Sustainable aviation
Sustainable aviation

Sustainable aviation

Tags
📌
Meet our investment in this space: Cosmic

TL;DR

  • Aviation is on track to account for 25% of all GHG emissions by 2050. To prevent this, the aviation sector must reach net-zero CO2 emissions AND reduce non-CO2 radiative forcings like water vapor and soot.
  • Most innovation focuses on new fuel sources for existing aircraft (such as hydrogen and biofuels), but significant technological, sourcing, and cost barriers exist.
  • With the right design, electrification can replace 50-80% of all air traffic, eliminating direct emissions and at lower maintenance and operational costs than a traditional aircraft.

The impact of air travel

Burning one kg of aviation fuel produces 3.16 kg of CO2. In 2018, the total CO2 emissions from air travel (passenger + freight) were 918 million tonnes—or 2.4% of global emissions. Air cargo is growing, but passenger transportation was 81% of all 2018 aviation emissions, or 747 million tonnes of CO2 from 38 million flights.

CO2 emissions, however, are only part of the problem. If you look at everything else that comes out of a jet engine, the warming effects are 3x higher than those of just the CO2.

Source:
Source: Lee et al., 2021

Water vapor

Aeroplane exhaust is made up of 30% water vapor, and water vapor is a potent GHG that amplifies global warming. As the earth’s temperature increases, more water evaporates into the atmosphere. Because water itself absorbs heat, the warming effect gets more potent, and the cycle continues.

image

Soot

Soot is black carbon particles produced by burning fuels, and jet engines leave a trail of these through the sky. In the atmosphere, soot particles absorb energy from the sun and release that as heat, but they also become condensation nuclei (aka cloud seeds), meaning a surface where water can condense. These particles create contrails along the path of the aircraft (the long white trails you see following planes in the sky).

Source:
Source: Kärcher, 2018
⚠️
The direct warming effect of contrails is larger than that of CO2 and NOx emissions.

Another problem is what happens when soot eventually settles onto the planet's surface, particularly on snow and ice. Snow and ice act like a mirror on Earth that reflects energy from the sun back into space, reducing its warming effects (this reflection is called albedo). When the soot settles, that normally reflective surface also becomes darker and absorbs more energy from the sun, causing local heating and speeding up melting.

Approaches to sustainable aviation

Rerouting

Current methods to reduce non-CO2 factors usually involve rerouting, and this can actually increase CO2 emissions. Directing an aircraft around cold air masses can reduce the number of contrails that form behind it, but this uses more fuel than taking the most direct route.

Aircraft design

Designing for aerodynamics can reduce fuel use by reducing drag and enhancing flow control (controlling airflow around the aircraft can help achieve greater lift at lower speeds). The holy grail of aircraft design is called “laminar flow,” where the air is not mixing, just smooth streams above and below. Most aircraft engineers optimize for turbulent flow rather than going for laminar because the laminar flow is very easily interrupted (it only takes a bug, a speck of dirt, ice crystals, or a scratch in the surface).

⚠️
Laminar flow cannot be retrofitted, so it must go into an aircraft during the design and engineering phases.

Some of the approaches to aircraft design include:

  • Engine placement: Having engines mounted under the wings can help improve stability, prevent wing bend, and reduce fire/explosion risk to passengers. Back-mounted engines are less susceptible to sucking in foreign objects (because they sit higher up). However, fuel needs to travel further to the engines, so there is more fuel pump consumption, and the weight distribution needs to be different to maintain the center of gravity at the wings.
  • Wing design: The difference in pressure above and below the wing is what causes an aircraft to lift—this ratio can be changed based on the design of the airfoil (the basic shape of the wing). The airfoil's shape is changed through flaps and slats that can be extended and retracted to balance lift and drag for takeoff and landing. Lift force will start to decrease at a certain point, so it’s a fine balance.
    • Source: [VIDEO] Understanding Aerodynamic Lift (
    Source: [VIDEO] Understanding Aerodynamic Lift (The Efficient Engineer, 2021)

    Blended wing design reduces the plane's surface area and drag so that fuel consumption can be reduced by up to 27%.

  • Reducing weight: Landing gear is currently 33-63% of an aircraft’s weight. Using new materials that are lighter (nanomaterials) and can serve multiple purposes (embedded structures) can reduce the overall weight of an aircraft.

Electric planes

“A small drone may fly on the energy of a few flashlight batteries, whereas a large airliner takes off with the energy of 30,000 Tesla cars.” - Viswanathan et al., 2022

Electrification is one of the best paths toward reducing the global warming potential of air travel, but today’s batteries are not powerful enough to propel today’s planes. Jet fuel has an energy density of 12,000 Wh/kg, whereas today’s batteries max out at around 300 Wh/kg. However, an electric powertrain* using batteries and electric motors is much more efficient than a jet fuel-based powertrain that uses gas turbines. The efficiency of a modern gas turbine engine is typically 28%, whereas all-electric powertrain clocks in at 90%, meaning the path to electric air travel will not require nearly as high of an energy density. Because of this efficiency advantage, the path to electric air travel will require a battery energy density of around 400-820 Wh/kg, which is likely possible by 2030.

⚠️
*Powertrain refers to all of the parts and processes that go into propelling a vehicle (be that a car or a jet engine). So the powertrain is everything that converts energy stored in fuel into power (or mechanical energy) and transfers that power into motion.

The benefits of electric air travel will generally be limited by how green the world’s energy grid becomes. If all of today’s planes were powered by the world’s current electricity grid (about 66% fossil fuel powered), the energy-related emissions could be higher than just burning jet fuel. So decarbonizing the grid to power electric aviation is very important. The good news is that renewables are becoming cheaper than fossil fuels, so deployment is increasing around the world.

There are two approaches to electrifying aviation:

  • Retrofitting: To convert an existing aircraft into an electric aircraft, issues of space and weight must first be solved. As fuel is burned off during a flight, the weight of a plane decreases, meaning the further it travels, the less fuel it requires. Batteries do not decrease in weight and come along with heavy coolants and energy-converting equipment. The heavier a plane is, the more energy it will require, and adding more energy via batteries increases the weight of the plane.
  • New Design: Starting from scratch allows engineers to optimize aircraft design around issues like the weight of batteries. Incorporating many of the elements listed in the aircraft design section above can offset the additional weight of batteries.

Alternative fuels

Aviation fuel must meet the requirements for freezing point, flash point, cost, and energy content in order to be viable. Beyond these requirements, the alternative fuel must be cost-effective to scale, and none of the alternatives listed below currently are.

  • Freezing point: The temperature where a liquid will freeze. Since planes travel at high altitudes where the temperature is low, any fuel must be able to remain in liquid form to be usable. This is extra important for flights across cold regions like the Arctic. The freezing point of any viable fuel should be between -40°C (for most travel) and -60°C (for travel over cold regions).
  • Flashpoint: The temperature at which a fuel will ignite. If fuel has too low of a flash point, it is too volatile and dangerous to use in aviation. Gasoline has a low flashpoint (about -43°C) and is not used in aviation, while kerosene is and has a high flashpoint (38°C).
  • Energy content: How much energy, in the form of heat, is released when a unit of fuel is burned.

Hydrogen

Hydrogen fuel will require a total overhaul of aviation infrastructure and, even at optimal conditions, will always have volumetric energy density levels lower than jet fuel. Hydrogen fuel will not work under regular atmospheric conditions. It requires extremely high pressure or extremely low temperatures (close to absolute zero) to maintain its energy density, both of which require energy input to maintain. Complicating matters further is that hydrogen can attack and destabilize the material surrounding it at high pressure. Pressurizing a fuel tank also requires bulky equipment that would replace passenger seats and is complicated and prone to failure.

  • Burning hydrogen fuel produces water vapor clouds, and remember that water vapor is a potent amplifier of global warming.

e-fuels

Hydrogen and CO2 can be combined to form synthetic kerosene, but the energy requirements are high. Energy inputs are needed to produce the hydrogen, capture the CO2, and then convert those materials into fuel. Ideally, this process will use renewable energy, and solar was recently proven viable as part of the EU project SUNlight-to-liquid.

  • Burning e-fuels still releases CO2 but is viewed as carbon neutral since it was not sourced from fossil carbon. There will still be non-CO2 emissions, so the net warming effect is probably still positive.
  • This is currently the only alternative fuel that is fully compatible - as is - with existing engines and for both short and long-haul flights.

Biofuels

These can be purposefully grown or made from agricultural waste and/or leftover cooking oils. They are sometimes also called Synthetic Aviation Fuels or Synthetic Biofuels. Biofuels are used in a blend (up to 50%) with regular kerosene.

  • Biodiesel is unusable as it has too high of a freezing point (1°C), so it needs to be further processed to be more similar to traditional jet fuel. They aren’t currently cost-effective to manufacture, and sourcing feedstock without contributing to land use change or food insecurity is problematic.
    • Increasing purposefully grown biofuels will most likely divert land away from food production, raising food prices and requiring more land to be converted to agriculture.
⚠️
The ReFuelEU aviation proposal from the EU would require fuel supplies to provide more than half of aviation fuel from sustainable and synthetic sources by 2030. A 50/50 blend has been shown to reduce soot in aircraft plumes by 5%.

Resources

Last updated: May 2023