Solid Oxide Fuel Cell

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Solid Oxide Fuel Cell
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INDEX

• History
• Technology
• Operation
• Advantages
• Limitations
• Applications
• Self-Test

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HISTORY
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• Both solid oxide and molten carbonate fuel cells
  are high temperature devices. The technical
  history of both cells seems to be rooted in
  similar lines of research until the late 1950s.
  Research into solid oxide technology began to
  accelerate in late 1950s at the Central
  Technical Institute in The Hague, Netherlands,
  Consolidation Coal Company, in Pennsylvania, and
  General Electric, in Schenectady, New York. A
  1959 discussion of fuel cells noted that problems
  with solid electrolytes discovered by
  Nernst(called Nernst Mass) included relatively
  high internal electrical resistance, melting, and
  short-circuiting due to semi conductivity. It
  seems that many researchers began to believe that
  molten carbonate fuel cells showed more
  short-term promise.

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• Not all gave up on solid oxide, however. The
  promise of a high-temperature cell that would be
  tolerant of carbon monoxide and use a stable
  solid electrolyte continued to draw modest
  attention. Researchers at Westinghouse, for
  example, experimented with a cell using zirconium
  oxide and calcium oxide in 1962.

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• More recently, climbing energy prices and
  advances in materials technology have
  reinvigorated work on SOFCs, and a recent report
  noted about 40 companies working on these fuel
  cells.

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TECHNOLOGY
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• A solid oxide fuel cell (SOFC) uses a hard
  ceramic electrolyte instead of a liquid and
  operates at temperatures up to 1,000 degrees C
  (about 1,800 degrees F). A mixture of zirconium
  oxide and calcium oxide form a crystal lattice,
  though other oxide combinations have also been
  used as electrolytes. The solid electrolyte is
  coated on both sides with specialized porous
  electrode materials.

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• At these high operating temperature, oxygen ions
  (with a negative charge) migrate through the
  crystal lattice. When a fuel gas containing
  hydrogen is passed over the anode, a flow of
  negatively charged oxygen ions moves across the
  electrolyte to oxidize the fuel. The oxygen is
  supplied, usually from air, at the cathode.
  Electrons generated at the anode travel through
  an external load to the cathode, completing the
  circuit and supplying electric power along the
  way. Generating efficiencies can range up to
  about 60 percent.

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SOFC Stacking

• A 40-cell SOFC stack with 16-cm diameter cells
  achieves 1.40 kW at 0.428 A/cm2 with 80 fuel
  utilization and an average cell voltage of 0.673
  V
• Four common SOFC stack configurations have been
  proposed and fabricated
• Flat-plate design
• The simplest way to envisage a SOFC as a
  single plate cell is by stacking components on
  top of each other. This design offers simple cell
  geometry and multiple fabrication options such as
  tape calendaring or tape casting.

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• Seal less tubular design
• The seal less tubular design consists of the
  cell components configured as thin layers on a
  tubular support closed at one end. The problems
  encountered previously with gas seals are now
  eliminated with this array, although the cell is
  still hampered by the limited gas flow through
  the tube and relatively long current path.
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• Segmented cell-in-a-series
• This design consists of segmented cells
  connected in electrical and gas flow series. The
  cells are either arranged as a thin banded
  structure on a porous support or filleted one
  into the other to form a tubular self-supporting
  structure. The major problem is low gas flow,
  which results from the thick support tube and the
  robust seals required at the ends of the tube

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• Monolithic
• The monolithic design is the newest SOFC stack
  concept. It consists of many cells fabricated as
  a single unit. The design has the potential to
  achieve high power density because of its compact
  and lightweight structure. Nevertheless, they are
  difficult to manufacture and have a higher
  likelihood of cracking during operation due to
  the expansion mismatches of the materials.
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OPERATION
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• An SOFC consists of two porous electrodes
  separated by a dense oxygen-ion conducting
  Electrolyte. Oxidant is reduced at the cathode
  side and fuel is oxidized at the anode. The
  difference in oxygen activity of the two gases at
  the electrodes provides a driving force for
  motion of oxide ions in the electrolyte. Oxide
  ions formed by dissociation of oxygen at the
  cathode under electron consumption migrate
  through electrolyte to the anode where they react
  with hydrogen to form water and release electron.

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• Electrolyte
• SOFC is based on the concept of an oxygen-ion
  conducting electrolyte where oxide ions (O2-)
  migrate from cathode to anode and react with the
  fuel to generate electricity. Oxide materials
  with fluorite crystal structure such as
  yttria-stabilized zirconia (YSZ), rare
  earth-doped ceria and rare earth-doped bismuth
  oxide have been widely investigated as
  electrolytes for fuel cell. Zirconia doped with
  8-10mole yttria (YSZ) is the most wide-used
  electrolyte for SOFC because it conducts only
  oxygen ions over a wide range of oxygen partial
  pressure.

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• Anode (Fuel electrode)
• The electrode must be stable in the reducing
  environment of the fuel, should be electronically
  conducting and must have sufficient porosity to
  allow transport of the products of fuel oxidation
  away from electrolyte-electrode interface.
• In this region the fuel oxidation reaction is
• O2- (s) H2 (g) H2O (g) 2e-

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• Cathode (Air electrode)
• The cathode is the site for the electrochemical
  reduction of oxidant. Therefore, the cathode
  material must be chemically and thermally stable
  in the oxidizing atmosphere. In addition, a
  potential cathode material should be reasonably
  compatible with other cell components.

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• Sealing materials
• Method of sealing the ceramic components to
  obtain gas-tightness is a major issue of SOFC.
  Glass ceramics are used as sealant, although
  migration of the silica component can still be a
  problem on anode and cathode.
• Interconnecting materials(External Circuit)
• As the name implies, the interconnecting
  material connects anode of one cell with cathode
  of another cell so that voltage output could be
  enhanced. Doped lanthanum chromite (LaCrO3) has
  been used as the interconnecting material since
  the 1970s

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• Catalyst (Nickel, Copper)
• The catalyst within the anode promotes release
  of free electrons from the cells fuel source.
  The catalyst within the cathode promotes the
  generation of oxygen ions from the cells oxygen
  source.

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• Hydrogen feed
• SOFC devices can internally reform some fuels
  to deliver hydrogen fuel, and they can be
  fabricated in a variety of shapes and form
  factors.
• Oxygen Feed
• At the cathode, the oxygen provided by the air
  or oxidant feed reacts with electrons in-bound
  from the external circuit and H protons coming
  through the electrolyte to form water. The water
  is expelled, along with any other compounds in
  the oxidant feed stream out through the cathode
  exhaust.

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• The Exhaust
• The SOFC exhaust exits the generator module at
  a temperature of between 800 degrees C and 850
  degrees C and in atmospheric pressure systems is
  passed through the exhaust gas heat recovery
  train. This heat can be adapted to generate
  process heat or hot water for a combined heat and
  power application

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ADVANTAGES
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High Electrical Efficiency

• SOFCs can achieve electrical efficiency of up to
  50 using natural gas and can also achieve this
  performance with other hydrocarbon fuels such as
  liquefied petroleum gas.
• As a result of its high operating temperatures,
  SOFCs can also be combined with heat recovery
  technologies such as heat exchangers to create a
  total system efficiency of up to 85.

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Fuel Flexibility

• SOFC is the most inherently fuel flexible of the
  fuel cell types

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High Reliability

• SOFCs are made from commonly available ceramic
  materials. SOFC technology has no moving parts or
  corrosive liquid electrolytes. They are expected
  to lead to electricity generation systems that
  are highly reliable and require low maintenance

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Solid Electrolyte

• One of the big advantages of the SOFC over the
  MCFC is that the electrolyte is a solid. This
  means that no pumps are required to circulate hot
  electrolyte, moreover there is no leakage problem
  with the electrolyte

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Broad product range capability

• SOFC technology can support distributed
  generation (DG) products such as generators and
  combined heat and power units in the capacity
  range from small residential to large industrial
  sizes, as well as automotive applications such as
  auxiliary power units. In DG applications, the
  low air and noise emissions of SOFC-based systems
  allow for ease of siting and permitting.
• The high operating temperature also results in
  high-grade exhaust heat, which can be utilised in
  a wide range of cogeneration applications.

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LIMITATIONS
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Solid Electrolyte

• Though the Solid Electrolyte cannot leak, it can
  break and provide a severe problem

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Manufacturing Cost

• Higher stack temperatures demand exotic
  materials, which add to manufacturing costs. Heat
  also presents a challenge for longevity and
  reliability because of increased material
  oxidation and stress.

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Stationary applications

• Unfortunately, the dominant SOFC developers aim
  at stationary applications Such ceramic solutions
  are indeed heavy, sluggish, expensive and fragile
  and must be operated at high temperatures. But
  totally different SOFCs are presently developed
  for mobile applications.

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APPLICATIONS
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RESIDENTIAL APPLICATIONSOFC micro-power plants
take away the dependence and limitations of the
electric distribution grid, in a remote
standalone package that can also provide heat for
the home. This lets the homeowner live just about
anywhere, in the mountains or deep woods, in the
desert or on an island.
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COMMERCIAL APPLICATIONSolid oxide fuel cell
(SOFC) power systems for commercial and
industrial applications are designed to provide
clean, highly efficient power for on-site
grid-support, grid-back-up or grid-independent
electrical generation needs.
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MILITARY APPLICATIONSOFC can be used as
uninterruptible power supplies (UPS), especially
in military application as back up power 
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IN COMMUNICATIONSFor high-priority carrier,
provider, corporate or government networks SOFC
provides high grade power