March 5, 2021, ©. Leeham News: We have discussed different auxiliary power generation principles for a hydrogen aircraft over the last weeks. We found a fuel cell auxiliary power system has many attractions, one being the possibility of making an elegant more-electric aircraft system architecture.
With or without such an architecture, the fuel cell alternative will save hydrogen consumption and cost compared to a hydrogen-converted APU alternative. What’s the value of the saving?
As we went through the different Auxiliary Power Unit (APU) alternatives for a hydrogen aircraft, we could see a fuel cell APU alternative had an efficiency about double that of a hydrogen-converted gas turbine APU.
We first calculate the gain from running a fuel cell APU instead of a hydrogen-converted APU during ground stops.
The APU is active during the landing (as a power backup), taxi in, the stop, and take-off. The total time per flight is 1 hour. The A320 APU fuel consumption is typically 120kg/hour during ground operation. We then assume 40kg hydrogen consumption for the gas turbine APU and 20kg for fuel cell APU, which give a daily consumption of 320kg versus 160kg with eight stops per day. Yearly consumption with 350 operations days results in 112t versus 56t hydrogen.
With a future hydrogen price of $3.00/kg (from EU report), we have a saving of $168,000 per year ($336,000 vs. $168,000) for the fuel cell alternative. For the 20 year lifetime of the aircraft, it’s a saving of $3.36m. For an OEM with an assumed production run of 2,000 aircraft (a reasonable assumption for a first-generation hydrogen airliner), his customer base cost gain is $6.7bn.
We now calculate the added gain, should the aircraft employ the more electric architecture we described here.
A typical yearly fuel burn for an A320neo class aircraft is 8,400t of Jet-A1. With a direct conversion of the A320neo engines to hydrogen, we assume an annual hydrogen consumption of 2,800t (in reality, the hydrogen aircraft has slightly different fuel burn due to changed weights, etc., but we ignore this here).
This gives us a yearly fuel cost of $8.4m, and a more electric architecture saves $84,000 for every percent gain in fuel consumption. We previously assumed a 2% lower fuel consumption with a more electric architecture, so we have a yearly cost saving of $168,000, or $3.36m over the aircraft’s lifetime.
For an OEM, the calculation is an additional customer cost gain of $6.7bn based on his 2,000 aircraft production run.
A fuel cell APU creates a cost-saving of $3.36m over an aircraft’s active life compared with one that is based on a hydrogen-fueled gas turbine. If we add a more electric system architecture, we can double this to $6.72m.
For an OEM with a 2,000 aircraft production run, the development of a fuel cell APU aircraft with a more electric system architecture could cost $10bn in additional development costs (which is way over what it should cost), and it would still be a good business case.