The APU Replacement: Fuel Cells. Or are they?

Airbus, DLR German Aerospace Centre, and Parker Aerospace are together studying the use of multifunctional fuel cell concept (MFFC) as a future replacement for today’s gas turbine based APUs.

The tail section of an A320, where part of a fuel cell system may be placed and tested in 2015, potentially replacing APUs.

A fuel cell essentially uses a constant flow of chemicals to produce electricity, heat, and gaseous by products. The gaseous by product is water, as the fuel is usually hydrogen and oxygen, and this makes fuel cells absolutely non-polluting.

A tank to contain liquid hydrogen (the non-freely available fuel component) and heat exchangers and associated fans to keep the generated heat from becoming excessive, are required. Airbus states, “Positioning of the fuel cell is planned in the cargo hold, while the system’s liquid hydrogen tank, heat exchangers and fans are to be located in the tail cone section."

The MFFC produces about 100kW of electrical power. A 2001 paper lists that approximately 182 Wats of power may be derived per kg of a fuel cell. This should make the fuel cells alone weigh about 550kgs, and may not include the weight of the hydrogen tanks. In contrast, the popular Honeywell APU, 131-9[A] which powers all 737NGs and 60% of all A320s, weighs only 160kg. In addition, the APU also supplies pneumatic power, which is used for air conditioning and pressurization in the event of loss of one or all (2 in the case of the A320) engine bleed systems on board the airplane. Using ATF as the source of fuel, no separate fuel systems are required, unlike fuel cells.

However, the APU is a device with moving mechanical components, and reliability is always impacted when something moves. The MTBUR, or the Mean Time Between Unscheduled Removals, is 8500 hrs. A fuel cell’s reliability is much, much higher, significantly improving dispatch reliability.

With the move towards more-electric architectures, such as the 787, there will no longer be any need for pneumatic air from the APU, but all systems, running off electric power, will pose a greater demand on the electric source, meaning more weight.

But there are hidden advantages. The gaseous by product, water vapour and a oxygen less air, has good uses: Fuel tank inerting. The water may be used to meet part of the 200 litre potable water tank on board the A320.

With the possibility of engine-less electric taxiA, fuel cells may be the future, but the question is: when will the weight of the unit be low enough for it to be a viable replacement? We’ll know in 2015, when Airbus may test it on an A320.

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