P1254413601rbyfF

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Electrolyzer Theory of Operation

• Deionized water flows into the positive side of
  the cell where it is dissociated by electrolysis
  into protons, electrons, and oxygen. The oxygen
  is carried away by the water flow. The positively
  charged protons are attracted to the negative
  side of the cell and they use the sulfonic acid
  ion groups embedded within the membrane as the
  path to travel through the solid material.
  Meanwhile, the electrons flow through the power
  supply to the negative electrode where they link
  up with the emerging protons to form a molecule
  of pure hydrogen gas

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U.S. Energy Dependence is Driven By Transportation
US Oil Use for Transportation

• Transportation accounts for 2/3 of the 20
  million barrels of oil our nation uses each day.
• The U.S. imports 55 of its oil, expected to
  grow to 68 by 2025 under the status quo.
• Nearly all of our cars and trucks currently run
  on either gasoline or diesel fuel.

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Hydrogen Economy or Hydrogen Energy System
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Why Hydrogen? Its abundant, clean, efficient,
and can be derived from diverse domestic
resources.
Biomass
.
Transportation
HIGH EFFICIENCY RELIABILITY
Hydro Wind Solar Geothermal
Nuclear
Distributed Generation
Oil
ZERO/NEAR ZEROEMISSIONS
Coal
With Carbon Sequestration
Natural Gas
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Types of Fuel Cells
Fuel Cell Type Electrolyte Anode Gas Cathode Gas Temp. Efficiency
Proton Exchange Membrane (PEM) Solid Polymer Membrane Hydrogen Pure or Atmospheric Oxygen 75 C 180 F 35-60
Alkaline (AFC) Potassium Hydroxide Hydrogen Pure Oxygen Below 80 C 50-70
Direct Methanol (DMFC) Solid polymer membrane Methanol solution in water Atmospheric oxygen 75 C 180 F 35-40
Phosphoric Acid (PAFC) Phosphorous Hydrogen Atmospheric oxygen 210 C 400 F 35-50
Molten Carbonate (MCFC) Alkali-Carbonates Hydrogen, methane Atmospheric oxygen 650C 1200 F 40-55
Solid Oxide (SOFC) Ceramic Oxide Hydrogen, methane Atmospheric Oxygen 800-1000 C 1500-1800 F 45-60
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PEM Fuel Cell Theory of Operation

• http//www.humboldt.edu/serc/animation.html

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1839 Sir William Grove - Welsh judge,
inventor, and physicist - realizes that
electrolysis (using electricity to split water
into hydrogen and oxygen) can be done in reverse
with the right catalyst, creating
electricity. The first fuel cell used hydrogen
and oxygen on platinum electrodes with sulfuric
acid as the electrolyte. In 1842, Grove
developed a bank of 50 fuel cells which he called
a gaseous voltaic battery Lack of power meant
limited development
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Francis T. Bacon, British engineer, develops high
pressure AFC through clever electrode
engineering. Proposed use in submarines. 1959
Bacon produced a 40-cell stack capable of 5 kW,
running at 40 atmospheres and 200C. The stack
powered a welding machine, circular saw, and
forklift. Aerospace company Pratt Whitney ( a
subsidiary of the later United Technologies
Corporation) licenses Bacons technology. In
1966, they deliver three 1.5 kW AFC stacks for
the Apollo spacecraft. Present-day Space
Shuttles use AFCs as well 1959 The
Allis-Chalmers Manufacturing company demonstrates
a tractor powered by a 15 kW stack.
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1950s General Electric Research
Laboratories begins development of the
PEMFC. 1960s Gemini space mission uses
PEMFCs produced by GE. Introduction of
Duponts Nafion membrane improves lifetimes
from 1000 hours to 50,000 hours. High
platinum loading and water management problems
cause stall in PEM development and lapse of GE
patents.
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Fuel Cell
Fuel cells are electrochemical devices that
convert the chemical energy directly into
electrical energy.
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• Electrolyte
• Conducts ions
• Electrical insulator
• External Circuit
• Conducts electrons

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1950s General Electric Research
Laboratories begins development of the
PEMFC. 1960s Gemini space mission uses
PEMFCs produced by GE. Introduction of
Duponts Nafion membrane improves lifetimes
from 1000 hours to 50,000 hours. High
platinum loading and water management problems
cause stall in PEM development and lapse of GE
patents.
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Fuel Cell
Fuel cells are electrochemical devices that
convert the chemical energy directly into
electrical energy.
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• Electrolyte
• Conducts ions
• Electrical insulator
• External Circuit
• Conducts electrons

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• The most common classification of fuel cells is
  by the type of electrolyte used in the cells and
  includes
• proton exchange membrane (polymer) electrolyte
  fuel cell (PEFC),
• alkaline fuel cell (AFC),
• phosphoric acid fuel cell (PAFC),
• molten carbonate fuel cell (MCFC),
• solid oxide fuel cell (SOFC).
• These fuel cells are listed in the order of
  approximate operating temperature, ranging from
  80C for PEFC, 100C for AFC, 200C for PAFC,
  650C for MCFC, and 800C to 1000C for SOFC.

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• Advantages of Fuel Cells
• Higher efficiency than heat engines,
  particularly at part load
• Zero emissions
• Good load-following Characteristics
• Quick start-up time (PEM)
• Opportunities for Cogeneration
• Do not require recharging

Disadvantages Hydrogen Storage Hydrogen
Infrastructure Cost
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Summary of Major Fuel Constituents Impact
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Electrochemical Reactions in Fuel cells
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Fuel Cell Reactions and the Corresponding Nernst
Equations
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Fuel cells and internal combustion engines share
similarities of form. Both fuel cells and
internal combustion engines use gaseous fuel,
drawn from an external fuel storage system. Both
systems use hydrogen-rich fuel. Fuel cells use
pure hydrogen or a reformate gas mixture.
Internal combustion engines typically use
hydrogen-containing fossil fuels directly,
although they could be configured to operate
using pure hydrogen. Both systems use
compressed air as the oxidant in a fuel cell
engine the air is compressed by an external
compressor. In an internal combustion engine, the
air is compressed internally through piston
action. Both systems require cooling, although
engines operate at higher temperatures than fuel
cells.
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In some respects, fuel cells and internal
combustion engines are fundamentally different.
Fuel cells react the fuel and oxidant
electrochemically whereas internal combustion
engines react the fuel and oxidant combustively.
Internal combustion engines are mechanical
devices that generate mechanical energy while
fuel cells are solid state devices that generate
electrical energy (although the systems used to
support fuel cell operation are not solid state).
Pollution is related to the fuel composition and
the reaction temperature. Fuel cell engines
operating on pure hydrogen produce no harmful
emissions those that operate on hydrogen- rich
reformate produce some harmful emissions
depending on the nature of the process. Internal
combustion engines operating on pure hydrogen can
be designed to produce almost zero harmful
emissions those that run on conventional fuels
produce significantly more pollution.
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Practical Thermodynamics
A logical first step in understanding the
operation of a fuel cell is to define its ideal
performance. Once the ideal performance is
determined, losses can be calculated and then
deducted from the ideal performance to describe
the actual operation.
Ideal Performance
The ideal performance of a fuel cell depends on
the electrochemical reactions that occur with
different fuels and oxygen
Low-temperature fuel cells require noble metal
electrocatalysts to achieve practical reaction
rates at the anode and cathode, and H2 is the
only acceptable fuel. With high-temperature fuel
cells, the requirements for catalysis are
relaxed, and the number of potential fuels
expands. Carbon monoxide "poisons" a noble metal
anode catalyst such as platinum (Pt) in
low-temperature
The ideal standard potential of an H2/O2 fuel
cell (Eo) is 1.229 volts with liquid water
product and 1.18 water with gaseous product.
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Gibbs Free Energy and Ideal Performance
The maximum electrical work (Wel) 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,
where n is the number of electrons participating
in the reaction, F is Faraday's constant (96,487
coulombs/g-mole electron), and E is the ideal
potential of the cell. If we consider the case of
reactants and products being in the standard
state, then
Thus,
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?G values for the reaction at various temperature
is
H2 ½ O2 H2O n2e
Form of water product Temp., oC ?Gf, Kj/mol
Liquid Liquid Gas Gas Gas Gas Gas Gas gas 25 80 80 100 200 400 600 800 100 -237.2 -228.2 -226.1 -225.2 -220.4 -210.3 -199.6 -188.6 -177.4
So, For example a hydrogen fuel cell operating at
25 oC has ?G -237.2
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Actual Performance
Large, complex computer models are used by
manufacturers to characterize the actual
operation of fuel cells based on minute details
of cell component design (physical dimensions,
materials, etc.) along with physical
considerations (transport phenomena,
electrochemistry, etc.).
Useful amounts of work (electrical energy) are
obtained from a fuel cell only when a reasonably
current is drawn, but the actual cell potential
is decreased from its equilibrium potential
because of irreversible losses as shown in the
Figure
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• Several sources contribute to irreversible losses
  in a practical fuel cell. The losses, which are
  often called polarization, overpotential or
  overvoltage (?), originate primarily from three
  sources
• activation polarization (?act),
• ohmic polarization (?ohm), and
• concentration polarization (?conc).
• These losses result in a cell voltage (V) for a
  fuel cell that is less than its ideal potential,
  E (V E - Losses). Expressed graphically as a
  voltage/current density characteristic
  (Activation region and concentration region more
  representative of low-temperature cells)

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Activation Polarization Activation polarization
is present when the rate of an electrochemical
reaction at an electrode surface is controlled by
sluggish electrode kinetics. In other words,
activation polarization is directly related to
the rates of electrochemical reactions. There is
a close similarity between electrochemical and
chemical reactions in that both involve an
activation barrier that must be overcome by the
reacting species. In the case of an
electrochemical reaction with ?act gt 50-100 mV,
?act is described by the general form of the
Tafel equation
Where a Electron transfer coefficient of the
reaction at the electrode being
addressed io Exchange current density R Gas
constant T Cell Temperature n No. of Moles
F Faraday constant
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Ohmic Polarization Ohmic losses occur because
of resistance to the flow of ions in the
electrolyte and resistance to flow of electrons
through the electrode materials. The dominant
ohmic losses, through the electrolyte, are
reduced by decreasing the electrode separation
and enhancing the ionic conductivity of the
electrolyte. Because both the electrolyte and
fuel cell electrodes obey Ohm's law, the ohmic
losses can be expressed by the equation
?ohm iR
Where i Current flowing through the cell, R
Total cell resistance, which includes
electronic, ionic, and contact resistance.
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Concentration Polarization As a reactant is
consumed at the electrode by electrochemical
reaction, there is a loss of potential due to the
inability of the surrounding material to maintain
the initial concentration of the bulk fluid. That
is, a concentration gradient is formed. Several
processes may contribute to concentration
polarization slow diffusion in the gas phase in
the electrode pores, solution/dissolution of
reactants/products into/out of the electrolyte,
or diffusion of reactants/products through the
electrolyte to/from the electrochemical reaction
site. At practical current densities, slow
transport of reactants/products to/from the
electrochemical reaction site is a major
contributor to concentration polarization
Where iL Limiting current
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Summing of Electrode Polarization Activation
and concentration polarization can exist at both
the positive (cathode) and negative (anode)
electrodes in fuel cells. The total polarization
at these electrodes is the sum of ?act and ?
conc, or
?anode ?conc, a ?act, a
and
?cathode ?conc, c ? act, c
The effect of polarization is to shift the
potential of the electrode (E electrode)
to a new value (V electrode) V electrode E
electrode ?electrode For the anode, V anode
E anode ? anode and for the cathode, V
cathode E cathode ? cathode
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Summing of Cell Voltage The cell voltage
includes the contribution of the anode and
cathode potentials and ohmic polarization V
cell V cathode V anode iR V cell E
cathode ? cathode (E anode ? anode) iR
Or V cell ?Ee ? cathode ? anode
iR Where ?Ee E cathode - E anode
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Maximum power occurs at approximately 0.5 to 0.6
V, which corresponds to relatively high current.
At the peak point, the internal resistance of the
cell is equal to the electrical resistance of the
external circuit. However, since efficiency drops
with increasing voltage, there is a tradeoff
between high power and high efficiency. Fuel cell
system designers must select the desired
operating range according to whether efficiency
or power is paramount for the given application.
It is never desirable to operate in the range
beyond where the power curve drops off.
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The shape of a polarization curve depends on the
operating temperature and pressure of the stack.
In general, a family of polarization curves can
be drawn that characterize the stack performance
over its entire operating envelope. In general,
any parameter variation that causes the
polarization curve to go up is beneficial since
this results in greater power and higher
electrochemical efficiency. The converse is also
true.
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Fuel Cell Performance Variables
Effect of pressure
Fuel cell polarization curves typically increase
with increasing operating pressure. Conversely,
the polarization curves decrease with decreasing
operating pressure. The reason for this is that
the rate of the chemical reaction is proportional
to the partial pressures of the hydrogen and the
oxygen. (Each gas within a gas mixture
contributes a partial pressure, the sum of which
makes up the total pressure.) Thus, the effect of
increased pressure is most prominent when using a
dilute oxidant (like air) or a dilute fuel (like
reformate). In essence, higher pressures help to
force the hydrogen and oxygen into contact with
the electrolyte. This sensitivity to pressure is
greater at high currents.
Although an increase in pressure promotes the
electrochemical reaction, it introduces other
problems. Fuel cell stack flow field plates work
better at low pressure since they exhibit smaller
flow-induced pressure losses. Fuel cell seals
operate under additional stress. Additional air
compression is required, which absorbs more of
the gross power. Other system components must be
re-designed accordingly some components must
increase in size and cost. Ultimately, increases
in pressure achieve diminishing returns when
considering both stack efficiency and overall
system consequences. Because of these factors,
PEM fuel cells are typically operated at
pressures no greater than a few atmospheres.
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• Fuel cell polarization curves increase with
  increasing operating temperature. Conversely, the
  polarization curves decrease with decreasing
  operating temperature. The reason for this is
  that
• higher temperatures improve mass transfer within
  the fuel cells
• results in a net decrease in cell resistance
• The accumulation of product water within the
  oxidant stream effectively limits operating
  temperatures to below 212 ºF (100 ºC). At this
  temperature, the water boils and the resulting
  steam severely reduces the partial pressure of
  the oxygen. This, in turn, drastically reduces
  cell performance due to oxygen starvation. This
  can damage the fuel cells and reduce their life.

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Fuel cell polarization curves increase with
increasing reactant gas stoichiometry.
Conversely, the polarization curves decrease with
decreasing reactant gas stoichiometry. The reason
for this is that higher stoichiometry increases
the chance that sufficient numbers of hydrogen
and oxygen molecules interact with the
electrolyte. Insufficient stoichiometry deprives
(or starves) the fuel cell stack of sufficient
reactants and may cause permanent
damage. Stoichiometry is the ratio of the amount
of gas present relative to the amount of that gas
that is needed to exactly complete the reaction.
This is much like the definition of specific
gravity where densities are indicated relative to
a reference substance. Thus, a stoichiometric
ratio of 1.0 provides exactly the correct number
of gas molecules to theoretically complete the
reaction. Stoichiometric ratios greater than 1.0
provide excess gas and ratios less than 1.0
provide insufficient gas. A stoichiometric ratio
of 2.0 provides exactly twice the number of gas
molecules as required.
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As gas stream stoichiometric ratio increases, the
resulting fuel cell voltage approaches its
terminal voltage asymptotically. Practical fuel
cell stacks are typically operated at a hydrogen
stoichiometric ratio of 1.4 and an air
stoichiometric ratio of 2.0 at rated load
additional gas provides little additional
benefit. Higher stoichiometric ratios are
required when operating at low power.
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Humidity Effects
Sufficient gas stream humidification is essential
to PEM fuel cell operation since water molecules
move with the hydrogen ions during the ion
exchange reaction. Insufficient humidification
water dehydrates the membrane and can lead to
cracks or holes in the membrane. This results in
a chemical short circuit, local gas mixing, hot
spots, and the possibility of fire. Conversely,
excess humidification water leads to condensation
and flooding within the flow field plates. This,
in turn, can result in a phenomenon known as cell
reversal where the affected cells produce a zero
or negative voltage. If a large enough negative
voltage occurs, the affected fuel cells start to
act like an electrolyzer. This produces a lot of
heat and can potentially destroy the cell. Cell
monitoring systems are typically installed to
detect cell reversal before cell damage occurs.
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Fuel cell systems are usually compared to
internal combustion engines and batteries and
offer unique advantages with respect to them.
Fuel cell systems operate without pollution when
run on pure hydrogen, the only by-products being
pure water and heat. When run on hydrogen-rich
reformate gas mixtures, some harmful emissions
result although they are less than those emitted
by an internal combustion engine using
conventional fossil fuels. To be fair, internal
combustion engines that combust lean mixtures of
hydrogen and air also result in extremely low
pollution levels that derive mainly from the
incidental burning of lubricating oil.
Fuel cell systems operate at higher thermodynamic
efficiency than heat engines. Heat engines, such
as internal combustion engines and turbines,
convert chemical energy into heat by way of
combustion and use that heat to do useful work.
The optimum (or Carnot) thermodynamic
efficiency of a heat engine is known to be
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Fuel Cell Efficiency
The efficiency of fuel cells compared with other
electric power generating systems, as a function
of power output, is shown in Figure below. Fuel
cell systems have higher thermal efficiencies,
particularly in small sizes and at intermediate
loads. It is this characteristic which provides
the main incentive for current fuel cell systems
development. Fuel cell power plants are able to
operate on reformed fossil fuels such as methanol
or natural gas. The improved thermal efficiency
of the fuel cell, compared to other fossil fueled
power generators, provides two direct benefits
fuel cost reduction and environmental pollution
reduction.
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Fuel Cell Stack Assembly
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Terrestrial fuel cell systems, for cars, buses or
stationary power plants, typically use compressed
air as the oxidant. A variety of fuels can be
used, but pure hydrogen is the simplest and most
efficient of these options. Hydrogen fuel suffers
from relatively low volumetric and gravimetric
energy storage densities, compared with liquid
fuels commonly used today. Moreover, there is a
lack of infrastructure for supporting the
transition to hydrogen fuel in global energy
markets. Fleet vehicles, such as transit buses
and taxi cabs, offer a good early market for
hydrogen bases fuel cell systems.
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In applications where ambient air is not
available, such as space or subsea environments,
pure oxygen can be used as the oxidant for fuel
cell operation. In these cases, oxygen must be
stored onboard the vehicle, either as compressed
gas or cryogenic liquid, contributing
considerable size and weight to the overall power
system. Fuel cells perform better on pure oxygen
than on air, exhibiting higher cell voltage and
overall efficiency. Moreover, removal of air
compression suggests that pure oxygen systems
will be quieter and have lower parasitic losses
than air systems.
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Fuel Cell Sample Calculations
Fuel flow rate Calculations
What hydrogen flow rate is required to generate
1.0 Ampere of current in a fuel cell? (This
exercise will generate a very useful conversion
factor for subsequent calculations.)
For every molecule of hydrogen (H2) that reacts
within a fuel cell, two electrons are liberated
at the fuel cell anode. This is most easily seen
in the PEMFC because of the simplicity of the
anode (fuel) reaction, although the rule of two
electrons per diatomic hydrogen molecule (H2)
holds true for all fuel cell types. The solution
also requires knowledge of the definition of an
Ampere (A) and an equivalence of electrons.
H2 2H 2e-
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The result of this calculation, 0.037605 kg H2
per kA (0.08291 lb H2 per kA), is a convenient
factor that is often utilized in determining how
much fuel must be consumed to supply a desired
fuel cell power output as illustrated below.
Required Fuel Flow Rate for 1 MW Fuel Cell

• A 1.0 MWDC fuel cell stack is operated with a
  cell voltage of 700 mV on pure hydrogen with a
  fuel utilization, Uf of 80.
• How much hydrogen will be consumed in lb/hr?
• What is the required fuel flow rate?
• What is the required air flow rate for a 25
  oxidant utilization, Uox?

We shall simplify the solution of this problem by
artificially assuming that the individual fuel
cells are arranged in parallel. That is, the fuel
cell module voltage is the same as the cell
voltage, and the fuel cell module current is
equal to the current of an individual fuel cell
times the number of fuel cells.
Recalling that power is the product of the
voltage and current, Power (P) I x
V Therefore, the current through the fuel cells
can be calculated as
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The quantity of hydrogen consumed within the fuel
cell is
Thus, the reader may find it more expedient and
less error prone to make the parallel arrangement
assumption when determining the mass flow
requirement of hydrogen, in spite of the actual
arrangement. the utilization of fuel in a PAFC
is defined as
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Therefore the required fuel flow rate can be
calculated as
To determine the air supply requirement, we first
observe that the stoichiometric ratio of hydrogen
to oxygen is 2 to 1 for H2O. Thus, the moles of
oxygen required for the fuel cell reaction are
determined by
If a 25 utilization is required, then the air
feed must contain four times the oxygen that is
consumed,
Because dry air contains 21 O2 by volume, or by
mole percent, the required mass flow rate of dry
air is,
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Fuel Cell Stack design Calculation

• Generic Fuel Cell - Determine the Required Cell
  Area, and Number of Stacks
• Given a desired output of 2.0 MWDC, and the
  desired operating point of 600 mV and
• 400 mA/cm2,
• How much fuel cell area is needed?
• Assuming a cell area of 1.00 m2 per cell and 280
  cells per stack, how many stacks are needed for
  this 2.0 MW unit?

a) Recalling again that power is the product of
the voltage and current, we first determine the
total current for fuel cell as
Because each individual fuel cell will operate at
400 mA/cm2, we determine the total area required
as,
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b) The number of required stacks and cells are
calculated simply as
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Hydrogen Production
The fundamental question underlying the use of
hydrogen as a fuel is, Where do we get it from?
Despite its abundance in the universe, hydrogen
does not occur freely on earth, as it reacts very
readily with other elements. For this reason, the
vast majority of hydrogen is bound into molecular
compounds. To obtain hydrogen means to remove it
from these other molecules. With respect to the
energy required, it is easy to remove hydrogen
from compounds that are at a higher energy state,
such as fossil fuels. This process releases
energy, reducing the amount of process energy
required. It takes more energy to extract
hydrogen from compounds that are at a lower
energy state, such as water, as energy has to be
added to the process.

• Hydrogen production Technologies
• Electrolysis
• 2. Non-Polluting renewable energy sources
• 2.1. Solar Cell
• 2.2. Wind Energy
• 2.3. hydroelectric
• 3. Reforming of Hydrocarbon

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1. Electrolysis
In electrolysis, electricity is used to decompose
water into its elemental components hydrogen and
oxygen. Electrolysis is often touted as the
preferred method of hydrogen production as it is
the only process that need not rely on fossil
fuels. It also has high product purity, and is
feasible on small and large scales. Electrolysis
can operate over a wide range of electrical
energy capacities, for example, taking advantages
of more abundant electricity at night. At the
heart of electrolysis is an electrolyzer. An
electrolyzer is a series of cells each with a
positive and negative electrode. The electrodes
are immersed in water that has been made
electrically conductive, achieved by adding
hydrogen or hydroxyl ions, usually in the form of
alkaline potassium hydroxide (KOH).
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Electrolyzers
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2. Renewable Hydrogen 2.1. Solar Cell
Solar electric power generation uses banks of
solar cells to convert the energy of the sun
directly into electrical power. Solar power is
only feasible in areas with significant amounts
of intense sunlight and requires large tracts of
land to generate sufficient levels of power. The
efficiency of solar cells currently ranges from 3
to 17.
Solar thermal power generation starts by
concentrating the suns heat into a fluid that
has a high heat-carrying capacity, such as oil.
The oils heat is transferred through heat
exchangers to water, generating steam for a steam
turbine generator. Like solar electric power
generation, solar thermal power generation is
only feasible in areas with significant amounts
of intense sunlight and requires large tracts of
land in order to generate sufficient levels of
power. The overall efficiency of converting
sunlight to electricity for these systems is
about 8 to 24 depending on the type of
technology used.
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2.2. Wind Energy
Wind power generation uses the energy of moving
air to turn turbines that in turn rotate
generators. Wind power is only feasible in areas
with favorable wind conditions and requires large
tracts of land in order to generate sufficient
levels of power. Wind has low energy density and
wind turbines operating at optimum conditions
(design speed) may obtain 30 efficiency at best.
Real-world operating conditions may reduce this
efficiency considerably.
2.3. Hydroelectric
Hydroelectric power generation uses the energy of
moving water to turn turbines that in turn rotate
generators. Hydroelectric power is only feasible
in areas with major rivers that undergo
significant changes in height. Most suitable
locations worldwide have already been developed.
Hydroelectric power is a cheap source of clean
power especially when utilizing excess, off-peak
power. The efficiency of hydroelectric power
generation can top 80. This is probably the
optimum form of renewable energy although the
environmental and ecological cost of dams is high.
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3. Reforming Fossil Fuels
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Fuel Characterization
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Typical Reformate composition
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Steam Reformers
Steam reformers are currently the most efficient,
economical and widely used technique of hydrogen
production. Steam reforming is based on the
principal that hydrogen-containing fuels
decompose in the presence of steam over
nickel-based catalysts to produce a mixture of
hydrogen and carbon monoxide. The output
products also contain some un reacted source fuel
and water. In addition, only light hydrocarbons
can be completely vaporized without leaving a
carbon residue. The carbon monoxide must be
converted to carbon dioxide using supplementary
processes.
A great benefit of steam reforming is that the
hydrogen present in the water is released during
the reaction and contributes to the overall
hydrogen yield. The steam reforming process
typically requires temperatures of 840 to 1700 ºF
(450 C to 925 C) and pressures of approximately
290 to 500 psig (20 to 35 barg). These
temperatures are achieved through combustion of a
portion of the reformate. These flame
temperatures are too low to form nitrous oxides,
which begin forming at temperatures above 2700 ºF
(1480 ºC).
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Partial Oxidation Reformer
Partial oxidation reformers react a lean mixture
of oxygen (air) with fuel to produce a mixture of
hydrogen and carbon monoxide. Since partial
oxidation reformers use oxygen from air, nitrogen
passes through the reactor along with the
reaction products, thereby diluting the fuel
stream. The output products also contain some
unreacted source fuel. The carbon monoxide must
be converted to carbon dioxide using
supplementary processes Unlike stream reformers,
partial oxidation reformers are typically used to
reform heavier hydrocarbons such as gasoline,
diesel and heavy oil. A form of partial oxidation
is used to gasify coal although the presence of
sulfur and large amounts of ash, both of which
must be removed, further complicates this
process. These processes do not use catalysts and
occur at 2100 to 2400 ºF (1150 to 1315 ºC) and on
the order of 880 psig (60 barg). Lighter
hydrocarbons, such as methane, can be partially
oxidized using catalysts at 1090 ºF (590 ºC).
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Autothermal Reformer
Autothermal reformers attempt to combine the
compactness and load following capabilities of
partial oxidation reactors with the efficiency of
steam reformers by combining the two reactors in
one unit.
In an autothermal reformer, fuel, steam and
oxygen (or air) are fed over a mixed catalyst bed
that supports both partial oxidation and steam
reforming reactions. The heat generated by the
partial oxidation reaction provides the heat
required for the steam reforming reaction, and
removes the need for an external burner or heat
source. This process requires careful thermal
integration and tight controls to ensure heat
balance and temperature matching between the two
reactions.
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Thermal Decomposition Reactors
Thermal decomposition reformers (or catalytic
crackers) use heat to break down source fuels
yielding high purity hydrogen (gt95) and solid
carbon.
Thermal decomposition reformers are very compact
with quick startup and load-following
characteristics but have the lowest thermal
efficiency of any of the reforming systems
(5565). The thermal efficiency is so low partly
because a great deal of the source fuels energy
remains trapped in the product carbon (rather
than in the lower energy forms of carbon monoxide
or carbon dioxide). Efficiencies would improve
significantly if the product carbon could be
burned for heat generation, but this would lead
to undesirable emission levels of nitrous oxides
and would substantially increase system
complexity.
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Reformate Purification
Steam, partial oxidation and autothermal
reformers all convert the carbon contained in the
source fuel into carbon monoxide. Depending on
the feedstock, the reformate stream can also
include sulfur compounds, liquid methanol other
contaminants. All of these compounds poison or
degrade fuel cell performance and must be removed
to very low levels. For example, alkaline fuel
cells can tolerate no more than 3 (by volume)
carbon monoxide, and PEM fuel cells can tolerate
no more than 50 ppm carbon monoxide. Other
reformate gases, such as nitrogen and carbon
dioxide, dilute fuel cell performance but do not
damage the cells. Reformate purification is a
two-stage process. In the first stage, the bulk
of the carbon monoxide is transformed into carbon
dioxide using a water/gas shift reaction. In the
second stage, the amount of carbon monoxide is
further reduced using selective oxidation or
methanation reactions, and/or the hydrogen is
extracted from the reformate stream using a
pressure swing adsorption process or by means of
metal separation membranes. The amount of
reformate purification chosen is a trade-off
between fuel cell longevity and performance, and
overall system complexity, size and cost.
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Water/Gas Shift Reaction
The water/gas shift reaction reacts carbon
monoxide with steam over a catalyst to produce a
mixture of hydrogen and carbon dioxide. The
water/gas shift reaction can be performed using
high temperature catalysts that support the
reaction in excess of 570 ºF (300 ºC), or using
low-temperature catalysts that support the
reaction to about 285 ºF (150 ºC). The water/gas
shift reaction is an exothermic process this
heat can be recovered and used in other parts of
the reforming process to improve the overall
thermal efficiency. Some shift reaction catalysts
also oxidize exothermically when exposed to air.
This can result in high temperatures that can
pose a fire hazard and/or damage the catalyst.
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Selective Oxidation
Selective oxidation is a chemical process that
reacts carbon monoxide with oxygen (air) over a
catalyst to produce carbon dioxide. The catalyst
bed facilitates both of the following competing
reactions CO ½O2 CO2 heat H2 ½O2
H2O heat The former reaction is
preferred and is selected by controlling the
temperature profile within the selective
oxidizer. Temperatures that are too high favor
the water producing reaction whereas temperatures
that are too low result in condensation. The
selective oxidation reaction is an exothermic
process this heat can be recovered and used in
other parts of the reforming process to improve
the overall thermal efficiency.
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Methanation Process
Methanation is a chemical process that reacts
carbon monoxide and carbon dioxide with hydrogen
to produce methane and water. The overall
reactions are CO 3 H2 CH4 H2O
heat CO2 4 H2 CH4 2 H2O heat These
reactions are the opposite of those that occur
during steam reforming of methane and are
therefore reversing the hydrogen production
initially accomplished. If this process were
performed on the reformate stream directly,
virtually all of the product hydrogen would be
consumed with no net yield. However, if the
carbon dioxide is removed prior to methanation
using some other means (such as pressure swing
adsorption), the remaining carbon monoxide can be
successfully reduced to very low levels with
little loss in overall hydrogen yield.
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Metal Separation Membrane Process
Metal separation membranes present an impermeable
physical barrier to all gases except hydrogen. To
function, the reformate must be delivered to the
membrane across a pressure gradient. Up to 85 of
the hydrogen then diffuses from the high pressure
(300 psig 20 barg) reformate stream to form a
low-pressure (30 psig 2 barg) stream of very
high purity (gt99.999). The remaining
high-pressure raffinate can be combusted to
provide heat for the reforming process. The
advantages of metal separation membranes are that
they provide high purity, undiluted hydrogen in a
compact, simple and reliable manner. Metal
separation membranes are also well suited to
thermal integration with steam reformers and
respond well to thermal transients.
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Fuel Processing Systems This section discusses
fuel processor system design and integration into
fuel cell power plants. Fuel selection primarily
determines the requirement for fuel processing.
In general, heavier and more complex fuels
require larger and more complex fuel processors,
which adds to system size and cost and reduces
overall system efficiency and reliability.
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Methanol Reformer Integration
Methanol is free of sulfur, olefins and
aromatics. A power plant using methanol would
differ from a hydrogen Based system only in the
addition of a reformer and a reformate purifier.
Methanol and water are vaporized and mixed prior
to the reformer unit. The reformer produces a
hydrogen-rich gas, which also contains carbon
dioxide, carbon monoxide, as well as unreformed
methanol and water vapor. A low temperature shift
reactor reduces CO levels and produces additional
hydrogen.
A selective oxidizer further reduces CO levels
(to about 10 ppm) for fuel cell consumption. The
shift reactor and selective oxidizer are
exothermic devices, producing heat, which can be
utilized in the reformer. Spent fuel cell anode
gas is combusted with air to provide most of the
heat for the reforming reaction. Exhaust gases
from the combustion process are comprised mainly
of CO2. Waste heat in the exhaust stream can be
recovered and utilized to produce stream for
injection into the reforming bed.
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Pd Membrane Reformer Integration
Thin metal-membranes may be utilized instead of
shift reactors and selective oxidizers for
purification of methanol reformate. These systems
produce essentially pure hydrogen, which
simplifies system design and increases overall
power plant efficiency. The use of thin
metal-membranes, however, requires the reformer
to be operated at elevated pressures in order to
provide a pressure gradient across the membrane
for hydrogen diffusion.
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Natural gas contains low levels of simple sulfurs
(mercaptans), mostly for odorant purposes, and no
olefins. Of course, natural gas compositions vary
according to location. For a natural gas fuel
processing unit, the addition of a low pressure
(10 bar) hydro desulfurization unit with limited
hydrogen recycle should be sufficient to ensure
long lifetime of the reformer catalyst.
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For the more complex liquid fuels, where olefin
formation and carryover of unsaturated compounds
is more likely to occur, a pre-reformer will be
required in addition to the desulfurizer. The
operating pressure and volume of the
hydrodesulfurizing unit will increase over the
simpler fuels, with component requirements being
worse for diesel systems than for those using
kerosene. The pre-reforming window for diesel
fuels will likely be smaller than that of
kerosene, leading to tighter control
requirements. A number of kerosene reforming
demonstration units exist, while diesel reforming
units are still in the proof-of-concept stage.
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High-pressure gas storage systems are the most
common and most highly developed methods of
storing hydrogen. Most existing fuel cell
vehicles use this form of hydrogen
storage. High-pressure hydrogen is stored in
cylinders, similar to those used for compressed
natural gas. Most cylinders have a cylindrically
shaped sidewall section with hemispherical end
domes, although new conformal designs use
multiple cylinders in tandem and distort the
cylindrical shape in order to increase the usable
volume.
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In general, the less metal used, the lower the
weight. For this reason, Type 3 cylinders are
usually used in hydrogen applications, and Type 4
cylinders will likely gain prominence in the
future. Specific weights depend on individual
manufacturers, but as a point of reference, a 3.5
ft3 (100 L) Type 1 (steel) cylinder weighs about
220 lb (100 kg) a Type 3 (aluminum/composite)
cylinder weighs about 143 lb (65 kg), and a Type
4 cylinder weighs about 66 lb (30 kg). Type 3
cylinders derive most of their strength from the
composite overwrap that is wound around the inner
liner. This composite consists of high-strength
fibers (usually carbon) that are wrapped around
the cylinder in many layers and glued together by
a resin such as epoxy.
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Liquid hydrogen storage systems overcome many of
the weight and size problems associated with
high-pressure gas storage systems, albeit at
cryogenic temperatures.
Liquid hydrogen can be stored just below its
normal boiling point of 424 ºF (253 ºC 20 K)
at or close to ambient pressure in a
double-walled, super-insulating tank (or
dewar). This insulation takes the form of a
vacuum jacket, much like in a Thermos bottle.
Liquid hydrogen tanks do not need to be as strong
as high-pressure gas cylinders although they do
need to be adequately robust for automotive use.
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• Fuel Cell Applications
• Portable fuel cell application
• Stationary application
• Transportation application
• Telecom commercial UPS application
• Home cogeneration application

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Ballard Power Systems 250 kW Natural Gas Fueled
Fuel Cell Power plant Field Trial - 9 systems
installed in North America, Europe Japan Over
3.5 million kWhs of electricity generated so
far
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Ballard Bus Engine Development Program (1993 -
2000)
Beginning in 2002 and throughout 2003, 30
Ballard heavy-duty fuel cell engines paired with
Ballard electric drive systems will be delivered
for use in buses in 10 European cities.
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Pushing the envelope ... The DaimlerChrysler Neca
r 5 was the first fuel cell car to cross
the United States. San Francisco to Washington,
D.C. May 20th June 4th, 2002 Methanol fueled
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