Now Boarding: Net-Zero Aviation

By Daniel Slesinski

POLICY | SCIENCE | PLANETARY HEALTH

Photo by Kevin Woblick on Unsplash

April 29, 2024

Whether you enjoy air travel or not, it’s increasingly common and persistently bad for the environment. Almost three million passengers fly in and out of US airports each day and global demand for air travel is expected to double by 2050. In addition to emitting pollutants that lower air quality, aviation is recognized as a hard-to-decarbonize sector and accounts for ~3% of global CO₂ emissions. Airlines have recently announced commitments to switch to ​“green” fuels, but as of today 95% of jet fuel is produced from fossil fuels. Why is that? 

The short answer is that today’s jet fuel has incredible energy density and is cheap to produce. Known as kerosene, it is one of the most energy-dense liquid fuels available – and is relatively cheap, making it difficult to displace with more environmentally friendly alternatives. It takes a lot of energy to fly a plane, and the high energy density of kerosene (11.9 kWh for each kg of jet fuel) means it packs a lot of power relative to the weight of the fuel itself. In addition, since kerosene is one of many products derived from crude oil (~10% of each barrel of oil is converted to kerosene, with the remainder used in other products spanning fuels for different vehicles, waxes and polishes, and even asphalt for roads and roofs – see figure below), its production is part of a massive global supply chain, helping to keep costs relatively low. This is particularly important in aviation given the industry’s high costs and relatively low margins. 

Fortunately, we at RA Capital are not the only ones to recognize the environmental challenges associated with flying; there are many innovations that can help decarbonize air travel, which can generally be categorized into three groups: 

1. electrifying aircraft
2. powering aircraft with hydrogen
3. replacing conventional jet fuel refined from oil with synthetic jet fuel made from sustainable alternatives (known as sustainable aviation fuel or SAF). 

A combination of market and technical factors make SAF the most promising long-term approach for decarbonizing aviation. Building new aircraft powered by electric motors and batteries sounds great in theory: low or no direct emissions, fewer air quality issues, greater efficiencies, and a familiar step toward the electrification trend we see with cars. However, aircraft have much greater requirements for range and carrying capacity relative to cars, and the weight of current batteries greatly limit their range of travel. Currently, state-of-the-art commercially available batteries can achieve energy densities of ~300 Wh for each kg of battery, which is just 2.5% of that of conventional jet fuel. As a result, fully electric aircraft will have a dramatically shorter range than conventional jets – likely limited to a few hundred miles – and such short-haul flights currently account for 5% of overall aviation emissions. In addition, building the charging infrastructure required in an airport full of small electric planes will not be an easy feat.

How about powering planes with hydrogen? Again, in theory, this sounds great: hydrogen is a clean-burning fuel with an impressive energy density (33.3 kWh for each kg of hydrogen), but as a gas it is diffuse, meaning 4 – 5x more space would be required for an aircraft to store the same amount of energy. Hydrogen-powered planes would not only need entirely new fleets of aircraft designed with much larger areas for fuel storage (not to mention cryogenic cooling for storing liquid hydrogen or pressurized tanks for storing liquid or gaseous hydrogen), they would also require an entirely new fueling infrastructure at each airport. Rolling out these changes could take decades, and billions to trillions of dollars would be required to move and store this difficult-to-transport fuel.

SAF’s clear market advantage is that, in contrast to electric or hydrogen-powered planes, it doesn’t require new aircraft or airport infrastructure. SAFs are carbon-neutral or carbon-negative jet fuels that can be produced a number of different ways from sustainable sources like municipal solid waste, plant material, or CO₂ captured from the sky mixed with hydrogen. Among all of these, there may not be a single ​“best” technical solution for producing SAFs; each production method has its own scalability potential, technological readiness, cost, and energy/​resource requirement. 

One approach to evaluate the relative resource intensity of SAF production pathways is to assess the land area that would be required to hypothetically replace all 95 billion gallons of jet fuel used annually with SAF. While making a number of simplifying assumptions (see figure), we calculated how many Texases worth of land would be required to create this amount of SAF from just four different pathways, each based on a specific raw material or ​“feedstock.”

On the low end, jet fuel made entirely from captured CO2 and green hydrogen (also known as e‑fuel) and powered entirely by solar panels would only need 0.23 Texases worth of land area.

On the high end, using trees (like the fast-growing and drought-tolerant species eucalyptus) as feedstock for jet fuel produced via gasification would require 15.46 Texases. While there are many other factors to consider when analyzing the potential of different SAF methods, this simplified approach helps to inform our view on scalability to address the needs of the overall sector.

Clean aviation is a hard nut to crack but, given the many benefits of air travel in our connected and globalized society, we can and must find ways to minimize its environmental harms. Our Planetary Health team here at RA Capital has identified over 100 companies focused on decarbonizing aviation and developing technologies that span advances in electric and hydrogen-powered aircraft as well as SAF production. If you are working on new technologies for SAF that can be produced with reduced resource intensity and at costs that can compete with conventional jet fuel, we would love to hear from you.