In a conventional combustion process of a turbine engine, jet fuel is mixed with pressurized air and ignited in the combustor. Hot gases expand, and the resultant force creates thrust and propels the aircraft forward. An alternate power source is the use of battery packs to deliver electrical power to the aircraft.
Batteries have 70 times less energy density than jet fuel, which makes them unviable for large-scale operations. Moreover, there is a significant difference between the performance of a battery used for research and what can be certified as a power source for airliners.
Fuel cells have a promising future when it comes to powering energy-efficient aircraft. Fuel cells generate electricity through an electrochemical reaction. Fuel cells require a constant supply of fuel and oxygen to provide continuous electrical power. For a small regional jet of approximately 72 passengers, such as the ATR72, a lightweight and efficient fuel cell system must be designed and installed onboard the aircraft.
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The question arises, should the fuel cell system be retrofitted on existing turbine engine-powered aircraft, or is a clean sheet design required? This article briefly highlights the two designs, along with their merits and demerits.
Retrofit design
In order to retrofit an existing 72-seater with fuel cell technology, the cabin space must be cleared. At least 30% of the seats (along with the cabin galleys and other floor space) must be removed. The area is used for the AC/DC converter, motor, and supporting digital systems.
At least 500 NM (925 km) of the range is lost due to fuel cell implementation. Stepping up to superconduction may only reduce its range by 250 NM (460 km). Moreover, the fuel cell technology's secondary effects, such as added aerodynamic drag, also reduce the aircraft efficiency. Cooling the fuel cells requires additional systems, which increases the overall mass of the aircraft.
While the loss of payload and range can be managed by altering operating conditions, the retrofit design cannot change the way the aircraft’s center of mass is designed. The installation of an eight to nine-ton retrofit system with its tank and fuel is therefore critical. The fuel cell installation mass cannot be placed in the wing or nacelles, which enables payload limitations due to the flow of stresses in the aircraft.
A clean sheet design
A clean sheet design project can optimize the aircraft’s configuration and structural characteristics to fit a fuel cell-based system. In a clean sheet design, the liquid hydrogen tank and fuel must be placed in the center fuselage. Based on that, the bending moments of the wings are designed for subsequent stress levels.
The air-cooled high-temperature superconducting systems minimize temperature issues without compromising the capacity or range of the aircraft. In comparison to battery-powered aircraft, the mass deficit for energy will be much lower.
Airbus focuses on clean sheet design to avoid messing with basic principles of physics while needing to retrofit the fuel cell technology onto an existing design. According to Guillaume Faury, CEO of Airbus,
We don’t need changed physics to make hydrogen aircraft work.
Hydrogen is an ideal candidate for future fuel cell technology. In an effort to launch a true zero-emissions airliner with useful operational characteristics, liquid hydrogen is considered for the fuel cell. Hydrogen has three times more energy density than jet fuel. When generated from renewable energy, hydrogen does not emit carbon dioxide. And, it can be used as a power source for short-to-medium haul aircraft.
What are your thoughts on the fuel cell design? Tell us in the comments section.