A fuel cell APU is completely noise-free and can be installed at any desired location on the aircraft.

Auxiliary power plant in the tail of the aircraft with open hood covers.
Photo: aaapp/Shutterstock

An Auxiliary Power Unit (APU) is a small turbine engine installed at the rear of the fuselage. The APU is similar (in function) to the main engine, but the exhaust is vented overboard rather than creating thrust to propel the aircraft forward. All large commercial aircraft have an APU onboard to provide electric power for aircraft systems and bleed air to start the main engines.

Gas Turbine APU

Like jet engines, the gas turbine APU uses air and fuel to function. Power from the battery spins the motor, and the air intake begins. The fuel and pressurized air mixture is ignited to produce power. The APU runs a generator that provides electrical power to the cockpit and cabin systems. The APU also provides pneumatic pressure for cabin air conditioning systems.

Aircraft manufacturers determine the APU requirements based on the aircraft size (cabin) and the amount of bleed air required to start the main engines. Extended-Range Twin-engine (ETOPS) flights require the APU to be tested using cold start procedures. This testing verifies the reliability of the APU if it is needed during primary engine failure.

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An AlliedSignal Honeywell APU.

One of the significant disadvantages of the APU is the noise it generates on the ground. Many airports worldwide restrict the use of APUs during night hours to prevent disturbance to neighboring communities.

Fuel Cell APU

A hydrogen-fueled aircraft can avoid using the noisy gas turbine APU and exchange it for a fuel cell APU. Unlike the combustion process, fuel cells generate electricity through an electrochemical reaction. Fuel cells require a continuous supply of fuel and oxygen and hence can provide continuous electrical power, which is a limitation in battery-supplied electric power generators.

The fuel-cell APU provides electricity to aircraft systems and oxygen-depleted air, which is an essential part of the hydrogen safety for the aircraft. Without a motor and a compressor, the fuel cell APU is based on the electrical aircraft system architecture.

It is noise-free and can be installed closer to where it is most needed rather than at the rear of the fuselage. The most suitable place for a fuel cell APU would be around the wing fairing (where the wings meet the fuselage). The wing fairing location would provide easy access to the Environmental Control System (ECS).

Render of Airbus' liquid-hydrogen refuelling station.

Photo: Airbus

Fuel cells produce more heat than electricity, which the ECS can use to keep the aircraft cabin warm when needed. Additionally, cabin galleys and lavatories can use the water produced by the fuel cell APU. Hydrogen is a strong candidate as the “fuel” for fuel-cell technology. Hydrogen-powered APUs would be environmentally clean as they do not generate carbon dioxide and nitrogen oxide.

In hydrogen-powered aircraft, the leaking heat from the hydrogen storage tanks (targeted to be less than 1%) can cause the liquid hydrogen to boil and cause heat influx. In a boil-off condition; the hydrogen can be routed to the fuel cell APU rather than dumped into the environment. A fuel cell APU’s cooling and heating capabilities can provide tremendous gains in future hydrogen-powered aircraft.

What are your thoughts on the comparison between gas turbine APU and the fuel cell APU? Tell us in the comments section.

Source: simpleflying.com

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