Rajan Wilcox
The aviation industry is facing increasing pressure to reduce its carbon footprint. With the world aiming to limit global warming by cutting carbon emissions, the industry must explore alternatives to fossil fuels. However, this is a challenging task due to the vast amounts of fuel required by large, long-range passenger planes. While biofuels and synthetic fuels are potential options, they face limitations such as low energy density and the need for significant land use. Hydrogen-based fuels have also gained attention, with companies like Airbus aiming to develop hydrogen-powered aircraft. Meanwhile, electric airplanes are on the horizon, but battery weight and energy remain obstacles for long-haul flights. As fossil fuel reserves deplete and prices rise, finding sustainable alternatives for the aviation industry becomes imperative.
| Characteristics | Values |
|---|---|
| Current reliance on fossil fuels | Kerosene is the most common fuel used by commercial aircraft |
| Alternative fuels | Biofuels, hydrogen, synthetic fuels, electrochemical reactions between water and captured carbon, sustainable aviation fuels (SAFs), and ammonia |
| Progress towards adoption of alternative fuels | Airbus plans to develop the world's first hydrogen commercial aircraft by 2035. A test flight of an A350 fuelled with 100% HEFA-SPK (a type of SAF) was successful in March 2021. |
| Challenges | Energy density, production costs, feedstock availability, operating temperature, infrastructure changes, and economic viability |
| Impact of aviation on climate change | Aviation is a relatively small contributor to global emissions but it is one of the fastest-growing sectors. A single flight can emit as much CO2 as many people do in a year. |
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What You'll Learn
Hydrogen-powered aircraft
The aviation industry is exploring alternative fuel sources as fossil fuel reserves are expected to deplete in the upcoming decades, causing crude oil prices to rise. Hydrogen-powered aircraft are a promising alternative to fossil fuels as they produce zero emissions and have a higher specific energy than usual liquid fuels.
Airbus, for example, has launched the ZEROe project, which aims to bring a hydrogen-powered commercial aircraft to market. ZEROe aircraft will feature an electric propeller propulsion system powered by hydrogen fuel cells, which transform hydrogen into electricity through a chemical reaction. Hydrogen fuel cells are not a new technology, but there are none commercially available that are large enough to power an aircraft while remaining within an acceptable weight range for flight. In 2023, the fuel cell demonstrator was powered on at 1.2 megawatts, and Airbus plans to launch its first commercial hydrogen-powered aircraft by 2040-2045.
Boeing is also exploring hydrogen-powered aircraft. In 2008, it converted a two-seat Diamond DA20 to run on a fuel cell, and in 2010, it unveiled its hydrogen-powered Phantom Eye UAV, which uses two converted Ford Motor Company piston engines. McKinsey & Company forecast that hydrogen aircraft will enter the market in the late 2030s and scale up through 2050, potentially accounting for a third of aviation's energy demand.
A UK study, NAPKIN (New Aviation, Propulsion Knowledge and Innovation Network), has also investigated the potential of new hydrogen-powered aircraft designs to reduce the environmental impact of aviation. The study proposed a range of hydrogen-fuelled aircraft concepts, from 7 to 90 seats, exploring the use of hydrogen with fuel cells and gas turbines to replace conventional aircraft engines. The findings suggest that hydrogen-powered aircraft could be commercially viable for short-haul and regional flights in the UK by the second half of the 2020s.
While hydrogen-powered aircraft show promise, there are challenges to their implementation. Hydrogen must be stored at extremely low temperatures, and hydrogen tanks must be housed in the fuselage or supported by the wing. Additionally, as of 2021, the majority of hydrogen production comes from fossil fuels, and less than 5% of all hydrogen produced is emissions-free. However, if hydrogen can be produced from low-carbon power sources such as wind or nuclear energy, its use in aircraft will significantly reduce greenhouse gas emissions.
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Biofuels as a viable alternative
The aviation industry has committed to reducing carbon emissions by 50% from their 2005 levels by 2050. Sustainable Aviation Fuel (SAF) is key to achieving this goal. SAF is a type of aviation biofuel that can be blended with fossil jet fuel to reduce carbon emissions. While biofuels are a promising alternative to fossil fuels, there are some challenges and considerations to keep in mind.
Biofuels have the advantage of requiring minimal to no changes to aircraft engines or airport infrastructure, making them a cost-effective option in the long run. For example, biofuels do not contain sulfur compounds and therefore do not emit sulfur dioxide. Additionally, biofuels made from wet waste feedstocks can divert waste from landfills, reducing methane emissions. NASA has determined that a 50% aviation biofuel mixture can cut particulate emissions caused by air traffic by 50-70%.
However, one challenge with biofuels is the issue of temperature stability. At extremely low temperatures encountered at high altitudes, biofuels tend to solidify, which could be addressed with heated fuel tanks. Another challenge is the availability of suitable feedstocks. Sustainable biofuels do not use food crops, prime agricultural land, or freshwater, which limits their sources. Additionally, the production cost of biofuels is currently higher than that of fossil jet fuel, which is a barrier to their wider adoption.
To address the cost issue, economies of scale could be achieved by refineries designed for continuous production. Additionally, airlines may include SAF consumption cost premiums within ticket costs to offset the higher production costs. In the future, as fossil fuel reserves deplete and crude oil prices rise, biofuels will become more economically competitive.
While biofuels show promise as a viable alternative to fossil fuels in aviation, further market development and investment in production capacity are needed to meet the demand and drive down costs. With ongoing research and development, biofuels can play a key role in reducing the carbon footprint of the aviation industry.
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Synthetic fuels
The aviation industry is responsible for more than 2% of global greenhouse gas emissions. As fossil fuel reserves are being exhausted, jet fuel prices are expected to rise in the coming decades. This makes it crucial to explore sustainable and renewable energy sources to power the aviation industry.
Synthetic aviation fuel (SAF) can be made with a variety of technologies that use physical, biological, and chemical reactions to break down biomass and waste resources and recombine them into energy-dense hydrocarbons. SAF has the potential to provide more than 400 million tons of biomass per year above current uses. It can be sourced from air-captured CO2, with companies like Prometheus Fuels working on this technology. Another method is to capture waste carbon monoxide from industrial processes and upgrade it with bacteria into ethanol for conversion into "alcohol-to-jet" SAF.
SAF has been deemed a key building block of aviation's roadmap toward a net-zero economy, with governments setting targets for its adoption. The US has set an ambitious target of 100% SAF usage by 2050, while the EU and the global aviation sector aim for over 60% by the same year. The first test flight of a large passenger airliner fuelled with 100% SAF was successfully conducted in March 2021, showcasing the viability of SAF as a replacement for fossil fuels in aviation.
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Electric airplanes
Electric aircraft have been in development for decades, with the first solar-powered flight taking place in 1979. Since then, there have been numerous successful solar-powered flights, including the Solar Impulse 2, which completed a round-the-world flight from Abu Dhabi in 2016, flying over 40,000 kilometres without fuel.
Solar power is not the only method of powering electric aircraft. Battery-powered aircraft have also been developed, such as the SoLong, which flew non-stop for 48 hours in 2005, and the Zephyr, a solar-powered unmanned aerial vehicle that holds the endurance record for an unmanned aerial vehicle of over two weeks.
Electric aircraft offer several advantages over their fossil fuel-powered counterparts. For example, the world's fastest electric airplane, the Long ESA, was found to be less expensive, have a higher maximum speed, and a higher rate of climb than gasoline-powered aircraft. Electric aircraft are also quieter and produce fewer emissions, with some aircraft, like the DA36 E-Star, reducing fuel consumption and emissions by up to 25%.
Despite these advantages, there are challenges to adopting electric aircraft for commercial use. One major challenge is the energy density of batteries. The max energy density for a battery is around 2mj/kg, while jet fuel is 45mj/kg, which limits the range and payload of electric aircraft.
To overcome this, hybrid electric aircraft have been developed, which take off and land using electric power but cruise under conventional piston or jet engine power. This allows for long flights while still reducing the carbon footprint. Additionally, biofuels, such as HEFA-SPK, are being explored as a way to reduce emissions without sacrificing range.
While there are challenges to be addressed, the development of electric and hybrid-electric aircraft is an important step towards reducing the aviation industry's carbon footprint and making commercial airplanes viable without fossil fuels.
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Solar power
Solar-powered aircraft are electric aircraft that can take the form of airplanes, blimps, or airships. They use solar cells to capture photons, which are converted into electrical potential to power the plane's motors. However, solar-powered planes today only capture about 10-20% of the sun's energy, resulting in a speed of approximately 50 miles per hour. This is significantly slower than commercial passenger jets, which typically travel at around 600 miles per hour.
There are several challenges associated with solar-powered flight and its implementation in commercial aviation. Firstly, the angle of capture for the sun to hit the panels is highly variable due to the constant movement of both the sun and the plane. This reduces the efficiency of energy capture compared to stationary solar panels. Additionally, solar-powered planes struggle to achieve the speeds of conventional jets due to the cubic relationship between speed and power requirements. Even with technological advancements to capture 100% of the sun's energy, estimated top speeds for solar-powered aircraft are around 100 miles per hour, which is still significantly slower than jet aircraft.
The design of solar-powered planes also presents challenges. These planes have enormous wingspans to accommodate lightweight solar cells, making them vulnerable to adverse weather conditions. The weight limitations of solar-powered aircraft further restrict their practicality for commercial use. A conventional jet aircraft like the Boeing 747-400 can carry around 400 passengers, whereas a solar-powered plane can only carry a single passenger with a similar weight capacity of 2 tons.
While solar-powered aircraft face challenges for widespread commercial use, they have been successfully employed in other applications. Solar Airship One, developed by Euro Airship, is an autonomous airship that uses electrolysis to store hydrogen, enabling it to stay airborne even when the sun isn't shining. It is planning a world tour in 2026, flying non-stop to 25 countries in 20 days. Additionally, solar-powered aircraft have been proposed for use in telecommunications, video/imagery, flight control, precipitation detection, and geopositioning applications. These aircraft can operate at altitudes of 20-100 kilometers for extended periods, making them suitable for unmanned missions and data collection for climate research or surveillance.
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Frequently asked questions
Some alternatives to fossil fuels for commercial airplanes include biofuels, hydrogen, and synthetic fuels.
Biofuels require minimal to no changes to the aircraft or airport infrastructure, and thus require the least investment.
Biofuels have been criticized for the large amounts of land required to produce sufficient quantities of fuel. Additionally, biofuels like bio alcohols and biodiesel do not provide enough energy to fuel large passenger airliners.
Hydrogen, when mixed with oxygen to create electricity, emits nothing but water vapour, unlike conventional jet fuels which release carbon dioxide.
Hydrogen has the issue of storage, as the storage tanks are heavy. Additionally, hydrogen is not a viable green source yet.
