Aviation Decarbonization Awaits Engineering Solutions

Industry by industry in transportation, the considerations for decarbonization vary. For example, marine and rail applications generally have moderate weight restrictions, although space requirements for fuel and powerplants matter, particularly in retrofits. Range and power requirements are additional considerations depending on routes and loads. Automotive and heavy truck applications have both space and weight considerations for new fuels and powerplants.

In this Insight article we consider perhaps the most difficult industry to decarbonize—aviation—where space and weight considerations must result in power systems that can fly with substantial loads.

CO2 emissions from the transportation sectors—air, road, rail, and water—are responsible for approximately 24% of global emissions. The challenges of size, weight, range, and cost suggest that reducing emissions could take much longer than in other sectors. Achieving meaningful emissions reductions will come from development of new technologies followed by a lengthy wait for existing fleets to turn over as they reach the end of their capital lives.

Global aviation contributes about 2.5% of global CO2 emissions. If passenger miles continue to increase at the currently estimated 5% per year, emissions from aviation will double within 14 years and could reach 8% - 11% of worldwide emissions as emissions in other areas diminish.

Of the total aviation emissions today, around 57% are from heavier wide-body aircraft, 36% from single-aisle aircraft, and the remaining 7% from regional jets (5%), general aviation (1%), and commuter planes (1%). Given each of these aircraft types have unique combinations of weight and average flight times, opportunities for decarbonization will vary, with larger aircraft needing the most change to decrease emissions.

Over the past 60 years, the aviation industry has focused on improving fuel efficiency, resulting in a roughly 50% reduction in fuel consumption per mile as highlighted in the following chart.

Fuel burn per kilometer

Source: ICAO

Currently the industry is working on new fuels, powerplants, propulsion systems, and aeronautical designs. The evolution and potential combination of these factors could enable the industry to reduce emissions longer-term. It is likely to take 10+ years before some of the potential new technologies are incorporated in aircraft designs and then another 20+ years for the world’s older plane fleets to turn over. Let’s look at some of the technological possibilities.

Sustainable Aviation Fuels

Fuel costs can represent 20% - 30% of airline costs, so alternative fuel costs matter to the industry. Alternative fuel options also have to compete with fossil fuel’s high energy density or energy content per pound. Other systems and sources of energy that add weight or are less efficient—which most currently are—detract from an aircraft’s potential load, flying distance, or both.

Most low-carbon fuel efforts center on sustainable fuels produced from vegetable oils, animal fats, grease, gases from crop residuals, forest waste, and corn grains, for example. Sustainable aviation fuels (SAF) resemble conventional jet fuel and emit carbon dioxide. However, SAF is considered more sustainable because it has a smaller carbon footprint considering its entire life-cycle emissions.

Producers estimate GHG emission reductions as much as 85% relative to conventional fuels, with some production pathways potentially delivering net-negative carbon. Research is also showing engines can burn 100% SAF but are currently approved at a 50/50 SAF mix with jet fuel. Because SAF can be mixed with jet fuel directly, no special pipelines or tankage infrastructure is required to adopt it.

Though SAF is one of the few available options to help lower commercial-aviation emissions, supply is limited and other industries such as marine shipping eye sustainable fuels as an alternative as well. Global jet-fuel consumption (fossil fuels) is ~7 million b/d (barrels per day), and the US market is ~1.65 million b/d. This contrasts sharply with a miniscule ~3 thousand b/d of US SAF production capacity in 2023 and a Biden administration goal of only 200 thousand b/d of production capacity by 2030. To enable the goal, Federal policy includes loan guarantees, R&D funding, and an interagency challenge. FactSet has identified 26 commercial SAF projects in the US that, if operational by 2030, would achieve ~196 thousand b/d, in line with the administration goal.

US SAF production capacity

Source: BTU Analytics, a FactSet company. Data as of October 2023.

At the moment, SAFs are three to four times more expensive than conventional fuel. To produce more over time on a scaled basis will require either or both:

• Development of expensive, carbon-emitting systems to collect waste streams

• Cultivation of large swaths of acreage for energy crops that will eventually release carbon

Hydrogen

Hydrogen is another fuel the aviation industry is experimenting with as a longer-term possibility. It requires special ground handling and larger storage space but is lighter than jet fuel. However, safety is a key consideration because hydrogen is highly flammable. Along with its production methods as the typical process today emits CO2.

Some predict that hydrogen “hubs” will eventually co-locate hydrogen production, carbon capture, sequestration, transportation, and end-use infrastructure. This would mean the expensive, specialized infrastructure for handling hydrogen can be limited in extent and that most hydrogen would be used by industries other than aviation. The Biden administration recently underwrote seven hydrogen hubs as part of its infrastructure investment plan.

Batteries

Various scientific papers have estimated that single-aisle and wide-body aircraft require batteries with a minimum energy density of 580 - 800 watt-hours/kg (Wh/kg). For comparison, current car battery energy densities range from 200 - 300 Wh/kg, and jet fuel energy density is approximately 12,000 Wh/kg.

In terms of what is possible for aviation, the National Academy of Sciences has estimated that current research may push battery energy densities to 400 - 600 Wh/kg in 20 years, potentially allowing for some low-to-mid-range aircraft to employ batteries at that time.

Four battery chemistries under research consideration are lithium-Ion, lithium-sulfide, solid state lithium-ion, and lithium-air.

Propulsion Systems

Various engine and motor configurations are also being investigated to help reduce aviation emissions.

A turboelectric system involves a gas turbine engine that drives an electric generator that provides propulsive power. No batteries are used. The downside is that converting energy this way decreases efficiency by 10% - 20%.

Adding a battery backup to a turboelectric system creates a hybrid propulsion system. These systems have the advantages of 1) allowing the turboelectric engine to constantly run at more efficient optimal speeds and 2) the battery being charged and discharged as needed throughout the flight. The additional weight of the battery system, however, tends to offset the gains in engine efficiency, so the hybrid approach might not be viable.

Both the turboelectric and hybrid power systems can be improved upon by adding additional propellers to the leading or trailing edges of the wings. In this way several design concepts use distributed power to 10 - 20 smaller motors powered by the gas turbine. The multiple motors take advantage of more surface airflows to reduce drag and increase propulsive efficiency. The design concepts are intriguing, with various performance improvements for turboelectric designs shown to achieve 12% - 15% lower take-off weight, 25% lower fuel consumption, and 25% improvements in total thrust with greater control and stability.

Conventional electric motors are not typically designed with weight considerations and can be limited by heat generated in the copper windings. They also offer less power than a gas turbine. One way to improve conductivity and reduce weight is to incorporate cooling equipment. Doing so adds weight, however, so the potential for this technology, which theoretically could generate more power per weight than a gas turbine, has been estimated to be 20 - 25 years out.

Conclusion

The introduction of multiple new smaller propellers and turboelectric configurations—which can be further advantaged by reimagined fuselage shapes—together with forecasts of SAF supply and developing battery technologies and concurrent material requirements all make for an interesting albeit long-term development arc for decarbonizing the aviation industry.

It takes 14 - 19 years for technology development in aircraft design, engine, and fuel combinations and about 10 years for a new airframe design to be certified for use in industry. On that basis alone, the development of much of what we might eventually see applied in the aviation industry will be slow moving.

Considering new fuels and successful design and propulsion efforts to decarbonize, fuel burn per mile and related emissions are expected to decrease 20% - 30% over the next 10 years. However, passenger miles are rising, which could threaten to offset the benefits of new technologies and efficiencies. Also, material is the required financial investment: trillions of dollars over the next 10 - 20 years and the potential impact on consumer ticket prices.

FactSet’s Carolyn Nuyen contributed to this article.

This blog post is for informational purposes only. The information contained in this blog post is not legal, tax, or investment advice. FactSet does not endorse or recommend any investments and assumes no liability for any consequence relating directly or indirectly to any action or inaction taken based on the information contained in this article.