Solid oxide fuel cells (SOFC) are energy conversion systems, which convert chemical to electrical energy directly. In essence fuel cells (SOFC included) are similar to batteries except that where batteries run down and become depleted, fuel cells are continually replenished with fuel and are able to provide a continuous supply of electric power.
How does a SOFC work?
SOFCs operate at high temperatures and employ ceramics as functional elements of the cell. Each cell is composed of an anode and a cathode separated by a solid impermeable electrolyte, which during operation conducts oxygen ions from the cathode to the anode where they react chemically with the fuel. The electric charge induced by the passage of the ions may then by collected and conducted away from the cell. Each cell generates a limited voltage but just as with the cells of a battery series connected stacks may be constructed to increase the voltage and hence the useful power that can be supplied. In order to do this each individual cell is connected electrically via an interconnect to its neighbour. In some designs the interconnect is also used to isolate the fuel and air supplies for each cell.
Design of a SOFC
Within the SOFC it is important that the fuel and air streams are kept separate, and that a thermal balance should be maintained to ensure that the temperature of operation remains within an acceptable range. Several designs of SOFC have been developed to accommodate these requirements; one option is shown schematically in Figures 1 and 2.
Figure 1. Planar SOFC construction - single cell unit.
Figure 2. Planar SOFC stack construction.
Intermediate versus high temperature systems
High Temperature SOFC
The attraction of solid oxide fuel cells (SOFC) is based on a number of aspects including the clean conversion of chemical energy to electricity, low levels of noise pollution, the ability to cope with different fuels, but most of all - high efficiency. The enhanced efficiency of SOFC in comparison with other energy conversion systems is born out of its high operating temperature, which in some designs may exceed 1000°C. In these cases the potential exists, by extracting the energy present in the high temperature exhaust gases - e.g. by using gas or steam turbines, to boost the overall efficiency of the SOFC system to ~70%.
Disadvantages of High Temperature SOFC
Material costs are high, particularly for interconnect and construction materials. Interconnects carry electrical current between individual cells in the stack and can also act as a separator between the fuel and oxidant supplies. In high temperature SOFC the interconnect may be a ceramic such as lanthanum chromite, or, if the temperature is limited to <1000°C, a sophisticated refractory alloy e.g. based on mechanically alloyed Y/Cr. In either case the interconnect represents a major proportion of the cost of the stack. Stack construction materials and balance of plant also need to be refractory enough to contain and manipulate the high temperature gas streams.
A potential drawback to the use of chromium containing ceramics and alloys is the volatility of the material, which can result in contamination of the stack components. This has an increased significance for future reclamation of materials and components from used stacks where the presence of a toxic material such as Cr6+ would require special disposal procedures.
Operation of a High Temperature SOFC
During operation the SOFC is at the same time a generator and a user of heat. Heat is generated through exothermic chemical reactions and ohmic losses whilst it is absorbed by the reforming reaction in which hydrocarbons are converted to usable hydrogen and CO. In a direct internally reforming SOFC (i.e. one in which the reforming takes place at the anode of the cell) it is possible to engineer the operating conditions to be thermally balanced, thereby minimising the need for external insulation and heating. Small-scale high temperature systems are not thermally self-sustaining and generally require an external heat source to initiate and maintain operation. In large-scale systems the heat generated is not fully absorbed by the reforming of fuel and the excess heat is available for downstream energy extraction processes such as turbines etc. Such a system is shown schematically in Figure 3. Consequently high temperature SOFCs generating excess heat are only fully viable as large-scale stationary units where downstream coupling to steam and gas turbines is feasible.
Figure 3. Integrated SOFC/turbine system schematic.
Intermediate Temperature SOFC
Operation of the SOFC at a reduced temperature can overcome some of these problems and bring additional benefits.
Advantages of Intermediate Temperature SOFC
• Operation at less than 700°C means that low cost metallic materials e.g. ferritic stainless steels can be used as interconnect and construction materials. This makes both the stack and balance of plant cheaper and more robust. (Balance of Plant is widely assumed to constitute 50% of the cost of the SOFC system). Using ferritic materials also significantly reduces the above-mentioned problems associated with Chromium.
• Lower temperature operation offers the potential for more rapid start up and shut down procedures.
• Reducing operating temperature simplifies the design and materials requirements of the balance of plant.
• Reducing the operation temperature significantly reduces corrosion rates.
In order to operate at reduced temperatures several changes need to be made to cell and stack design, cell materials, reformer design and operation, and operating conditions.
As the temperature of operation of the SOFC is reduced the ionic conductivity of the electrolyte decreases and the parasitic losses due to the conductivity of the electrodes and interconnects increase, a combination which results in a rapid deterioration of the performance of the SOFC. This can be overcome in two ways:
• By changing the geometry of the cell
• By changing the materials used
Reducing the thickness of the electrolyte in the cell compensates for its reduced ionic conductivity at low temperature but in practice the thickness reduction required to accommodate, say a 200°C reduction in operating temperature, leads to impracticably thin membranes. A number of designs have been put forward in which the electrolyte is physically supported on one of the electrodes (electrode supported design). This structure, a thin dense layer on the surface of a very porous support is difficult to achieve and expensive thin film deposition techniques such as CVD are often needed to manufacture these systems. Even so, the mechanical strength of the structure (defined by the porous electrode) is often poor and the handling of the structure through subsequent processing and assembly is difficult.
Maintaining the geometry of the cell so that an ‘electrolyte supported’ design can be used inevitably means that different materials will be needed for the electrolyte, and because of the need to maintain compatibility and performance, the cathode and anode as well. Several materials options exist all of which have benefits and drawbacks. The most developed intermediate temperature ‘package’ is:
Gadolinia doped Ceria (CGO)
LSCF (a four component oxide based on La, Sr, Co, and Fe oxides)
As an electrolyte material CGO possesses a much higher ionic conductivity than zirconia, which is commonly used in high temperature systems. Unfortunately however its electrical conductivity is also higher and using CGO electrolyte inevitably carries the penalty of increased parasitic ohmic losses which limit the performance of the cell.
Advantages of SOFC over other Fuel Cells
One of the main attractions of SOFC over other fuel cells is their ability to handle more convenient hydrocarbon fuels - other types of fuel cell have to rely on a clean supply of hydrogen for their operation. Because SOFCs operate at high temperature there is the opportunity to reform hydrocarbons within the system either indirectly in a discrete reformer or directly on the anode of the cell. Reducing the operating temperature makes internal reforming more difficult and less efficient, and can mean that more active (and inevitably more expensive) reforming catalysts are required.
On balance however, the above drawbacks and technical penalties associated with intermediate temperature SOFCs are outweighed by the potential cost savings associated with the replacement of expensive interconnect ceramics and alloys with a commercially available ferritic stainless steel.
The opportunities for application of high and intermediate temperature SOFCs range from large scale distributed power generation to small-scale domestic heat and power
Table 1. Potential areas of application for high and intermediate temperature SOFC’s.
• Centralised power generation (multi MW)
• Distributed power generation (up to 1 MW)
• Combined heat and power (CHP) plants (100 kW to 1 MW)
• Domestic CHP (up to 10kW)
• Leisure (1 to 5 kW)
• Military and aerospace (5 to 50 kW)
• Transport (up to 50kW)
Application of high temperature SOFC’s will focus on large-scale static systems (i.e. generating surplus high-grade exhaust heat) where downstream energy extraction systems such as steam and gas turbines are feasible.
The applications for intermediate temperature SOFC’s will be the smaller scale more cost sensitive areas such as distributed domestic power generation where the low grade heat emitted by the system can be used for hot water generation or space heating. These low cost methods for utilising the waste heat from the intermediate temperature system also allow the SOFC’s to be mobile to meet leisure and military demands.