In a fuel cell stack several single cells are electrically connected in series and supplemented with the necessary components for medium supply (fuel gas, air, partially separated cooling) to achieve a unit with the desired electrical power output. This cell stack is then completed with additional components (fuel gas supply and preparation, afterburner, control and power electronics) to a complete system, where fuel is taken from a tank and electricity and heat are dissipated according to the application boundary conditions. H.C. Starck is a manufacturer of fuel cells, Staxera GmbH is a manufacturer of fuel cell stacks and Webasto AG integrates the stacks into fuel cell systems. A fuel cell stack represents approximately 30-50% of the total cost of a system, and the individual cells comprise approximately one third of the stack costs.
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The efficiency of SOFC stacks is dependent on the overall system, the system size and the available fuel. Diesel APU systems with less than 5kW power will achieve energy efficiencies of 25-30 percent. Stationary natural gas systems with less than 5kW power should reach electrical efficiencies of 35% and a total energy efficiency of 80-90%.
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Currently, stacks are built in modular way up to 1kW electrical power. A concept for stacks up to 5kW is under way.
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The first demonstrated application of the Staxera stack technology is the world’sfirst diesel APU system developed by Webasto AG in Neubrandenburg, Germany. To date the diesel APU system has demonstrated 30-50 normal start/stop cycles and accumulated some hundred hours of operation. System partners are also developing propane and natural gas systems. In laboratory tests, single stacks have exceeded some thousand hours of operation with virtually no degradation.
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For most fuels types, SOFC systems require both a reformer and afterburner. However, these components are clearly simpler than for a standard engine, because hydrogen and carbon monoxide are fuel gases for SOFC and a complex cleaning of the gaseous fuel is not necessary. In addition, the stack and reformer operate at approximately the same temperature. Webasto has developed the reformer and afterburner technology for the SOFC on the basis of their existing car and truck heater components.
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Lifetimes of 4000 hours have been reported by others for planar SOFC systems. The Fraunhofer IKTS uses fundamentally similar components as these systems. Newly developed elements are the contact layers and joining technologies between ceramic fuel cells and metal interconnector plates in the stack, the single most important element which prevents degradation and early failure. The ceramic contacting interlayers have so far shown no degradation in endurance testing.
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The key challenges to industrialize the SOFC stack are:
- Increase maximum power and power density of the fuel cell stack;
- To understand the failure mode and degradation mechanism of all components and materials used (SOFC cell, seals, metallic interconnectors);
- Design optimization to get high gas utilization and equal thermal load during heat up and operation and for external environmental effects;
- To establish a capable manufacturing and assembly process and process optimization for high reliability and low cost;
- To establish a prototype manufacturing facility including industrial manufacturing, supply chain and quality management.
Staxera GmbH is a 50/50 joint venture between Webasto and H.C. Starck aiming at faster commercialisation of SOFC Technology in Europe.
Both partners can combine their individual strengths within Staxera GmbH and focus on their key competencies in the parent companies. This means the workload is shared in the optimum way to accelerate technological progress on any step in the value chain. This will allow Staxera GmbH to become a prototype stack manufacturer as quickly as possible and to steer the
technical development of SOFC stacks:
- Within the first years Staxera GmbH will employ 6 to 8 specialists. 10-20 IKTS employees are involved in a multi-year cooperation agreement between Staxera and IKTS.
- Funding of Staxera GmbH is only one element of the Webasto and H.C. Starck effort. Each industrial partner will invest more than 10 million Euros within the coming years.
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IKTS has pursued research in SOFC related areas since 1992. In 2003 IKTS was the only partner able to supply a functional stack to Webasto for system testing. After this milestone, the three-sided cooperation agreement between H.C. Starck, Webasto and IKTS was signed. The fast progress achieved towards a low cost stack concept lead to the decision to take the cooperation next step and to found a separate company for industrial manufacturing.
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Our first objective is to proceed as fast as possible. The use of available infrastructure and resources and the proximity of the R&D team help us to maintain speed in the program. Staxera will commence investing in our own manufacturing equipment so we can build up to 1000 stacks per year. This capacity limit won’t be surpassed before 2008. Of course serious manufacturing will not be done inside IKTS. The cooperation between IKTS and Staxera is limited to planar SOFC stacks.
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H.C. Starck is the preferred supplier of SOFC cells. In addition they have competency in specialist powders and other ceramic materials. Webasto specifies the SOFC stacks based on their system requirements, which they develop together with lead customers and system partners for specific applications. In addition, they contribute experience in system integration, sheet metal processing and assembly. This includes CAD design, FE and CFD simulation as well as quality tools – the complete tool set used in the automotive supply industry.
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Commercial availability will be dictated by our system partners, when they integrate and sell SOFC stacks in their systems. Initial niche product launches are aimed for the years 2008 to 2010. Systems will be sold by truck, mobile home and residential heating system manufacturers to their respective customers.
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World wide approximately 20 companies are active in the area of high temperature fuel cells, and focus on similar market segments as Staxera. 5 of these companies can be regarded as close competitors who can deliver planar solid oxide fuel cells. We compare ourselves with these companies; in the areas of innovation, development status, and business strategy.
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We at Staxera can only give rough estimates. At launch, we expect a final customer price of about 8000 EUR per system. However it is clear that system prices must decrease at a later date, as the technology becomes established in the market.
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A low temperature fuel cell (Polymer Exchange Membrane or PEM) is perfectly suited for clean hydrogen fuel. Preprocessing of fossil fuels like natural gas or propane to produce hydrogen for use in such a fuel cell is theoretically possible. However, we are convinced that this process is too complex and thus too expensive for commercial applications. Many system components are not available today.
In contrast, the high temperature solid oxide fuel cell, not yet as advanced as current PEM stacks, will enable dramatically less complex and more cost efficient systems with high reliability for many fuels other than hydrogen. The application as the primary drivetrain for cars is not in focus, due to the limited dynamics and turn down ratios of SOFC systems.
Our SOFC stacks must be integrated into a complete system before they can be installed in a house. In this regard, the direct sale of a SOFC stack to an end-customer is not possible.
Staxera will supply SOFC stacks to Vaillant for their micro-CHP development program. The resulting micro-CHP system will be suitable for direct installation in a house.
The selection of end customers to participate in a field test program is up to Vaillant.
Further details about Vaillant’s development program are available in their press release:
www.vaillant-group.com |
