Innovative thermal management in fuel cell air compressors

Fuel cell systems play a key role in decarbonization, especially in commercial vehicles and aircraft. One application in aviation is the electric drive of the turbocharger, which supplies air to a fuel cell. The HABICHT project achieved a significant increase in system power density by maximizing the speed. This was achieved with innovative thermal management, the use of new materials, and an optimized electromagnetic design of the electric motor. In addition to Fraunhofer LBF, the institutes IISB, IFAM, and SCAI were also involved in the project.

High-speed electric drive for fuel cell air compressors

The HABICHT project aimed to develop a high-speed electric drive for fuel cell air compressors with a previously unachieved power-to-weight ratio. The goal is to achieve a significant improvement on the current state of the art by increasing the power density to 30 kW/kg. The planned drastic increase in power density means that the heat generated by power losses must be reliably dissipated from a very compact drive. Analysis of the motor's heat replacement diagram showed that at the operating point of 150,000 rpm (80 kW), around 1 kW of heat loss occurs as copper losses in the stator windings. At the same time, magnetic losses occur within the stator laminations. The heat generated in this process must also be reliably dissipated.

Simulation analysis of cooling concepts in combination with material development

The focus of the work at Fraunhofer LBF was on the simulation analysis of different cooling concepts in combination with the development of highly thermally conductive duromer-based resin formulations. Extensive simulations were carried out to determine the minimum required thermal conductivity of the casting compound. A CAD model of a stator mock-up was transferred to FEM and combined with a self-developed method for anisotropic thermal simulation. Numerical simulation of different cooling concepts has shown that when using a cooling channel, the thermal conductivity of the casting compound must be at least 2.75 W/mK. However, a cooling channel is not necessary if the thermal conductivity is 5.7 W/mK or higher.

Duromers ensure better thermal conductivity

In the context of developing thermally conductive duromer molding compounds, two main approaches were pursued: On the one hand, formulations were developed that are characterized by increased intrinsic thermal conductivity. It was found that both the type and number of hydrogen bonds and the degree of cross-linking influence the achievable thermal conductivity. On the other hand, highly filled molding compounds were developed and optimized in order to explore the limits between sufficiently high thermal conductivity and processing.

The comparison of the achievable thermal conductivities with the simulation results showed that a cooling channel can be dispensed with. Based on the contributions of Fraunhofer LBF, an overall simulation model was developed and transferred to a functional demonstrator at IISB.

Sponsors and partners

Funded by the Fraunhofer Society's internal programs, funding number PREPARE 840072.

Publication

• Effects of hydrogen bonding and cross-linking on thermal conductivity of amorphous epoxy network - ScienceDirect 
  F. Schönberger, M. Fatemeh, E. Chalwatzis

    Curable resin formulation for the production of a thermosetting plastic,     corresponding thermosetting plastic and use of resin formulation and     thermosetting plastic

    DE 102023102175 A1.