Scottish fuel-cell specialist Hy-Hybrid Energy is working with an undisclosed partner on a hybrid-electric propulsion system for aircraft that combines a gas turbine with a battery and a solid oxide fuel cell (SOFC) that operates on jet fuel.
Current fuel cell aircraft propulsion development programs are employing proton exchange membrane (PEM) fuel cells that use hydrogen as the fuel. The potential benefit of Hy-Hybrid’s system is the ability to bring low-carbon propulsion to longer-haul commercial aircraft before a hydrogen infrastructure is available at airports.
Among the challenges in developing fuel-cell propulsion systems is that existing low-temperature PEM technology requires high-purity hydrogen to avoid contaminating the catalysts in the fuel cell stacks. Such cells also operate below 100C (212F), making it difficult to remove waste heat. Managing the water produced by PEM cells is also a challenge, to avoid flooding or drying out.
Solid oxide fuel cells offer fuel flexibility and operate at high temperatures, up to 1,000C. This improves thermal management, eliminates the water management challenge and provides the ability to use the existing jet fuel infrastructure.
Glasgow-based Hy-Hybrid is studying the use of microtubular SOFC in a hybrid configuration. Conventional SOFCs have issues with long startup and shutdown times and thermal shock caused by the high heating rates. Microtubular SOFCs offer higher volumetric power density while their smaller size reduces startup times and thermal gradients, making them more robust to rapid thermal cycling.
The project will look at the use of a microtubular SOFC, gas turbine and battery in a hybrid drivetrain that can select the most appropriate energy source for the phase of flight, from takeoff through climb, cruise and descent to landing, while optimizing overall efficiency.
NASA has studied the use of SOFCs in a hybrid-electric aircraft propulsion system under the FuelLeap project (for Fostering Ultra Efficient, Low-Emitting Aviation Power). The technology generated hydrogen for the fuel cell by reforming traditional aviation fuels. The integrated system was estimated to produce electricity from hydrocarbon fuels twice as efficiently as an equivalent conventional aircraft engine.
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