study on fuel cells
TRANSCRIPT
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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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[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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