F
MICHAEL W. ELLIS,
MICHAEL R. VON SPAKOVSKY, AND DOUGLAS J. NELSON
At the beginning of the 21st century,
fuel cells appear poised to meet the power needs of a variety of applications.
Fuel cells are electrochemical devices that convert chemical energy to
electricity and thermal energy. Fuel cell systems are available to meet the
needs of applications ranging from portable electronics to utility power
plants. In addition to the fuel cell stack itself, a fuel cell system includes a fuel processor and
subsystems to manage air, water, thermal energy, and power. The overall system
is efficient at full and part-load, scaleable to a wide range of sizes,
environmentally friendly, and potentially competitive with conventional
technology in first cost. Promising applications for fuel cells include
portable power, transportation, building cogeneration,
and distributed power for utilities. For
portable power, a fuel cell coupled with a fuel container can offer a higher
energy storage density and more convenience than conventional battery systems.
In transportation applications, fuel
cells offer higher efficiency and better part-load performance than
conventional engines. In stationary power applications, low emissions permit
fuel cells to be located in high power density areas
where they can supplement the existing utility grid. Furthermore, fuel cell
systems can be directly connected to a building to provide both power and heat
with cogeneration efficiencies as high as 80%.
II. FUEL CELL TYPES
A.
Proton Exchange Membrane Fuel
Cells (PEMFC)
The proton exchange membrane fuel cell
is one of the most promising and certainly the best known of the fuel cell
types.The PEMFC is often considered as a potential replacement for the internal
combustion (IC) engine in transportation applications. The PEMFC consists of porous
carbon electrodes bonded to a very thin sulphonated polymer membrane. As illustrated
in Fig. 2(a), this membrane electrode assembly (MEA) is sandwiched between two
collector plates, which provide an electrical path from the electrodes to the
external circuit. Flow channels cut into the collector plates distribute reactant
gases over the surface of the electrodes. Individual cells consisting of
collector plates and MEAs are assembled in series to form a fuel cell stack..
B. Direct Methanol Fuel Cells (DMFC)
Like the PEMFC, direct
methanol fuel cells use a polymer membrane as the electrolyte. In the DMFC,
however, the fuel is methanol, which is dissolved in liquid water and supplied to
the anode. Since it is a liquid, methanol is easy to transport, and since the
methanol is used directly in the stack, there is no need for a fuel processor.
However, because the reaction rate for methanol on currently available
catalysts is slow, DMFCs have relatively low efficiencies and power densities. can
be competitive with batteries in terms of storage density.
C. Phosphoric Acid Fuel Cells (PAFC)
Phosphoric acid fuel cells were the first
fuel cells to be commercially available. The PAFC consists of porous carbon
electrodes surrounding a porous matrix that retains the liquid phosphoric acid electrolyte.
Phosphoric acid fuel cells operate with efficiencies that are comparable to
PEMFCs but at power densities that are lower.
D. Molten Carbonate Fuel Cells (MCFC)
Molten carbonate fuel cells are typically
designed for mid-size to large stationary (or shipboard) power applications.
the MCFC consists of nickel and nickel-oxide electrodes 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 PAFC 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.
E. Solid Oxide Fuel Cells (SOFC)
Solid oxide fuel cells operate at the
highest temperatures, 800 C–1000 C (1500 F–1800 F), of all fuel cell
systems.These high temperatures simplify system configuration by permitting
internal reforming and also facilitates the development of cogeneration systems
as well as hybrid power systems that use fuel cells as topping cycles for gas
turbines and/or steam cycles. SOFCs are attractive to large utility-scale
applications, which benefit from the high efficiencies obtained by combining
SOFC systems with gas turbines and/or steam cycles.
III. FUEL CELL SYSTEMS
A. System Configuration
a fuel cell system is composed of six basic
subsystems: the fuel cell stack discussed in the preceding section, the fuel
processor, air management, water management, thermal management, and 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 systems.
B. Fuel Processor
Since most fuel cells use hydrogen as a
fuel and most primary energy sources are hydrocarbons, a fuel processor is 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. C. 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.
E. 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 surroundings. Low temperature fuel
cell systems are cooled by either air or a circulating liquid.
F. Power Management
The final component of the fuel cell system
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.
IV. FUEL CELL SYSTEM CHARACTERISTICS
Fuel cell systems promise to provide a
number of advantages when compared to conventional power systems. These
advantages include modularity, high efficiency across a broad range of load
conditions, and low environmental impact. These advantages coupled with
projected cost reductions will make fuel cells attractive in a variety of applications.
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
ormore.
V. FUEL CELL SYSTEM APPLICATIONS
A. 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 soldiers deployed
in the field. Fuel cells based on DMFC technology or PEMFC technology are well suited
for many of these applications. In
portable power applications, the fuel cell would be incorporated into the
electronic device..
B. Stationary Power
Conventional on-site back-up power systems based
on diesel engines are generally operated only in the event of an emergency.
With their high efficiency, low noise, and minimal emissions, fuel cell systems
can operate continuously to supplement or replace utility power. This approach
can make better use of the capital investment in an on-site generation
system.Utilities are also interested in fuel cells for centralized and
distributed generation due to their high efficiency, modularity, and low
environmental impact. Distributed generation is attractive in remote areas or
areas where electrical demand has grown beyond the limits of the utility grid.
In these areas,it may be more economical to add generating capacity near the
load than to try to upgrade the utility grid. Fuel cell systems can be used in
these applications because their low environmental impact permits them to be
sited in a variety oflocations where traditional power plants would be
unacceptable.
VI. CONCLUSION
Fuel cell systems promise to provide
benefits in a variety of applications. Systems based on PEMFC and DMFC technology
promise to make power more portable and convenient. Fuel cell systems based on
PEMFC technology promise to provide a more efficient, cleaner technology for the
automotive industry. All four technologies, PEMFC, PAFC, MCFC, and SOFC, are
likely to be applied in building cogeneration applications. With cogeneration
efficiencies as high as 80%, these applications promise to reduce energy use and
environmental impact. Many research and development organizations, manufacturers,
and regulatory agencies are working to insure that fuel cell systems fulfill
their promises in each of these areas .
