energy from a fuel
into oxygen or another
What is a Fuel Cell?
oxidizing hydrocarbons
A fuel cell is a device that converts the such as sometimes used.
chemical electricity through a chemical Fuel cells a constant
reaction with agent. Hydrogen is the most
source of electricity
common fuel, but natural gas and alcohols
continually for
like methanol are are different from batteries
in that they require fuel and oxygen to run,
but they can produce as long as these inputs
are supplied.
Space Programs
to capsules. Since
Where were they first used?
then,
The first commercial use of Fuel cells was in
Fuel cells are
NASA generate power for probes, satellites andused industrial and
space fuel cells have been used in many otherareas. They are used
applications. for primary and backup power for buses,
commercial, residential buildings and in remote
or inaccessible to power fuel cell vehicles,
including automobiles, airplanes, boats,
motorcycles and submarines.
rklifts,
Different types of Fuel Cells:
There are many types of Fuel cells, but he all consist of an Anode
(negative side), a Cathode (positive side) and an Electrolyte that
allows charges to move between the two sides of the Fuel cell.
Phosphoric Acid Fuel Cell
Phosphoric Acid Fuel Cells (PAFC) are commercially available around
the world. Hundreds of Fuel cell systems have been installed in
H
ospitals, Schools, Offices, Utility power plants etc. in 19 nations
across t he globe. PAFC's generate electricity with 40% more efficiency
and the 8 5% steam the fuel cell produces us used for Cogeneration.
use liquid Phosphoric acid as the electrolyte and operate at 450 F.
main advantage of such a cell is that it uses impure Hydrogen as
Fuel.
Polymer Electrolyte Membrane
deliver high-power density and offer the advantages of low weight
volume, compared with other fuel cells. PEM fuel cells use a solid
polymer as an electrolyte and on porous carbon electrodes containing
a platinum catalyst. They need only hydrogen, oxygen from the air, and
to operate and do not require corrosive fluids like some fuel
They are typically fueled with pure hydrogen supplied from
storage tanks or on-board reformers.
Fuel cells are used primarily for transportation applications and
stationary application. Due to their fast startup time, low sensitivity to
orientation and favorable power-to-weight ratio, PEM cells are
particularly suitable for use in passenger vehicles, such as and buses.
Alkaline Fuel Cells
Alkaline Fuel Cells (AFCs) were only one of the first fuel cell
technologies developed, and they were the first type widely used in
the
Space Program to produce electrical energy and water on-board
crafts. These fuel cells use a solution of Potassium Hydroxide in as the
electrolyte and can use a variety of non-precious metals as Catalysts
as the anode and cathode. High-temperature AFCs operate at
temperatures between 100 C and 250 C. However, newer AFC designs
OPerate at lower temperatures of roughly 230 C to 70 C.
disadvantage of this fuel cell type is that it is easily poisoned by
carbon dioxide (C02).
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The
fact, even the small amount of C02 in the air can affect this cell's
operation, making it necessary to purify both the hydrogen and oxygen
used in the cell. This purification process is costly. Susceptibility to
poisoning also affects the cell's lifetime (the amount of time before it
must be replaced), further adding to cost.
Solid oxide fuel cells
solid oxide fuel cells (SOFCs) use a hard, non-porous ceramic compound
the electrolyte. SOFCs are around 60% efficient at converting fuel to
electricity. In applications designed to capture and utilize the system's
waste heat (co-generation), overall fuel use efficiencies could top 85%.
High-temperature operation has disadvantages. It results in a slow
startup and requires significant thermal shielding to retain heat and
protect personnel, which may be acceptable for utility applications but
for transportation. The high operating temperatures also place
stringent durability requirements on materials. The development of
low-cost materials with high durability at cell operating temperatures is
key technical challenge facing this technology.
Molten carbonate fuel cell
Molten carbonate fuel cells (MCFCs) are currently being developed for
natural gas and coal-based power plants for electrical utility, industrial,
military applications. MCFCs are high-temperature fuel cells that
an electrolyte composed of a molten carbonate salt mixture
Suspended in a porous, chemically inert ceramic lithium aluminum
oxide matrix. Because they operate at high temperatures of 6500C
(roughly 1,2000F), non-precious metals can be used as catalysts at the
ode and cathode, reducing costs.
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Unlike alkaline, phosphoric acid, and PEM fuel cells, MCFCs do not
require an external reformer to convert fuels such as natural gas and
biogas to hydrogen. At the high temperatures at which MCFCs
operate, methane and other light hydrocarbons in these fuels are
converted to hydrogen within the fuel cell itself by a process called
internal reforming, which also reduces cost.
Molten carbonate fuel cells are not prone to poisoning by carbon
monoxide or carbon dioxide — they can even use carbon oxides as
fuel — making them more attractive for fueling with gases made
from coal. Because they are more resistant to impurities than other
fuel cell types, scientists believe that they could even be capable of
internal reforming of coal, assuming they can be made resistant to
impurities such as sulfur and particulates that result from
converting coal, a dirtier fossil fuel source than many others, into
hydrogen.
Anode Reaction: 2C03 2- + 21-12 21-120 + 2C02 + 4e
Cathode Reaction: 2C02 + 02+
4e- + 2C03 2
Overall Reaction: 2 H 2 + 02 o
Direct methanol fuel cells
Most fuel cells are powered by hydrogen, which can be fed to the
fuel cell system directly or can be generated within the fuel cell
system by reforming hydrogen-rich fuels such as methanol,
ethanol, and hydrocarbon fuels. Direct methanol fuel cells
(DMFCs), however, are powered by pure methanol, which is usually
mixed with water and fed directly to the fuel cell anode.
Direct methanol fuel cells do not have many of the fuel storage
problems typical of some fuel cell systems because methanol has
a higher energy density than hydrogen—though less than
gasoline or
diesel fuel. Methanol is also easier to transport and supply to the public
using our current infrastructure because it is a liquid, like gasoline.
DMFCs are often used to provide power for portable fuel cell
applications such as cell phones or laptop computers.
o Reversible Fuel Cells
A Regenerative Fuel cell or Reverse Fuel cell (RFC) produces electricity
from hydrogen and oxygen and generate heat and water as
byproducts, just like other fuel cells. However, reversible fuel cell
systems can also use electricity from solar power, wind power, or other
sources to split water into oxygen and hydrogen fuel through a process
called electrolysis. Reversible fuel cells can provide power when
needed, but during times of high power production from other
technologies (such as when high winds lead to an excess of available
wind power), reversible fuel cells can store the excess energy in the
form of hydrogen. This energy storage capability could be a key
enabler for intermittent renewable energy technologies.
Design and Operation
Fuel cells come in many varieties; however, they all work in the same
general manner. They are made up of three adjacent segments: the
anode, the electrolyte, and the cathode. Two chemical reactions occur
at the interfaces of the three different segments. The net result of the
two reactions is that fuel is consumed, water or carbon dioxide is
created, and an electric current is created, which can be used to
power electrical devices, normally referred to as the load.
The most important design features in a fuel cell are:
The electrolyte substance. The electrolyte substance usually
defines the type of fuel cell.
The fuel that is used. The most common fuel is hydrogen.
The anode catalyst breaks down the fuel into electrons and ions.
The anode catalyst is usually made up of very fine platinum
powder. The cathode catalyst turns the ions into the waste
chemicals like water or carbon dioxide. The cathode catalyst is
often made up of nickel but it can also be a nanomaterial-based
catalyst.
A typical fuel cell produces a voltage from 0.6 V to 0.7 V at full rated
load. Voltage decreases as current increases, due to several factors:
• Activation loss
• Ohmic loss (voltage drop due to resistance of the cell components
and interconnections)
• Mass transport loss (depletion of reactants at catalyst sites under
high loads, causing rapid loss of voltage).
TO deliver the desired amount of energy, the fuel cells can be
combined in series to yield higher voltage, and in parallel to allow a
higher current to be supplied. Such a design is called a fuel cell stack.
The cell surface area can also be increased, to allow higher current
from each cell. Within the stack, reactant gases must be distributed
Uniformly over each of the cells to maximize the power output.
Theoretical maximum efficienc
The energy efficiency of a system or device that converts energy is
measured by the ratio of the amount of useful energy put out by the
system to the total amount of energy that is put in or by useful
output energy as a percentage of the total input energy. In the case
of fuel cells, useful output energy is measured in electrical energy
produced by the system. Input energy is the energy stored in fuel.
According to the U.S. Department of energy, fuel cells are generally
between 40-60% energy efficient. This is higher than some other
systems for energy generation. For example, the typical internal
combustion engine of a car is about 25% energy efficient. In
combined heat and power systems, the heat produced by the fuel
cell is captured and put to use, increasing the efficiency of the
system up to 85-90%.
The theoretical maximum efficiency of any type of power generation
system is rarely reached in practice, and it does consider any other
steps in power generation, such as producti transportation and
storage of fuel and conversion of the ele ricity into mechanical power.
However, this calculation allows the mparison of different types of
types of power generation. The maximum theoretical energy efficiency
Of a fuel cell is 83%, operating at low power density and using pure
hydrogen and oxygen as reactants. According to the world Energy
council, this compares with maximum theoretical efficiency of 58%
for internal combustion engines. While these efficiencies are not
approached in the most real world applications, high temperature
fuel cells can theoretically be combined with gas turbines to allow
stationary fuel cells to come closer to the theoretical limit. A gas
turbine would capture heat from the fuel cell's operational
efficiency. This solution has been predicted to increase total
efficiency to as much as 70%.
In practice
The tank- to- wheel efficiency of a fuel cell vehicle is greater than 45%
at low loads and shows average values of about 36% when a driving
cycle like the NEDC is used as test procedure. The comparable NEDC
value for diesel vehicle is 22%. In 2008 Honda released a
demonstration fuel cell electric vehicle with fuel stack claiming a 60%
tank-to-wheel efficiency.
It is also important to take losses due to fuel production transportation,
and storage into account. Fuel cell vehicles running on compressed
hydrogen may have power plant-plant-to-wheel efficiency of 22% if the
hydrogen is stored as high pressure gas, and 17% if it is stored as liquid
hydrogen. Fuel cell cannot store energy like battery, except as hydrogen,
but in some applications, such as stand-alone power plants based on
discontinuous sources such as solar and w•nd power, they are combined
with electrolyzers and storage syste o form a an energy storage system.
Most hydrogen, however, • produced by steam in methane reforming,
and so most hydr en production emits carbon dioxide. The overall
efficiency of s plants, using pure hydrogen and pure oxygen can be "from
35 up to 50 percent", depending on gas density and other conditions.
While a much cheaper lead-acid battery might return 90%, the
electrolyzer/fuel cell system cans tore indefinite quantities of hydrogen,
and is therefore better suited for long-term storage.
Solid-oxide fuel cells produce exothermic heat from the
recombination of the oxygen and hydrogen. The ceramic can run as
hot as 800 degrees Celsius. This heat and can be captured and used
to heat water in a micro combined heat and power application.
When the heat is captured total efficiency can reach 80-90% at the
unit, but does not consider production and distribution losses. CHP
units are being developed today for the European home market.
Professor Jeremy P .Meyers, in the electro chemical society journal
interface in 2008, wrote, while fuel cells are efficient relative to
combustion engines, they are not as efficient as batteries, due to
primarily to the inefficiency of the oxygen evolution reaction (and
the oxygen evolution reaction, should the hydrogen be formed by
electrolysis of water).......They make the most sense for operation
disconnected from the grid, or when fuel can be provided continuously.
For applications that require frequent and relatively rapid
startups......Where zero emission are a requirements, as in enclosed
spaces such as warehouse, and hydrogen is considered an ac table
reactant, a( PEM fuel cell) is becoming an increasingly attr Ive choice (if
exchanging batteries is inconvenient)"
Applications
Power - Stationary fuel cells are used for commercial, industrial and
residential primary and backup power generation. Fuel cells are very
useful as power sources in remote locations, such as spacecraft, remote
Weather stations, large parks, communications centers, rural locations
including research stations, and in certain military applications. A fuel cell
system running on hydrogen can be compact and lightweight, and have
no major moving parts and do not involve combustion, in ideal COnditions
they can achieve up to 99.9999% reliability. This equates to less than one
minute of downtime in a six-year period.
Since fuel cell electrolyzer systems do not store fuel in themselves, but
rather rely on external storage units, they can be successfully applied in
large-scale energy storage, rural areas being one example. There are
many different types of stationary fuel cells so efficiencies vary, but
most are between 40% and 60% energy efficient. However, when the
fuel cell's waste heat is used to heat a building in a cogeneration
system this efficiency can increase to 85%. This is significantly more
efficient than traditional coal power plants, which are only about one
third energy efficient. Assuming production at scale fuel cells could save
20-40% on energy costs when used in cogeneration systems. Fuel cells
are also much cleaner than traditional power generation; a fuel cell
power plant using natural gas as a hydrogen source would create less
than one ounce of pollution for every 1000 kW-h produced, compared
to 25 pound of pollutants generated by conventional combustion
systems. Fuel cells also produce 97% less nitrogen oxide Issions than
conventional coal-fired power plants.
Cogeneration
Combined heat and power fuel cell systems, including Micro
combined heat and power systems are used to generate both
electricity and heat for homes, office building and factories. The
system generates constant electric power and at some time produces
hot air and water from the waste heat. Micro CHP is usually less than
5 kWe for a home fuel cell or small business. The waste heat from fuel
cells can be diverted into the ground providing further cooling while
the waste heat during winter can be pumped directly into the
building. The university of Minnesota Owns the patent right to this
type of system.