Effects of SOFC Pressurization and Investigation of SOFC Gas Turbine Hybrids for Aerospace Applications

Trevor Kramer – Dissertation Defense

The introduction of electrified aviation can both lower fuel costs and reduce carbon emissions for the aviation sector. This is due to the higher energy conversion efficiency that can be found in hybrid electric drivetrains and in some cases, coupled with the use of a cleaner fuel. A solid oxide fuel cell gas turbine (SOFC-GT) hybrid system boasts thermal efficiencies of 60-70% if properly sized, placing it near the front of the pack of possible electrified aviation power/propulsion systems. A crucial component to the success of an SOFC-GT is the robust operation of the fuel cell component. There exists a lack of publications regarding the pressurized operation of tubular anode-supported SOFCs, a required operating condition of the SOFC-GT. This dissertation begins to fill the lack of experimental data of tubular SOFCs under pressurized conditions through polarization curves and feed gas composition variation. An electrochemical SOFC model is then fit to the experimental data to separate the individual electrode overpotential contributions. It was determined that the cathode electrode was generating the dominant voltage loss over wide ranges of pressures due to the anode loss holding more sensitivity towards pressurization.

When modeling the SOFC, five unique gas diffusion models were investigated and compared due to the pressurization dominantly effecting the gas diffusion process.

It was determined that Fick’s law can successfully capture the one-dimensional gas diffusion if the diffusion coefficient is adjusted by Knudsen and multi-component gas mixture corrections, even compared to a more widely utilized model such as the dusty gas formulation. The fitted electrochemical model captures both the measured polarization behavior and diffusion behavior, allowing for it to be the genesis of reduced order models, enabling rapid calculation of an SOFC’s off-design performance.

The reduced order SOFC model, a unique mass and volume optimizer for an SOFC system, and a thermodynamic model of the SOFC-GT system were combined into a single model to estimate the performance of an SOFC-GT system. To evaluate the performance, four different aircraft concepts were investigated, applying the SOFC-GT to urban air mobility concepts and future electrified single isle tube and wing aircraft. It was found that the SOFC-GT can produce a more efficient and more energy dense system compared to battery-based hybrids.