The results of a European study into the impact of a widebody jet flown with 100% sustainable aviation fuel show a substantial 56% reduction in the number of contrail ice crystals produced by unblended SAF compared to Jet A-1 fuel. The project, involving Airbus, Rolls-Royce, the German Aerospace Centre (DLR) and SAF producer Neste, was the third stage of the Emission and Climate Impact of Alternative Fuels (ECLIF) programme, with in-flight and ground emissions tests taking place in 2021. They worked together to quantify the reduction in engine soot particles produced by a Rolls-Royce powered Airbus A350 fuelled by Neste and trailed by a DLR research aircraft. In certain weather conditions, airborne vapour freezes around soot particles emitted by aircraft engines, creating cloudy canopies of ice crystals which can trap heat in the atmosphere.

Prior to the A350 test flights using 100% SAF – a campaign designated as ECLIF3 – airborne research was conducted in 2015 (ECLIF1) to characterise the emissions of synthetic fuels, followed in 2018 by flight tests with NASA (ECLIF2) to demonstrate that 50/50 blends of conventional jet fuel and SAF could reduce the climate damage caused by aircraft condensation trails.  

The ECLIF3 tests were conducted over the Mediterranean and southern France using the first Airbus A350 aircraft built by the airframer and powered by two Rolls-Royce XWB-84 engines. Conventional Jet A-1 was used as the reference fuel, while the comparative SAF was a mix of hydro-processed esters and fatty acids and synthetic paraffinic kerosene (HEFA-SPK).

In all conventional jet fuels, naturally-occurring aromatics serve as a vital sealant within the aircraft engine to help prevent fuel leaks. But this compound is slow-burning and emits soot particles which contribute to contrail crystals that can remain for several hours in cold, humid conditions at altitudes of eight to 12 kilometres, and can have a local warming or cooling impact depending on the position of the sun and underlying surface, with a warming effect predominating globally. Many types of SAF are free of aromatic compounds.

During the ECLIF3 programme, which involved multiple flights, the A350 testbed departed Toulouse Blagnac airport in France while DLR’s research jet, a Falcon 20-E, flew from the agency’s base in Oberpfaffenhofen, Germany, meeting at multiple points over the Mediterranean and southern France. The research aircraft was equipped with instruments to assess exhaust gases, volatile and non-volatile aerosol particles, and contrail ice particles produced by the A350, which it followed at various distances to capture data on emissions and condensation trails produced by both conventional Jet A-1 and Neste’s SAF.  

DLR used global climate model simulations to estimate how contrails could change the energy balance in the earth’s atmosphere, an effect known as radiative forcing. The tests revealed a 26% reduction in the overall climate impact of contrails when 100% SAF was used.

“The results from the ECLIF3 flight experiments show how the use of 100% SAF can help us to significantly reduce the climate-warming effect of contrails, in addition to lowering the carbon footprint of flying – a clear sign of the effectiveness of SAF towards climate-compatible aviation,” explained Markus Fischer, DLR Divisional Board Member for Aeronautics.

“We already knew that sustainable aviation fuels could reduce the carbon footprint of aviation,” added Mark Bentall, head of Research and Technology Programme, Airbus. “Thanks to the ECLIF studies, we now know that SAF can also reduce soot emissions and ice particulate formation that we see as contrails. This is a very encouraging result, based on science, which shows just how crucial sustainable aviation fuels are for decarbonising air transport.”

Alexander Kueper, Neste’s VP, Renewable Aviation Business, said SAF was recognised widely as a crucial means of mitigating the climate impacts of aviation. “The results from the ECLIF3 study confirm a significantly lower climate impact when using 100% SAF due to the lack of aromatics in Neste’s SAF used, and provide additional scientific data to support the use of SAF at higher concentrations than the currently approved 50%.”

And Rolls-Royce’s Director of Research and Technology, Alan Newby, said the use of SAF at higher blend ratios would be a key factor in enabling aviation to achieve its target of net zero CO2 emissions by 2050. “Not only did these tests show that our Trent XWB-84 engine can run on 100% SAF,” he said. “The results also show how additional value can be unlocked from SAF through reducing non-CO2 climate effects as well.”

The ECLIF3 research team says its programme is “the first in-situ evidence of the climate impact mitigation potential of using pure, 100% SAF on a commercial aircraft,” and has reported the findings of its tests in the Copernicus journal Atmospheric Chemistry and Physics (ACP) as part of a peer-reviewed scientific process. The ECLIF3 programme also includes members of the National Research Council of Canada and the University of Manchester.

Last year, Boeing, NASA and United Airlines conducted contrail research in the US using a Boeing 737 MAX twinjet with one engine powered by 100% SAF and the other by conventional jet fuel, and Virgin Atlantic partnered with Rolls Royce, Boeing, Air BP and Virent to perform a trans-Atlantic crossing using a Boeing 787 powered purely by SAF, with a blend of 88% recycled waste fats and oils and 12% sustainable aromatic kerosene. UK-based technology group SATAVIA also worked with 12 airlines over 65 flights to test its DECISIONX route optimisation software, which uses atmospheric modelling data to assist carriers in planning flight paths which avoid conditions in which contrails can form. The company said its tests avoided more than 2,200 tonnes of carbon dioxide equivalent (CO2e), or an average of more than 40 tonnes per flight, with little impact on aircraft fuel burn or flight distances.

Photo (S. Ramadier): Airbus A350 fuelled with 100% Neste SAF trailed by DLR’s Falcon 20-E research jet

Tony Harrington

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Rolls-Royce and European low-cost carrier easyJet have performed the first-ever operation of a prototype aircraft engine powered by green hydrogen. The ground test, conducted at a military aircraft testing site in Boscombe Down, England, was hailed by the companies as “a new aviation milestone” and a major step towards the introduction of zero emission hydrogen propulsion systems for aircraft. The test was conducted using a converted Rolls-Royce AE 2100-A engine and followed the recent establishment by Rolls-Royce and easyJet of a partnership to research hydrogen propulsion for aircraft such as the Airbus A320-family of narrowbody jets operated by the airline. The engine test also coincided with other initiatives designed to progress hydrogen-powered aviation. In Hamburg, Lufthansa Technik has just converted a decommissioned A320 to test ground processes for future hydrogen-powered aircraft, while hydrogen propulsion company ZeroAvia has partnered with the UK’s AGS Airports to investigate hydrogen fuelling infrastructure. 

The engine used for the Rolls-Royce and easyJet test was a modified version of a powerplant typically used by high-speed turboprop aircraft, including the SAAB 2000 regional airliner and the Lockheed C130J military transporter. The companies are now planning more rig tests, ahead of a full-scale ground trial using a Rolls-Royce Pearl 15 jet engine, a new powerplant designed to extend the range of Bombardier Global 5500 and 6500 corporate jets. The longer-term aim is to undertake flight tests and eventually to develop hydrogen engines for larger planes. Green hydrogen for the Boscombe Down test was provided by the European Marine Energy Centre and generated by wind and tidal power at its test facility on Eday, part of the Orkney Islands that lie north of the Scottish mainland.

Grazia Vittadini, Chief Technology Officer, Rolls-Royce, described the engine test as “an incredible start” to the new partnership with easyJet.  “The success of this hydrogen test is an exciting milestone. We are pushing the boundaries to discover the zero carbon possibilities of hydrogen, which could help reshape the future of flight,” she said.

Johan Lundgren, easyJet’s CEO, said his airline was committed to supporting the research “because hydrogen offers great possibilities for a range of aircraft, including easyJet-sized aircraft. That will be a huge step forward in meeting the challenge of net zero by 2050.”

Both organisations have signed up to the UN-backed Race to Zero campaign that commits them to achieve the net zero carbon emissions target.

In Hamburg, an Airbus A320 operated by Lufthansa for 30 years has been converted into the Hydrogen Aviation Lab (HAL), a mobile laboratory designed to test maintenance and ground handling processes for future aircraft powered by hydrogen. The initiative is a collaboration between Lufthansa Technik, which has converted the jet into a research platform, Hamburg Airport, an early adopter of low-or-no carbon practices, and two major research groups, the German Aerospace Center (DLR) and Hamburg’s ZAL Centre for Applied Aeronautical Research. It was funded by Hamburg’s Ministry of Economic Affairs and Innovation and IFB Hamburg, the city’s investment and development bank.

While this particular jet will never fly again, it will be equipped in coming months with test systems, an internal tank for liquid hydrogen and an onboard fuel cell compatible with ground-based hydrogen infrastructure, to help prepare both airlines and airports for new zero-emission aircraft. The testbed plane will be towed between the Lufthansa Technik base and locations on the airport as part of the study of future ground management processes. Research will include integration of hydrogen fuel systems into existing airport infrastructure, safe and efficient refuelling of aircraft with liquid hydrogen, cooling and insulation of the fuel, and inert storage of hydrogen.

“We’ve enabled a unique project,” said Michael Westhagemann, Hamburg’s Senator for Economic Affairs. “It will make a valuable contribution to enabling the use of hydrogen as a fuel for aviation. The focus on maintenance and refuelling procedures should provide us with insights that will be important for developing hydrogen infrastructure. This real-world lab lets us add a crucial building block to Hamburg’s strategy to make aviation more sustainable. We are following two strategic goals – the development of a hydrogen economy in Hamburg and the decarbonisation of the mobility industries. We are very pleased to make this world-first project possible through the Special Aviation Fund.”

The Hydrogen Aviation Lab jet will also be used for research into predictive maintenance methods for future generations of aircraft, with a ‘digital twin’ of the decommissioned A320 to be used to help predict failures of hydrogen components and systems, and enable timely responses. 

In another research project, aero-hydrogen propulsion pioneer ZeroAvia has partnered with AGS Airports, which owns and operates Aberdeen, Glasgow and Southampton airports in the UK, to investigate the development of hydrogen fuel infrastructure, regulatory requirements and resources needed to deliver zero-emission flights. Their focus will be on short-haul hydrogen-powered flights from Aberdeen and Glasgow.

“In recent months, we have stepped up our work with airports significantly to better understand the operational needs and requirements for hydrogen as a fuel,” said Arnab Chatterjee, ZeroAvia’s VP Infrastructure. “Working with the team at AGS allows us to plan for some of the commercial routes that we will be able to support in a little over two years’ time, and to do so in the setting of a major international airport.”

The CEO of AGS Airports, Derek Provan, said the development of hydrogen-propulsion was becoming “an increasingly viable option” for regional and short-haul aircraft. “As a regional airport group serving the highlands and islands of Scotland as well as the Channel Islands from Southampton, AGS will be the perfect testbed for hydrogen flight,” he said. “Through our partnership with ZeroAvia we’ll address some of the challenges associated with the generation, delivery and storage of hydrogen on site, and how we can prepare our infrastructure to support zero emission flights.”

Photo: Ground testing of the converted Rolls-Royce AE 2100-A regional aircraft engine

The unprecedented heatwave sweeping the UK during the 2022 Farnborough International Airshow was a timely, if unwelcome, prod to the aviation sector that it must continue raising its game in the collective fight to mitigate the growing impact of global warming. Established and emerging aerospace players, from Airbus to ZeroAvia, used the biggest air show since the start of the pandemic to promote and progress deals, partnerships and initiatives designed to help deliver net zero emissions by 2050. In addition to more than 300 orders for new-technology aircraft, Farnborough showcased a range of developments on new propulsion systems and fuels, the growing trend to convert fossil-fuelled aircraft to zero emission power and continued strong growth in the urban air mobility sector, reports Tony Harrington. As countries met in Montreal to discuss a long-term target to reduce emissions from international aviation, the UK government released at the air show its eagerly-awaited Jet Zero Strategy to decarbonise the British aviation sector.

Having recently unveiled plans to use an A380 superjumbo as a testbed for its ZEROe hydrogen propulsion programme, Airbus announced it would convert a second A380, this time to be used in a collaboration with engine manufacturer CFM International to test new open-architecture powerplants. This engine technology, known as RISE (Revolutionary Innovation for Sustainable Engines), features large external fans which are expected to drive significant operating efficiencies and cut emissions by 20%.

Airbus UpNext, a subsidiary of the airframer, also announced a partnership with the German Aerospace Center (DLR) to study contrails created by hydrogen-powered engines. Through a new project called Blue Condor, two modified Arcus gliders will be deployed, one powered by a conventional kerosene combustion engine, the other hydrogen combustion. A chase aircraft will follow each of these craft to assess and compare their contrails at high altitude, in what will be the first in-flight tests by Airbus using a hydrogen engine.

To further support its hydrogen ambitions, Airbus has invested an undisclosed amount in Hy24, described as the world’s largest clean hydrogen infrastructure investment fund, focused on supporting large-scale green hydrogen infrastructure projects. “Since 2020, Airbus has partnered with numerous airlines, airports, energy providers and industry partners to develop a stepped approach to global hydrogen availability,” said Karine Guenan, VP ZEROe Ecosystem, Airbus. “Joining a fund of this magnitude demonstrates Airbus’ continuously active role in infrastructure investments for the production, storage and distribution of clean hydrogen worldwide.” 

Rolls-Royce and European low-cost airline easyJet also announced a hydrogen propulsion programme, the H2Zero Partnership, to jointly pioneer the development of hydrogen combustion engine technology suitable for a range of aircraft, including narrowbody airliners, from the mid-2030s. This collaboration, which combines Rolls-Royce’s engine expertise and easyJet’s operational experience, will start later this year with engine tests on the ground and ambitions by both companies to also progress to flight tests.

“In order to achieve net zero by 2050, we have always said that radical action is needed to address aviation’s climate impact,” said Johan Lundgren, CEO of easyJet. “The technology that emerges from this programme has the potential to power easyJet-size aircraft, which is why we will also be making a multi-million-pound investment into this programme. In order to achieve decarbonisation at scale, progress on the development of zero-emission technology for narrowbody aircraft is crucial. Together with Rolls-Royce, we look forward to leading the industry to tackle this challenge head-on.”

Boeing, which announced more than 200 aircraft orders at the show, has become a founding member of the University of Sheffield Energy Innovation Centre to explore various methods of producing sustainable aviation fuel, and bringing it to market. During the air show, the aircraft OEM revealed it was advancing its partnership with the University of Cambridge on the Aviation Impact Accelerator (AIA), an international group of practitioners and academics convened by the university. AIA develops interactive evidence-based models, simulations and visualisation tools for decision-makers and the wider engaged public to understand the pathways to net zero flight. The outcomes and key learnings will eventually be integrated into Boeing’s Cascade data modelling tool, which provides real-time visualisation of carbon emission reductions in aviation, and also announced during the show. The model assesses the full lifecycle impacts of renewable energy by accounting for the emissions required to produce, distribute and use alternative energy carriers such as hydrogen, electricity and SAF. Boeing said it plans to utilise the tool with airline operators, industry partners and policymakers to inform when, where and how different fuel sources intersect with new airplane designs.

The company also expanded a long-standing collaboration with Japan’s Mitsubishi Heavy industries to study electric and hydrogen propulsion, development of green hydrogen, new feedstocks and technologies for development of SAF, carbon capture and conversion, sustainable materials and new aircraft design concepts. As well, Boeing announced a $50 million investment in AEI HorizonX, a partnership it established with private equity group AE Industrial Partners to support transformative aerospace technologies.

“In order for the aviation industry to meet its net zero carbon emissions commitment by 2050 it will take all of us collaborating and investing in scientific research and testing,” said Boeing’s VP of Global Sustainability Policy, Brian Moran.

Boeing also announced a new partnership with Alder Fuels to expand production of SAF around the world. Using Boeing aircraft, the companies will test and qualify Alder-derived SAF, advance policies to expedite aviation’s energy transition.

Meanwhile, Virgin Atlantic, Corendon Dutch Airlines and Albawings have selected Boeing’s Jeppesen FliteDeck Advisor to optimise operational efficiency and reduce fuel consumption across their fleets of Boeing aircraft. During a three-month trial on its 787 Dreamliners, Virgin Atlantic found the digital solution delivered cruise fuel savings of 1.7%, saving around 1,900kg of CO2 per flight.

Hydrogen propulsion pioneer ZeroAvia secured an additional $30 million from new investors including Barclays Sustainable Impact Capital, NEOM, a sustainable regional development in Saudi Arabia, and the impact technology fund AENU, as well as additional capital from International Airlines Group, an existing investor and parent of airlines including British Airways, Iberia, Aer Lingus, Vueling and LEVEL. “Our new investors are each looking at our journey through a different lens,” said Val Miftakhov, founder and CEO of ZeroAvia, “but all energised by our mission to enable zero-emission flight using hydrogen-electric engines.”

ZeroAvia, Universal Hydrogen and Ampaire announced during the air show a total of 55 firm orders for kits to convert commuter or turboprop aircraft from fossil fuels to zero-emission electric or hydrogen propulsion, while Swiss aero-battery manufacturer H55 launched a partnership with Canadian training group CAE and Piper Aircraft to convert to battery-electric power two-thirds of CAE’s fleet of Piper Archer training aircraft. Ampaire also flagged in excess of 200 orders on the horizon for its Eco Caravan and Eco Otter aircraft, re-engined variants of the Cessna Caravan and De Havilland Twin Otter regional aircraft.

GKN Aerospace revealed during the show that advances in fuel cell technology could enable hydrogen-electric propulsion to be scaled up more quickly than previously thought. The company had assumed that hydrogen propulsion was easiest to introduce for aircraft seating around 19 passengers, but now believes the use of cryogenic cooling technology can expedite deployment of the technology to power aircraft seating 96 or even more passengers, and reducing both CO2 and non-CO2 emissions.

Norway’s Widerøe Zero, the sustainability arm of regional airline Widerøe, signed a MoU with Embraer to help develop the airframer’s new Energia family of zero emission aircraft, with four variants ranging from 19 to 50 seats, while Collins Aerospace has completed the preliminary design of a 1-megawatt motor and controller to power a hybrid-electric demonstrator aircraft for the engine manufacturer Pratt & Whitney Canada.

Collins and Pratt & Whitney also launched a new electric propulsion concept, the Scaleable Turboelectric Powertrain Technology demonstrator (STEP-tech), to power novel aircraft including high-speed electric vertical take-off or landing craft (eVTOL), unmanned aerial vehicles (UAV) and small-to-medium commercial aircraft, while new deals, developments and partnerships were announced in the eVTOL segment by companies including Germany’s Lilium, Embraer’s Eve, UK-based Vertical Aerospace, French start-up Ascendance Flight Technologies and a tie up between Rolls Royce and Hyundai Motor Group’s air taxi division, Supernal.

GE Aviation announced a milestone for its own electric engine programme, conducting the world’s first test of a hybrid-electric propulsion system in simulated high-altitude conditions. Using NASA’s Electric Aircraft Testbed (NEAT) in Sandusky, Ohio, GE assessed a pair of hybrid electric systems, one to simulate an aircraft’s left engine, the other its right engine, in conditions expected when flying at 45,000 feet. The test simulated the electrical loads needed to optimise engine performance, while propelling and powering an aircraft at that altitude.   

Mohamed Ali, VP and GM of Engineering for GE Aerospace, said: “We’re making aviation history by developing the technology to help make hybrid electric flight possible for everyday commercial air travel. We just passed a key milestone by successfully concluding the world’s first test of a high power, high voltage electric system at altitude conditions. This is one of many milestones in our journey with NASA towards demonstrating a hybrid electric aircraft engine system for a more sustainable future of flight.”

A small Spanish airline, AlbaStar, was identified at Farnborough as the European launch customer for the US-made WheelTug electric taxiing system, which enables aircraft to be manoeuvred around airports without using external tractors or their own engines. Using a small electric motor installed within the nosewheel, pilots can control all ground movements by their aircraft, including reversing from airport aerobridges. AlbaStar, which operates six Boeing 737 jets, estimates that in a year the WheelTug system could eliminate 1 million kilograms of CO2 and nitrogen oxide emissions from the airline’s operations. The WheelTug system is due to be introduced into service in mid-2023.

Image (Embraer): Norwegian airline Widerøe has signed a MoU with Embraer to help develop the airframer’s new Energia family of zero emission aircraft

SWISS and the Lufthansa Group have entered into a strategic collaboration with solar aviation fuel pioneer Synhelion, which will enable SWISS to become the first airline to use sun-to-liquid fuel. Synhelion has developed a key technology for producing sustainable aviation fuel using concentrated solar heat to manufacture syngas that can then be synthesised into kerosene using standard industrial processes. Last October, the company announced it had received funding worth €3.92 million ($4.3m) from the Energy Research Program of the German Federal Ministry for Economic Affairs and Energy, which will be used towards building the world’s first industrial plant for solar fuels in North Rhine-Westphalia, Germany. The facility will cover the entire process from concentrated sunlight to synthetic liquid fuel on an industrial scale, with the end products being solar kerosene and solar gasoline. SWISS is set to become the first customer for the solar kerosene in 2023 and under the collaboration will support the development of another commercial facility in Spain.

“Our team-up with Synhelion is founded on our shared vision to make carbon-neutral flying in regular flight operations possible through the use of solar fuel,” said SWISS CEO Dieter Vranckx. “In partnering with them, we are supporting Swiss innovation and are actively pursuing and promoting the development, the market introduction and the scaling-up of this highly promising technology for producing sustainable fuels.”

Together with the Lufthansa Group and sister airline Edelweiss, SWISS has been working with Synhelion on solar fuels since 2020. The airline said it would be substantially increasing its use of SAF in the next few years to help achieve its climate objectives, although it acknowledges that in view of the limited availability of biofuels, alternatives will be required.

“This is why we are actively supporting the development of solar fuels,” said Vranckx. “We want to be a pioneer in their use, so our involvement with Synhelion is a key element in our long-term sustainability strategy.”

Synhelion evolved from the Swiss Federal Institute of Technology (ETH Zurich) in 2016 as a clean energy company with an aim to decarbonise transportation. In 2019, it demonstrated the feasibility of its technology based on process heat from concentrated sunlight under real operating conditions in a small pilot plant with ETH Zurich. The following year, it tested a second prototype with artificial sunlight at the German Aerospace Centre’s (DLR) Synlight facility and in 2021 partnered with consulting and engineering company Wood on a test facility for the production of syngas, which was set up on DLR’s solar tower in Jülich, North Rhine-Westphalia, to demonstrate the technology on an industrial scale.

The project to build the industrial production plant at Brainergy Park Jülich is being carried out by Synhelion Germany, DLR and the Solar Institute Jülich of Aachen University of Applied Sciences. Synhelion Germany was formed last year following the acquisition of Heliokon, an expert in concentrated solar power founded in 2016 and a spin-off from DLR. In addition to the German government funding, Synhelion raised a further 16 million Swiss francs ($17.4m) in a Series B funding round last November. A paper by members of Synhelion, ‘Drop-in fuels from sunlight and air’, was published in the journal Nature the same month.

“We believe in a globalised world connected by climate-friendly mobility,” commented Dr Philipp Furler, Synhelion’s co-founder and CEO. “Our next-generation carbon-neutral solar kerosene is an economically and ecologically viable substitute for fossil fuels. The commitment of SWISS and the Lufthansa Group underlines the aviation sector’s keen interest in our solar fuel.”

Top photo: The Very High Concentration Solar Tower of IMDEA Energy was built as part of the EU’s Horizon 2020 ‘Sun-to-Liquid’ project. It is located in Móstoles, a suburb of Madrid, Spain. Synhelion has been renting this facility, featuring a 1,000m² solar field, to develop and test its solar fuel technology on a medium scale. (source: Synhelion)

MRO and technical aircraft services provider Lufthansa Technik has joined with Hamburg Airport in an initiative to design and test maintenance and ground processes for handling liquid hydrogen (LH2) that is expected to be used in powering future aircraft. The two-year research project, which has received funding from the city of Hamburg, will also involve the German Aerospace Center (DLR) and the Center for Applied Aeronautical Research (ZAL). As part of the project, a decommissioned Airbus A320 aircraft will be converted into a stationary, fully functional field laboratory equipped with LH2 infrastructure at Lufthansa Technik’s base at the airport. In the first phase, due to be completed by the end of this year, the partners will identify areas for developing and elaborating the concept for subsequent testing with the aim next year to jointly implement a pioneering demonstrator to be operational by 2022. Elsewhere in Germany, start-ups Deutsche Aircraft and H2FLY have agreed to work together on developing hydrogen fuel cell technology for commercial regional aircraft.

As well as contributing to the Hamburg project operational expertise in the maintenance and modification of commercial aircraft, Lufthansa Technik says it is also able to incorporate a customer perspective through its airline customers. In parallel, DLR will be creating a virtual environment to achieve digital and highly accurate mapping of the defined fields of development and says the platform will aim to provide inspiration for the next generation of aircraft through parameterised and highly accurate virtual models. DLR adds that it has long-standing and cross-sector experience with hydrogen.

“The aircraft of the future are lighter, more efficient and fly with alternative propulsion concepts, and hydrogen will play an important role in this. We need to learn – promptly and in detail – the ground requirements for aircraft and their maintenance of real-world operation with hydrogen,” explained Dr Markus Fischer, DLR Deputy Board Member Aeronautics. “In the project, we are using this data and experience to develop digital models for ground processes that can then be used directly in the design of future-oriented and yet practical aircraft configurations.”

Hamburg Airport CEO Michael Eggenschwiler said climate-friendly flying with hydrogen technology was only possible through a perfect fit with ground infrastructure. “Close coordination is required here and we as an airport are pleased to be able to contribute our know-how to this important project, from questions over storage and distribution to the refuelling process on the apron.

“At the airport, we also rely on hydrogen as a technology of the future for our ground transport. This project offers us the chance to identify and make the best possible use of synergy effects between gaseous hydrogen, such as that used for refuelling our baggage tractors, and liquid hydrogen for aircraft refuelling.”

ZAL, meanwhile, will provide its know-how in fuel cell technology and digital process mapping. “The development of a field laboratory and a digital twin are important components of Hamburg’s Green Aviation Technology Roadmap,” said ZAL CEO Roland Gerhards. “They were developed together with the members of the Hamburg Aviation Cluster last year to strengthen Hamburg’s competence in research and development in a European context.”

The project is the largest single initiative in a City of Hamburg’s special programme to mitigate the economic impact of the coronavirus pandemic on the aviation industry.

“Hamburg is not just one of the three largest aviation clusters in the world – last year the city also developed a clear vision of becoming a major hydrogen metropolis,” said Michael Westhagemann, the city’s Senator for Economics and Innovation. “The port, the energy sector, industry and the entire mobility sector are involved and are preparing for this groundbreaking technology.

“With this project, we are now also making an essential contribution to the transformation of aviation into a climate-neutral mobility solution of the future. The clear goal is to build up a hydrogen economy in Hamburg that will occupy a leading position internationally.”

Responded Lufthansa Technik CEO Dr Johannes Bussmann: “I am very grateful for the foresight of the city of Hamburg and its generous funding of this project.

“There is no alternative to the transformation of our industry towards climate-neutral aviation. With this project, we want to tackle this enormous technological challenge at an early stage – for the entire MRO industry as well as for us. We are building up know-how today for the maintenance and ground processes of tomorrow.”

Meanwhile, the partnership between German start-up H2FLY, which is developing hydrogen fuel cell systems for aircraft, and a new German aircraft manufacturer Deutsche Aircraft will aim to convert a Dornier 328 aircraft for a first hydrogen flight in 2025.

The programme is expected to validate climate neutral regional air travel with up to 40 seats and the teams are planning to equip the demonstrator with a 1.5MW hydrogen system which they say will make it the most powerful hydrogen-electric powered aircraft to date. The companies will work together on integrating the power system into the aircraft as well as defining the specific technical and certification requirements for fuel cell systems in EASA’s large aircraft class.

H2FLY grew out of a partnership between DLR and the University of Ulm and its four-seater hydrogen-electric powered HY4 has already undertaken over 70 take-offs in flight campaigns. With its range of up to 750km, the company believes regional markets can be developed.

“Hydrogen fuel cell technology provides an opportunity for us to completely eliminate carbon and NOx emissions from regional flights and the technology to make that happen is closer than most people think,” said Prof Dr Josef Kallo, co-founder and CEO of H2FLY.

“Over the last 16 years we have worked hard to demonstrate our technology on smaller aircraft, completing record breaking flights based on six powertrain generations. We are pleased to be taking that to the next level with Deutsche Aircraft as we scale our efforts up to regional aircraft.”

Deutsche Aircraft says it is putting climate change at the heart of its design philosophy. “We are looking forward to partner with companies that not only share our passion for the environment but also have the technical expertise to ensure climate-optimised aviation stays safe and reliable,” said Martin Nüsseler, Deutsche Aircraft’s CTO.

The company says the higher propulsive efficiency of propeller-powered aircraft will drive a change in propulsion technology that will result in further reductions in fuel consumption and emissions.

“Combining modern propeller aircraft design with zero carbon energy sources is central to achieving climate-neutral air transportation,” added Nüsseler.

Commenting on the project, Thomas Jarzombek, Member of the German Bundestag and Coordinator of the Federal Government for German Aerospace, said: “From 2035 onward, hybrid-electric flying has to be the new standard in Germany. The German government will continue to support this path to innovation with its R&D funding programme, aiming to let the vision of zero-emission aircraft become a reality.”

Top image: Airbus

The aviation sector’s efforts to mitigate anthropogenic climate change have predominantly focused on CO2 emissions. However, the non-CO2 effects created by aircraft flying at cruise altitude have been found to be larger than the impact from CO2 alone, and estimated at two thirds of the net radiative forcing from aviation, but this has been largely unaddressed by the airline industry and policymakers. This is mainly due to uncertainties over the scientific understanding through a lack of reliable data and how best to overcome the effects through aircraft operations, as well the inability to agree on what policy measures should be introduced to deal with the issue. Susan van Dyk reports on a recent conference hosted by the Royal Aeronautical Society’s Greener by Design Specialist Group, in collaboration with German aviation research organisation DLR, which presented the latest scientific studies on non-CO2 impacts and potential strategies and policies for mitigation.

Non-CO2 climate effects come from NOx emissions, soot from fuel combustion, water vapour and formation of persistent contrails (contrail cirrus) which contribute to increased cloudiness. Unlike CO2 impacts, non-CO2 effects depend on the location and time of emissions and this plays a role in selecting suitable mitigation steps. The climate impact of aviation from non-CO2 effects can be mitigated through various measures, including new engine technologies to reduce emissions, use of sustainable aviation fuel and air traffic management (ATM) strategies.

Contrail cirrus formation has the largest positive net (warming) effective radiative forcing (ERF) effect, followed by CO2 and NOx emissions. Contrail cirrus alone contributes >50% of the net ERF impact and is therefore an important target for mitigation of non-CO2 effects. Contrails are formed by aircraft through condensation of the water vapour from fuel combustion. While most contrails disappear relatively quickly, persistent contrails form cirrus clouds under certain temperature and humidity conditions. The cirrus clouds have some cooling effect by reflecting the sunlight, but also has a warming effect by trapping heat radiating from the earth’s surface.

Mitigation through flight diversion aims at avoidance of ice supersaturated regions (ISSR) where contrail cirrus formation has the greatest impact. The net warming effect of contrail cirrus is also greatest between 3pm until 6am and flight diversion is focused on this time period. Contrail avoidance through diversion of flights and ATM measures could represent a fast way (hours or days) to reduce the impact of aviation on the earth radiation budget.

Flight diversion and contrail avoidance is the current focus of live trials in the Maastricht Upper Area Control (MUAC). Dr Rudiger Ehrmanntraut, Senior Project Manager at Eurocontrol, explained that vertical diversion, 2000 ft above the ISSR or 2000 ft below the ISSR during the time period of 4pm to 6am, is the main approach during this project to avoid persistent contrail formation. A crucial prerequisite, he said, is the accurate prediction of ISSRs and potential areas of persistent contrail formation using meteorological data.

Dr Marc Stettler from Imperial College London has been carrying out research on the extent of flight diversion that will be required. His work has demonstrated that only 2.2% of flights account for 80% of the total net energy forcing due to contrails. Therefore, total fleet diversion is not necessary to achieve a significant impact, he argued. By diversion of 1.7% of flights, contrail radiative forcing can be reduced by 59%. If flight diversion is used simultaneously with cleaner engines, the contrail radiative forcing can be reduced by 92% (which amounts to 57% of the total radiative forcing).

However, flight diversion increases fuel consumption and CO2 emissions. Professor Ian Poll from Cranfield University quantified this fuel penalty for avoiding ISSRs and concluded the overall extra fuel burn was insignificant. If an aircraft is diverted 2000 ft above the ISSR, there is a 1.5% increase in fuel, and 1.4% when diverted 2000 ft below. However, this fuel penalty is only for the duration of the diversion, not the whole flight. A reduction in altitude results in a reduction in NOx emissions as an added benefit.

A significant contributor to formation of persistent contrails is the creation of soot as a result of fuel combustion. Studies at DLR by Professor Christiane Voigt’s research group show that 80% of soot particles form ice particles, which leads to persistent contrails. Lower soot formation will impact contrails and contrail cirrus formation and reduce some of the climate impacts of non-CO2 emissions. Reduction of soot particles causes a non-linear decrease in radiative forcing and it has been demonstrated that a 50-70% reduction in ice particles leads to 20-40% reduction in contrail radiative forcing. According to Voigt’s research, the main cause of soot formation is the aromatics in jet fuel, so reducing soot emissions can be achieved by decreasing the aromatic content. She said an effective way of reducing the aromatic content is through the use of sustainable aviation fuel (SAF). Currently available SAF, based on hydrotreatment of fats, oils and greases (called HEFA), is almost entirely composed of paraffins and cycloparaffins with virtually zero aromatics.

ASTM jet fuel specifications require aromatics at a minimum of 8.5% by volume and a maximum of 25%. A minimum aromatic content is considered essential to maintain seal integrity in the engine and aircraft and prevent seals from leaking. Current certification under ASTM D7566 only permits a 50% blend of HEFA with conventional fossil jet fuel and the blend must contain at least 8.5% aromatic content by volume. An earlier study, published in Nature in 2017, was carried out with a 50% blend of HEFA and demonstrated a reduction in the particle number and mass emissions immediately behind the aircraft by 50-70%. It concluded the effect was mainly due to reduced aromatics content and reduced soot particle formation.

Recent research as part of the ECLIF3/ND-MAX campaign, a collaboration between DLR and NASA, has also confirmed a strong reduction in soot emission indices by using low aromatic fuels. According to Voigt, her research has shown naphthalenes are more efficient soot precursors compared to monocyclic aromatics and decreased naphthalene content decreased the soot formation. She emphasised that stronger reductions in aromatics in jet fuels are needed to reduce persistent contrails and this will require changes in certification standards to permit fuel with lower than 8.5% aromatics. In addition, certification should also permit the use of 100% alternative jet fuels rather than the current maximum of 50%.

Rolls-Royce and Airbus recently announced what they say is the world’s first in-flight emissions study using 100% sustainable aviation fuel (see article). The ongoing Emission and Climate Impact of Alternative Fuels (ECLIF) project studies the impact of 100% SAF on aircraft emissions and performance. Paul Madden, Energy Emissions Expert from Rolls-Royce, told the conference there had been no operability issues or seal leakage problems as a result of using 100% SAF in the aero engine manufacturer’s engines, although he conceded this may not be the case in older engines.

Madden also explained some of the modified engine operations that Rolls-Royce is implementing, such as the lean burn engine, which can improve NOx emissions and soot formation. NOx emissions are not reduced through the use of SAF, and cleaner burning engines will be the main approach for mitigation.

Stephen Arrowsmith, Chief Expert on Environmental Protection at the European Aviation Safety Agency (EASA), highlighted the potential policy measures that could be implemented to address non-CO2 impacts as published in a recent EASA report (see article). The EU Emissions Trading System (EU ETS) currently only addresses CO2 emissions through policy.

The report divided policy options into three categories: financial/market-related, fuel and ATM. Financial measures suggested include a monetary charge levied on aircraft NOx emissions on one side and/or the inclusion of such emissions under the EU ETS on the other. Potential fuel-related measures could encompass the reduction of aromatics within fuel – leading to cleaner fuel burn and reduced nvPM emissions – and the mandatory use SAF. Both measures would target soot emissions and associated formation of contrail cirrus clouds.

Proposed measures in the ATM category are avoidance of ice-supersaturated areas and a climate charge. Flight diversions to avoid ice supersaturated areas would reduce the formation of contrail-cirrus clouds, while a climate charge would address all non-CO2 effects (NOx, water vapour, soot, sulphates, contrails). Several research issues would have to be addressed before these measures could be implemented, said Arrowsmith. Fuel-related measures could potentially be implemented in the short term (2-5 years), while other measures will likely take longer to implement (5-8 years) due to outstanding research questions, he believes.

The conference succeeded in highlighting the important, but somewhat overlooked, climate impact of non-CO2 effects from aviation. It demonstrated this impact can be significantly mitigated through modified air traffic management and use of sustainable aviation fuels with low aromatic content.

Photo: NASA’s ‘airborne laboratory’ flies close behind the DLR A320 Advanced Technology Research Aircraft (ATRA), flying through the Airbus’ exhaust plume. On board, scientists measure the composition of the exhaust stream and analyse the effects of biofuels like HEFA on the formation of soot particles and ice crystals (credit: DLR/NASA/Friz)

About the author

Susan van Dyk

A Research Associate at the University of British Columbia in Canada, Susan is a scientist and independent researcher specialising in biofuels and renewable energy, with a particular focus on sustainable aviation fuels. She has published numerous peer-reviewed papers and been the author of multiple reports on drop-in biofuels and SAF. She has worked on a number of Canadian biojet initiatives. Susan can be contacted through her website https://www.svdconsulting.ca/

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A European project to understand the emissions performance of sustainable aviation fuel (SAF) when unblended with fossil jet fuel has started with the first clearance test flight of a Rolls-Royce Trent XWB-powered Airbus A350 aircraft. One of the two engines was fuelled with an unblended SAF mixture made from hydroprocessed esters and fatty acids (HEFA) produced in the EU and supplied by Neste. The test flight is the first in a series scheduled this month to analyse the safety of 100% SAF and next month a German Aerospace Center (DLR) Falcon 20E ‘chase aircraft’ equipped with sensors will follow 50 metres behind the A350 test aircraft to measure the emissions directly from the SAF-fuelled engine exhaust. Although SAF has demonstrated its capability to reduce CO2 emissions, little research has been carried out on other aircraft engine emissions and the ‘Emission and Climate Impact of Alternative Fuels’ (ECLIF3) project will measure other climate-relevant emissions and assess the volume and consistency of contrails. The project will also carry out ground testing to measure particulate emissions in local airport environments. Both Airbus and Boeing are keen to see the current maximum 50% SAF blending limit raised.

“SAF is one of the aviation industry’s best low-carbon solutions with an immediate impact on CO2 emissions today,” said Steven Le Moing, Airbus New Energy Programme Manager. “This research project will help us to better understand the impact of unblended SAF on the full scope of aircraft emissions, while supporting SAF’s future certification for blends that exceed today’s maximum of 50%.”

Airbus Flight Test Engineer Emiliano Requena Esteban reported the first test flight of its kind to use a commercial passenger jet went “exceptionally well” and had shown no perceptible difference in engine behaviour between jet fuel and SAF.

Other emissions from aircraft engines to be measured by the study include carbon monoxide, nitrogen dioxide, water vapour, soot and aerosol and sulphate aerosol particles. Additional measurement and analysis for the characterisation of the particulate-matter emissions during the ground testing will be delivered by the UK’s University of Manchester and the National Research Council of Canada.

“Decarbonising aviation is not just about reducing CO2 emissions,” said Le Moing. “At Airbus, our priority is to deal with the complete climate-impact challenge, which includes overall greenhouse gases and other aircraft emissions. Our decarbonisation plan focuses on accelerating technology development to this end, in complement to a dynamic deployment of SAF.”

The flight and ground tests will compare findings from the unblended HEFA fuel against those of standard kerosene and low-sulphur kerosene. Neste has supplied 117 tonnes of neat SAF for the entire test campaign, which was made from EU-sourced used cooking oil. The SAF refining process was carried out at Neste’s Finnish biorefinery in Porvoo, transported by ship to Rotterdam for the final processing step and then brought by truck to Toulouse.

“We’re delighted to contribute to this project to measure the extensive benefits of SAF compared with fossil jet fuel and provide the data to support the use of SAF at higher concentrations than 50%,” said Jonathan Wood, Neste’s VP Europe, Renewable Aviation. “Independently verified analysis has shown 100% Neste MY Sustainable Aviation Fuel delivering up to 80% reduction in GHG emissions when all life-cycle emissions are taken into account. This study will clarify the additional benefits from the use of SAF.”

DLR has previously conducted research on analytics and modelling, as well as performing ground and flight tests using alternative fuels with its Airbus A320 ATRA research aircraft in 2015 and 2018, together with NASA.

“By investigating 100% SAF, we are taking our research on fuel design and aviation climate impact to a new level,” said Dr Patrick Le Clercq, ECLIF Project Manager at DLR. “In previous research campaigns, we were already able to demonstrate the soot reduction potential of between 30 and 50% blends of alternative fuels and we hope this new campaign will show this potential is now even higher.”

Added Simon Burr, Director, Product Development and Technology, Rolls-Royce Civil Aerospace: “In our post-Covid-19 world, people will want to connect again but do so sustainably. For long-distance travel, we know this will involve the use of gas turbines for decades to come. SAF is essential to the decarbonisation of that travel and we actively support the ramp-up of its availability to the aviation industry. This research is essential to support our commitment to understanding and enabling the use of 100% SAF as a low-emissions solution.”

Initial results from the ground and flight tests are expected later in 2021 and more complete results in 2022.

In January, Boeing said raising the SAF blending limit was necessary for the industry to achieve its long-term reduction goals and pledged to ensure its aircraft were able to fly on 100% SAF by 2030 (see article).

Top photo (S. Ramadier): The Airbus A350 test aircraft is refuelled with 100% SAF before its first clearance test flight. Video below shows the refuelling and the aircraft taking off.