INTERVIEW WITH GEORG OBERHOLZER FROM CELEROTON AG
In INN-BALANCE Celeroton develops a novel air-turbo compressor. How will the compressor contribute to increasing the efficiency of the whole fuel cell system? Please tell us more about the advantages of the air-turbo compressor in comparison to other compression systems.
Georg Oberholzer (GO): The efficiency of a fuel cells stack significantly depends on the air mass flow and the pressure the compressor can provide to the cathode of the stack. In a fuel cell system, the air compressor, compared with all Balance of Plant (BoP) components, is the main contributor to the parasitic losses. Therefore, it is crucial for the overall system performance to not only provide the required air mass flow to operate the fuel cell stack in its optimum, but also to provide this air flow at highest possible efficiency.
The efficiency of any compressor is strongly dependent on its operating point, namely the combination of air mass flow and pressure. In INN-BALANCE, the aerodynamic of the developed compressor is matched to the characteristics of the applied fuel cell stack from the project partner PowerCell such that the stack can be operated at its highest efficiency in all operating points while maintaining highest efficiency of the air turbo compressor and thus, optimizing the efficiency of the overall system.
"The high efficiency, the oil-free air supply and the compactness of the system make the turbo compressor the optimal solution for the automotive fuel cell application." Georg Oberholzer
In addition to being efficient, the air supply also needs to be completely oil-free and cope with the limited space available in an automotive application. The high-speed turbo compressor technology combined with the gas bearing technology developed by Celeroton fulfils all this criteria and provides a highly compact and efficient oil-free solution for an automotive fuel cell system. Other compression systems such as displacement compressors operate typically at much lower efficiency and require a significant effort and complexity to seal the air from any oil contamination as well as demand more of the limited space. The high efficiency, the oil-free air supply and the compactness of the system make the turbo compressor the optimal solution for the automotive fuel cell application.
INN-BALANCE pursues a manufacturing-oriented approach, meaning the design of components will take into account manufacturing requirements and costs. Regarding the air compressor prototype construction, how did/does Celeroton implement this approach?
GO: The design of the air turbo compressor incorporates technology previously applied, proved and improved in smaller systems of Celeroton. The yearlong experience with the smaller systems allowed Celeroton to build up on this knowhow and to consider cost reduction measures already in the design of the turbo compressor prototype. For the first prototype, technical performance is certainly in the foreground. However, once the performance has been tested and proved, a redesign of the compressor will be conducted for allowing alternative manufacturing technologies and to further lower the costs of the system. In this redesign, Celeroton has the possibility to work together with the project partner BROSE and to access their enormous and broad knowhow in manufacturing of automotive components.
Currently, Celeroton together with BROSE is conducting various performance tests with the compressor prototype. What are the particular objectives of these tests?
GO: The automotive requirements defined in INN-BALANCE are very demanding for the whole fuel cell system and for the turbo compressor in particular. All these requirements have been considered during the design of the turbo compressor. Now that the compressor has been realized, the next step is to verify the performance of the compressor and the fulfilment of the requirements.
A first test is to verify the aerodynamic performance of the turbo compressor and to compare the actual measurement results with the calculated compressor map. Further tests include the testing of the wide range of cooling water temperature, testing of the high altitude operation of the compressor and the high demands for vibration robustness specific to automotive applications. Once the performance of the compressor has been proved, BROSE will continue to test the compressor on a test bed in combination with the other components of the cathode module. Furthermore, BROSE will test a novel control algorithm for the mass flow and pressure which allows highly dynamic and precise control of the operating conditions of the fuel cell stack.
Celeroton will also provide customized electronics for driving the turbo-compressor system. What are the particular challenges for designing the electronics?
GO: The turbo compressor is operating at rotational speeds above 150,000 revolutions per minute. These high speeds in combination with the power demand of the compressor present a significant challenge for the design of the electronics, as well on the level of the hardware but also in respect to the control of the motor and the software implementation thereof.
A further challenge is given by the system architecture. The electronics for driving the turbo compressor will be supplied directly from the fuel cell stack voltage to omit an additional power conversion step and to increase the system efficiency. This system architecture provides, however, some challenges for designing the electronics. The electronics needs to be able to operate at nominal output power over a wide range of input voltage while maintaining a high efficiency. Furthermore, the electronics needs to be able to operate at limited power from an auxiliary power supply for starting up the fuel cell stack when no voltage is present yet at the fuel cell stack.