Stage : Equivalent Circuit Model for PEM Fuel Cells st

Filename: Model 1.sch

79C-1

2 Stage : DC-DC Boost Converter for PEM Fuel Cells nd

Filename: 2nd stage (final).sch

80C-2

3 Stage : Control Circuit after the second stage rd

Filename: Finalmodel.sch

81C-3

All Stages : 3 stages combined model for PEM Fuel Cells

Filename: Combinedmodel.sch

82C-4