Getting renewable energy in the sky with better aircraft designs
November 18, 2021
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  • A long-term issue for the airline industry is the need to tackle their fossil emissions, which contribute to >2% of global emissions.

    How do we get this industry to net zero, and is it even possible? The conventional “tube and wing” design can only be optimized so far. While novel designs are showing promise to “break the mold” of conventional aircraft and offer a way to incorporate lower energy density propulsion systems.

    Solutions have been put forward by way of renewable or “green” fuels as well as re-imagining existing molds. New airframe designs not only improve aircraft efficiency but have the side benefit of increased cabin volume, providing ample space for sustainable propulsion schemes, such as electric-fuel hybrid planes or even fully electric systems.

    This would offer a way to de-fossilize long-distance flights of 1,500 km or more, which are estimated to make up 80% of the total industry’s emissions. Innovation continues to be needed even though the average fuel burn per passenger per km traveled for conventional “tube and wing” aircraft has decreased 50% over the past 5 decades.

    Are electric planes a future possibility?

    To begin, fully electric, long-distance flight is the future hope for aviation. However, electric planes still lie in the proverbial “10 years away” category. This is due to commercial Li-ion batteries low energy-to-weight ratio, which lies at ~270 Wh kg-1 where 800 Wh kg-1 is estimated to be required for long-distance flights.

    Despite this, pure electric flight has already been achieved for short distance flights. Amazingly, the cost of electricity for the MagniX Cesna Caravan electric plane is ~$6 USD for a 160 km local flight compared to ~$400 USD if powered by a liquid fuel. Electrification is not only a “greener” option for airlines, but there is a cost advantage for their bottom line.

    An interim solution, until fully electric planes get off the ground, could be a hybrid model where high-energy fuels can be used for takeoff and batteries used for cruising. These electric-fuel hybrids have the advantage due to the fuel’s chemical energy and further, gaseous or liquid chemical fuels coupled with fuel cell electricity could continuously charge batteries, as is done in hybrid-electric vehicles.

    Renewably derived hydrogen or high-energy fuels, such as methanol, dimethyl ether, or oxymethylene dimethyl ethers, could replace conventionally used kerosene in planes. However, hydrogen is not yet viable as it needs heavy containment equipment while high-energy chemicals need more auxiliary equipment to extract the hydrogen for use.

    Lack of space makes electrifying a challenge

    A significant challenge with electrifying conventional “tube and wing” designs is the lack of storage volume for alternative propulsion types. For example, for batteries to supply the equivalent fuel energy for the world’s largest aircraft, the Airbus A380, it would require batteries weighing 30-fold the liquid fuel mass and many times the volume due to their low energy-to-mass and volume ratio. Thus, hybrid designs are an appealing technology for at least the foreseeable future.

    Innovative aircraft proposals like Boeing’s and NASA’s X-48, which uses a blended wing body airframe design similar to the U.S. military aircraft B-2 bomber, offer solutions to conventional “tube and wing” designs that have been prevalent since the inception of commercial aviation.

    They have a 15% takeoff weight reduction and 27% fuel efficiency advantage compared to conventional designs. As a result of the more uniform lift profile of the X-48’s airframe, it avoids the need for additional, weighty reinforcement materials required where the wings attach to plane’s body. Fuel efficiency improvements of 5% or more can be achieved by avoiding the need for stabilizing tail fins, and the lighter, more uniform shape can be made from lighter composite materials, which allows for more internal volume.

    This increased volume can be used for hydrogen tanks, battery racks, structural battery packs, electric-fuel hybrid setups, and can seat up to 450 passengers in a less densely packed arrangement. One drawback to the blended-wing designs is the need for computer-controlled level stabilization during flight, whereas the conventional designs with a tail fin have “fail-safe” or self-correcting stabilization. Although initial tests were conducted on small scale versions a decade ago, Boeing and NASA are still planning a larger blended wing body demonstrator.

    Renewable jet fuels could bridge the gap

    While these new airframe and propulsion schemes are being sorted out, today the airline industry is looking to renewable jet fuel to achieve their ambitious climate targets. The International Air Transport Association (IATA) aims to procure 2% or 7.3 billion liters of sustainable aviation fuel by 2025.

    Within the budding renewable jet industry, which wants to capitalize on this initiative, are start-up companies such as Climeworks and Synhelion, both spin-offs of ETH Zurich. They have offered airlines, like Lufthansa Group, another option for powering their conventional aircraft during this transition period by promising a “green” jet fuel. They use ambient CO2 capture and water to generate syngas via a thermochemical process that splits these feedstocks using heat from the sun. This is then further used to produce “green” kerosene.

    There are a number of other companies in this arena that are now able to deliver low-carbon jet fuel using feedstocks and innovative technologies as diverse as waste vegetable oils via hydroprocessed esters and fatty acids (≥10 companies including Total and Neste), industrial waste gases via alcohol to jet (Lanzatech), agriculture and forestry residue/biomass, municipal solid waste, discarded cotton clothing via gasification and Fischer-Tropsch synthesis (≥7 companies including Enerkem and Japan Airlines), captured ambient CO2 via photothermal syngas and Fischer-Tropsch synthesis (Synhelion and Climeworks), and captured CO2 via power-to-liquids (≥5 companies including Sunfire and Copenhagen Airport). The success and long-term viability of these solutions depends on how cost effective and net-neutral or negative carbon balance they can be to compete with up and coming electric-fuel hybrids.

    Incentivizing sustainable fuels

    Several European countries are also calling for a sustainable aviation fuel mandate that will require a certain percentage of consumed jet fuel to be renewably sourced. With net-zero by 2050 on the table for many countries, this industry will be an essential element in the coming decades while other technologies become available.

    As of March 2021, however, the U.S. aviation industry group, Airlines for America, suggests the North American market is not ready for a mandate due to fears of fewer producers monopolizing it and keeping fuel prices high. Notwithstanding an aversion to change, credit schemes are being explored in the US to incentivize sustainable aviation fuel and help it compete with conventional jet fuel.

    For example, a similar approach to the successful $1 USD biodiesel blenders tax credit for vehicles is has been considered in addition to some additional credits, such as a fossil emission tax credit, to help it compete with fossil jet fuel. The International Civil Aviation Organization (ICAO), which supports international diplomacy and cooperation in air transport, is expected to commit to 10% sustainable aviation fuels by 2030 at its 41st General Assembly next year.

    Finding the right balance between incentivizing the production of sustainable aviation fuels while gradually phasing out fossil fuels, while not favoring one technology over the other will be a nuanced task for governments, regulators, and industry groups. Taking the first step, the introduction of blending mandates by the ICAO is seen a key step to help bolster this upstart industry and navigate the way for continued innovation.