Flying to a green future

Introduction

The air travel industry aims to be climate-neutral by 2050. While one crucial step along the way to ‘green flight’ is to get alternatives to fossil fuels ready for market, that alone will not be enough: A 360-degree approach is needed which also encompasses airport infrastructures – and which factors in the production of electricity from renewable sources.

Aviation has been growing unchecked for several decades and is believed to be one of the drivers of climate change. Even after the collapse triggered by the coronavirus pandemic, experts expect passenger and freight volumes to return to a constant uptrend in the long term. In 2020, one international study involving the German Aerospace Center (DLR) concluded that the air travel industry accounts for 3.5 percent of anthropogenic global warming. Seen in this light, it is obvious that the industry must play its part in reaching climate policy targets. Work is currently underway on many and varied ways to do so: There are new aircraft designs to reduce fuel consumption, climate-optimised flight paths, sustainable aviation fuels (SAFs) and battery electric and hydrogen-based propulsion systems, to name but a few examples.

Initial installation for solar fuel

As things stand, the fastest way to a green future appears to lie in the direction of SAFs, and a variety of source materials and methods present themselves as candidates for production – from biokerosene to synthetic power-to-liquid fuels. The latter can be produced from renewable energy sources, hydrogen, the CO₂ in the air and/or industrial exhaust gases. The advantage? Existing aircraft can run on these sustainable fuels without the need for technical refitting. On the downside, however, the required quantities of SAFs are not yet available, although pilot projects are up and running at various locations. One of these is being implemented by Swiss start-up Synhelion in the German city of Jülich, where work on building the world’s first solar fuel production plant recently commenced. The method developed by Synhelion uses concentrated sunlight to produce carbon-neutral kerosene. A field array of movable mirrors captures sunlight and reflects it as a bundled ray to a tower fitted with a thermochemical reactor. Here, at temperatures of over 1,000 °C, the solar fuel is generated from water and from both methane and CO₂ from the organic waste from a local paper mill. The initial aim is to produce several thousand litres a year. Future installations of this size in sunnier regions will be able to churn out as much as 150,000 litres per year. The Lufthansa Group has reached a strategic partnership agreement with the start-up and plans to fly the aircraft of its subsidiary Swiss with synthetic kerosene as of this year.

It will take longer than that before green hydrogen can be used as jet fuel. Extensive research and development work is still needed both to convert hydrogen to power in fuel cells for electric motors and to modify aircraft engines so that they can be powered by hydrogen directly. Aircraft manufacturer Airbus has announced that the world’s first emission-free commercial airliner will take wing by 2035 and is backing hydrogen as its primary source of energy. One of the biggest challenges is that liquid hydrogen has to be stored at minus 253 °C. It also needs very well insulated tanks and a lot of space, as it has a larger volume than kerosene. Right now, the company is working on three concepts, each based on differing strategies and designed for varying passenger numbers and flying ranges. To try out this technology, a converted A380 fitted with four hydrogen tanks and a test engine in the tail should take to the air as of the end of 2026. At the same time, Airbus wants to work with gas producer Air Liquide and airport operator VINCI Airports to develop the infrastructure needed to (re)fuel aircraft with hydrogen. The pilot site is Lyon Saint-Exupéry Airport in France

The DLR Institute of Solar Research operates two solar towers in Jülich as well as a field of around ten hectares with more than 2,000 movable mirrors called heliostats. With these, sunlight is concentrated and directed towards the towers for fuel production.

The close-up shot shows the solar receiver by Synhelion, which provides the necessary process heat for the production of solar fuels.

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60,000 solar panels at Groningen Airport

The people in charge of Groningen’s Eelde Airport in the Netherlands were quick to grasp that air travel can only become greener if airports make the necessary infrastructure available. Right in the middle of the airport, a solar park comprising more than 60,000 panels has therefore been set up, producing electricity that flows into the public grid. Additional solar modules on the roof of an arcade generate the power needed for the airport’s own operations. Looking ahead, the same solar power could also be used to produce synthetic kerosene or charge e-aircraft. Moreover, the airport is also currently working on a project to produce green hydrogen that will in future supply ground equipment which currently runs on fossil fuels. Groningen’s Eelde Airport sees itself as a testing ground and wants to show that sustainable energy can be generated even in a challenging environment subject to strict security requirements. Such commitment is desperately needed: In the shape of the “Toulouse Declaration”, governments and the aviation industry in early 2022 set themselves the goal of decarbonizing air travel in Europe by 2050 and reducing net CO₂ emissions to zero.

There will never be a silver bullet, a single solution that solves every problem, as Professor Rolf Henke well knows: “Some years ago, the industry was still heavily focused on electric propulsion”. Henke spent numerous years in his career at Airbus before making a transition in 2006 to join the Technical University of Aachen as a professor specializing in aerospace engineering. “Now, more alternatives are opening up, and each one will play its part and find its niche,” the aviation expert reports.

On the subject of electric flight, Henke expects batteries to continue to improve, such that the first small e-aircraft could go into short-haul service before the end of the decade. Aircraft with fuel cells will take a few years longer. Research is needed most around the storage of the cryotanks that will be needed for direct hydrogen propulsion and for fuel management. “When you are taking technologies that bring so much change to the aircraft to maturity, the main thing is to have all actors pulling in the same general direction.” Henke himself stresses that sustainable fuels are the way to achieve the fastest progress – provided they can be made available in sufficient quantities. “SAFs will definitely be part of the solution," the professor says. “But before the coronavirus crisis, global aviation was consuming a billion litres of kerosene a day. So, we would need to build refineries for synthetic fuels on a huge scale to even come close to meeting demand.” Henke is working on the assumption that, in the future, a toolbox with different options will be available: “Every aircraft will then get the propulsion and energy storage systems that it needs to optimise operations. The airports too will have to adapt accordingly by providing the appropriate infrastructure. And along the way, they must make sure that they too become climate neutral.

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With the output of the solar modules at Groningen Airport Eelde, approximately 6,200 households can currently be supplied with electricity per year.

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Roadmap to emission-free airports

Over the next three years, the German Aerospace Center (DLR) will be exploring the best way to make this work under the aegis of the THOR research project, whose objective is to develop a roadmap to emission-free airports while also serving as an interface to science and industry. “Airports have reached varying stages along the road to a greener future,” says Steffen Loth of the DLR Institute of Flight Guidance. “The fact is, everyone is now aware that this issue will become even more important in the future.” At the present time, it is impossible to say exactly what awaits the individual airports. “The aircraft manufacturers are currently looking at what types of propulsion should be deployed in the future. Certain requirements will probably be passed to the airports from these quarters. Airports could conceivably specialise by aligning their infrastructure with certain types of propulsion: “Step by step, we will see who implements what, and that will be very airport-specific.” How much energy will be needed when and where in complex airport systems? What will this mean for the infrastructure and the operating processes around aircraft, passenger and freight handling? Which solutions will deliver how much savings potential, and what interdependencies will exist? Not all these questions have clear answers as yet. “We need holistic projects such as THOR to achieve greater clarity,” Steffen Loth says. He also highlights the importance of testing the practical value of theoretical concepts at pilot airports, as is already being done in Groningen and Lyon. Professor Henke agrees that translating theory into practice is a critical factor if the green transformation is to succeed: “Whichever type of propulsion we are talking about, we need many more real-world laboratories, and it is high time we started doing airborne tests of the new technologies,” the expert insists. For that, he says, we need the political will, faster licensing procedures and a funding landscape to match: “An aviation authority along the lines of the German and European space agency would be an important step to get things moving in the right direction. If we want to make rapid progress, we need to do something different now.

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Fuel consumption of different types of airplanes

In recent decades, air travel emissions per seat and per 100 km have shrunk continually. Further reductions are promised by the DLR programs EXACT (Exploration of Electric Aircraft Concepts and Technologies) and KuuL (a play on the word “cool” derived from the German acronym for “climate-friendly ultra-efficient long-haul flight”). DLR’s stated aim is clear: to achieve emission-free air travel.

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Flying to a green future

Introduction

Initial installation for solar fuel

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60,000 solar panels at Groningen Airport

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Roadmap to emission-free airports

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60,000 solar panels at Groningen Airport

Roadmap to emission-free airports

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