– Power generation
A fuel cell is an energy converter that transforms a fuel directly into electricity. Various systems exist today that differ depending on the type of fuel used, the electrolyte, the power output, the operating temperature and their efficiency
High-temperature fuel cells (SOFC)
According to the state of the art today, a SOFC can reach an efficiency rate of 70 %. It is therefore a technically attractive option for low-loss energy generation. The attractiveness of the SOFC is based on the fact that it possible to work with fuels with relatively low levels of purity. This is possible on account of the operating temperatures that reach up to 1000 °C.
The central functional unit of a SOFC is the oxide ceramic solid electrolyte with the associated electrodes. The oxide ceramic is mostly a ZrO2 (Y-FSZ) doped with around 8 mol% Y2O3. With this Y2O3 content, in the binary system the maximum conductivity for O2- ions is reached, and at the same time the cubic high-temperature phase is stable from melting point to room temperature. The thickness of the electrolyte lies in the order of 10 µm. The air-side cathode in state-of-the-art systems generally consists of a perovskite of the general composition La1-xSrxMnO3 with thermal expansion adapted to the ceramic. Between this electrode and the ceramic, a diffusion barrier is inserted, which generally consists of Gd-doped CeO2. The anode-side electrode is a porous cermet material consisting of nickel and Y-FSZ covered by a dense Y-FSZ layer.
SOFC-based systems for energy generation are now ready for series production. Applications for such systems include, for example, decentralized energy supply of public institutions and private households, emergency power generators as well as mobile applications in motorized vehicles, ships and aircraft. In cogeneration systems, efficiencies of 90 % seem possible.
Another application for the SOFC is in principle operation with reverse function. This enables the electrolysis of water.
Molten carbonate fuel cell (MCFC)
An MCFC is generally operated at 600 – 700 °C with a eutectic mix of Li2CO3 and K2CO3. The CO32- ion is used for transport of the electric charge.
Owing to the pronounced corrosive nature of such melts, only sufficiently corrosion-resistant materials are suitable for plant components in direct contact with them.
One type of material that has proven effective for more than 10 years in this application is high-purity, dense-sintered Al2O3 ceramics. In the following, the data of the ceramics’ properties important for this application are summarized.
- Purity: ≥99,5 %
- Density: >98 % of the theoretical value
- Maximum crystallite size in the microstructure: 50 µm
- Mean bending strength at room temperature: 350 MPa
- Mean compressive strength at room temperature: 3500 MPa
- Modulus of elasticity: 380 GPa
- High edge strength
- High shape stability at high temperature
- Thermal stability to well above 1500 °C
- High thermal shock resistance
- High resistance to Li-K-carbonate melts
- Low wetting of hard-machined surfaces by Li-K-carbonate melts
- Specific electrical resistance: >1014 Ω*cm
- Dielectric strength: >30 kV
Fuel cells of this type can achieve efficiencies of 70 %. Numerous systems are installed today worldwide, e.g. as cogeneration units, sometimes with power outputs in the mid- to high MW range.
Stationary hot gas turbine
A hot gas turbine operated at a temperature level of 1500 °C is exposed to particular thermal, thermomechanical and corrosive stresses in the area of the combustion chamber, which severely limits the possible selection of materials suitable for this application. Especially the use of oxide- and non-oxide ceramic fibre composites for stationary and moving plant components is conducive to the economically efficient operation of the plant as they enable the realization of such operating temperatures.
Selected characteristics that are technically important for this application and that can be realized with such ceramic materials are listed in the following:
- Low weight for moving components
- High mechanical resilience
- High thermal stability
- High thermal shock resistance
- High resistance to hot gas corrosion
- High resistance to microstructural changes in operating conditions
- High resistance to geometric form changes at high temperature
- High damage tolerance
- Application of force, form-fit and substance-to-substance joining methods for ceramic-ceramic and ceramic-metal composites.