Fuel cell systems play a significant role in decarbonization, especially in commercial and aviation vehicles, which require high-capacity energy storage systems with the lowest possible weight. One application in aviation, for example, is the substitution of auxiliary power units. The HABICHT project is aimed at achieving a significant increase in system power density. The focus is on maximizing the speed of the electric drive of the turbo compressor, which is used to supply air to the fuel cell. This is achieved through innovative thermal management with regard to cooling design and materials.

Development of intelligent materials

The HABICHT project aims to develop a high-speed drive for fuel cell air compressors with an unprecedented power-to-weight ratio. This targets applications such as aviation, where any additional weight can make the difference between the success or failure of a new technology. The air compressor, which is usually electrically driven, is used to supply the oxygen required in the fuel cell system and therefore directly influences its efficiency. By increasing the power density to 30 kW/kg, the aim is to achieve a significant improvement over the current state of the art. In addition to Fraunhofer LBF, the following Fraunhofer institutes are also involved in the project:

• Fraunhofer Institute for Integrated Systems and Device Technology (IISB), which is focusing on the design of the overall system
• Fraunhofer Institute for Manufacturing Technology and Advanced Materials (IFAM) focusing on rotor development
• Fraunhofer Institute for Algorithms and Scientific Computing (SCAI), which is responsible for carrying out the multiphysics simulation of the entire system.

A project advisory board made up of industrial customers as well as material and process suppliers supports the consortium.

Fraunhofer LBF is focusing on optimized cooling in the stator area. This includes the cooling of the stator laminated core and the winding including the winding heads. The targeted drastic increase in power density has the direct consequence that the heat generated by power dissipation must be reliably dissipated from a very compact drive. Current solutions are not sufficient to cool such high-power drives. Therefore, it is necessary to develop a new cooling concept that combines different aspects of heat dissipation. At the same time, this cooling concept must be suitable for large-scale production and function reliably during the service life of the drive.

Customized materials with a function

With the aim of achieving the project goal, Fraunhofer LBF scientists are pooling their expertise in the synthesis and formulation of polymers as well as component design and plastics processing in optimally coordinated design, material and manufacturing processes.

The electromagnetic simulations of the stator revealed local thermal hot spots from which the temperature needs to be dissipated to ensure reliable operation of the overall system. As a result, various, partly interlinked, cooling concepts were derived, which were initially investigated using simulations to evaluate the suitability of the different concepts. The results have shown that a combination of specially designed local cooling, state-of-the-art jacket cooling and thermally conductive epoxy resins can ensure cooling of the stator.

In the following step, the researchers worked out how to implement this type of cooling in practice. In order to be able to evaluate the technical feasibility of the process, an initial geometry demonstrator was set up in cooperation with partner institutes, which allows the positioning and realization of the simulated cooling concepts to be investigated in addition to electrical aspects. This demonstrator is shown in Figure 1.

Objective: cost-effective and suitable for mass production

For commercial-scale use, it is necessary that the overall system can be mass produced and cost-effectively implemented. For the insertion of the cooling channels and potting with the thermally conductive resin, the next phase of the project will therefore focus exclusively on processes that allow this type of series production. These include, for example, vacuum potting, duromer injection molding and transfer molding.

When implementing the cooling concept with these processes, the process-dependent material properties in the later component, the infiltration of the resin, the avoidance of heat transfers and trapped air, the coefficients of thermal expansion in the overall system and the sealing of the cooling channels play a decisive role.

Application-specific experimental investigations will be carried out at subcomponent level in the next phase of the project with the aim of achieving implementation in the overall system. These will allow early validation of partial aspects. The findings are then used, for example, in the targeted adaptation of resin systems and in the modeling of thermomechanical behavior in relation to service life. The knowledge gained will be used to optimize methods for designing plastic-based cooling structures and developing strategies for designing application-specific and process-optimized resin systems. The long-term goal is to transfer these strategies to other application areas.

Sponsors and partners

Funded within the framework of internal programs run by the Fraunhofer-Gesellschaft, grant number PREPARE 840072