study on fuel cells

7/30/2019 study on fuel cells

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cognomen for any form of energy production with non-toxic

(pollutant free) by product; in more colloquial terms green-tech is

an earth friendly process of production. In a green-tech

production method the application of one or more of

environmental science, green chemistry, environmental

monitoring, and electronic devices to model, monitor and

conserve the natural environment and resources, as well as to

curb the negative impact of human application of scientific

knowledge. [2]

Fuel cell technology is a form of green-tech that has shown

much promise and is a possible replacement for fossil fuel

systems of power generation. As already stated the fuel cell

converts hydrogen or hydrogen containing fuels, directly into

electrical energy. 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. For a fuel cell, in the burning of a hydrocarbon, as the

hydrogen content of the fuel being fed to the fuel cell increases,

the formation of water becomes more significant while there is a

resulting proportional emission reduction of carbondioxide.

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1.1 STATEMENT OF STUDY

Within the last century many developments have lead to the

much needed research to find a new means of power generation,

problems that have lead to such endeavors includes:

GLOBAL WARMING: This is a rise in the average temperature of

the earth due to rising level of green house gases .Heat engines

and most conventional methods of power generation emit

greenhouse gases; inadvertently speeding up global warming.

POLLUTIONOF THEENVIRONMENT: One of the drawbacks of most

methods of power generation is the release of different pollutants

into the environment. Some examples of such pollutants include

hot water, toxic waste, carbonmonoxide and green house gases,

radio active by products etc.

SEARCH FOR HIGHER LEVELS OF EFFICIENCY: For machines the

total input energy is not used for work, some portion of it (input

energy) is lost as losses. In a bid to conserve the available

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resources, higher levels of efficiency are sort after in the design of

machines.

SUSTAINABILITY OF ENERGY RESOURCES: After World War II

scientists became highly aware of the need to conserve fossil fuel,

since then many efforts have been made to find a more

sustainable energy source, or at least, a more conservative

method of using energy resources.

1.2 PURPOSE OF STUDY

This research work is aimed at studying and describing fuel

cell technology. This includes taking into perspective the following

features of a fuel cell:

History and developments in fuel cell technology

Working principles of a fuel cell

Types of fuel cells and various applications most suited for

the various types.

Fuel cell systems.

Performance and efficiency of fuel cells

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Fuel cells are referred to as the replacement for heat

engines; this work looks into the cause of such claims, the

feasibility, and final economic impact.

1.3 SIGNIFICANCE OF STUDY

The various conclusions drawn from this study will act

as a theoretical backing for the implementation of fuel cell

technology in the Nigerian electrical infrastructure, as applies to

distributed generation, cogeneration, and sustainable power in

the country.

1.4 SCOPE OF THE STUDY

The study covers all the fundamental knowledge and

concepts upon which the fuel cell is built upon, as well as a

theoretical approach to the technology itself. Various charts and

equations are used in the write-up, all of which are gotten from

trusted and reputable sources.

1.5 DEFINITION OF TERMS

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COGENERATION: This is a process where by a power

production unit simultaneously generates both electricity

and useful heat.

POWER DENSITY: This is the amount of power (time rate of

energy transfer) per unit volume in energy transformers,

expressed as 3m

W .

PART-LOAD: This is the partial load value of a system. It is a

load value less than the fuel load but greater than no load.

STANDARD POTENTIAL: A measure of individual potential of

a reversible electrode at standard state i.e. solute

concentration of 1 mol dm-3

, temperature of 250

c (298k), and

pressure of 1 atm.

ELECTRODES: This an electrical conductor used to make

contract with a nonmetallic part of a circuit i.e.

semiconductor, electrolyte, or vacuum. The electrodes are

either;

Anode: This is the electrode at which electrons leave a cell

and oxidation occurs.

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Cathode: This is the electrode at which electrons enter the

cell and reduction occurs.

ELECTROLYTES: This is a liquid, gel, or solid which contains

ions and can be decomposed by electrolysis.

CHEMICAL KINETICS: This is the rate of chemical processes

(chemical reaction) with regards to changes brought about

by environmental conditions (pressure, temperature,

volume, light etc.)

OXIDATION: Loss of electrons or an increase in oxidation

state of a molecule, atom or ion

REDUCTION: Gain of electrons or a decrease in oxidation

state of a molecule atom or ion.

MEMBRANE: A selective barrier that allows the movement of

some selected particles or chemical through it, while

blocking out others.

CATALYST: A reagent that changes the rate of a chemical

reaction without being consumed by the reaction.

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ENTROPY: Measure of a systems thermal energy per unit

temperature that is unavailable for doing work.

EXOTHERMIC: Reaction process that releases energy from a

system in the form of heat.

ENTHALPY: This is the measure of the total energy of a

thermodynamic reaction

ACTIVATION ENERGY: The energy that must be overcome in

order for a chemical reaction to occur.

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CHAPTER TWO

LITERATURE REVIEW

2.1 HISTORY OF FUEL CELLS

The principle of the fuel cell was discovered by German

Scientist Christian Friedrich schonbein in 1838 and published in

one of the Scientific Magazines of the time [2]. Based on this work,

the actual first running fuel cell was developed by Sir William

Grove in 1839. The principle was discovered by accident during

an electrolysis experiment. When Sir William disconnected the

battery from the electrolyser and connected the two electrodes

together, it was observed that current was flowing in the other

direction (opposite direction) consuming gases of hydrogen and

oxygen. He called this device a gas battery. This fuel cell he

made used many similar materials to todays phosphoric-acid fuel

cell. The set up consisted of platinum electrodes placed in test

tubes of hydrogen and oxygen, immersed in a bath of dilute

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sulphuric acid. It generated voltage of about 1V. However due to

problems of corrosion of the electrodes and instability of the

materials the cell (gas battery) was not very practical. As a result

their was little research and further development of fuel cells for

many years to follow.

Significant work on fuel cells began again in the 1930s, by

Francis Bacon, a chemical engineer at the university of

Cambridge. In the 1950s Bacon Successfully produced the first

practical fuel cell, which was an alkaline version. It used an

alkaline electrolyte (Molten KOH) potassium hydroxide instead of

dilute sulphuric acid. The electrodes were constructed of porous

sintered nickel powder so as to let the gases diffuse through the

electrodes and be in contract with the aqueous electrolyte on the

other side. This design greatly increased the surface area of

contact between the electrodes, gas, and the electrolyte. Thus

increasing the power density of the fuel cell, the chemical

reactions were [1].

Anode: 2H2 + 4OH- 4H2O + 4e

-

Cathode: O2 + 4e- + 2H20 4OH

-

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Overall reaction 2H2+ O2 2H20

Fuel cell research picked up again after the Bacon module

was inspected and seen to have potential. Fuel cells under went

some more changes and where found very viable for space travel

applications. For space applications, fuel cells have the advantage

over conventional batteries in that they produce several times

more energy per equivalent unit of weight. In 1960s, international

fuel cells in Windsor, Connecticut, USA, developed a fuel cell

power plant for the Apollo spacecraft, which provided both

electricity as well as drinking water for the astronauts on their

journey to the moon. The fuel cell could supply 1 to 5 kW of

continuous electrical power. The fuel cell which was used on the

Apollo mission lasted over 10000 hours of operation after 18

missions without a single-in-flight incident. It should be known

that there were no back-up batteries on the space shuttle, thus

the fuel cells must be highly reliable. The reliability of fuel cells

used for space missions (such as the Apollo shuttle or the orbiter

space shuttle) is as high as 99%.

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The fuel cells that where used for space missions where the

alkaline fuel cells type. Compared with the 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 anions in an alkaline

environment. This made alkaline fuel cells ideal for space

applications. However, for terrestrial use, the alkaline fuel cells

where not the best of choices. This is due to the carbondioxide

poisoning of the electrolyte. Carbondioxide 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 carbondioxide reacts with

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

reducing the efficiency of the fuel cell by reducing the hydroxide

ion concentration of the electrolyte [1]. An example of such a

reaction is as thus (using KOH as the case study alkaline).

2KOH + CO2 K2 CO3 + H20

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Because of the complexities involved in isolating carbondioxide

from the alkaline electrolyte in fuel cells, most fuel cell developers

have focused on new types of electrolytes that are non-alkaline.

2.2 THE CHEMISTRY OF A SINGLE CELL

In a fuel cell, two half-cell reactions take place

simultaneously, an oxidation reaction (loss of electrons) at the

anode and a reduction reaction (gain of elections) at the cathode.

These two reactions known as a redox reaction (reduction-

oxidation) are what fuel cell works on to form water from

hydrogen and oxygen.

As an electolyser, the anode and cathode are separated by

an electrolyte, which allows ions to be transferred from one side

to the other. The electrolytes (which for PEM and PAEC are acids)

which is supported with a membrane and normally uses platinum

as an electrode catalysis, is wedged between the anode and the

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cathode and the hydrogen and oxygen gases allowed to flow over

their various electrodes. The normal chemical reactions for a PEM

fuel, cell is:

Anode reaction: H2 2H+ + 2e-

Cathode reaction: 1/2O2 + 2e- + 2H+ H2O (l)

Overall reaction: H2 + 1/2O2 H2O (l)

At the anode, the hydrogen molecules first come into

contact with a platinum catalyst on the electrode surface. The

hydrogen molecules brake apart, bonding to the platinum surface

forming weak H-Pt (Hydrogen Platinum) bonds. As the hydrogen

molecule is now broken the oxidation reaction can proceed. Each

hydrogen atom releases its election, which travels around the

external circuit to the cathode (it is this flow of electrons that is

referred 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 catalysis

site free for the next hydrogen molecule.

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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

hydrogen proton atoms (these have traveled through the

electrolyte 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.

The process is exothermic and leads to the formation of

water from the hydrogen and oxygen gases, along with heat

energy given off to the environment. The reaction has an

enthalpy of 286kj of energy per mole of water formed. The free

energy available to perform work decreases as a function of

temperature [2].

2.3 WHY FUEL CELLS?

Very extensive competitive efforts to build practical fuel cells

started after World War 1 but came to an end in the mid-nineteen

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thirties without much practical results. This was mainly due to the

arrival of the improved heat engine (the engine made much

advancement during the First World War), in spite of the

efficiency limit set by the Carnots cycle, the heat engine had

gone through the process of mass production during the war and

thus manufacturing processes leading to its eventual

commercialization were very favorable. After the Second World

War scientists became strongly aware of the need to preserve

fossil fuels by obtaining higher energy conversion efficiencies. As

time went on the negative effects of the gases and by products of

various methods of power production became apparent, thus

giving the already fundamental problem of finding green-tech

substitutes more momentum. In the twenty first century the

impact of technology on nature is now a highly controversial

topic. The world has turned to finding substitutes for power

production that have minimal and if possible no effect on the

environment. So far, fuel cells have shown the greatest potential,

why with its high efficiency and practically zero emission

characteristics, it seems to be the sure bet for future power

production.

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Since fuel cells convert chemical energy directly, to electrical

energy the intermediate steps of producing heat and mechanical

work typical of other more conventional power generation

methods are avoided, fuel cells are not limited by thermodynamic

limitation of heat engines such as the Carnot efficiency. Also

unlike batteries the reductant and oxidant in fuel cells must be

replenished to allow continuous operation at optimal power

output thus re-charging is not necessary.

2.4 TYPES OF FUEL CELLS

Fuel cells are characterized generally by the type of

electrolyte used in the stack. The most promising fuel cell types

are:

Proton exchange membrane fuel cells (PEMFC)

Direct methanol fuel cells (DMFC)

Phosphoric acid fuel cells (PAFC)

Molten carbonate fuel cells (MCFC)

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Solid oxide fuel cells (SOFC)

For all cells except the DMFC, the net cell reaction is

H2 +2

1O2 H20

Although these five major fuel cell structures have similar

structure and net reaction, they are very different with respects

to operating characteristics, materials of construction, and

potential application. The following sections discuss the

characteristics of each fuel cell type.

2.4.1 PROTON EXCHANGE MEMBRANE FUEL CELL

The proton exchange membrane fuel cell is one of the most

promising and certainly the best known of the fuel cell types. The

PEMFC consists of porous electrodes bonded to a very thin

sulphonated polymer membrane; this membrane electrode

assembly is sandwiched between two collector plates, which

provide an electrical path from the electrodes to the external

circuit. Flow channels cut into the collector plate distribute

reactant gases over the surface of the electrode. Individual cells

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consisting of collector plates and membrane electrode assembly

are assembled in series to form a fuel cell stack.

Like other fuel cells, the PEMFC is very efficient. The

efficiency for a PEMFC stack operating on hydrogen and

pressurized air at typical operating current conditions is

approximately 50%. The PEMFC also provide a very high power

density. Automotive fuel cell systems based on the PEMFC

technology have power density as high as 1.35KW/liter [3], which is

comparable to that of the internal combustion engine. This power

is produced while the cell is operated at a relatively low

temperature ranging between 600c to 800c. This low temperature

of operation permits the fuel cell to reach operating temperature

quickly. The combination of high efficiency, high power density,

and rapid start-up makes the PEMFC curative as a replacement

for conventional automobile engines.

Unfortunately, the low temperatures of the PEMEC leads to

very slow chemical kinetics, precious metal catalysts, typically

platinum, must be used at the electrodes to facilitate the

reactions. As recent as 10 years ago the cost of the catalyst alone

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was as high as N29500 per kilowatt electrode. This makes the

PEMFC too expensive for most applications (3).

The most commonly used electrolyte for the proton

exchange membrane fuel cell is nifon which is normally produced

and cut into sheets of the ranged of 50-175m (equivalent to the

thickness of 2-7 sheets of paper) it basically consists of

polytetrafluoroethylene chains (Teflon) which acts as the

backbons of the membrane; Attached to the Teflon chain are side

chains ending with sulphonic acid (HSO3) group. The chemical

structure is as shown

20

F F F F F F F F F F

C C C C C C C C C CF F F F F O F F F F

F C F

F C F

O

F C F

F C F

O = S = O

O-H+


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Fig, 1 structure of a sulfonated flouroethylene.

An interesting feature of this molecule is that, where as the

long chain molecules are highly hydrophobic (repeal water) the

sulphonate side chain is highly hydrophilic. For the membrane to

conduct ions efficiently the sulphonate chains must absorb large

quantities of water, when this is done the hydrogen ions of the

sulphonated group can move freely enabling the membrane to

transfer hydrogen ions, in the form of hydronium ions from one

side of the membrane to the other [1]. One major advantage of the

polymeric solid electrolyte used in a PEMFC is that the solid

electrolyte forms a thin electronic insulator and a barrier for gases

between electrodes, allowing fast proton transport and high

current while still allowing the fuel cell to operate in any special

position [4].

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Fig 2. Proton exchange membrane fuel cell.

As can be seen from the above diagram the fuel cell stacking

for the PEMFC is almost universally the planar bipolar type [5].

PROS AND CONS OF THE PEMFC

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PROS

The PEMFC has a solid electrolyte which provides excellent

resistance to gas cross over.

The PEMFCs low operating temperature allows rapid start-up

The use of exotic materials used in other fuel cell types is

not required in PEMFC

PEMFCs give off a by product of pure water (exhaust) when

the fuel used is strictly hydrogen.

When compared to other fuel cells, PEMFC technology has a

very high current density, while most technologies operate up

to approximately 1amp/cm2 the PEMFC can operate up to

4amp/cm2.

No corrosive fuel hazards are connected with PEMFC.

PEMFC has a very rapid response to load changes.

CONS

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Due to the low temperature of operation there is little, if

any, heat available from the fuel cell. Thus PEMFC are not a

good choice for co-generation.

Water management is another significant challenge in

PEMFC design as engineers must balance ensuring sufficient

hydration for the electrolyte against flooding the electrolyte.

The cost of production for PEMFCs is quite high, considering

the use of platinum catalysts.

The low temperature of operation leads to a higher

activation energy needed for the redox reaction to take place.

2.4.2 DIRECT METHANOL FUEL CELLS

Like the PEMFC, the direct methanol fuel cell uses a polymer

membrane as the electrolyte. However in the DMFC the fuel used

is methanol which is dissolved in water and delivered to the

anode. Since methanol is a liquid, it is easy to transport, and since

the methanol is used directly in the stack there is no need for a

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fuel processor. The reactions that occur in a typical DMFC is as

follows:

Anode reaction: CH2OH + H2O 6H+ + 6e- + CO2

Mobile ion: H+

Cathode reaction: 3/2O2 + 6H+ + 6e- 3H20

The main problem with the DMFC is that the reaction rate of

methanol is slow. Thus DMFC has a relatively low efficiency and

power rating. In other to compensate for such problems other

catalysts in addition to platinum are required on the anode side of

the membrane to break the methanol bond in the reaction, this

forms carbondioxide hydrogen ion and a free electron. Though

the problem of slow reaction seems taken care of the extra cost

that is required to make such reactions speed up out ways the

very problem that the catalysis try to solve[3].

However the DMFC seems to be a sustainable replacement

for batteries in small portable power applications where the

simplicity of the DMFC system and the portability of the liquid

methanol fuel out weigh the relatively low efficiency [3].

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PROS AND CONS OF THE PEMFC

PROS

Methanol fuel used in DMFC is liquid, thus transport and

supply to anode is simplified.

Methanol fuel can easily be stored in a storage tank, just like

gasoline.

DMFC has relatively high storage density.

CONS

Reaction rate of methanol is slow, thus the efficiency of DMFC

is relatively low

Methanol is soluble in the polymer membrane, so it can easily

cross over to the cathode where it reacts without producing

any electricity.

Extra cost is incurred by the added catalysts needed for

operation.

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2.4.3 PHOSPHORIC ACID FUEL CELL

The phosphoric acid fuel cell was the first fuel cell

technology to be commercialized. There have only been minor

changes in the design of the PAFC. The conventional porous

electrodes were polyterafluoroethylene-bound platinum black,

and loading was about 9 mg pt/cm2. In the last two decades

however platinum supported on carbon black has replaced the

platinum-black in porous polytetrafluoroethylene-bound electrode

structure.

The operating temperature of a PAFC is about 2000c with an

acid concentration of about 100% H3PO4. The present day PAFC

consists of porous carbon electrodes surrounding a porous matrix

(silicon carbide) that retains the liquid phosphoric acid electrolyte,

the PAFC structure resemble the PEMFC structure in terms of

electrode material. A fluid such as air, water, oil is circulated

between the collector plates to cool the stack assembly. [3]

Phosphoric acid fuel cells operate with efficiencies

comparable in PEMFCs but at power densities that are lower. The

operating temperature of the PAFC is approximately 2000C, and

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although the present practice is to operate at atmospheric

pressure, the operating pressure of PAFCs can surpass 8atm. This

is due to various results from tests which have confirmed an

increase in power plant efficiency when pressure is applied.

However it must be noted that though pressurization increases

efficiency, it complicates the power-unit, thus resulting in higher

cost. The economic trade-off favors simpler, atmospheric

operation for commercial units. [5] Another very important issue

with pressurization in PAFCs is that pressure promotes corrosion.

The phosphoric acid electrolyte, H3PO4, produces a vapour. The

vapour, which forms over the electrolyte, is corrosive to cell

locations other than the active cell layer. The limit at which

corrosion occurs in a PAFC is at a voltage of 0.8v/cell. If voltage

rises above such a value the H3P04 vapour will lead to massive

corrosion. An increase in cell total pressure causes the partial

pressure of the H3PO4 vapour to increase, thus causing increased

corrosion in the cell. [5]

One of the most advantageous characteristics of the PAFC is

its operating temperature. Apart from the fact that higher

temperature of operation lead to faster reactions and decreased

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activation energy for the redox reaction process, the heat that is

generated during cell operation can be harnessed for co-

generation.

PROS AND CONS OF PAFC

PROS

PAFCs are more tolerant to CO than other fuel cell types,

they tolerate about one percent of CO as a dilutent

The operating temperature is high enough to speed up

reactions but still low enough to allow the use of common

construction materials.

The waste heat from PAEC can be readily used in most

commercial and industrial cogeneration applications.

CONS

Although less complex than PEMFC, PAFCs still need

extensive fuel processing, including typically a water gas shift

reactor to achieve good performance.

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The highly corrosive nature of phosphoric acid requires the

use of expensive materials in stack.

2.4.4 MOLTEN CARBONATE FUEL CELL (MCFC)

Molten carbonate fuel cells are typically designed for mid-

sized to large stationary power applications.

The MCFC consist of nickel and nickel oxide electrode

surrounding a porous substrate which retains the molten

carbonate electrolyte. Collector plates and cell separator plates

are typically fabricated from stainless, steel, which can be formed

less expensively than the carbon plates in the PEMFC and PCFC

cells. Thermal energy produced within the cell stack is transferred

to the reactant and product gases, and a separate cooling system

is not usually required. [3]

The half cell electro chemical reactions are;

At the anode: H2+ CO32- H20 + CO2 + 2e

-

At the cathode: 1/2O2 + CO2 + 2e- CO3

=

Overall reaction: H2 + 1/2O2 + CO2(cathode) H2O + CO2(anode)

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Fig 3: Molten carbonate fuel cell

The mobile reaction in the MCFC is the CO32- ion, unlike in the

PEMFC and PAFC where H+ is the mobile ion.

The MCFC differ from PAFCs in many ways because of its

operating temperature which is approximately 6000-7000. At this

temperature (6500c), precious metal catalysts are not required for

the fuel cell reactions. In addition, the heat available from the

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stack can be used to produce steam and hot water in building co-

generation applications. Furthermore, at this temperature, fuel

gases other than hydrogen can be used by reforming the fuel

within in the cell stack in a process called internal reforming.

For example with the proper catalyst, carbon monoxide

introduced into the anode compartment of the fuel cell will react

with the water produced by the fuel cell, this will in turn lead to

production of hydrogen and carbondioxide through the water gas

shift reaction:

CO + H2O CO2 + H2

For the reason above molten carbonate fuel cells are being

developed mainly for natural gas and coal-based power plants,

since it can be seen that the MCFC operates more efficiently with

CO2 containing bio-fuel derived gases.[5] Since the mobile ion is

CO32- performance losses on the anode due to fuel dilution is

compensated by cathode side performance enhancement

resulting from CO2 enrichment.

The MCFCs ability to undergo the process of internal

reforming simplifies a lot of matters. For example, internal

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reforming can be accomplished with carbon monoxide and simple

hydrocarbon fuel such as methane (This is simpler to obtain than

pure hydrogen), though heavier hydrocarbons still have to

undergo external fuel processing.

Obviously the increased operating temperature of the MCFC

brings along with it various advantages, however the higher

operating temperature places severe demands on the corrosion

stability and life of cell components, particularly in the aggressive

environment of the molten carbonate electrolyte. [5]

PROS AND CONS OF THE MCFC

PROS

No expensive electro-catalysts are needed as the nickel

electrodes provide sufficient activity [5].

Both CO and certain hydrocarbons can be used as fuel for

the MCFC, as they are converted to hydrogen within the stack

(internal reforming). [5]

The high temperature waste heat allows the use of a

bottoming cycle to further boost the system efficiency. [5]

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problems. The cell is constructed with two porous electrodes that

sandwich an electrolyte. Airflows along the cathode, when an

oxygen molecule contacts the cathode/electrolyte interface; it

acquires electrons from the cathode. The oxygen ion diffuses into

the electrolyte material and migrates to the other side of the cell

where they contact the anode. The oxygen ion encounters the

fuel at the anode/electrolyte interface and reacts catalytically,

giving off water, carbondioxide, heat, and elections. The electrons

transport through the external circuit, providing electrical energy.

The reactions can be summarized as:

Cathode reaction: 21 O2 + 2e

- O2-

Anode reaction: H2 + O2- H2O + 2e

-

Mobile ion: O2-

The most common cell configuration for the SOFC is of

tubular geometry. In such a configuration as that shown in figure

4 the cathode is a hollow tube constructed in such a way as to

support the electrolyte. The anode then surrounds the electrolyte,

encasing both the electrolyte and cathode. Fuel enters the cell

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from the outer surface and air enters the cell from the inner

surface. [3]

Fig 4: Tubular stacked solid oxide fuel cell.

SOFC allow conversion of a wide range of fuels, including

various hydrocarbon fuels. The relatively high operating

temperature allows for highly efficient conversion of power,

internal reforming, and high quality by-products of heat for co-

generation. Both simple and hybrid SOFC system have

demonstrated among the highest efficiencies of any power

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generation system, combined with minimal pollutant emissions

and low greenhouse gas emissions. These capabilities have made

SOFC an attractive emerging technology for stationary power

generation in the 2KW to 100MW capacity range. [5]

Development efforts for SOFC are focused on reducing

manufacturing cost, improving system integration and lowering

the operating temperature to the range of 5500c-7500c. The lower

operating temperature would still provide the advantage of

reforming while still reducing the material problems associated

with high operation temperature.

2.5 FUEL CELL PERFORMANCE

Theoretically, the maximum electrical work obtainable in a

fuel cell operating at constant temperature and pressure is given

by the change in Gibbs free energy ( G) of the electrochemical

reaction:

G=H - TS

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Where H is the enthalpy change and S is the entropy change. [5]

In the fuel cell , the reaction is exothermic thus the system

change in enthalpy is the energy released as heat. On the other

hand S is the internal energy used by the system. The entropy

change does not give energy to the surrounding. For a fuel cell

system using cogeneration methods the Gibbs free energy can be

harnessed to its fullest. The following discussion expatiates on the

efficiency, and losses that are involved during the operation of a

fuel cell.

2.5.1 FUEL CELL VOLTAGE

The standard potential E0 is a quantitative measurement of

the maximum cell potential i.e. the open circuit voltage. For a

hydrogen-oxygen cell, in which there is a transfer of two electrons

by each water molecule. The standard potential E0 = 1.229v if the

produced water is in liquid and EO = 1.8V if the produced water is

in gaseous state. These values are obtained at a temperature of

298k (250c, which is the approximate room temperature value)

and pressure of 1 atmosphere. [4]

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The potential (E0) is the change in Gibbs free energy

resulting from the reaction between hydrogen and oxygen. The

difference between the 1.229v and 1.18v for the standard

potential of water in liquid state and water in gaseous state

respectively is the Gibbs free energy change of Vaporization of

water at standard conditions. [5]

2.5.2 LOSSES IN FUEL CELLS

Although the theoretical values of voltage for a fuel cell is

1.229v, in practice the cell potential is significantly lower than

this. This is due to some losses in the system even when no

external load is connected. Moreover, when a load is connected to

the fuel cell, the voltage in the terminals decreases still due to a

number of factors; these include polarization losses and

interconnection losses. 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.

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Platinum forms an excellent catalyst; however there is much

research under way in search of better and less expressive

materials. A limiting factor to power density available from a cell

is the speed at which the reaction 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 cross over and internal currents: fuel crossover and

internal currents are a result of the 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

resistance of 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 inter-

connections.

Concentration losses: These are also referred to as mass

transport, thee losses result from the reduction of the

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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 build up of

water at the cathode, catalyst sites can become clogged,

restricting oxygen access. It is therefore important to remove this

excess water, hence the term mass transport. [1]. Another way of

looking at this is that concentration losses are caused by the

diffusion of ions through the electrolyte which produces an

increase in the concentration gradient, diminishing the speed of

transport. The relation between the voltage of the cell and the

current density is voltage of the cell and the current density is

approximately linear up to a limit value, beyond which the losses

grow quickly. [4]

The fuel cell voltage of a simple cell can be expressed as

VFC = E0 - Vohm Vact Vconc.

Where Vfc = Voltage of a simple fuel cell

E0 = Standard potential

Vohm = Ohmic losses

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Vact = Activation losses

Vconc = concentration losses

2.5.3 FUEL CELL STACKING

In practice, successions of cells are connected in series in

order to provide the necessary voltage and power output,

constituting a fuel cell stack. Generally the stacking involves

connecting multiple unit cells in series via electrically conductive

interconnects. Different stacking arrangements have been

developed, these include:-

PLANAR- BIPOLAR STACKING: The most common fuel cell

stack design is the so-called planar-bipolar arrangement

individual unit cells are electrically connected as shown in figure

5. Because of the configuration of a flat plate cell the interconnect

becomes a separator plate with two functions:

To provide an electrical series connection between adjacent

cells, specifically for flat plates cells

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To provide a gas barrier that separates the fuel and oxidant

of adjacent cells.

In many planar-bipolar designs, the interconnect also

includes channels that distribute the gas flow over the cells. The

planar-bipolar design is electrically simple and leads to short

electronic current pats, this helps to minimize cell resistance.

Fig 5: Planar-bipolar stacking.

TUBULAR CELLS STACKING: This is used especially for high

temperature fuel cells; stacks with tubular cells have been

developed. Tubular cells have the significant advantages in

sealing and in the structural integrity of the cells. In the earliest

tubular designs the current is conducted tangentially around the

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tube-interconnections between the tubes are used to form

rectangular arrays of tubes. Alternatively, the current can be

conducted along the axis of the tube, in which case

interconnection is done at the end of the tubes.

2.6 THEORETICAL AND REAL FUEL EFFICIENCY

The efficiency of any energy conversion device is the ratio

between the useful energy output and the energy input. In a fuel

cell, the useful energy output is the generated electrical energy

and the energy input is the energy content in the mass of

hydrogen supplied. The energy content of an energy carrier is

called the higher heat value which will be represented as HHHV.

The HHHV of hydrogen is 286.02kjmol-1 or 141.0mjkg-1. This is the

amount of heat that may be generated by a complete combustion

of 1kg of hydrogen.[4]

Assuming that all the Gibbs free energy of hydrogen, G, can

be converted into electrical energy, the maximum possible

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(theoretical) efficiency of a fuel cell, taking G of hydrogen as

237.34 kjmol-1, would be

%8302.286

34.237 ===

HHVHG

The Gibbs free energy G is used to represent the available

energy to do external work. The change in Gibbs free energy is

negative because in a fuel cell reaction energy is released. [4]

Using faradays constant the voltage generated by both G

and HHHV can be calculated. Dividing by 2f, where 2 is the

number of electrons per molecule of H2 and F is faradays number,

then.

VG = 23.12 = FG

V H = 482.12 =

fH

Using these values the fuel cell efficiency can be expressed as a

ratio of two potentials

=

==

=FH

FG

HG

2/

2

482.1

FCV

Where VG = the generated voltage

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VG = the thermo neutral potential

The efficiency of fuel cells are very high but in practice

efficiencies of 83%are not so feasible. The lower efficiency of the

practical fuel cell is mainly due to losses and various auxiliary

systems attached to the fuel cell. These take up a portion of the

total output power and so lower the real efficiency value. In

practice when the power consumed by incorporate auxiliary

systems is taken into account. Then the equation for power

becomes

=2PH

PP auxfc

Where Paux = power consumed by auxiliary system

PH2 =power input

P fc =power output of fuel cell.

In real life the fuel cell efficiency is between the ranges of

47% to 50%. This is quite high when compared to other forms of

power generation. Plus, with co-generation methods efficiency

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can get to values between 70% and 80%. That is one reason that

makes fuel cells unique.

2.7 FUEL CELL SYSTEMS

Although a fuel cell produces electricity a fuel cell system

requires the integration of many components beyond the fuel cell

stack itself, for the fuel cell will produce only DC power and

utilizes only certain processed fuels. Various system components

are incorporated into power systems to allow operation with

conventional fuels, tie into ac power grids, or to utilize rejected

heat to achieve high efficiency. The basic features of a fuel cell

system are illustrated in figure 6.

Fig 6.fuel cell system schematics

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The figure indicates that a fuel cell system is composed of six

basic subsystems:

The fuel cell stack

Fuel processor

Air management

Water management

Thermal management

Power conditioning subsystems

The design of each subsystem must be integrated with the

characteristics of the fuel cell stack to provide a complete system.

Optimal integration of these subsystems is key to the

development of cost effective fuel cell system. Seeing as the fuel

cell stack has been discussed in previous chapters; the rest of

the subsystems will be looked into. [3]

2.7.1 FUEL PROCESSOR

Since most fuel cells use hydrogen as a fuel and most

primary energy sources are hydrocarbons, a fuel processor is

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required to convert the source fuel to a hydrogen rich fuel stream.

The complexity of the fuel processor depends on the type of fuel

cell system and the composition of the source fuel. For low

temperature fuel cells such as PEMFCs and PAFCS, the fuel

processor is relatively complex and usually includes a

desulphurizer, a stream reformer or partial oxidation reactor, shift

converters, and a gas clean-up system to remove carbon

monoxide from the anode gas stream. The development of a

compact economical reformer to supply hydrogen rich fuel for low

temperature fuel cells in building applications and automotive

applications is a formidable challenge in higher temperature fuel

cells such as MCFC and SOFC the fuel processing for simple fuels

such as methane many consist simply of a desulphurizing and

preheating the fuel stream before introducing it into the internally

reforming anode compartment of the fuel cell stack. More

complex fuels may require additional steps of clean-up and

reforming before they can be used even by the higher

temperature cells. For all types of fuels, the higher operating

temperature associated with the MCFC and SOFC systems provide

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better thermal integration of the fuel cell with the fuel processor.

[3]

2.7.2 AIR MANAGEMENT

In addition to fuel, the fuel cell requires an oxidant, which is

typically air. Air is provided to the fuel cell cathode at low

pressure by a blower or at high pressure by an air compressor.

The choice of whether to use low or high pressure air is a

complicated one. Increasing the pressure of the air improves the

kinetics of the electrochemical reactions and leads to higher

power density and higher stack efficiency. Furthermore, in PEMFC

stacks, increasing the air pressure reduces the capacity of the air

for holding water and consequently reduces the requirements for

humidification. On the other hand, the power needed to compress

the air to high pressure reduces the net available power from the

cell system. Some of this energy can be record by expanding the

cathode exhaust through a turbine before expelling it to the

atmosphere. Nevertheless, the air compressor typically uses more

power than any other auxiliary device in the system. Furthermore,

while the fuel cell stack performance actually improves at low

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power, the performance of the air compressor is usually poor at

very low loads. Currently most fuel cell stacks design call for

operating pressures in the range of 1-8atm. To achieve high

power densities and to improve water management, most

automotive fuel cell systems based on PEMFC technology are

operated at pressures of 2-3atm. [3]

2.7.3 WATER MANAGEMENT

Water is required for a variety of purposes in a fuel cell

system. The fuel reforming processes require water to react with

hydrocarbon fuels in the fuel steam reforming reaction. In PEMFC

systems the reactant gases must be humidified in order to avoid

drying out the fuel cell membrane. Water is available from the

fuel cell reaction, but it must be removed from the exhaust gas,

stored and pumped to a pressure suitable for the various

operations.in automotive applications it is critical that the system

operates in such a way that water condensed from the exhaust

streams are sufficient for reforming and reactant humidification.

Otherwise the vehicle must periodically be recharged with water

as well as fuel

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2.7.4 THERMAL MANAGEMENT

A fuel cell stack releases thermal energy at a rate that is

roughly equivalent to the electrical power that it produces. This

thermal energy can be used for a variety of purposes within the

fuel cell system, transferred externally to meet the thermal needs

of a particular application, or rejected to the surrounding. Low

temperature fuel cell systems are cooled by either air or a

circulating liquid. In some low temperature, low power (below

200w) system, the excess air flowing over the cathode is

sufficient to transfer thermal energy from the cell. In larger low

temperature systems, additional flow channels are provided

within the cell stack and either air or liquid coolant (typically

deionized water is circulated through the channels to remove

thermal energy. It a liquid coolant is used the stack is made to be

more compact. Furthermore, with a liquid coolant, it is easier to

transfer energy for other purposes such as space heating or water

heating in cogeneration applications. In high temperature such as

the MCFC and SOFC, and fuel cell stack operates at such high

temperature that all the thermal energy from the cell reaction can

be transferred to the reactant gases without heating the exhaust

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beyond the operating temperature limit of the stack. Thermal

energy from the stack exhaust can also be used to preheat the

incoming air stream. Thermal energy that is not needed for

reforming or air preheating can be used to make stream or hot

water for cogeneration in a heat recovery boiler. Proper

integration of the fuel cell system is essential to insure that

thermal energy available from the stack is used for the most

appropriate application. [3]

2.7.5 POWER MANAGEMENT

The final component of the fuel cell systems is the power

management system. This system converts the electricity

available from the fuel cell to a current and voltage that is

suitable for a particular application and supplies power to the

other auxiliary systems. Fuel cell stacks produce direct current at

a voltage that varies with load. A switching power converter is

used to match the voltage produced by the fuel cell to the

application and to protect the fuel cell from over current or under

voltage conditions. If the application requires alternating current,

the electricity is processed through an inverter, which constructs

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single or three-phase wave form as required by the application. If

the application involves interconnection with the utility grid, then

the power management system must also be able to synchronize

the frequency of the fuel cell system power with the utility power

and provide safety features to prevent the fuel cell system from

feeding power back into the utility grid if the grid is offline.

The IEEE is currently developing IEEE 1547 (standards for

Distributed Resources Interconnection with Electric power

Systems). This standard being developed so as to address

synchronization issue in distributed generation of which fuel cells

are one of such.

2.7.6 FUEL CELL SYSTEM CHARACTERISTICS

Fuel cell systems promise to provide a number of

advantages when compared to conventional power systems.

These advantages couple with projected cost reductions will make

fuel cells attractive in a variety of applications.

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The major component of a fuel cell system, the fuel cell

stack, is composed of individual fuel cells assembled in repetition.

Thus, the fuel cell stack is modular and can be constructed in

sizes ranging from a few watts, to a megawatt or more. Other

components of the fuel cell system, particularly the fuel

processor, do not scale as well as the stack. However, even fuel

cell systems incorporating fuel processors can be constructed to

meet a variety of applications with power needs as small as 10kw.

Across the entire range of applicable sizes, fuel cell systems offer

attractive electrical conversion efficiencies. Furthermore, the fuel

cell system efficiency for various fuel cell systems ranges from

40% to 50% for simple systems in a broad range of sizes. Few

small to medium-sized conventional systems can achieve

efficiencies comparable to those provided by fuel cell systems at

design conditions. Furthermore, no conventional systems can

maintain efficiencies comparable to fuel cell systems at part-load

operation. More complex fuel cell systems can yield even higher

efficiencies for a combined system consisting of a pressurized

SOFC with the exhaust gas driving a gas turbine, the overall

efficiency can be up to 60% if not more. [3] The combined gas

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turbine/steam cycle is the only conventional cycle that can

approach, at least at design load, a level of 60% efficiency. Since

fuel cells can operate at high efficiency even in relatively small

sizes, they are attractive in small-scale generation and

cogeneration applications such as buildings. By producing

electricity and thermal energy for applications such as water

heating or space heating, fuel cells can offer cogeneration

efficiencies as high as 80% (from a first law of thermodynamics

standpoint).

Fuel cells are also attractive because of their low

environmental impact relative to conventional systems. The fuel

cell stack itself operates on hydrogen giving water as its by

product. Emissions of currently regulated pollutants such as

carbon monoxide, nitrous oxides, oxides of sulpur, and

particulates are well below current are quality regulations and

typically nearly non existent. Even carbondioxide which is

produced when hydrocarbon fuels are used and reformed are well

below any conventional system emission value.

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In addition to minimizing emissions of regulated pollutants,

fuel cell systems are also relatively quiet and unobtrusive so that

the overall impact on the environment is small. This permits fuel

cells to be located in a variety of settings that would not be

acceptable for conventional power plants. [3]

In a number of areas, including response time, useful life

maintainability and cost, fuel cell systems promise to exhibit

performance comparable to existing system. Fuel cells which are

already operating at their design temperature typically respond

quickly to load changes with demonstrated response rates on the

order of 0.3% to 10%. [3]. The rate of response is more of a

function of the auxiliary systems than the fuel cell stack it self.

For high temperature fuel cells such as the MCFC, the time taken

to reach optimal operating temperature can be significant and

these systems tend to be more appropriate for generating power

for building applications and large-scale transportation

applications. While the low temperature fuel cells, PEMFC, can

reach its operating temperature quickly making it a more suitable

fuel cell type for automotive power application.

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Maintenance tasks for the fuel cell system typically focuses

on the auxiliary systems including rotating machinery (tans,

compressors, pumps). The stack itself has no moving parts and is

not field serviceable. The stack simply consumes fuel and

produces heat and electricity throughout its useful work life. At

the end of its useful life, the fuel cell stack can be removed and

recycled while a new stack can be installed in its place.

Demonstration projects, primarily conducted with PAFC systems,

confirm that service and maintenance issues associated with the

stack are almost nonexistent while those that exist with the

support systems have declined as the system technology has

matures. [3]

2.8 FUEL CELL SYSTEM APPLICATIONS

2.8.1 PORTABLE POWER

Portable power typically refers to systems that can be

transported by a person and that can generate power of a few

watts to a few hundred watts. Examples include power for

camping and recreational vehicles, power, for portable electronic

devices such as computers and cellular phones, and power for

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soldiers deployed in the field. File cells based on DMFC technology

of PEMFC technology are well suited for many of these

applications. DMFC systems are particularly attractive because,

as a liquid, methanol can be conveniently transported. In portable

power applications, the fuel cell would be incorporated into

electronic devices. A small container of methanol or a cylinder of

compressed hydrogen would be inserted into an inlet port. Air

would be supplied to the fuel cell by natural convection or a very

small blower. When the fuel is depleted, the fuel container would

be removed and a new one stalled in its place. Recharging would

not be necessary and carrying extra canisters would be lighter

and expensive than transporting extra batteries.

2.8.2 TRANSPORTATION

Arguably, the major driving force behind recent interest in

fuel cell technology is the potential for using fuel cells in

transportation applications including personal vehicles.

Automakers in North America, Europe, and Japan have invested

several billion dollars in advancing the state of PEMFC technology

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with the goal of producing a fuel cell power plant that provides

the efficiency and low emission characteristic of fuel cells at low

cost that is competitive with the existing internal combustion

engine. Today many of the technical objectives that are related to

the fuel cell stack have been met or a close to being met and

current development efforts are focused on decreasing cost and

resolving issues related to fuel supply and systems integration.

Proton exchange membrane fuel cell systems operating on

hydrogen and having power densities as high as 1.35KW/liter

have been demonstrated. A good example of a fuel cell powered

vehicle is the Honda FCX clarity (FCX stands for fuel cell

experimental). The Honda FCX has a rated power of 100KW

(134hp) which is gotten from a vertical flow hydrogen fuel cell

stack that supplies electrical energy on demand. As in electric

cars, waste energy from braking and deceleration are captured by

the motor/Generator and stored in a battery (in the case lithium

ion unit) [2]. The FCX provides quite steady acceleration and high

torque. The range of a full hydrogen tank is EPA certified at about

386km. [6] The main problem that has limited the use of fuel

vehicles is the problem of storage. Hydrogen gas has to be stored

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at a temperature of -2480c, this introduces many complexities

additional cost, and obvious safety issues. Apart from material

cost and storage, fuel cell powered transportation is best means

of carrying on with human comfort and machinery with out

damaging the environment (emissions).

Cost reduction efforts include development of improved

materials used for the membrane electrode assembly, better

design of the gas flow channels, and development of less

expensive materials and methods of fabrication for the collector

plates.

2.8.3 STATIONARY POWER

In many respects stationary power applications are even

more favourable for fuel cell systems than transportation

application. In stationary applications, most systems will operate

continuously so the time to reach operating temperature from a

cold start is not typically an important criterion. Thus, higher

temperature systems including MCFC and SOFC systems can be

considered in addition to PAFC and PEMFC systems. Another great

feature of fuel cell in stationary power applications is that the fuel

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source is likely to be natural gas. Natural gas is primarily methane

which is a light hydrocarbon and relatively easy to reform. In

addition, a distribution infrastructure for natural gas is already in

place. Promising stationary applications include premium power

systems; cogeneration systems for residency, commercial

buildings, and industrial facilities, as well as distributed power

generation for utilities.

Many facilities that house data processing centers or

telecommunications equipment require very high quality power.

With their high efficiency, low noise, minimal emissions, fuel cell

systems can operate consciously to supplement or replace utility

power.

Even without the need for backup power, fuel cell systems

can be attractive when both heat and electricity are required. For

example during the summer, a fuel cell serving a residence can

provide electricity for lights, appliances, and air conditioning while

supplying thermal energy for heating water this is a simple form

of cogeneration.

CHAPTER THREE

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METHODOLOGY

The research works methodology is centered on calculation,

collation, and analysis of data which has be gotten from various

journals and online resources.

3.1 DATA ACQUISITION

Data used in the following parts was gotten from the named

online journals and resources.

(1) WWW.IEEE.org [/ieee/xplore/member home.]

(2) WWW.Wikipedia.org/Wiki/

(3) WWW.fuelcells.org [charts and articles.]

(4) United States department of defense fuel cell demonstration

program[http://www.dodfuelcell.com]

(5) Fuel cell Handbook, 5th ed; U.S Department of Energy.

(6) Description of PEM fuel cell systems [ieee report], diego

ferold; and Marta Basualdo

Charts gotten from these sources include

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Honda FCX clarity comparison to fuel injection heat engine

vehicles of equivalent specifications.

U.S Department of Energy comparison of fuel cell

technologies table.

3.2 FUEL CELL MATHEMATICAL MODEL

Based on the principles described on fuel cell performance, A

mathematical model for fuel cell performance assessment is

given below. The model is based on electrochemical engineering

fundamentals and has been developed on the following

assumptions.

Fuel and oxidant are perfect gases

Fuel is H2 and oxidant is O2

Temperature and pressure are uniform along the electrodes

The conversion of energy occurs isothermally and in

constant volume.

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The following steps are identified for modeling fuel cells. The step

are used to analyze the voltage and current of a PEMFC but is

applicable to all types of FC.

STEP1

Define the chemical reaction equations and the corresponding

stoichiometric coefficients.

H2 + 1/2 02 H20

Thus VH2 = 1, VO2 = 21 ,VH2O = 1

STEP 2

Define the half cell reactions and find valency (electron count.)

Anode: H2 2H+ + 2e-

Cathode: 1/2O2 + 2e- + 2H+ H2O

Valancy = Z = 2 (number of electrons)

STEPS 3

Establish operating temperature of cell (TFC) and partial pressures

of H2, O2 and H20

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STEP 4

Establish equilibrium constant, K, at the operating temperature, of

cell T:

For H2 + 21 02 H20

K = [pressure of H20]

STEP 5

Calculate standard fuel cell EMF and actual cell emf:

Using change in Gibbs free energy of H2 and O2 reacting to give

water

G[H2,02] = 237.14 [source wikipedia.com]

But,

G = nFEmf

Where n = number of electrons

F = faradays constant (96,485.33c/gmolelecron)

Emf = electromotive force generated

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G = Gibbs free energy

Substituting values,

237.14 = 2 x 96485.33 x Emf

EMF = 237.14/2 x 96485.33 = 1.22888 1.229

Emf = E0cell = standard potential = 1.229

Calculate electromotive force of cell using Nernst equation [8]

Ecell = Eocell

ZF

RTln(k)

R = universal gas constant, 8.314

T = absolute temperature

Z = electrons

F = faradays constant 9.648533 x 104 cmol-1

K = equilibrium constant

E0

= open circuit voltage: 1.229

Thus for a fuel cell, using H2 as fuel and O2 as oxidant

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Ecell = 1.229 +F

RT

2= ln

][

21]][[

0

2

022

PH

PPH

Note : logba = logb [ a1 ]

Since R = 8.314 JK-1 mol -1, T = Tfc, Z = 2,

F = 9.648533X104 cmol-1

The constants can be calculate and replace

Thus

Ecell = 1229 + 4.3085 x 10-5 Tfc (In

][

21]][[

0

2

022

PH

PPH)

STEP 6

Determine fuel rate in gm-moles/sec

STEP 7

Determine the exchange current density, which is one out of

important factors of efficiency.

The current density is proportional to the catalyst, area, electrode

area, partial pressure of the reactant, temperature and activity

energy. It is derived as

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I0 = I0R aclc ( ref

r

r

P

P)r exp

ref

c

T

T

RT

E1

Where, IOref

reference exchange current density

ac is the catalyst specific area

lc is the catalyst leading

r is the reaction order with respect to the reactant

Prrefis the reference pressure

Tref is the reference temperature

With above values calculated power (IV) can be calculated

3.3 DATA COLLATION

Table 1

EERE fuel cell comparison chart from U.S

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Department of energy: updated on 5 Oct 2011 at 6:45 [2]

Table 2

Honda FCX clarity comparisons chart. [9]

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CHAPTER FOUR

DISCUSSIONS

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TRANSPORTATION

Fuel cells are likely the next step in transportation. Taking

table 2 of chapter 3 into consideration it can be seen that fuel cell

cars give of more power and handle part-land better than their

heat engine counterparts. Using the Honda FCX clarity as a case

study, the fuel cell vehicle has a mileage rating estimate of 56km

in both city and high way journeys. This is a lot better than the

milage range of 22km-30km of its counterparts. The main

advantage of fuel cell vehicles over other transportation means is

their efficiency, which ranges from 40% to 60% which is a far

better rating that the 25%-30% range rating of current heat

engine powered vehicles.

The most suited type of fuel cell for transportation is the

PEMFC, with a power rating of 100-500kw, Low operating

temperature of 900c, and high system efficiency of up to 50% the

PEMFC seems to be destined to take over the transportation

industry, so much so that billions are being pushed into research

to find means, materials and production process to bring the cost

of PEMFC cells to as low as N8000/kw (10)

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DISTRIBUTED GENERATION

Distributed generation is an approach that employs small-

scale technologies to produce electricity. Due to the fact that fuel

cells have low emissions and high efficiency, with the added

advantage that they can be coupled to an already existing power

grid, and undergo cogeneration. Fuel cells are so reliable for

distributed generation applications that there are more than 300

DG fuel cell installation worldwide. From the data given in table 1

chapter 3, it is easy to see that the most convenient fuel cell type

for distributed generation would be SOFC. With its high efficiency

of 60% (stand alone) to 85% (cogeneration) the SOFC gives the

best output power for DG applications. Another great advantage

of the SOFC is that the operating temperature of the cell is high

enough to be harnessed for steam generation, thus giving it extra

advantage of being used to power an added on steam turbine

system (hybrid power system layout). The main problem with the

SOFC is that of corrosion and breakdown. If better materials can

be found for the fabrication of the stack, the SOFC will become a

major player in the stationary power generation sector. Tests are

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being carried out all over the world to see which steps can be

taken to improve on the SOFC.

CHAPTER FIVE

5.1 PRINCIPLE FINDINGS

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potential in making power more portable and convenient. Fuel cell

systems based on PEMFC technology promise to make more

efficient, cleaner means of providing power in the auto mobile

industry. MCFC and SOFC are likely to be applied in building

cogeneration systems. With cogeneration efficiencies as high as

80%, these applications promise to reduce energy use and

environmental impact. Many research developments and

regulatory agencies are working to insure that fuel cell systems

fulfill there potentials. Fuel cells are the way to power production

of the future, so much so that the new world trade center is to run

on fuel cell units rated to generate a total of 4.8MW for the

towers, the cells are said to run on natural gas. This further goes

to show that fuel cells are a convenient, conceivable, and reliable

means of power generation of the future.

5.3 RECOMMENDATIONS

The following recommendations are made;

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The implementation of fuel cells into the Nigerian grid

system. As natural gas (70%-90% methane and other light

hydrocarbons) is one of the resources available in the

country, fuel cells technology will have the resources needed

to sustain it.

From the Electric Power Sector Reform (EPSR) Act of 2005,

government made emphasizes on the role of distributed

generation electricity in the overall energy mix. Fuel cells

infrastructure should have high priority as they are well

suited for distributed generation and hybrid plant designs .

Fuel cells such as the SOFC have high power densities

meaning that they can serve more consumers without

having large infrastructural areas allocated to them. They

thus will act to reduce the demand on the national grid and

also help to break up the single transmission grid system of

the country into smaller autonomous grid systems.

Fuel cell technology if not run as base load plants in the

country can act as peak load suppliers or power outage

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regulators by being used as stand by systems in various

plants.

The cogeneration properties of fuel cells mean that they can

be installed at already existing power generating

infrastructure. This means that fuel cell technology can be

coupled into the Nigerias already existing power system,

thus helping provide more power and help utilize the energy

being wasted at current plants so as to gain higher efficiency

levels.

REFERENCES

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[1] Introduction to fuel cells, brain cook, (2012, June 14,)

[online]: Available: http://www.IEEE.org/xplore

[2] Fuel cells (2013, Sept 21) [online]; Available:

http://www.wikipedia.com

[3] Fuel cells consistent energy for the future (2012, Sept 14)

[online]: Available: http//www.IEEE.org/xplore

[4] PEM fuel cells, Diego Feroldi and Marta Basualdo (2012,

Sept12) [online]: Available; http;//www.IEEE.org/xplore

[5] Fuel cell handbook, 7th ed.; Morgantown. WV: EGBG technical

services, inc., for U.S. Department of Energy, Office of Fossil

Energy, National Energy technology laboratory, November

2004.

[6] Honda fcx-clarity (2012, Sept.26) [online]: available:

http;//www.automobiles-honda.com/fcx-clarity/reviews

[7] Fuel cells: modeling, control and applications Bei Gou, woon

Ki Na. CRC press, 2010

[8] Nernst equation (2012,Sept.29) [online]: Available:

www.wikipedia.com/Nernstequaation.htm.

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http://www.ieee.org/xplorehttp://www.wikipedia.com/http://www.wikipedia.com/Nernstequaation.htmhttp://www.ieee.org/xplorehttp://www.wikipedia.com/http://www.wikipedia.com/Nernstequaation.htm


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[9] Fuel cells 2000 Home page (2012, May.20) [online]:

Available: http://.fuelcells.org.

[10] US DOD fuel cell demonstration program Home page (2012,

may) [online]: http://www.dod fuelcell.com

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http://www.dod/http://www.dod/

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