March 18, 2022, ©. Leeham News: In our series, we have now seen the major limitations batteries as an energy source impose on an airliner and that hybrids work but don’t bring any advantages for an airliner.
The alternatives are to use an energy source with a higher energy density and combine it with an efficient propulsion system. Sustainable Aviation Fuel, SAF, has the same high energy density as today’s Jet fuel and hydrogen’s density is three times higher than Jet fuel.
Figure 1 gives a graphical view of the energy volume and mass densities of the different fuels discussed for aviation. The graph is scientifically correct in using Joule and MegaJoule (MJ) as the measure for energy. Few have the feeling for a Joule or MegaJoule as it’s not a daily life term, but a Joule = a WattSecond. Divide a MegaJoule with 3.6 to get kWh.
We see the hopeless position of lithium-ion batteries both from an energy volume and mass density. Kerosene (Jet fuel) is very efficient from a volume perspective and good on mass density.
But hydrogen beats our Jet fuel on energy mass density three to one. Its problem is an almost four to one deficit in energy volume density for LH2 and six to one for H2 in 700bar bottles.
Sustainable Aviation Fuel (SAF) has the huge advantage its a “drop-in” substitute for our kerosene-based Jet fuel. The SAF can be produced in two ways;
Hydrogen is today produced by steam reforming of natural gas, but this is a process that produces CO2. The result is called Grey hydrogen.
Green hydrogen can be produced by the electrolysis of water. It requires electrical energy, but so does Synfuel-based SAF, Figure 2.
Both production methods start with producing hydrogen through electrolysis. We must add carbon atoms in a Fisher-Tropsch (FT) process to create SAF. To make the SAF green, these carbon atoms must come from the atmosphere as these will later be released as CO2 in the combustor of the aircraft engines.
Figure 2 also shows the energy efficiencies when generating fuel energy. As SAF is a product of further hydrogen processing, it’s more expensive to produce than gaseous hydrogen (H2 in the figure) or liquid hydrogen (LH2).
On the other hand, hydrogen is more complicated and energy-consuming to package for distribution and get to the aircraft in the desired form. It requires a whole new infrastructure for the production, distribution, and fueling of the airliner. It also requires a changed fuel and propulsive system for the aircraft.
SAF, being a “drop-in” hydrocarbon fuel like today’s Jet fuel, can use existing infrastructure with minor changes/adaptations.
The straightforward solution for greener air transport is Sustainable Aviation Fuel, SAF. Its problems are a limited biomass supply and a higher production cost for Synfuel-based SAF.
Hydrogen promises lower cost per kg but requires a new ecosystem, both on the production and distribution side and for the aircraft.
Which of these alternatives will prevail? The probable answer is both. The question then remains, in what proportions? It will be the subject of our next articles.