April 1, 2022, ©. Leeham News: Last week, we looked at how to store hydrogen in an aircraft. We could see the gaseous storage of hydrogen is too heavy other than for demo systems and extreme short-haul. For practical airliners, liquid hydrogen is the solution.
Now we look at what this means for the aircraft fuel system and how to configure a suitable Auxiliary Power Unit, APU.
As discussed before, liquid hydrogen (LH2), which is cryogenically stored at -253°C, must have tanks that minimize the heat influx through the tank walls. The heat that leaks in will cause the LH2 to boil. At boil-off, LH2 absorbs surrounding heat, which keeps the rest of the LH2 below -253°C.
The tanks are made as close to a sphere as possible as this exposes the least surface per held mass of LH2. The tank is made as a Dewar vacuum flask with additional insulation on top (to limit the boil-off, should the tank lose its vacuum), Figure 2. The boil-off in a well-designed tank would be below one percent per day.
The tank holds a mixture of LH2 with H2 gas on top. It has a fill pipe (D) and an LH2 propulsion system feed (E). To keep the pressure low (at about 1.2 bars), the gaseous H2 is piped out of the tank via a pressure-regulation valve (A).
LH2 is routed to the propulsion system (B) via pumps and valves. Before its usage in either a fuel cell or H2 burning gas turbine, it must be converted to a gas form in a heat exchanger at the propulsion system. The gas conversion is a performant heat sink that can be used by the propulsion system to manage heat problems. We will discuss this more in the propulsion system articles.
To not vent the boil-off H2 to the atmosphere (which is entirely OK, H2 is a harmless substance), a hydrogen-fueled airliner will skip the noisy gas turbine APU and exchange it for a fuel cell APU.
Figure 3 shows a block schema for a hydrogen fuel system. The schema is taken from Airbus’ CRYOPLANE study report from 2003. We have modified it to show a three-tank system in the fuselage (called Passive tanks in the study) with engine feeder tanks (called Active tanks) close to the studies’ H2 burning gas turbine engines (we will look at both H2 gas turbine and fuel cell propulsion). We also replaced the studies’ gas turbine APU with a fuel cell one.
A fuel cell APU has several advantages over a gas turbine unit:
As a fuel cell APU doesn’t produce pressurized air (without adding a motor and compressor for this purpose), it assumes a “more electrical” aircraft system architecture like the Boeing 787 Dreamliner (where engine starts, de-icing, ECS air supply are all done by electric means).
With a fuel cell APU, the H2 boil-off is routed to the APU and not wasted in the atmosphere. Any additional H2 supply needed is added through a heat exchanger (C in Figure 2).
An LH2 fuel cell APU’s cooling and heating capability hints at gains with tight integration with the ECS in future LH2 aircraft.
A hydrogen fuel system must meet the safety standards for airliner applications. Airbus CRYOPLANE study looked at the safety aspects and concluded:
Hydrogen poses its specific safety aspects to be considered in design and operation. However, the overall safety level will not be worse than for kerosene aircraft.
For the fuel system, some of the considerations: