February 26, 2021, ©. Leeham News: Last week we discussed auxiliary power generation for a hydrogen aircraft and found that a fuel cell system had many attractions.
However, it’s more challenging to develop than a hydrogen-converted APU, and we were asked to work through this case as well.
Figure 1. The principal parts of a single-aisle APU. Source: United Technologies.
Hydrogen APU as auxiliary power?
The carbon-fueled APU is primarily used on the ground when the engines are not running. It supplies the aircraft with bleed air through the load compressor, Figure 1, and electricity through the AC generator. Electric-driven pumps then provide hydraulic power.
The installation is in the rear tailcone as the unit is noisy and requires air intakes and a muffled exhaust pipe, Figure 2.
Figure 2. An A320 APU installation. Source: Airbus.
The APU is a simple gas turbine with one centrifugal compressor stage, giving at best a pressure ratio of 8:1. The shaft power out efficiency is around 20%-25%. The efficiency of the bleed air from the load compressor is then below 20% as a load compressor is about 80%-85% efficient.
Figure 3 shows our ballpark power and weight table for our 165 seater for the hydrogen APU or fuel cell auxiliary power. We now assume a classical aircraft architecture with engine bleed air and auxiliary gearboxes with alternators, hydraulic pumps, and air/electric starters. Wing de-ice is bleed air.
Figure 3. An APU versus Fuel Cell auxiliary power comparison. Source: Leeham Co.
Once again, it’s not an exact comparison, just a check where we compare “grosso modo.”
We can see in Figure 2 the APU has a rather elaborate installation in the aircraft’s tail. But the fuel cells at the wing roots are no easy installation as well. They are pressure, temperature, and humidity sensitive, so we most likely must provide a pressurized and temperated compartment with a regulated humidity level. We assume the two installations are similar in terms of weight and space requirements.
The fuel cell alternative is 65kg heavier, but it wins the efficiency race with a margin. If we assume a fuel cell efficiency of 45%, it’s more than double the efficiency of the bleed and electrical power delivery of the APU which both are below 20% (we use a 90% efficiency for the APU AC generator). Over the lifetime of the aircraft, this will count, especially with a higher LH2 price than today’s fuel prices.
As before, the fuel cell produces H20 and O2 as output that we can use as onboard water and emergency oxygen.
We see the fuel cell is still the more attractive alternative, but it will be costlier in development. Converting an APU to hydrogen fuel is straightforward. It was successfully done in the Airbus Cryoplane project.
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