A modern turbojet passenger aircraft is around 80% more fuel efficient per passenger kilometre than its predecessors thanks to technological advancements such as improved aerodynamic compressor design in engines and the introduction of carbon fibre-reinforced polymers into wing design. Despite this, emissions levels of the aviation industry are steadily increasing with air transport carbon dioxide emissions in the European Union rising by 2-3% each year since 2014. According to EASA, it's the aviation industry's desire to fly larger aircraft longer distances that is a major reason for this steady growth.
In a recent blog post 'Aviation Emissions – Virtue Signalling or Real Action?', Paul Campbell discussed the 2019 ACI Annual Assembly. He highlighted comments by Dag Falk-Petersen, CEO of the Avinor Group, and Dalton Phillips, CEO of DAA, who both pointed to the need to share more widely the sustainability advances aviation is making. This got us thinking - what are those advances, and do they go far enough?
A key stumbling block to environmentally sustainable air transport is our dependence on hydrocarbons to power aircraft. It is the combustion of these hydrocarbons which leads to the emission of carbon dioxide, water vapour and nitrous oxides that contribute directly to the greenhouse effect. Methods to bypass the need for the combustion of hydrocarbons, such as the use of cryogenic hydrogen, or all-electric aircraft are being researched. But these are some way off widespread implementation. There has however been considerable recent progress in the development of Sustainable Aviation Fuels (SAFs) derived from sources other than petroleum.
SAFs can be obtained from biological materials such as biomass or recycled waste, a method championed by Virgin Atlantic and British Airways in partnership with LanzaTech and Velocys respectively. Currently, these fuels need to be mixed with conventional aircraft fuels and as such still produce some emissions from combustion; however, across the entire fuel lifecycle there is a net reduction of carbon dioxide emissions thanks to the biomass absorbing carbon dioxide during its lifetime.
Government policies around Europe have also further incentivised airlines to move towards SAFs. The European Union has provided €464 million to aid in the development and production of advanced biofuels. These funds are part of the Horizon 2020 Programme (to be replaced by Horizon Europe in 2021). In addition, the use of SAFs has been included by ICAO as a means of meeting CORSIA carbon offset requirements, although concerns have been raised about this.
The transition of aircraft from combustion powered to partially or totally battery powered is another avenue through which the industry is trying to tackle its emissions. This year marked the first delivery of a commercial all-electric aircraft called Alice, launched by Israeli company Eviation. The aircraft can carry 9 passengers up to 650 miles and could completely revolutionise regional travel and island-hopping in certain parts of the world. Alice has no direct combustion emissions and has the potential to be carbon-neutral depending on the electricity source.
Could a concept like Alice be used to replace our current commercial transport aircraft like the A320 and the 737? At this moment in time, the answer is no. The main issue being the energy density of current batteries. Kerosene, the fuel used to power jet aircraft, is about 43 times more energy dense than the state-of-the-art lithium ion batteries meaning a battery powered aircraft would need a far heavier battery pack to provide the same power as kerosene. In the absence of any further advancements in battery technology, all-electric aircraft will be limited to short regional flights.
The energy density of batteries has not entirely derailed the electrification of aircraft and industry is now pursuing hybrid- and more-electric aircraft as a means of reducing air transport emissions. In 2021, Airbus, Siemens and Rolls Royce, supported by the UK government, are set to demonstrate their hybrid aircraft called the E-Fan X. This project involves replacing one of the four gas turbine engines on a British Aerospace RJ100 with a 2-megawatt electric motor. Most promising, however, is the more-electric aircraft, a concept which has been under development for many years and is even present on some modern-day transport aircraft. The more-electric aircraft replaces the traditional heavy power systems, such as the hydraulic system used to position flight controls and the landing gear, with electrically driven systems. This brings a variety of benefits to the aircraft including reduced maintenance costs of electrical systems, but most importantly brings a valuable weight saving across the aircraft leading to reduced fuel burn and emissions. Back in 2015, studies predicted that the market for more-electric aircraft would be worth over $10 billion by 2021. Today, that estimate looks overly conservative given that other studies estimate the market in 2018 alone to have been worth $30 billion.
The aviation industry has also backed up its progress with some lofty commitments to minimise its environmental impact. These include the Air Transport Action Group's promise to cut net aviation carbon emissions to 50% of 2005 levels by 2050 and the Flightpath 2050 commitment to a 75% reduction in carbon dioxide emissions per passenger kilometre. Meeting these goals will be difficult without a transition away from traditional fuels and power sources. The advances made to date are indeed something to talk about, but they clearly don't go far enough: The industry shows no sign of decline, and emissions continue to rise. The search for new solutions must not only continue, but accelerate.
Author, Alex Sawyers has just completed an 11-week summer internship with Helios. He is studying an integrated Master's in Aerospace Engineering at the University of Bath and will graduate in the Summer of 2020.
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