For the SOFC20 project, the Fraunhofer Institute for Ceramic Technologies IKTS designed the hot box with eight fuel cell stacks in series.
The initial test of the newly developed hot box was completed successfully in March 2012. During the test, the stack module achieved a power output of 5.5 kW. Following a further test using simulated natural gas reformate, the stack module will be installed in a specially developed system environment at AVL List GmbH, where it will be tested.
PLANSEE–s contribution to this project is the world–s largest SOFC interconnects with an integrated protective coating manufactured using powder metallurgy and with dimensions of 130 x 150 mm. These components connect the individual fuel cells and distribute fuel gas and air in the system. With the MK351, PLANSEE has developed an interconnect from chromium (Cr), iron (Fe) and yttrium (Y) – known as a CFY alloy – specially for use in combination with electrolyte supported cells made from fully stabilized zirconia. The coefficient of expansion of this chromium-based interconnect is perfectly matched to that of the electrolyte. Unlike proprietary, protected designs, the MK351 interconnect is available to all potential stack manufacturers.
The technology group SCHOTT, as a glass specialist among other things leading in the development and manufacture of complex glass-to-metal seals, and Fraunhofer IKTS are developing new joining materials that are fully compatible with the CFY interconnects in terms of their thermo-mechanical properties. Optimum, long-term sealing using suitable sealing glasses is a prerequisite for achieving high levels of efficiency with the fuel cell stack.
As well as coordinating the overall project, the Fraunhofer IKTS is responsible for the construction of the stack, sealing and characterization of the stacks.
The fact that the newly developed MK351 stack design has only a few individual components allows the manufacturing process to be simple and easily automated. Some of the components used are already commercially available or are being manufactured by subcontractors or those involved in the project on a pilot scale.
The insights derived from the tests and characterizations are fed into the development and optimization of the individual components. The intended high power density of the cells and cyclic stability required for the field of application can only be achieved if the components are perfectly harmonized.
AVL is developing the fuel cell system from gas conditioning right up to the AC inverter and is responsible for integrating the stack module. The company has developed a large number of peripheral components especially for this project, such as the ignition boiler or the residual anode gas recycling blower.
The Jülich Research Center provides particular support for system design and qualification of the desulfurization materials to be used.
What is a SOFC?
A Solid Oxide Fuel Cell, or SOFC for short, is used at operating temperatures of between 650 and 1000°C. In an ESC SOFC (Electrolyte Supported Cell), the electrolyte not only conducts the oxygen ions but also ensures the mechanical stability of the cell. Unlike other types of fuel cell, the SOFC is not only able to convert hydrogen to water with the help of oxygen, but can also convert CO to CO2. This electrochemical reaction results in the generation of predominantly electrical power from the energy contained in the gases.
The high operating temperature of the SOFC offers a crucial advantage: Hydrogen and CO can be produced within the fuel cell system from natural gas, biogas or other gases containing hydrocarbons, by means of what is known as a shift reaction.
Metallic interconnects connect the anode and cathode of the individual cells electrically and distribute fuel gas and air to the active areas. Several cell layers stacked on top of one another create a cell stack. The SOFC stacks are housed in a hot box which shields the rest of the fuel cell system from the high temperature and contains ducts for fuel gas, air, electric conductors and sensors.
The peripheral components (BoP) provide the stack module with all the requisite media. Among other things, the BoP includes a fuel gas desulfurization unit, all the control systems for fuel gas and air, heat exchangers, fans and an AC inverter to convert the direct current produced by the fuel cell into alternating current.