Solid oxide fuel cell

IT-SOFC

2009-12-18T14:44:00.003+05:30

Due to their high efficiency, solid oxide fuel cells (SOFC) not only conserve valuable natural resources but also assist in reducing pollution and greenhouse gas (GHG) emissions. However, long-term degradation problems associated with the high-temperature operation and relatively high manufacturing costs remain the main challenges for the commercialization of this technology. Lowering the operating temperature to an intermediate temperature (IT) range (400–700 °C) not only improves reliability but also lowers the cost, time and energy. This will extend its application domain to residential power and portable devices. However, with the current state-of-the-art SOFC materials, it is not possible to obtain sufficient power in the IT range. This is because of high ohmic losses and electrode polarization, which have a detrimental effect on performance and efficiency. Thus, there is a need to develop materials that show improved properties in the IT range.

High ionic conductivity solid oxide electrolytes are critical for the development of SOFCs that can successfully generate reasonable power at IT range. In recent years, doped ceria electrolytes have emerged as a potential candidate due to their higher ionic conductivity than that of the conventional yttria stabilized zirconia (YSZ) at intermediate temperatures. Among doped ceria materials, Gd_0.10Ce_0.90O_2−δ (GDC) is widely accepted to exhibit the high ionic conductivity in the IT range. However, recent research shows that co-doping based on Sm³⁺ and Nd³⁺ leads to further enhancement in the ionic conductivity in ceria systems 91[See Ref.]. Optimization of dopant concentration in the Sm_x_/2Nd_x_/2Ce_1−xO_2−δ system resulted in the development of Sm_0.075Nd_0.075Ce_0.85O_2−δ (SNDC), which exhibits 30% higher grain ionic conductivity than that of GDC at 550 °C in air.

The current–voltage performance of the cell was measured at intermediate temperatures with 90 cm³ min⁻¹ of air and wet hydrogen flowing on cathode and anode sides, respectively. At 650 °C, the maximum power density of the cell reached an exceptionally high value of 1.43 W cm⁻², with an area specific resistance of 0.105 Ω cm². Impedance measurements show that the power density decrease with decrease in temperature is mainly due to the increase in electrode resistance. The results confirm that Sm_0.075Nd_0.075Ce_0.85O_2−δ is a promising alternative electrolyte for intermediate temperature solid oxide fuel cells.

Ref.: Jin Soo Ahn, Shobit Omar, Heesung Yoon, Juan C. Nino and Eric D. Wachsman, “Performance of anode-supported solid oxide fuel cell using novel ceria electrolyte”, Journal of Power Sources, Volume 195, Issue 8, 15 April 2010, Pages 2131-2135

SOFC in Micro-combined heat and power system (CHP)

2009-07-19T21:24:00.002+05:30

Micro-combined heat and power (CHP) holds great potential for lowering energy cost and CO2 emissions in the residential housing sector. Of the various micro-CHP technologies, fuel cells and in particular solid oxide fuel cells (SOFC), show great promise due to their high electrical efficiency and resulting low heat-to-power ratio that is better suited to residential applications. However, fuel cells are still under development and the capital cost of units available today remains high.

Microgeneration is the generation of zero or low-carbon heat and/or power by individuals, small businesses and communities to meet their own needs. A range of microgeneration technologies exist that either harness energy from the environment (i.e. renewables, such as small scale wind turbines, water turbines, heat pumps, solar thermal collectors, solar photovoltaics, etc.) or generate heat and power from a fuel (i.e. micro-CHP, such as internal combustion engines, Stirling engines and fuel cells)

The key economic driver for micro-CHP is the ability to meet on site electricity demand. This ability is influenced by (a) the presence of on site electricity demand and (b) thermal constraints. The thermal constraints relate to the fact that the micro-CHP system cannot operate when there is insufficient thermal demand, thereby limiting the ability of the system to meet on site electricity demand. Therefore, the presence of significant thermal demand is important, as is the development of fuel cell micro-CHP systems with a very low heat-to-power ratio. Fuel cells have a relative advantage over other micro-CHP technologies in this regard in that their high electricity efficiency implies they are more likely to achieve a low heat-to-power ratio if appropriately designed.

The drivers for environmental performance of fuel cell micro-CHP were found to be almost identical to the economic drivers. The only distinction is that on site electricity demand is not important for CO2 reduction, because credit for displaced CO2 exists regardless of whether the electricity is consumed on site or exported to the grid.

For fuel cell micro-CHP to be successful, it is essential that we overcome the key technical challenges of improving the dynamic efficiency and durability of operating systems, as well as lowering the capital cost of systems.

Ref: Adam Hawkes, Iain Staffell, Dan Brett, Nigel Brandon, "Fuel cells for micro-combined heat and power generation", J. Energy Environ. Sci.(www.rsc.org/ees), 2009, Vol. 2, pp 729–744

SOFC: micro-tubular

2009-07-04T12:22:00.003+05:30

Micro-tubular solid oxide fuel cells (MT-SOFCs), pioneered by K. Kendall in the early 1990s, are a variety of SOFCs that are on the scale of millimetres compared to their much larger SOFC relatives that are typically on the scale of tens of centimetres. The main advantage of the MT-SOFC, over its larger predecessor, is that it is smaller in size and is more suitable for rapid start up. This may allow the SOFC to be used in devices such as auxiliary power units (APU, automotive power supplies, mobile electricity generators and battery re-chargers.

2008-09-30T18:51:00.000+05:30

SOLID OXIDE FUEL CELL (SOFC)

Dr J D Bapat

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The solid oxide fuel cell or SOFC, is so called because it uses solid oxide electrolyte as a conducting medium for oxygen ions. The major advantages of SOFC are multi-fuel capacity, non-requirement of expensive catalysts, high quality exhaust heat useful for cogeneration and wide applications. The major disadvantages are high temperature operation, limited unit cell size due brittle ceramic components and high capital cost-to-output ratio. The conduction of oxygen ions through the electrolyte takes place due to the oxygen vacancies in the crystal structure generated due to the defect reactions. The maximum thermodynamic or the open circuit potential of the SOFC is given by the Nernst equation. The actual potential is less than that predicted on thermodynamic considerations, on account of the irreversible losses or overpotentials occurring due to the resistance to the transport of gaseous species through the porous electrodes (concentration overpotential), activation energy barriers for the electrochemical reaction (activation overpotential), resistance to the transport of ions or electrons through the electrolyte or electrodes (ohmic overpotential), leakage electronic current through the electrolyte (leakge overpotential) and the interfacial resistances at the material boundaries (interface overpotential). All the overpotentials are the functions of current density. The mathematical correlations commonly used to estimate these overpotentials are given. The trends reveal that increasing working temperature results in extended operating current range and reduced irreversible losses, but reduced open circuit potential also. The cathode contributes most to the cell irreversible losses. The four factors, commonly used to evaluate the performance of fuel cells, namely fuel utilisation factor, air ratio, power output and fuel efficiency are defined. The two ways to utilise fuel have been discussed, namely the reforming and the direct oxidation of hydrocarbons. The direct oxidation yields maximum open circuit potential but the process is still under development. The internal steam reforming of fuels is preferred over external reforming, on account of significant reduction in losses and capital, operating costs. However there are drawbacks, namely possibility of carbon deposition at the anode deteriorating the cell operating performance and life and rapid endothermic cooling leading to large temperature gradients across the cell and the interconnect. The carbon deposition can be prevented by maintaining the steam-to-fuel ratio above the limiting value calculated on the basis of thermodynamic equilibrium considerations. However such steam-dilution of fuel leads to the reduction in its chemical potential. The problem of large temperature gradients is tackled, to some extent, by partial pre-reforming of fuels or by reducing the anode’s catalytic activity towards reforming reaction. The oxidative reforming, in which a mixture of hydrocarbon fuel and stoichiometrically deficient quantity of oxygen is fed to the anode, is being studied for use in small size SOFC. The Nernst potential reduces with fuel utilisation. It is possible to improve the cell efficiency maintaining the fuel utilisation at low levels and increasing the fuel recycle ratio. However this measure increases the cost and complexity of the fuel cell system. The alternative simpler approach is the multistage oxidation of fuel, in which partially utilised fuel from one stack is fed to the next stack. The direct oxidation of hydrocarbons at high temperature, on the conventional Ni-based anodes suffers from the drawback of carbon formation. Alternate anode materials are being tried to tackle this phenomenon. The fuels which could be used in SOFC, besides hydrocarbons, are natural gas and other hydrocarbons, methanol, ethanol and biogas. The selection of the most appropriate fuel for SOFC for the given application is a multi-criteria task involving both qualitative and quantitative parameters. Based on the overall considerations, various fuel options could be arranged in the following order of preference: methane >ethanol>biogas>gasoline. The two principal design configurations of SOFC are planar and tubular designs. The operation at elevated pressure yields higher cell power output. In a megawatt capacity plant, such SOFC can successfully replace the combustor in a gas/steam turbine power generation system and achieve fuel efficiency up to 70%. The individual SOFC are arranged in stack. The interconnect connects the cathode of one cell to the anode of the other, while protecting it from the reducing atmosphere. A power generation unit may consist of groups of fuel cell stacks of a particular size and output, held in containment vessels. The tubular SOFC is more useful for stationary power generation, whereas the planar design for auxiliary power units, due to relatively higher power density. The requirements for the selection of materials are given. The cathode is porous and possesses high electronic and small ionic conductivity and also catalytic activity towards oxygen molecule dissociation and reduction. The ABO₃ oxide system of perovskite structure satisfies most of the requirements. The commonly used SOFC cathode material is a mixture of strontium substituted lanthanum manganite (LSM) and the yttria substituted zirconia (YSZ). The anode materials are also porous and require high stability in reducing atmosphere. The transition metals best fit the requirement, as they have high catalytic activity towards hydrocarbon reforming or oxidation. The Ni-YSZ cermets are commonly used as anodes. The Cu-ceria anodes have been reported to effectively tackle the problem of carbon deposition. The YSZ is commonly used electrolyte material at present. The advantages in reducing the operating temperature of SOFC are stated. The mixed oxide electrolytes made from ceria doped with yttria, gadolinia and samaria show high oxygen ion conductivity at low temperatures, but suffers from the drawback of Ce⁴⁺ reduction to Ce³⁺ at low oxygen partial pressures, obtaining at the anode. Promising results have also been obtained on ceria based multi-phase (heterogeneous) materials. The doped lanthanum gallate is another promising electrolyte material under investigation. At operating temperatures below 700 ⁰C, it is possible to use stainless steel, which is comparatively inexpensive and readily available material, as the interconnect. The SOFC is the means of pollution free, sustainable power generation of the future but its success will depend upon the effective dealing of certain aspects like reducing capital cost-to-output ratio, developing low cost large volume production techniques, developing materials and techniques for low temperature, multi-fuel operation.

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