v>EPA
United States
Environmental Protectior
Agency
United States Environmental Protection Agency
August 2013
Renewable Energy 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 of reactants and oxygen 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 cells 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, cadmium, or other metal.
Once these reactants are depleted, they must be
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. An electrolyte separates the anode
and cathode sides of the fuel cell. The electrolyte
varies depending on the type of fuel cell being
used. On the anode side, hydrogen diffuses and is
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 (L1A1O2) matrix.
These cells have fuel-to-electricity efficiencies of
between 60% and 85%, meaning 60% to 85% of
the energy generated by the chemical reaction can
be harnessed as useable power. MCFCs operate at
about 1,200° F (650° 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 kilowatts
(kW) to 2.8 megawatts (MW).
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% efficiency. In
addition, 85% 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
400 kW
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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 yttrium (the oxide
of the element yttrium) instead of a liquid
electrolyte. Operating temperatures are around
1,800° F (1,000° C). This high-temperature
operation removes the need for precious-metal
catalyst, thereby reducing cost. Power
generating efficiencies are around 60%. In
addition, 85% of the steam by-product from the
chemical reaction can be used for cogeneration
activities thus reducing the use of commercial
electric power. Siemens has developed a solid
oxide fuel cell system capable of producing 200
kW of electricity. Figure 1 shows a molten
carbonate fuel cell diagram.
MOLTEN CARBONATE FUEL CELL
Electrical Current
— fr>O,
Hydrogen In
H2
Water and
Heat Out
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1
CO-
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r^O
II
e- V
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e-
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•
=j
e-
Tej
CO?
3
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Anode
Electi
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,
\
Oxygen In
i
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4
Figure 1: Molten carbonate Fuel Cell Diagram
ADVANTAGES AND 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 liquefied. 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 the 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 the flexibility to use more types of
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fuels. The higher temperature also allows 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 higher temperatures is that
they enhance corrosion and the breakdown of the
cell components.
COST
Fuel cell manufacturers currently publish a
commercial entry price of about $2,400 per 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
cost of manufacturing reliable fuel cells is
decreasing. As this technology becomes more
commercial available, the costs of fuel cells will
rapidly decline. An independent panel report
published for the Department of Energy, Hydrogen
Program in 2008 estimates the potential
manufactured cost for an 80 kW system to be in
the range of $60 per kW to $80 per kW. This 2008
estimate for fuel cell systems for transportation
was based on an annual production of 500,000
units.3
CASE STUDIES
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 volts. 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%.
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 British Thermal Units (BTUs) per hour of
waste heat. The electrical efficiency was 43%- to
44% and the thermal efficiency was 59% to 64%.
The Palmdale Water Reclamation Plant (WRP),
Los Angeles County, CA, is a 9.5 MOD
wastewater treatment plant. The plant's digesters
produce about 11,500 cubic feet per day of biogas
containing 55% methane. A 250 kW molten
carbonate fuel cell system was installed at a capital
cost of $1.9 million. The cells use 70% to 80% of
the total gas produced and they generate 225 kW
electricity. The cogeneration system includes a
waste-heat recovery unit that utilizes the waste
heat to provide heating to the digesters. The net
thermal and electrical efficiency was calculated to
be 73%. Based on a 90% operating capacity and a
2004 retail electricity costs of $0.128 per kWh, the
annual savings in energy costs was calculated to be
about $227,000.
The other wastewater treatment plants that have
operational biogas based fuel cell systems are Tulare
WWTP, Dublin San Ramos WWTP, Riverside
WWTP, Rialto WWTP and Turlock WWTP, all of
which are located in the state of California.
REFERENCES
1. Rastler, Dan. King County Carbonate Fuel Cell
Demonstration Project., Electric Power Research
Institute (EPRI), February 2005.
2. Robert J. Remick, Fuel Cells on Bio-Gas,
Michigan Water Environment Association
Biosolids and Energy Conference, 2009.
3. Fuel Cell System Cost for Transportation - 2008
Cost Estimate, National Renewable Energy
Laboratory, Golden, Colorado.
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4. Mark McDannel and Ed Wheless, The Power
of Digester Gas, Water Environment and
Technology, June 2008.
5. http://en.wikipedia.org/wiki/Fuel Cell
6. http://hydrogendoedev.nrel.gov/pdfs/45457.pdf
7. http://www.fuelcells.org/types/basic.html.
8. http ://dnr.metrokc. gov/wtd/fuelcell/.
9. http://www.eere.energy.gov/hydrogenandfuel
cells/fuelcells/fc_types.html#phosphoric.
10. http://fossil.energy.gov/programs/powe
rsystems /fuelcells/fuelcells solidoxide.html
11.Auxiliary and Supplemental Power Fact Sheet:
Viable Sources (EPA 832-F-11-009), Office of
Wastewater Management, Revised October 2007.
Some of the information presented in
this fact sheet was provided by the
manufacturer or vendor and could not
be verified by the EPA.
The mention of trade names, specific
vendors, or products does not
represent an actual or presumed
endorsement, preference, or
acceptance by the EPA or federal
government.
Stated results, conclusions, usage, or
practices do not necessarily
represent the views or policies of
the EPA.
Environmental Protection Agency
Office of Wastewater Management
EPA 832-F-13-014
August 2013
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