As the aviation industry works toward net-zero carbon emissions by mid-century, one thing is clear: sustainable aviation fuels (SAFs) are expected to be the primary decarbonization lever. These fuels are favorable because they can replace fossil jet fuel with minimal changes to aviation operations and infrastructure. Meanwhile, zero-emission aircraft powered by hydrogen or electricity—which can be more efficient than fossil-fueled counterparts and emit no carbon dioxide or air pollutants during operation—typically play a secondary role in decarbonization roadmaps.  

This blog lays out why SAFs are forecast to play a leading role in the decarbonization of aviation. While my colleague recently explained the current limitations of zero-emission planes, this blog also offers policy considerations to support their uptake as part of a multi-pronged approach toward net-zero emissions. 

Ease of adoption and aircraft performance 

As a “drop-in” alternative to fossil jet fuel (Jet A), SAF is easily used in today’s aircraft. Most engines are certified to use a 50% blend of SAF and fossil jet fuel without any modifications. Newer aircraft engines are being certified to use 100% SAF, and even for an engine not certified this way, synthetic aromatics can be added to ensure a 100% SAF-fueled flight is compatible with the aircraft.  

Additionally, using SAF—no matter which kind—does not impact aircraft performance and does not require investing in the extensive new electric charging, hydrogen refueling, and hydrogen storage infrastructure at airports that would be needed for zero-emission aircraft. Hydrogen use also requires developing new safety procedures around its handling, storage, and delivery. These are expensive endeavors that can be challenging to implement. 

Aircraft require lots of energy to overcome the gravitational and aerodynamic forces required for flight. Consequently, the energy source needs to be light and compact to reduce those forces. As shown in Figure 1, liquid hydrocarbon fuels, a term used to encompass Jet A, bio-derived SAF, and synthetic fuels, have the perfect combination of specific energy (43 MJ/kg) and energy density (about 34 MJ per liter) for use in aircraft. Current lithium-ion batteries at the pack level achieve 0.9 MJ/kg and 1.8 MJ/L, far worse than liquid hydrocarbon fuels on both counts. Although hydrogen has a specific energy nearly three times as high as jet fuel (120 MJ/kg), it occupies 4–7 times more volume per unit energy (5–8.5 MJ/L), depending on the method of storage.  

Because of the modest aircraft performance that can be achieved with today’s electric propulsion systems, battery electric aircraft are expected to be limited to the commuter segment of the aviation market carrying 9 passengers less than 100 miles. Additionally, turboprop aircraft retrofit with fuel-cell propulsion are expected to have 26% fewer seats and 15% less range than the original jet-fueled aircraft. Commuter and turboprop aircraft service a small fraction of the total aviation market. Hydrogen combustion propulsion could power a narrowbody-sized aircraft, but hydrogen’s voluminous nature would limit the amount that can be carried on board, resulting in a reduction in the range of the aircraft.  

Why zero-emission planes remain a long-term goal 

If SAF has such clear advantages in implementation and performance, why consider alternatives at all? The answer is that zero-emission aircraft offer substantial benefits in resource efficiency, and that’s important for decarbonization. Compared with an aircraft using fossil jet fuel or SAF, zero-emission aircraft require an estimated one-half to one-third of the energy to service each passenger kilometer. We estimate that regional aircraft retrofit with fuel-cell propulsion systems would use 30% less energy than their hydrocarbon-fueled alternatives. The exception are hydrogen combustion aircraft, which are expected to use ~10% more energy than aircraft burning jet fuel or SAF.  

The resource consumption extends beyond energy use, because there can be extensive land-use implications with SAF depending on the feedstock used to produce it. For example, an acre of land used to grow corn, a feedstock for alcohol-to-jet (ATJ) SAF, would likely produce 30 times more energy if it were instead used to harness solar energy. One flight between Chicago O’Hare airport (ORD) and San Francisco airport (SFO) would require over 9 acres of land to fuel using corn ATJ fuel, while hydrogen- and e-kerosene-powered aircraft would take less than 1 acre of land to source the energy needed to complete the same flight (Figure 2). Multiple analyses have suggested that the limited availability of arable cropland greatly constrains the potential for crop-based SAF to provide a meaningful share of global aviation fuel demand. 

With recent setbacks, and without active policy intervention, it’s likely that zero-emission aircraft will play a smaller role than the 13% carbon emissions reduction that was attributed to them in our previous Vision 2050 net-zero roadmap for aviation, published in 2022. Fortunately, there are regulatory approaches that may help to change this. For example, the ReFuelEU Aviation regulation includes provisions for counting hydrogen and electricity use toward the SAF blending mandates. This is a simple and effective addition to SAF policy that makes the mandate technology-neutral and incentivizes the use of hydrogen and electricity in aviation. That this provision is absent from most of the other mandates implemented is a missed opportunity.

Additionally, more fiscal support programs like the EU Innovation Fund could be established to support research and development and early capital expenditure for hydrogen and electric aviation. Indeed, a strong combination of incentives and mandates could create the policy environment required to increase the uptake of zero-emission technologies. This combination could help zero-emission aircraft overcome current turbulence and make them viable options in the medium- and long-term.