United States
Environmental Protection
Agency
Auxiliary and Supplemental Power
Fact Sheet: Fuel Cells
DESCRIPTION
This fact sheet describes the use of Fuel Cells as
Auxiliary and Supplemental Power Sources
(ASPSs) for wastewater treatment plants (WWTPs).
A fuel cell is an electrochemical device similar to a
battery. While both batteries and fuel cells generate
power through an internal chemical reaction, a fuel
cell differs from a battery in that it uses an external
supply that continuously replenishes the reactants in
the fuel cell. A battery, on the other hand, has a
fixed internal supply of reactants. The fuel cell can
supply power continuously as long as the reactants
are replenished, while the battery can only generate
limited power before it must be recharged or
replaced. Fuel cells have been a popular choice as
an ASPS in recent years, because they are highly
efficient and emissions-free. Although there are
many different types of fuel cells, each of which
uses its own specific set of chemicals to produce
power, only molten carbonate fuel cells (MCFC),
phosphoric acid fuel cells (PAFC) and solid-oxide
fuel cells (SOFC) can generate enough energy to
power a typical WWTP. Each of these types of fuel
cell is appropriate for use as either a supplemental
power source or an auxiliary power source.
A fuel cell contains hydrogen on its anode
(negatively charged electrode) side and oxygen on
its cathode (positively charged electrode) side. In
contrast, conventional batteries consume solid
reactants, such as Lead (Pb), Cadmium(Cd) or other
metal. Once these reactants are depleted, they are
discarded or recharged. Batteries can be
regenerated either with electricity or by replacing
the electrodes. In a fuel cell, reactants flow in and
reaction products flow out. This makes continuous
long-term operation feasible as long as these flows
are maintained.
PEJU FUEL CELL
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Fuel Cell Schematic
An electrolyte separates the anode and cathode
sides. The electrolyte varies depending on the type
of fuel cell being used. On the anode side,
hydrogen diffuses to the anode catalyst where it
dissociates into protons and electrons. The protons
are conducted through the membrane to the
cathode catalyst, but the electrons are forced to
travel in an external circuit because the membrane
is electronically insulating. This external circuit is
the power supplied by the fuel cell. On the
cathode side, oxygen molecules react with the
electrons, which have traveled through the external
circuit, and with hydrogen ions to form water.
Molten carbonate fuel cells (MCFCs) use an
electrolyte composed of a molten carbonate salt
mixture suspended in a porous, chemically inert
ceramic lithium aluminum oxide (LiAlO2) matrix.
These cells have fuel-to-electricity efficiencies of
between 60 and 85 percent, meaning 60 to 85
percent of the energy generated by the chemical
reaction can be harnessed as useable power.
MCFCs operate at about 1200° F (650° degrees C).
This high temperature is needed to achieve
sufficient conductivity of the electrolyte. Since
they operate at extremely high temperatures non-
precious metals can be used as catalysts at the
anode and cathode, reducing costs. MCFCs are
available in the range of 10 kW to 2 MW.
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Phosphoric acid fuel cells (PAFCs) use liquid
phosphoric acid as an electrolyte; the acid is
contained in a Teflon-bonded silicon carbide
matrix, and porous carbon electrodes containing a
platinum catalyst. Operating temperatures range
from 300° to 400° F (150-200° C). PAFCs generate
electricity at more than 40 percent efficiency. In
addition, 85 percent of the steam by-product from
the chemical reaction can be used for cogeneration
activities, such as heating onsite buildings and
keeping WWTPs operating at optimal temperatures,
thus reducing the use of commercial electric power.
Existing PAFCs have outputs up to 200 kW.
Solid oxide fuel cells (SOFCs) are primarily used in
big, high-power applications, including industrial
and large-scale central electricity generating
stations. A SOFC system usually uses a hard
ceramic material of solid zirconium oxide and a
small amount of ytrria (the oxide of the element
yttrium) instead of a liquid electrolyte. Operating
temperatures are around 1800 degrees F (1000° C).
This high temperature operation removes the need
for precious-metal catalyst, thereby reducing cost.
Power generating efficiencies are around 60
percent. In addition, 85 percent of the steam by-
product from the chemical reaction can be used for
cogeneration activities thus reducing the use of
commercial electric power. Existing SOFCs cells
have output around 100 kW.
ADVANTAGES & DISADVANTAGES
A common misconception of fuel cells is that
hydrogen is the source of energy. Hydrogen is not
the primary source of energy; it is only an energy
carrier and must be manufactured using energy from
other sources. Some critics argue that the energy
needed to create the fuel in the first place may
reduce the ultimate energy efficiency of the system
to below that of the most efficient gasoline internal
combustion engines. This is especially true if the
hydrogen has to be compressed to high pressures or
liquified. Most types of fuel cells can operate on a
wide variety of fuels including hydrogen, carbon
monoxide, natural gas, propane, landfill gas, diesel,
and simulated coal gasification products. In some
cases, such as at a WWTP, methane (natural gas)
from anaerobic digesters can be reused in the fuel
cell instead of flaring off the excess. Other
advantages of fuel cells include few moving parts,
modular design and negligible emission of
pollutants.
The high operating temperature serves as a big
advantage for the MCFC. This leads to higher
efficiency, since breaking of carbon bonds occurs
much faster at higher temperatures. Other
advantages include flexibility to use more types of
fuels and the ability to use inexpensive catalysts.
A major disadvantage of MCFCs is that high
temperatures enhance corrosion and the breakdown
of cell components.
One of the main advantages of PAFCs is they can
use impure hydrogen as a fuel, removing the need
of pretreatment of the fuel supply. Also the PAFC
technology is the most mature. Utilization of
steam by-products for cogeneration can improve
the overall economic value of the technology.
Disadvantages of PAFCs include the need for
expensive platinum as a catalyst, relatively low
current and power generation compared to other
types of fuel cells, and their generally larger and
heavier size.
Advantages of SOFCs are similar to those of the
MCFCs, including higher operating temperatures.
These high operating temperatures imply higher
efficiency and flexibility to use more types of
fuels. The higher temperatures also allow SOFCs
to use inexpensive catalysts. Utilization of steam
by-products for cogeneration can improve the
overall economic value of the technology. One
disadvantage of these high temperatures is that
they enhance corrosion and the breakdown of cell
components.
COST
Fuel cell manufacturers currently publish a
commercial entry price of about $2,400/kW.
Initial price does not include installation, balance
of systems costs, or other miscellaneous costs that
can drive the entry price up by 30% to 50%.
Manufacturers believe that the entry price where
fuel cells could compete successfully with other
small power generators would have to be roughly
half of the current price. This would allow for
more competition for smaller scale installations.
Through improved manufacturing techniques,
higher efficiency and increases in production the
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cost of manufacturing reliable fuel cells is
decreasing. As this technology becomes more
commercial available, costs of fuel cells will rapidly
decline.
CASE STUDY
King County Wastewater Treatment Division in
Renton, Washington, installed a 1 MW molten
carbonate fuel cell power plant to reduce energy
costs to the treatment plant. The output is tied to a
transformer to step-up voltage to 13,000 V. The
fuel cell system was chosen because of its high
efficiency and low emissions. This cell is operated
using methane from the anaerobic digesters. King
County uses the electricity produced by the fuel
cells to supplement its energy needs, which also
reduces the facility's power costs by 15 percent.
The estimated installed cost for the MCFC system
was approximately $22.8 million, including the
waste heat recovery system. The waste heat
recovery unit for the exhaust is sized for 1.7 million
Btu per hour of waste heat. At 45% electrical
efficiency and at rated heat recovery, the net
thermal efficiency of the plant is expected to be
around 68%. The waste heat can also be returned to
the digester loop.
http ://dnr.metrokc. gov/wtd/fuelcell/
http: //www. eere. energy. gov/hy drogenandfuel cell
s/fuelcells/fc_types.html#phosphoric.
Rastler, Dan. King County Carbonate Fuel Cell
Demonstration Project, Electric Power Research
Institute (EPRI), February 2005.
The mention of trade names or commercial
products does not constitute endorsement or
recommendation for use by the U.S.
Environmental Protection Agency.
EPA 832-F-05-012
Office of Water
March 2006
Large Fuel Cell Application
REFERENCES
http://en.wikipedia.org/wiki/Fuel cell
http://www.fuelcells.org/basics/types.html
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