9.a modular fuel cell, modular dc dc converter

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A Modular Fuel Cell, Modular DC – DC Converter Concept

for High Performance and Enhanced Reliability

ABSTRACT

Fuel cell stacks produce a dc output with a 2:1 variation in output voltage

from no-load to full-load. The output voltage of each fuel cell is about 0.4 V at full-load, and

several of them are connected in series to construct a stack. An example 100 V fuel cell stack

consists of 250 cells in series and to produce 300 V at full load requires 750 cells stacked in

series. Since fuel cells actively convert the supplied fuel to electricity, each cell requires proper

distribution of fuel, humidification, coupled with water/thermal management needs. With this

added complexity, stacking more cells in series decreases the reliability of the system. For

example, in the presence of bad or mal performing cell/cells in a stack, uneven heating coupled

with variations in cell voltages may occur. Continuous operation under these conditions may not

be possible or the overall stack output power is severely limited. In this paper, a modular fuel

cell powered by a modular dc – dc converter is proposed. The proposed concept electrically

divides the fuel cell stack into various sections, each powered by a dc – dc converter. The

proposed modular fuel cell powered by modular dc – dc converter eliminates many of these

disadvantages, resulting in a fault tolerant system. A design example is presented for a 150-W,

three-section fuel cell stack and dc – dc converter topology. Experimental results obtained on a

150-W, three-section proton exchange membrane (PEM) fuel cell stack powered by a modular

dc – dc converter are discussed.

INTRODUCTION

FUEL CELLS are electrochemical devices that process H2 and oxygen to generate electric

power, having water vapor as their only by-product. The voltage resulting from the reaction of

the fuel and oxygen varies with the load, and ranges from 0.8 V at no-load to about 0.4 V for



full-load. Due to their low output voltage, it becomes necessary to stack many cells in series to

realize a practical system. For low-power applications, the number of cells that needs to be

connected in series is small, but as power increases, the number of cells that are required in the

stack increases rapidly [1], [2]. An example 100 V fuel cell stack consists of 250 cells in series

and to produce 300 V at full-load requires 750 cells stacked in series. A conventional fuel cell

system (Fig. 1) consists of a stack of cells and a dc – dc converter to step-up its terminal voltage

and compensate for its no-load to full-load variation [3] – [5]. Since this fuel cell structure is

equivalent to connecting several voltage sources in series, each with its own internal impedance

[6], [7], the output power of the stack is limited by the state of the weakest cell. The state of a

cell can be inferred from the voltage across its terminals, which is affected by parameters such as

fuel and air pressure, and membrane humidity. Furthermore, if a stack contains malfunctioning

or defective cells, the whole system has to be taken out

of service until major repairs are done.

In order to circumvent these problems, a modular fuel cell powered by a modular

dc – dc converter (Fig. 2) is proposed in this paper. The proposed modular concept electrically

divides the fuel cell stack into various sections, each powered by a dc – dc converter. This

modular fuel cell powered by modular dc – dc converter eliminates many of the disadvantages,

resulting in a fault tolerant system. A design example is presented for a 150-W, three-section fuel

cell stack and dc – dc converter topology. Experimental results obtained on a 150-W, three-section

proton exchange membrane (PEM) fuel cell stack powered by a modular dc – dc converter are

discussed.



F ig. 1 Uti li ty scale fuel cell stack and dc – dc/dc – ac converter

F ig. 2 Proposed modular fuel cell and modular dc – dc converter concept

The proposed system has the fol lowing advantages.

1) The power generated by different sections in the modular fuel cell stack can be independently

controlled by each dc – dc converter.

2) Extra heating in underperforming sections of the stack, due to their larger internal impedance,

can be reduced by limiting their load current, thus reducing the internal losses in the fuel cell.

3) If a section of the stack is faulty, the dc – dc converter controlling the faulty section can be

disenabled and/or bypassed, while the rest of the system can continue operation at reduced

power.



4) If the proposed modular stack is employed in automotive systems, under faults, the driver can

steer the vehicle to a safe location at reduced power, since faulty stack sections can be shut

down.

Different modular topologies have been explored in the past for application on

photovoltaic (PV) systems aimed mainly to reduce the need of having long strings of panels

[8].However, for the case of fuel cell systems, due to their physical construction; long stacks of

cells are not avoidable. Instead, the main concern is overheating and loss of output power due to

the presence of bad cells in the stack. Thus, a different converter structure, control scheme, and

modified stack structure become necessary.

WHAT IS A FUEL CELL?

A fuel cell by definition is an electrical cell, which unlike storage cells can be continuously fed

with a fuel so that the electrical power output is sustained indefinitely (Connihan, 1981). They

convert hydrogen, or hydrogen-containing fuels, directly into electrical energy plus heat through

the electrochemical reaction of hydrogen and oxygen into water. The process is that of

electrolysis in reverse.

Because hydrogen and oxygen gases are electrochemically converted into water, fuel cells have

many advantages over heat engines. These include: high efficiency, virtually silent operation

and, if hydrogen is the fuel, there are no pollutant emissions. If the hydrogen is produced from

renewable energy sources, then the electrical power produced can be truly sustainable. The two

principle reactions in the burning of any hydrocarbon fuel are the formation of water and carbon

dioxide. As the hydrogen content in a fuel increases, the formation of water becomes more

significant, resulting in proportionally lower emissions of carbon dioxide (Fig. 1). As fuel use

has developed through time, the percentage of hydrogen content in the fuels has increased. It

seems a natural progression that the fuel of the future will be 100% hydrogen.



Figure 1. Trends in the use of fuels. As fuel use has developed through time, the percentage of

hydrogen content in the fuel has increased

HISTORY OF FUEL CELLS

2.1 The “Gas Battery”

Sir William Grove (1811-96), a British lawyer and amateur scientist developed the first fuel cell

in 1839. The principle was discovered by accident during an electrolysis experiment. When Sir

William disconnected the battery from the electrolyzer and connected the two electrodes

together, he observed a current flowing in the opposite direction, consuming the gases of

hydrogen and oxygen (Fig. 2). He called this device a „gas battery‟. His gas battery consisted of

platinum electrodes placed in test tubes of hydrogen and oxygen, immersed in a bath of dilute

sulphuric acid. It generated voltages



Figure 2. The principle of an electrolyzer, shown left; of a fuel cell, shown right. (Larminie,

2000).

of about one volt. In 1842 Grove connected a number of gas batteries together in series to form a

„gas chain‟. He used the electricity produced from the gas chain to power an electrolyzer,

splitting water into hydrogen and oxygen (Fig. 3). However, due to problems of corrosion of the

electrodes and instability of the materials, Grove‟s fuel cell was not practical. As a result, there

was little research and further development of fuel cells for many years to follow.



F igure 3. Grove’s ‘gas battery’ (1839) produced a voltage of about 1 volt, shown left. Grove’s

‘gas chain’ powering an electrolyzer (1842), shown right. (Photo courtesy of Berry, 2000)

The “Bacon fuel cell”

Significant work on fuel cells began again in the 1930s, by Francis Bacon, a chemical engineer at

Cambridge University, England. In the 1950s Bacon successfully produced the first practical fuel

cell, which was an alkaline version (Fig. 4). It used an alkaline electrolyte (molten KOH) instead

of dilute sulphuric acid. The electrodes were constructed of porous sintered nickel powder so that

the gases could diffuse through the electrodes to be in contact with the aqueous electrolyte on the

other side of the electrode. This greatly increased the contact area contact between the electrodes,

the gases and the electrolyte, thus increasing the power density of the fuel cell. In addition, the

use of nickel was much less expensive than that of platinum. Alkaline fuel cell:



Anode reaction: 2 H2 + 4 OH- - (Eq. 2.1)

Cathode reaction: O2 + 4 e- - (Eq. 2.2)

Bacon’s laboratory, at the Department of Chemical Engineering, Cambridge University (1955)

A fuel cell stack can be seen being assembled on the left of the photograph (Photo cour tesy of

Department of Chemical Engineer ing, University of Cambridge).



Fuel cells for NASA

For space applications, fuel cells have the advantage over conventional batteries, in that they

produce several times as much energy per equivalent unit of weight. In the1960s, International

Fuel Cells in Windsor Connecticut developed a fuel cell power plant for the Apollo spacecraft.

The plant, located in the service module of the spacecraft, provided both electricity as well as

drinking water for the astronauts on their journey to the moon. It could supply 1.5 kilowatts of

continuous electrical power. Fuel cell performance during the Apollo missions was exemplary.

Over 10,000 hours of operation were accumulated in 18 missions, without a single in-flight

incident (IFC). In the 1970s, International Fuel Cells developed a more powerful alkaline fuel

cell for NASA‟s Space Shuttle Orbiter (Fig. 5). The Orbiter uses three fuel cell power plants to

supply all of the electrical needs during flight. There are no backup batteries on the space



NASA Space Shutt le Orbi ter fuel cel l . One of th ree fuel cells aboard the Space Shuttle. These

fuel cell s provide all of the electr ici ty as well as drinking water when Space Shuttle is in f li ght.

I t produces 12 kil owatts electr icity, and occupi es 154 li tres.

shuttle, and as such, the fuel cell power plants must be highly reliable. The power plants are

fuelled by hydrogen and oxygen from cryogenic tanks and provide both electrical power and

drinking water. Each fuel cell is capable of supplying 12 kilowatts continuously, and up to 16

kilowatts for short periods. The Orbiter units represent a significant technology advance over

Apollo, producing about ten times the power from a similar sized package. In the Shuttle

program, the fuel cells have demonstrated outstanding reliability (over 99% availability). To

date, they have flown on 106 missions and clocked up over 82,000 hours of operation (NASA).

Alkaline fuel cells for terrestrial applications

Compared with other types of fuel cells, the alkaline variety offered the advantage of a high

power to weight ratio. This was primarily due to intrinsically faster kinetics for oxygen reduction

to the hydroxyl anion in an alkaline environment. Therefore alkaline fuel cells were ideal for

space applications. However, for terrestrial use, the primary disadvantage of these cells is that of

carbon dioxide poisoning of the electrolyte. Carbon dioxide is not only present in the air but also

present in reformate gas, the hydrogen rich gas produced from the reformation of hydrocarbon

fuels.

In the poisoning of an alkaline fuel cell, the carbon dioxide reacts with the hydroxide ion in the

electrolyte to form a carbonate, thereby reducing the hydroxide ion concentration in the

electrolyte. This reduces the overall efficiency of the fuel cell. The equation of carbon dioxide

reacting with a potassium hydroxide electrolyte is shown below:

2 KOH + CO2 → K2CO3 + H2O (Eq. 2.4)

Because of the complexity of isolating carbon dioxide from the alkaline electrolyte in fuel cells

for terrestrial applications, most fuel cell developers have focused their attention on developing

new types using electrolytes which are non-alkaline. These fuel cells include: solid oxide fuel

cells (SOFC), phosphoric acid fuel cells (PAFC), molten carbonate fuel cells (MCFC) and

polymer electrolyte membrane (PEM) fuel cells.



The PEM fuel cell

In the early 1960s, General Electric (GE) also made a significant breakthrough in fuel

cell technology. Through the work of Thomas Grubb and Leonard Niedrach, they invented and

developed the first polymer electrolyte membrane (PEM) fuel cell. It was initially developed

under a program with the US Navy‟s Bureau of Ships and U.S. Army Signal Corps to supply

portable power for personnel in the field. Attracted by the possibility of using GE´s PEM fuel

cell on the Apollo missions, NASA tested its potential to provide auxiliary power onboard its

Gemini spacecraft. The Gemini space program consisted of 12 flights in preparation for the

Apollo missions to the moon. For lunar flights, a longer power source was required than could be

provided by batteries, which had been used on previous space flights. Unfortunately, the GE fuel

cell, model PB2, encountered technical difficulties prior to launch, including the leakage of

oxygen through the membrane. As a result the Gemini missions 1 to 4 flew on batteries instead.

The GE fuel cell was redesigned and a new model, the P3, successfully operated on the Gemini

flights 6 to 12. The Gemini fuel cell power plant consisted of two fuel cell battery sections, each

capable of producing a maximum power of about 1000 watts (NASA).

2.5.1 Bal lar d Power

A further limitation of the GE PEM fuel cell at that time was the large quantity of platinum

required as a catalyst on the electrodes. The cost of PEM fuel cells was prohibitively high,

restricting its use to space applications. In 1983, Geoffrey Ballard a Canadian geophysicist,

chemist Keith Prater and engineer Paul Howard established the company, Ballard Power. Ballard

took the abandoned GE fuel cell, whose patents were running out and searched for ways to

improve its power and build it out of cheaper materials (Koppel, 1999). Working on a contract

from the Canadian Department of National Defence, Ballard developed fuel cells with a

significant increase in power density while reducing the amount of platinum required. From

these developments it was recognized that fuel cells could be made smaller, more powerful and

cheaply enough to eventually replace conventional power technologies. Ballard Power has since

grown to become recognized as a world leader in PEM fuel cell technology, developing fuel cells



with power outputs ranging from 1 kilowatt, for portable and residential applications, through to

250 kilowatts for distributed power. Ballard has formed alliances with a wide range of

companies, including DaimlerChrysler, Ford, Coleman and Ebara Power in Japan.

Los Alamos National Laboratory

In the late 1980s and early 1990s Los Alamos National Laboratory and Texas A&M University

also made significant developments to the PEM fuel cell. They also found ways to significantly

reduce the amount of platinum required and developed a method to limit catalyst poisoning due

to the presence of trace impurities in the hydrogen fuel (Los Alamos National Laboratory).

3. FUEL CELL APPLICATIONS

3.1 Transportation

The California Low Emission Vehicle Program, administered by the California Air Resources

Board (CARB), has been a large incentive for automobile manufacturers to actively pursue fuel

cell development. This program requires that beginning in 2003, ten percent of passenger cars

delivered for sale in California from medium or large sized manufacturers must be Zero

Emission Vehicles, called ZEVs. Automobiles powered by fuel cells meet these requirements, as

the only output of a hydrogen fuel cell is pure water.

The NECAR 5 (Fig. 6) is the latest prototype fuel cell automobile by DaimlerChrysler. This

automobile is fuelled with liquid methanol which is converted into hydrogen and carbon dioxide

through use of an onboard fuel processor. The vehicle has virtually no pollutant emissions of

sulphur dioxide, oxides of nitrogen, carbon monoxide or particulates, the primary pollutants of

the internal combustion engine. The efficiency of a fuel cell engine is about a factor of two

higher than that of an internal combustion engine and the output of carbon dioxide is

considerably lower.

The NECAR 5 drives and feels like a “normal” car. It has a top speed of over 150 km/hr (90

mph), with a power output of 75 kW (100 horsepower). It is also believed that this vehicle will

require less maintenance. It combines the low emission levels, the quietness and the smoothness



associated with electric vehicles, while delivering a performance similar to that of an automobile

with an internal combustion engine.

In April 1999 the California Fuel Cell Partnership was developed. Founding members included

DaimlerChrysler, the California Air Resources Board, the California Energy Commission,

Ballard Power, Ford, Shell and Texaco. The primary objective was to help commercialize fuel

cell technology for vehicles through joint demonstration programs by the partners. Since then

new participants have included General Motors, Honda, Hyundai, Nissan, Toyota, Volkswagen,

British Petroleum, Exxon Mobil, Xcellsis, US Department of Energy and US Department of

Transportation. To date seven of the world‟s ten leading auto manufacturers have announced that

they plan to introduce fuel cell automobiles beginning in the 2003 to 2005 timeframe. There are

also plans for buses, trucks and trains all powered with fuel cell engines. In 2000, Ballard

completed a two-year program testing six fuel cell buses, three in Vancouver, British Columbia

and three in Chicago. The design and maintenance requirements of fuel-cell vehicles as well as

public acceptance were included in the study. The results of the tests were exemplary. Thirty

new buses powered by Ballard‟s fuel cell will be introduced to 10 European cities beginning in

2002 for additional field testing. The resulting data will be used to further develop a commercial

fuel cell bus

Distributed power generation

Electrical energy demands throughout the world are continuing to increase. In Canada the

demand is growing at an annual rate of approximately 2.6%. In America the rate is about 2.4%

(IEA., 1997), and in developing countries it is approximately 6% (Khatib., 1998). How can these

energy demands be met responsibly and safely? Distributed power plants using fuel cells can

provide part of the solution. Distributed or “decentralized” power plants, contrasted with

centralized power plants, are plants located close to the consumer, with the capability of

providing both heat and electrical power ( a combination known as “cogeneration”). Heat, the

by-product of electrical power generation, is transferred from the fuel cell to a heat exchanger.

The exchanger transfers the heat to a water supply, providing hot water to local customers.

The overall efficiency of a cogeneration system can be in excess of 80 percent, comparatively

high compared to a system producing electricity alone. An increase in efficiency naturally



corresponds to a decrease in fuel consumption. Distributed power plants have many additional

advantages. For example, they can provide power to a remote location without the need of

transporting electricity through transmission lines from a central plant. There is also an

efficiency benefit in that the cost of transporting fuel is more than offset by the elimination of the

electrical losses of transmission. The ability to quickly build up a power infrastructure in

developing nations is often cited. Using fuel cell power plants obviates the need for an electrical

grid.

Grid-connect applications

Distributed power plants can provide either primary or back-up power. In primary

applications they can provide base-load power, operating virtually continuously from the

consumption of natural gas, reducing the demand from the electrical grid. This not only

decreases the cost of displaced power, but can also result in a reduction of demand charges

imposed by the utility. Should the power plant provide an excess of electricity, the excess can be

fed back into the electrical grid, resulting in additional savings. In case of a power outage on the

grid, a distributed power plant can continue to provide power to essential services; eliminating

the need for both an uninterruptible power supply (UPS), presently handled by lead-acid battery

banks, and a stand-by generator, for extended periods of power outage. An additional quality of a

fuel cell power plant for UPS applications is that the average “down time” is anticipated to be

low, 3.2 to 32 seconds per year versus typically nine hours for a conventional battery-bank UPS

(HDR Engineering). For industries where UPS systems are critical, such as banking, minimizing

down time is of up most importance.

Non-grid connect applications

Other applications for fuel cell distributed power plants are also possible e.g. stand-alone

back-up power generators. The fuel cell plant can be started in seconds, supplying power for as

long as required from stored hydrogen, producing electrical power cleanly and virtually silently.

Shown in Figure 7 is a prototype fuel cell distributed power plant, by Ballard Power. This unit

provides 250 kilowatts of electricity and an equivalent amount of heat. This is enough power for



a community of about 50 homes, or a small hospital or a remote school. This particular unit

incorporates a fuel processor so that natural gas can be used as a fuel. The fuel processor

converts the natural gas, through the process of reformation, into a hydrogen-rich gas composed

primarily of hydrogen and carbon dioxide. The hydrogen is used by the fuel cell and the carbon

dioxide is released into the atmosphere Eventually as an infrastructure for hydrogen develops,

these units could be powered with hydrogen directly without the need of a fuel processor. Ballard

Power is presently field testing five of these units in the United States, Germany, Japan and

Switzerland, with four more units planned for 2002. Testing is expected to continue until 2004

after which commercial introduction is planned (Ballard Power).

Residential Power

Fuel cell power plants are also being developed by several manufacturers to provide electricity

and heat to single-family homes. Fuelled by either natural gas or propane, these plants will be

able to supply base-load power or all of the electricity required by a modern-day home. Ballard

Power has developed a one-kilowatt fuel cell designed to supply both base-load electrical power

as well as heat to a dwelling. This unit can also be fuelled by natural gas. It does not provide

enough power to supply the total electrical demands of a residence, but it does shift a portion of

the demand from the electrical grid to natural gas. The electrical efficiency of this fuel cell

system is rated at 42% and the heat efficiency is rated at 43%. Therefore the combined

cogeneration efficiency of the system can be as high as 85%. This particular generator is targeted

at the Japanese residential market. Ballard‟s goal is to commence sales of these units in 2004.

Plug Power, based in Latham, New York has developed a new fuel cell power plant that supplies

seven kilowatts of electrical power to the home plus heat, using either natural gas or propane as

the fuel (Fig. 8). This is enough power to supply the electrical needs of a modern energy efficient

house. At present, these units are designed to be used in parallel with the grid. This means the

fuel cell will supply base-load power and the utility grid will handle momentary power surges.

Should the electric grid fail, the fuel cell operates as a back-up generator providing power for the

home‟s cr itical requirements. Second generation products will be designed to run independent of

the grid. During 2000, Plug Power installed and tested 52 systems in the field and accumulated



over 133,000 hours of system run-time. General Electric which is marketing Plug Power‟s unit,

has announced that commercial introduction of this home fuel cell power plant is expected in

2002 (Plug Power)

Portable Power

Several manufacturers are also developing fuel cell power supplies for portable applications,

providing a few watts up to several kilowatts of electricity (Fig. 9). Fuelled by stored natural gas,

propane, methanol or hydrogen gas, portable fuel cells may one day replace both gasoline and

diesel-engine generators for portable applications as well as conventional batteries for uses such

as remote lighting, laptop computers and mobile phones.

Di rect methanol fuel cell s for portable power

Direct methanol fuel cells were invented and initially developed at the Jet Propulsion Laboratory

in Pasadena, California. They were designed to supply electricity for field troops in the Armed

Forces and for applications with NASA (Fig. 11). The direct methanol fuel cell has the

advantage over the hydrogen fuel cell in that they can use a liquid fuel i.e. methanol without the

need for external reforming. Liquid fuel is easy to store and has a high energy density compared

to compressed hydrogen. At present, the direct methanol fuel cell suffers from relatively low

efficiency and high cost, owing to required platinum loading compared to that of the hydrogen

fuel cell. However, as this improves, it is expected that the direct methanol fuel cell will play a

leading role in providing power for portable and possibly transportation applications.

THE SCIENCE OF THE PEM FUEL CELL

4.1 The Chemistry of a Single Cell



In a PEM fuel cell, two half-cell reactions take place simultaneously, an oxidation

reaction (loss of electrons) at the anode and a reduction reaction (gain of electrons) at the

cathode. These two reactions make up the total oxidation-reduction (redox) reaction of the fuel

cell, the formation of water from hydrogen and oxygen gases. As in an electrolyzer, the anode

and cathode are separated by an electrolyte, which allows ions to be transferred from one side to

the other (Fig. 12). The electrolyte in a PEM fuel cell is a solid acid supported within the

membrane. The solid acid electrolyte is saturated with water so that the transport of ions can

proceed.

Diagram of a single PEM fuel cell When an electrical load is attached across the anode and

the cathode of the fuel cell a redox reaction occur s. The work ing voltage produced by one cell

in thi s process is between 0.5 and 0.8 volts, depending on the load. To create practical working

voltages, individual fuel cel ls are stacked together in series to form a fuel cel l stack.

PEM Fuel Cell:



- (Eq. 3.1)

Cathode reaction: ½O2 + 2e-

At the anode, the hydrogen molecules first come into contact with a platinum catalyst on the

electrode surface. The hydrogen molecules break apart, bonding to the platinum surface forming

weak H-Pt bonds. As the hydrogen molecule is now broken the oxidation reaction can proceed.

Each hydrogen atom releases its electron, which travels around the external circuit to the cathode

(it is this flow of electrons that is refered to as electrical current). The remaining hydrogen proton

bonds with a water molecule on the membrane surface, forming a hydronium ion (H3O+). The

hydronium ion travels through the membrane material to the cathode, leaving the platinum

catalyst site free for the next hydrogen molecule.

At the cathode, oxygen molecules come into contact with a platinum catalyst on the electrode

surface. The oxygen molecules break apart bonding to the platinum surface forming weak O-Pt

bonds, enabling the reduction reaction to proceed. Each oxygen atom then leaves the platinum

catalyst site, combining with two electrons (which have travelled through the external circuit)

and two protons (which have travelled through the membrane) to form one molecule of water.

The redox reaction has now been completed. The platinum catalyst on the cathode electrode is

again free for the next oxygen molecule to arrive.

This exothermic reaction, the formation of water from hydrogen and oxygen gases, has an

enthalpy of -286 kilojoules of energy per mole of water formed. The free energy available to

perform work decreases as a function of temperature. At 25º C, 1 atmosphere the free energy

available to perform work is about -237 kilojoules per mole. This energy is observed as

electricity and heat.

The Polymer Electrolyte Membrane (PEM)

The membrane material used in a PEM cell is a polymer. PEMs are generally produced in large

sheets. The electrode catalyst layer is applied to both sides, and is cut to the appropriate size. A



single PEM sheet is typically between 50 to 175 microns thick, or around the thickness of 2 to 7

sheets of paper.

A common PEM material used today is Nafion®. Developed in the 1970s by Dupont, Nafion

consists of Polytetrafluoroethylene (PTFE) chains, commonly known as Teflon® forming the

backbone of the membrane. Attached to the Teflon chains, are side chains ending with sulphonic

acid (HSO3) groups (Fig. 13). A close-up view of the membrane material shows long, spaghetti-

like chain molecules with clusters of sulphonate side chains (Fig. 14). An interesting feature of

this material is that whereas the long chain molecules are hydrophobic (repel water), the

ulphonate side chains are highly hydrophylic (attract water).

For the membrane to conduct ions efficiently the sulphonate side chains must absorb large

quantities of water. Within these hydrated regions, the hydrogen ions of the sulphonic acid

groups can then move freely, enabling the membrane to transfer hydrogen ions, in the form of

hydronium ions from one side of the membrane to the other



Chemical structure of a PEM fuel cell membrane Long chains of PTFE (Teflon® ) with side

chain ending with sulphonic acid (HSO3) (Source: L armin ie & Dicks, February

Close-up of a PEM fuel cell membrane. Di agram shows long spaghetti -li ke chain molecules of

Teflon surr ounding clusters of hydrated regions around the sulphonate side chains. The ef lon

chains form the backbone of the membrane. The hydrated regions around the sulphonate side

chains become the electrolyte.

Cell Voltage and Efficiency

If the fuel cell was perfect at transferring chemical energy into electrical energy, the ideal cell

voltage (thermodynamic reversible cell potential) of the hydrogen fuel cell would be at 25º C, 1

atmosphere, 1.23 volts. As the fuel cell heats up to operating temperature, around 80º C the ideal

cell voltage drops to about 1.18 volts. However there are many limiting factors that reduce the

fuel cell voltage further. The voltage out of the cell is a good measure of electrical efficiency; the



lower the voltage, the lower the electrical efficiency and the more chemical energy is released in

the formation of water and transferred into heat.

The primary losses that contribute to a reduction in cell voltage are:

Activation losses. Activation losses are a result of the energy required to initiate the reaction.

This is a result of the catalyst. The better the catalyst the less activation energy is required.

Platinum forms an excellent catalyst however there is much research underway for better

materials. A limiting factor to power density available from a fuel cell is the speed at which the

reactions can take place. The cathode reaction, (the reduction of oxygen) is about 100 times

slower than that of the reaction at the anode, thus it is the cathode reaction that limits power

density.

Fuel crossover and internal currents. Fuel crossover and internal currents are a result of

fuel that crosses directly through the electrolyte, from the anode to the cathode without releasing

electrons through the external circuit, thereby decreasing the efficiency of the fuel cell.

Ohmic losses. Ohmic losses are a result of the combined resistances of the various

components of the fuel cell. This includes the resistance of the electrode materials, the resistance

of the electrolyte membrane and the resistance of the various interconnections.

Concentration losses (also referred to as “mass transport”). These losses result from the

reduction of the concentration of hydrogen and oxygen gases at the electrode. For example,

following the reaction new gases must be made immediately available at the catalyst sites. With

the buildup of water at the cathode, particularly at high currents, catalyst sites can become

clogged, restricting oxygen access. It is therefore important to remove this excess water, hence

the term mass transport

DIRECT METHANOL FUEL CELL

A direct methanol fuel cell also uses a PEM membrane. However, other catalysts in addition to

platinum are required on the anode side of the membrane to break the methanol bond in the

reaction forming carbon dioxide, hydrogen ions and free electrons. As with the hydrogen fuel

cell, the free electrons flow from the anode of the cell through an external circuit to the cathode



and the hydrogen protons are transferred through the electrolyte membrane. At the cathode the

free electrons and the hydrogen protons react with oxygen to form water.

Di rect Methanol Fuel Cell :

- (Eq. 4.1)

Cathode reaction: 3/2O2 + 6H+ + 6e-

6. WHERE WILL THE HYDROGEN COME FROM?

One of the most important questions to be asked is: where the hydrogen will come from? A very

interesting study published by the Pembina Institute, based in Calgary, Alberta, compared total

carbon dioxide emissions of fuel cell vehicles using hydrogen produced from a variety of

methods (Fig 15). The results clearly show that the choice as to which method will be used to

produce the hydrogen will be a critical environmental decision. For example, if hydrogen is

produced from the electrolysis of water and the electrolysers are powered from the electrical

grid, whereby the electricity is produced from a coal burning power station, then there will be no

reduction in carbon dioxide emissions compared with the levels of the present day internal

combustion engine. In fact, there will be an increase in metals & pollutants into the environment.

If on the other hand the electrolyzer is powered from a renewable energy source, through use of a

solar panel, a wind turbine or a hydroelectric turbine, there will be no emissions of carbon

dioxide. The choice has yet to be made as to which method of hydrogen production will

dominate as the fuel cell industry grows.

Carbon dioxide emissions (kilograms) per 1000 km



1 Car with internal combustion engine

2 Fuel cell car with hydrogen produced from Alberta electric grid (coal generation)

3 Fuel cell car with onboard gasoline reformer

4 Fuel cell car with onboard methanol reformer

5 Fuel cell car using hydrogen from natural gas (distributed from urban retail outlets)

6 Fuel cell car using hydrogen from natural gas (made at large refineries)

Graph comparing carbon dioxide emissions of cars, using different types of fuel sources

Reformation of hydrocarbon fuels

For the short term, because of the abundance of natural gas, the availability of methanol

and propane, and the lack of a hydrogen infrastructure, it is expected that hydrocarbon fuels will

be the dominant fuels for stationary fuel cell applications. For as long as these fuels are cheaplyavailable, reformation of a hydrocarbon fuel is the most cost efficient method of producing

hydrogen. In the reformation of a hydrocarbon fuel however, there is an emission of carbon

dioxide. Although carbon dioxide is not considered a pollutant, controversy exists that man-made

emissions may contribute to global warming.



Renewable Energy Systems

Hydrogen can be produced sustainably with no emission of carbon dioxide from

renewable energy systems. An example of such a system is the use of a solar panel, a wind

turbine or a micro-hydro generator to convert the radiant energy of sunlight into electrical power,

which drives an electrolyzer. The electrolyzer breaks apart water producing hydrogen and

oxygen gases. The hydrogen is stored for use by the fuel cell and the oxygen is released into the

atmosphere. Thus when the sun shines, the wind blows or the water flows, the electrolyser can

produce hydrogen. A power system incorporating hydrogen from renewable sources and a fuel

cell is a closed system, as none of the products or reactants, water, hydrogen and oxygen are lost

to the outside environment. The water consumed by the electrolyzer is converted to gases. The

gases are converted back to water. The electrical energy produced by the solar panel is

transferred to chemical energy in the form of gases. The gases can be stored and transported, to

be reconverted back to electricity These systems are truly sustainable, for as long as there is

sunlight there can be electrical power, available where and when required



Electri cal power fr om renewable energy sources. I n the past, the limit ing factors of r enewable

energy have been the storage and tr ansport of that energy. Wi th the use of an electrolyzer , a

method of storing and transporti ng hydrogen gas, and a fuel cell, electr ical power fr om

renewable energy sour ces can be delivered where and when requi red, cleanly, eff icientl y and

sustainably.

BENEFITS AND OBSTACLES TO THE SUCCESS OF FUEL CELLS AND THE

DEVELOPMENT OF A HYDROGEN-BASED ECONOMY

Benefits

• Fuel cells are efficient.



They convert hydrogen and oxygen directly into electricity and water, with no combustion in the

process. The resulting efficiency is between 50 and 60%, about double that of an internal

combustion engine.

• Fuel cells are clean.

If hydrogen is the fuel, there are no pollutant emissions from a fuel cell itself, only the

production of pure water. In contrast to an internal combustion engine, a fuel cell produces no

emissions of sulphur dioxide, which can lead to acid rain, nor nitrogen oxides which produce

smog nor dust particulates.

• Fuel cells are quiet.

A fuel cell itself has no moving parts, although a fuel cell system may have pumps and fans. As

a result, electrical power is produced relatively silently. Many hotels and resorts in quiet

locations, for example, could replace diesel engine generators with fuel cells for both main

power supply or for backup power in the event of power outages.

• Fuel cells are modular.

That is, fuel cells of varying sizes can be stacked together to meet a required power demand. As

mentioned earlier, fuel cell systems can provide power over a large range, from a few watts to

megawatts.

• Fuel cells are environmentally safe.

They produce no hazardous waste products, and their only by-product is water (or water and

carbon dioxide in the case of methanol cells).

• Fuel cells may give us the opportunity to provide the world with sustainable electrical

power.



Obstacles

At present there are many uncertainties to the success of fuel cells and the development of a

hydrogen economy:

• Fuel cells must obtain mass-market acceptance to succeed.

This acceptance depends largely on price, reliability, longevity of fuel cells and the accessibility

and cost of fuel. Compared to the price of present day alternatives e.g. diesel-engine generators

and batteries, fuel cells are comparatively expensive. In order to be competitive, fuel cells need

to be mass produced less expensive materials developed.

• An infrastructure for the mass-market availability of hydrogen, or methanol fuel initially,

must also develop.

At present there is no infrastructure in place for either of these fuels. As it is we must rely on the

activities of the oil and gas companies to introduce them. Unless motorists are able to obtain fuel

conveniently and affordably, a mass market for motive applications will not develop.

• At present a large portion of the investment in fuel cells and hydrogen technology has

come from auto manufacturers.

However, if fuel cells prove unsuitable for automobiles, new sources of investment for fuel cells

and the hydrogen industry will be needed.

• Changes in government policy could also derail fuel cell and hydrogen technology

development.

At present stringent environmental laws and regulations, such as the California Low Emission

Vehicle Program have been great encouragements to these fields. Deregulation laws in the utility



industry have been a large impetus for the development of distributed stationary power

generators. Should these laws change it could create adverse effects on further development.

• At present platinum is a key component to fuel cells.

Platinum is a scarce natural resource; the largest supplies to the world platinum market are from

South Africa, Russia and Canada. Shortages of platinum are not anticipated, however changes in

Government policies could affect the supply.

II. MODULAR FUEL CELL STACK

Fuel cell stacks are constructed by stacking several individual cells, which is

equivalent to connecting many voltage sources in series, each with its own internal impedance.

The fuel and oxygen input lines to each cell in the stack are connected in parallel in order to

ensure that the pressure on the anode and cathode of every cell is kept at a similar level. This is

done by means of manifolds that connect the fuel and oxygen lines to the actual cells in the

stack. The voltage produced by each cell in the stack, as well as its internal impedance, is afunction of fuel pressure, membrane humidity, and state of the catalyst. The fuel pressure on

each cell is, in theory, constant due to the input manifold, but in reality, it may drop due to water

condensation or other obstructions. Cells receiving a lower pressure will produce a reduced

voltage. The membrane humidity may vary from cell to cell depending on the heat distribution

within the fuel cell. Cells with a drier membrane will produce less voltage than cells with a more

moisturized membrane, and this will produce a voltage closer to its nominal. All these reasons

contribute to an uneven voltage distribution through the fuel cell stack. As an example, Fig. 3

shows the V – I characteristic measured from different cells in a 24-cell 12-V/150-W H2 – air

PEM fuel cell stack with an active area of 50 cm2 . The characteristics were obtained at room

temperature (20◦C) with a fuel pressure of 2 lbf/in2 ; the maximum cell temperature was

measured to be 62 ◦C.



It is clear from Fig. 3 that the voltage produced by each cell in the stack differs

from adjacent cells. Further, a set of cells is shown to produce less voltage when compared to a

F ig. 4 Individual cell P – I



F ig. 5 Modular fuel cell simpli fi ed equivalent cir cuit

Healthier group Fig. 4 shows the power (in watts) generated by each individual cell in this test

stack. Comparing the maximum power points P1, P2, and P3 in Fig. 4, it becomes evident that

underperforming cells in the fuel cell can produce less power than the cells that are in good

operating condition. Although, as shown in Fig. 3, fuel cells exhibit a nonlinear behavior in their

voltage – current characteristic, it is possible to use a linearized model to predict their behavior. In

steady state, the simplest electrical model that can be constructed consists of a Thevenin voltage

source (Vc ) in series with a resistor ( Rc ) [Fig. 5(a)] whose values are functions of fuel pressure,

humidity, and catalyst state, as discussed before. From this equivalent circuit model, the power that a single cell in the stack can supply can be calculated by



Additionally, from circuit theory, the maximum power that can be extracted from such an

electric circuit is given by

From (1) and (2), it is apparent that increasing the load of the fuel cell beyond the maximum

power point given by (2) results in increased losses and reduced output power, as can be

observed from Fig. 4. Therefore, the load current should be limited to avoid going beyond the

maximum power point to a value given by

Further, since in a conventional system all the cells in the stack are connected

in series, the load current in every cell has to be limited to the maximum current that the weakest

cell in the system can supply, which is around 8 A in the case shown in Figs. 3 and 4.

Considering this fact, the maximum output power of the stack can be calculated by

If the load current exceeds the limit given by (3) for a long period of time, the

stack overheats due to the additional internal losses in the mal performing cells, and operation of

the fuel cell has to be discontinued. However, from Fig. 4, it is clear that healthier cells, which

have a smaller internal resistance and larger open circuit voltage, can supply higher load currents

(10 or 12 A). From this analysis, it appears that to avoid limiting the current in healthy cells due

to the presence of bad cells in the system, a modular approach is more suitable. This modular

system is shown in Fig. 5(b), where additional terminals on each cell allow loading cells



independently, and therefore, healthier cells can generate more power than in the conventional

series-connected approach. For this modular case, the maximum power that can be generated in

the stack is given by the sum of the maximum powers of all cells, and can be calculated by

To compare both approaches, traditional and modular, the maximum power that can

be produced by each is evaluated. For this, let us consider a stack constructed with three cells

and with the equivalent circuit parameters as shown in Table I, where V a and Ra are the base

voltage and resistance of the system. Using the parameters of Table I and (3), we can find that in

the case of the conventional approach, the load current should be limited to 0.32 V a /Ra ; this is

due to the internal impedance



F ig. 6. (a) Schematic drawing of the modular fuel cell stack. (b) Prototype of the 12-V, 150-W,

24-cell modular fuel cell stack.

of cell 3, which is the weakest cell in this stack. Thus, according to (4), the maximum power that

can be produced in this case is 0.534 V 2 a /Ra . Now, for themodular approach where each cell

is loaded independently [Fig. 5(b)] using (5), the maximum power that the system can generate is

0.568 V 2 a /Ra . In other words, the same stack can produce 6.4% more power if the cells are

loaded independently.

From these results, it is shown that to optimize the operation of the stack, one should be

able to control the current flowing through each individual cell in the stack [Fig. 5(b)]. But such

an approach proves to be impractical as well as uneconomical. A more convenient approach is to

divide the stack in sections of five to ten cells, as shown in Fig. 2, which is done by installing



additional electric terminals in the stack (Fig. 6). Having access to these additional terminals

allows loading each section differently, which, in turn, allows maximizing the power generated

by the stack.

This has the obvious advantage of increasing the overall reliability of the system.

Fig. 7 shows the V – I characteristic of each of the three sections in the prototype fuel cell stack

measured at room temperature with a fuel pressure of2 lbf/in2. It can be observed that the

performance of each of the sections is quite different. The nominal current of the stack (12 A)

can only be drawn from sections 1 and 3. On the other hand, section 2 can only supply a

maximum current of 9 A before its voltage collapses.

Fig. 8 shows the power produced by each section in the fuel cell stack as function of

the load current. If a traditional approach is used, to avoid overheating, the current in the stack

should be limited to a value given by its weakest section (section 2, 9 A),

F ig. 7 V – I characteri stics of the proposed modular fuel cell stack with three sections.



F ig. 8 Output power produced by each section of the modular fuel cell stack

but due to the modular construction of the system, the other sections can be operated at different

load currents to optimize their operation. Table II shows a comparison of the power that this

prototype stack can produce if operated in a traditional or modular fashion. These results are

obtained from Fig. 8, and the results for the traditional approach are obtained by multiplying the

section voltages by the maximum current that the weakest section can produce (9 A). On the

other hand, the power for the modular approach is calculated from the maximum power point for

each of the sections in Fig. 8. It can be seen from Table II that by loading each section of the proposed modular stack differently, the fuel cell can produce 10% more power than using a

conventional stack. Moreover, this result shows that despite having underperforming cells in the

system, the power generated is close to the stack nominal.

DC-DC Power Converters

Dc-dc power converters are employed in a variety of applications, including

power supplies for personal computers, office equipment, spacecraft power systems, laptop

computers, and telecommunications equipment, as well as dc motor drives. The input to a dc-dc

converter is an unregulated dc voltage Vg. The converter produces a regulated output voltage V,

having a magnitude (and possibly polarity) that differs from Vg. For example, in a computer off-



line power supply, the 120 V or 240 V ac utility voltages is rectified, producing a dc voltage of

approximately 170 V or 340 V, respectively. A dc-dc converter then reduces the voltage to the

regulated 5 V or 3.3 V required by the processor ICs. 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.

Pulse-width modulation (PWM) allows control and regulation of the total output voltage. This

approach is also employed in applications involving alternating current, including high-

efficiency dc-ac power converters (inverters and power amplifiers), ac-ac power converters, and

some ac-dc power converters (low-harmonic rectifiers). A basic dc-dc converter circuit known as

the buck converter is illustrated in Fig. 1. A single-pole double-throw (SPDT) switch is

connected to the dc input voltage V g as shown. The switch output voltage

.



The buck converter consists of a switch network that reduces the dc component of voltage, and a

low-pass filter that removes the high-frequency switching harmonics: (a) schematic, (b) switch

voltage waveform

vs(t ) is equal to V g when the switch is in position 1, and is equal to zero when the switch is in

position 2. The switch position is varies periodically, such that vs(t ) is a rectangular waveform

having period T s and duty cycle D. The duty cycle is equal to the fraction of time that the switch

is connected in position 1, and hence 0 D 1. The switching frequency f s is equal to 1/T s. In

practice, the SPDT switch is realized using semiconductor devices such as diodes, power

MOSFETs, IGBTs, BJTs, or thyristors. Typical switching frequencies lie in the range 1 kHz to 1

MHz, depending on the speed of the semiconductor devices. The switch network changes the dc

component of the voltage. By Fourier analysis, the dc component of a waveform is given by its

average value. The average value of vs(t ) is given by

The integral is equal to the area under the waveform, or the height V g multiplied by the time

DT s. It can be seen that the switch network reduces the dc component of the voltage by a factor

equal to the duty cycle D. Since 0 D 1, the dc component of V s is less than or equal to V g.

The power dissipated by the switch network is ideally equal to zero. When the switch contacts

are closed, then the voltage across the contacts is equal to zero and hence the power dissipation is

zero. When the switch contacts are open, then there is zero current and the power dissipation is

again equal to zero. Therefore, the ideal switch network is able to change the dc component of

voltage without dissipation of power.

In addition to the desired dc voltage component V s, the switch waveform vs(t ) also contains

undesired harmonics of the switching frequency. In most applications, these harmonics must be

removed, such that the converter output voltage v(t ) is essentially equal to the dc component V =

V s. A low-pass filter is employed for this purpose. The converter of Fig. 1 contains a single-

section L-C low-pass filter. The filter has corner frequency f 0 given by



The corner frequency f 0 is chosen to be sufficiently less than the switching frequency f s, so that

the filter essentially passes only the dc component of vs(t ). To the extent that 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 that (ideally) do not dissipate power. The conversion ratio M ( D)

is defined as the ratio of the dc output voltage V to the dc input voltage V g under steady-state

conditions:

For the buck converter, M ( D) is given by

M ( D) = D (4

This equation is plotted in Fig. 2. It can be seen that the dc output voltage V is controllable

between 0 and V g, by adjustment of the duty cycle D. Figure 3 illustrates one way to realize the

switch network in the buck converter, using a power MOSFET and diode. A gate drive circuit

switches the MOSFET between the conducting (on) and blocking (off) states, as commanded by

a logic signal (t ). When (t ) is high (for 0 < t < DT s), then MOSFET Q1 conducts with negligible

drain-to-source voltage. Hence, vs(t ) is approximately equal to V g, and the diode is reverse-

biased. The positive inductor current iL(t ) flows through the MOSFET. At time t = DT s, (t )

becomes low, commanding MOSFET Q1 to turn off. The inductor current must continue to flow;hence, iL(t ) forward-biases diode D1, and vs(t ) is now approximately equal to zero. Provided that

the inductor current iL(t ) remains positive, then diode D1 conducts for the remainder of the

switching period. Diodes that operate in the manner are called freewheeling diodes.

Since the converter output voltage v(t ) is a function of the switch duty cycle D, a control



system can be constructed that varies the duty cycle to cause the output voltage to follow a given

reference vr. Figure 3 illustrates the block diagram of a simple converter feedback system. The

output voltage is sensed using a voltage divider, and is compared with an accurate dc reference

voltage vr. The resulting error signal is passed through an op-amp compensation network. The

analog voltage vc(t ) is next fed into a pulse-width modulator. The modulator produces a switched

voltage waveform that controls the gate of the power MOSFET Q1. The duty cycle D of this

waveform is proportional to the control voltage vc(t ). If this control system is well designed, then

the duty cycle is automatically adjusted such that the converter output voltage v follows the

reference voltage vr, and is essentially independent of variations in vg or load current

Reali zation of the ideal SPDT switch using a transistor and fr eewheeli ng diode. In additi on, a

feedback l oop is added for r egulation of the output voltage .

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 [1-5]. Figure 4 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 IGBTs, BJTs, or thyristors can be substituted if desired. 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 V g. 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. The

single-ended primary inductance converter (SEPIC) can also either increase or decrease the

voltage magnitude. However, it does not invert the polarity. The conversion ratio is

M ( D) = D/(1 – D).



Several basic dc-dc converters and their dc conversion rati os M(D ) = V/Vg .

Transformer isolation

In the majority of applications, it is desired to incorporate a transformer into the

switching converter, to obtain dc isolation between the converter input and output. For example,

in off-line power supply applications, isolation is usually required by regulatory agencies. This

isolation could be obtained by simply connecting a 50 Hz or 60 Hz transformer at the power



supply ac input terminals. However, since transformer size and weight vary inversely with

frequency, incorporation of the transformer into the converter can make significant

improvements: the transformer then operates at the converter switching frequency of tens or

hundreds of kilohertz. The size of modern ferrite power transformers is minimized at operating

frequencies ranging from several hundred kilohertz to roughly one egahertz. These high

frequencies lead to dramatic reductions in transformer size. When a large step-up or step-down

conversion ratio is required, the use of a transformer can allow better converter optimization. By

proper choice of the transformer turns ratio, the voltage or current stresses imposed on the

transistors and diodes can be minimized, leading to improved efficiency and lower cost.

Multiple dc outputs can also be obtained in an inexpensive manner, by adding

multiple secondary windings and converter secondary-side circuits. The secondary turns ratios

are chosen to obtain the desired output voltages. Usually, only one output voltage can be

regulated, via control of the converter duty cycle, so wider tolerances must be allowed for the

auxiliary output voltages. Cross regulation is a measure of the variation in an auxiliary output

voltage, given that the main output voltage is regulated perfectly. The basic operation of

transformers in most power converters can be understood by replacing the transformer with the

simplified model illustrated in Fig. 8. The model neglects losses and imperfect coupling between

windings; such phenomena are usually considered to be converter nonidealities. The model

consists of an ideal transformer plus a shunt inductor known as the magnetizing inductance LM.

This inductor models the magnetization of the physical transformer core, and hence it must obey

all of the usual rules for inductors. In particular, volt-second balance must be maintained on the

magnetizing inductance. Furthermore, since the voltages of all windings of the ideal transformer

are proportional, volt-second balance must be maintained for each winding. Failure to achieve

volt-second balance leads to transformer saturation and, usually, destruction of the converter.

The means by which transformer volt-second balance is achieved is known as the transformer

reset mechanism.

T here are several ways of incorporating transformer isolation into any dc-dc

converter. The full bridge, half-bridge, forward, and push-pull converters are commonly used

isolated versions of the buck converter. Similar isolated variants of the boost converter are

known. The fly back converter is an isolated version of the buck-boost converter. Isolated



variants of the SEPIC and Cuk converter are also known. The full-bridge, forward, and fly back

converters are briefly described in this section

Modeling a physical transformer such that its basic operation within an isolated dc-dc converter

can be understood: (a) transformer schematic symbol, (b) equivalent circuit model that includes

magnetizing inductance LM and an ideal transformer

There are several ways of incorporating transformer isolation into any dc-dc converter. The full

bridge, half-bridge, forward, and push-pull converters are commonly used isolated versions of

the buck converter. Similar isolated variants of the boost converter are known. The flyback



converter is an isolated version of the buck-boost converter. Isolated variants of the SEPIC and

Cuk converter are also known. The full-bridge, forward, and flyback converters are briefly

described in this section.

The fu ll bridge transformer-isolated buck converter

Waveforms of the ful l bri dge circui t of F ig.



Ful l -bridge buck-deri ved converter

The full-bridge transformer-isolated buck converter is sketched in Fig. 9. Typical waveforms are

illustrated in Fig. 10. The transformer primary winding is driven symmetrically, such that the net

volt-seconds applied over two switching periods is equal to zero. During the first switching

period, transistors Q1 and Q4 conduct for time DT s. The volt-seconds applied to the primary

winding during this switching period are equal to V g DT s. During the following switching period,

transistors Q2 and Q3 conduct for time DTs, thereby applying – V g DT s volt-seconds to the

transformer primary winding. Over two switching periods, the net applied volt seconds is equal

to zero. In practice, there exist small imbalances such as the small differences in the transistor

forward voltage drops or transistor switching times, so that the average primary winding voltage

is small but nonzero. This nonzero dc voltage can lead to transformer saturation and destruction

of the converter. Transformer saturation under steady state conditions can be avoided by placing

a capacitor in series with the transformer primary. Imbalances then induce a dc voltage

component across the capacitor, rather than across the transformer primary. Another solution is

the use of current programmed control; the series capacitor is then omitted. By application of the

principle of volt-second balance to the output filter inductor voltage, the dc load voltage can be

shown to be

V = nDV g

So, as in the buck converter, the output voltage can be controlled by adjustment of the transistor

duty cycle D. An additional increase or decrease of the voltage V can be obtained via the

physical transformer turns ratio n.

The full bridge configuration is typically used in switching power supplies at power levels of

approximately 750 W or greater. At lower power levels, approaches such as the forward

converter are preferred because of their lower parts count. Four transistors and their associated

drive circuits are required. The utilization of the transformer is good, leading to small

transformer size. The transformer operating frequency is one-half of the transistor switching

frequency.



.

The forward converter , a single-transistor isolated buck converter

Forward converter

The forward converter is illustrated in Fig. 11. This transformer-isolated converter is also based

on the buck converter. It requires a single transistor, and therefore finds application at power

levels lower than those encountered in the full bridge circuit. The maximum transistor duty cycle

is limited in value; for the common choice n1 = n2, the duty cycle is limited to the range D < 0.5.

The transformer is reset while transistor Q1 is in the off state. While the transistor conducts, the

input voltage V g is applied across the transformer primary winding. This causes the transformer

magnetizing current to increase. When transistor Q1 turns off, the transformer magnetizing

current forward biases diode D1, and hence voltage – V g is applied to the second winding. This

negative voltage causes the magnetizing current to decrease. When the magnetizing current

reaches zero, diode D1 turns off. Voltsecond balance is maintained on the transformer windings

provided that the magnetizing current reaches zero before the end of the switching period. It can

be shown that this occurs when



For the common choice n2 = n1, this expression reduces to

Hence, the maximum duty cycle is limited. If this limit is violated, then the transistor off time is

insufficient to reset the transformer. There will then be a net increase in the transformer

magnetizing current over each switching period, and the transformer will eventually saturate.

The converter output voltage can be found by application of the principle of inductor volt-second

balance to the output filter inductor L. The result is

This expression is subject to the constraint given in Eq. (12).

F igure 12 A two-transistor version of the forward converter

A two-transistor version of the forward converter is illustrated in Fig. 12. Transistors

Q1 and Q2 are controlled by the same gate drive signal, such that they conduct simultaneously.

After the transistors turn off, the transformer magnetizing current forward-biases diodes D1 and



D2. This applies voltage – V g across the primary winding, thereby resetting the transformer. The

duty cycle is again limited to D < 0.5.

This converter has the advantage that the transistor peak blocking voltage is

limited to V g, and is clamped by diodes D1 and D2. This circuit is quite popular in power

supplies having 240 Vac inputs

Figure 13. The fly back converter, a single-transistor isolated buck-boost converter

F ly back converter

The flyback converter of Fig. 13 is based on the buck-boost converter. Although the two-winding

magnetic device is represented using the same symbol as the transformer, a more descriptive

name is “twowinding inductor.” This device is sometimes also called a “flyback transformer.”

Unlike the ideal transformer, current does not flow simultaneously in both windings of the

flyback transformer. Rather, the flyback transformer magnetizing inductance assumes the role of

the inductor of the buck-boost converter. The magnetizing current is switched between the

primary and secondary windings.

When transistor Q1 conducts, diode D1 is reverse-biased. The primary winding then

functions as an inductor, connected to the input source V g. Energy is stored in the magnetic field

of the flyback transformer. When transistor Q1 turns off, the current ceases to flow in the



primary winding. The magnetizing current, referred to the secondary winding, now forward-

biases diode D1. Energy stored in the magnetic field of the flyback transformer is then

transferred to the dc load. Application of the principle of inductor volt-second balance to the

transformer primary winding leads to the following solution for the conversion ratio of the

flyback converter:

Thus, the conversion ratio of the flyback converter is similar to that of the buck-boost converter,

but with an added factor of n. The flyback converter has traditionally been used in the high-

voltage power supplies of televisions and computer monitors. It also finds widespreadapplication in switching power supplies at the 50 W to 100 W power range. This converter has

the advantage of very low parts count. Multiple outputs can be obtained using a minimum

number of added elements: each auxiliary output requires only an additional winding, diode, and

capacitor. However, in comparison with buck-derived transformer-isolated converters such as

the full bridge and forward circuits, the flyback converter has the disadvantage of poor cross

regulation

III. PROPOSED MODULAR DC – DC CONVERTER

To take advantage of the modular fuel cell stack, an appropriate dc – dc converter and

control scheme are required. The converter should have as many independently controllable

inputs as there are sections in the stack. In addition, since the positive terminal of one section in

the stack also serves as the



Fig. 9 Proposed modular dc – dc converter

Negative terminal for the next section, the converter should provide isolation

between its input and output to avoid circulating currents. A converter meeting these

specifications can be constructed by using an arrangement of isolated dc – dc converter modules,

where the inputs of each module are connected across each of the sections of the stack and their

outputs are connected in series in order to add the output voltages of the different modules, thus

obtaining a higher output voltage. Such a modular dc – dc converter is shown in Fig. 9, where the

converter is composed of three push – pull modules.

As discussed earlier, another advantage of constructing a fuel cell stack with

several sections is that faulty portions of the stack can be bypassed, while the rest of the stack

can continue operation. To implement this function, each of the modules used to construct the

dc – dc converter should be able to stop extracting power from the section they are connected to

and set its output impedance to zero. This function can be accomplished by removing the gatingsignals to the transistors. In addition, it is necessary to add a switch ( Sx ) at the output of each

module to short-circuit the output capacitor of the module and bypass it. In order to optimize the

power extraction from each of the sections in the fuel cell, an appropriate control scheme needs

to be devised. From Figs. 7 and 8, it can be observed that the voltage across the terminals of each



section in the stack is a good indication of how much power it can generate; thus, this

information can be used to better distribute the power extracted from each section.

A section producing a higher voltage can generate more power than a section that

produces a lower voltage. Therefore, by controlling the load current on each section in the stack

as a function of the voltage, they produce results in healthier sections supplying more power than

underperforming sections. This, in turn, reduces internal losses and improves the overall

efficiency of the system. Since the outputs of the modules are connected in series, their output

currents io are identical. Now, if the modules are constructed by push – pull converters, the input

current of every module is given by (6), where Dn is the duty cycle of the nth module and N 1

and N 2 are the transformer primary and secondary turns

Fig. 10. Proposed control scheme.

Thus, the input current of each module can be controlled by setting an appropriate duty

cycle. The duty cycle for each module is calculated as shown in Fig. 10. In this block diagram,

the output voltage of the converter is maintained constant to the value set by VO, ref. The output

of the main voltage loop compensator is then used to calculate the required duty cycle for each

dc – dc converter by multiplying it with the corresponding reference signal for each module.



These reference signals are calculated by taking into account the voltage produced by each of the

sections in the stack and the number of modules that compose the dc – dc converter. Each of the

reference signals is calculated by the weighting function shown as

Where VSn is the voltage produced by the “nth” section in the fuel cell stack, VSi is the voltage

produced by the “ith” section in the stack, and NAC is the total number of active sections in the

stack. Thus, the reference signal for the “nth” module is given by the ratio between the voltage

produced by the “nth” section in stack and the total voltage produced by the stack. The number

of active sections is defined by all the sections that produce a voltage above a minimum value.

Now, if one of the sections produces a voltage below this threshold level, then that section can be

considered faulty. Thus, it cannot produce power and needs to be discarded. In this case, the

controller reduces NAC by 1 and sets the reference signal to the respective module to zero.

Additionally, this has the effect of increasing the reference signals of the remaining modules to

compensate for the loss of one stack section.The implementation of this control scheme can be carried out by combining digital and

analog controllers. The calculation of the reference signals for each of the modules is done

digitally by means of a DSP. The reference signals are then feed to analog controllers located on

each of the dc – dc modules.

IV. EXPERIMENTAL RESULTS

To verify the operation of the proposed fuel cell stack and converter, a laboratory

prototype was built. The test system is composed of a 12-V/150-WH2 – air PEM modular fuel

cell stack consisting of three sections of eight cells, each with an active area of 50 cm2 , and a

modular dc – dc converter composed of three push – pull modules. The dc – dc converter is designed

to supply a 22-V load; thus, if all the sections in the fuel cell produce the same voltage across



their terminals, each module needs to provide one-third of the total output voltage Vo and output

power.

F ig. 11 Modul ar fuel cell section voltages dur ing a load transient

However, since the dc – dc converter has to be designed for continue operation under

the condition of having faulty sections, each module is designed in order to provide the total

output voltage Vo of the converter and one-third of its output power, i.e., 22 V and 50 W. The

dc – dc converter and modules are connected as shown in the schematic in Fig. 9. This prototype

system was tested at room temperature and with an H2 supply at a pressure of 2 lbf/in2

As a first test, a load change is applied at the output terminals of the converter to

verify that the controller adjusts the loading of each section according to their relative health.

Fig. 11 shows the voltage across each of the sections in the prototype stack when the output load

of the system increases from 40% to 90%. During this test, the voltage at the output terminals of

the converter was maintained constant at 22 V. In this figure, Ch. 1 shows the voltage in section

3 (Vs3 ), Ch. 2 the voltage in section 1 (Vs1 ), and Ch. 3 the voltage in section 2 (Vs2 ). As can

be seen from Fig. 11, initially, the voltages of the three sections were 6.4, 5.6, and 6.2 V,

respectively. After the load increases, the controller adjusts the module reference signals in order



to maintain the output voltage of the converter constant. And thus, the three section voltages

drop due to the increase in the output load to 6, 5.5, and 5 V.

Fig. 12 shows the current drawn from each section in the stack before and after the

load change. As can be observed from these results before the load change, the voltage supplied

by section 3 was the highest in the stack; consequently, the current and power supplied by it are

the highest. On the other hand, the voltage produced by section 2 in the prototype stack is the

lowest, and therefore, the current drawn from it is less than the other two sections.

After the transient, the voltage across each section drops due to the increase in

output load, and the currents drawn from the three sections increase to maintain the output

voltage of the system constant; however, their magnitudes are different. As can be seen from

Figs. 11 and 12, the section producing the highest voltage carries a larger share of the output

power and the weakest section produces a smaller portion of the load.

The other functionality offered by the proposed converter is the ability to discard a

section of the fuel cell if the controller detects that the voltage across its terminals drops below a

certain

F ig. 12. Modular fuel cell section curr ents dur ing a load transient



Fig. 13 Operation of the modular fuel cell modular dc – dc converter under a fault Note

that in section 2, current is reduced to zero, while the output voltage continues to be regulated

Threshold level The prototype fuel cell is rated for 12 V at full load, and the

nominal voltage of each of the sections at full-load is 4 V. Therefore, if a section is faulty, its

terminal voltage will fall below this value. For this reason, the threshold level in the controller

was set to 3.8 V. Fig. 13 shows the behavior of the system when a faulty section is detected,

where Ch. 1 corresponds to the current drawn from section 3 ( Is3 ), Ch. 3 the output voltage of

the dc – dc converter (Vo ), and Ch. 4 the current drawn from section 2 ( Is2 ).

In this case, section 2 in the stack is faulty, and it has to be discarded to avoid stack

shutdown due to overheating. As can be seen in Fig. 13, once the fault condition is detected, the

current drawn from the faulty section (section 2, Ch. 4) falls to zero In order to maintain the



output voltage of the system constant, the currents drawn from the remaining sections in the

stack ( Is1 and Is3 ) have to increase. This can be observed from Fig. 13, where the current

supplied by section 3 ( Is3 ) increases from 5 to 7 A after section 2 is discarded. The increase in

the magnitude of the currents drawn from the remaining sections is regulated in terms of their

relative health as determined by the converter control. As can be seen from these results, the

system can continue operation despite having a faulty section; thus, the modular approach

exhibits higher reliability than the traditional approach. To further verify the effectiveness of the

proposed approach, thermal images of the fuel cell stack operating in both single stack and

modular stack modes were taken. Fig. 14(a) shows a thermal image of the prototype stack

operating in conventional mode. Load current in this case is 7.25 A, and the voltage of the

F ig. 14 Thermal comparison of conventi onal and modular fuel cell



Stack was measured to be 12 V, which is the nominal voltage of the fuel cell for full-load. The

power generated by the stack operating under this condition was measured to be 87 W. It can be

observed that the temperature distribution is quite uneven due to the presence of bad cells in

section 2, while sections 1 and 3 show a lower temperature indicating that they are underused.

The result of reconfiguring the stack for modular operation and the use of the proposed dc – dc

converter is shown in Fig. 14(b). In this case, the voltage across each of the sections was

regulated by the dc – dc converter modules to 4 V, i.e., the nominal voltage for each section. The

currents drawn from sections 1 – 3 in the stack were measured to be 10, 6, and 9 A, respectively.

Thus, the power generated by the fuel cell in this case is 102 W. As can be seen from Fig. 14(b),

the temperature distribution within the stack in this case is even, indicating full utilization of the

three sections. Moreover, due to the use of the modular approach, the fuel cell generates 15%

more power than in the conventional case [Fig. 14(a)].

V. CONCLUSION

In this paper, a modular fuel cell stack and dc – dc converter concept has been

presented. It has been shown that the standard fuel cell stack can be reconfigured into several

sections with smaller cell count, each supplying an isolated power module in the dc – dc

converter, resulting in a high-performance system. The proposed system has been shown to be

fault tolerant and can continue to operate at a reduced power level under fuel cell or power

module faults. Experimental results on a 12-V/150-W system demonstrate that under normal

operation, the proposed system is capable of producing 10% additional power when compared to

the traditional approach. In addition, experimental results also confirm the operation of the

system under stack failure.



MAIN CIRCUIT



CONTROL CIRCUIT



RESULTS

9.a modular fuel cell, modular dc dc converter

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