Climate neutral aviation? Now boarding!

We are aiming for climate neutral aviation. And we believe it’s possible. What’s our drive? We know that climate change is real. And although aviation contributes a lot to welfare and wellbeing of people around the world, aviation contributes to climate change and if we don’t intervene this contribution will grow exponentially. Therefore, we need to take climate action in aviation and we need to take it now. We believe TU Delft is well positioned to come up with technological solutions that will make a significant contribution to a climate neutral aviation.  

Henri Werij 
Dean Faculty of Aerospace Engineering TU Delft

How do we work on making aviation sustainable?

Reduce energy consumption

We make aircraft and aircraft engines as efficient as possible. Our revolutionary Flying-V aircraft design for example aims at saving 20% on fuel consumption compared to state-of-the-art long-haul aircraft, like the Airbus A350, just as a result of improved aerodynamics and reduced weight. But lightweight aircraft materials and new systems for Air Traffic Management, such as a Single European Sky, so that aircraft can always fly the shortest distance possible, will also save energy.

Sustainable energy

We want aviation to run completely on a mix of sustainable energy and propulsion technologies. We for example work on electric and electric/hybrid aircraft. But also on the sustainable production of green aviation fuels, such as synthetic kerosene, LNG and hydrogen. By re-using CO2 for the production of green fuels, we make aviation CO2 neutral. All options need to be on the table. Sustainable fuels are better suited for long-haul flights and electric aircraft for shorter routes.

Sustainable aviation operations

This varies from sustainable airports – in healthy environments - to sustainability-focused Air Traffic Management systems and fleet management (we can fly more on electric aircraft if they make more stops on the way to recharge). We also study the effect of new aircraft and fuels on the earth’s atmosphere. By doing so we can optimize route and cruise altitude to minimize climate effects, and to greatly reduce air and noise pollution.


Sustainability Stories

Can ancient algae help replace chromium-6 in coatings?

Timelapse: corrosion protection of the letters ‘TUDelft’ What seemed like a wild idea in 2014, using the external skeletons of algae to prevent corrosion, has now been shown to provide long term protection of aluminium used in airplanes. In a few years’ time, it may provide a safe and environmentally-friendly replacement for the use of chromium-6. “Because of its toxicity, the European Commission has forbidden the use of chromium-6,” says Paul Denissen, PhD researcher in the Novel Aerospace Materials group at the faculty of Aerospace Engineering. “Use of chromium-6 is only still tolerated in situations where good alternatives are lacking, for example to protect airplanes against corrosion.” He explains that the aluminium alloy most used in aviation is especially susceptible to corrosion because of the copper that has been added to increase material strength. Typically, multiple boundary layers are applied to protect this aluminium against weathering. One of these layers is a primer coating loaded with chromium-6. “Our research focusses on using the external skeletons of a sort of algae to develop an environmentally friendly alternative for the use of chromium-6 in this layer.” Challenging chromium-6 Chromium-6 is a so-called active corrosion-inhibitor. When a treated surface is damaged, for example by scratching, the chromium-6 atoms will be released from the primer layer. They will create a thin layer of chromium oxide on the exposed metal surface, preventing further corrosion. After their release, chromium-6 atoms can continually redistribute themselves, providing continuous protection of the damaged area. “There are a number of alternative corrosion-inhibitors that are also very good at creating a protective barrier,” Denissen explains. “Unlike chromium-6, however, they can oxidize only once, and the protective layer they create is not permanent. Long-term protection therefore requires the continuous release of these inhibitors. More importantly, these alternative inhibitors may already chemically react with the primer coating at the time of its fabrication or application, thereby weakening their anti-corrosive power.” Quite some challenges to overcome, with a possible solution coming from the world of algae. Various shapes of the external skeleton of diatom algae Source : Pill-box protection Diatoms are a group of microalgae that have been roaming the earth for more than 100 million years. These single cell organisms come in various sizes, ranging from one to a few tens of micrometres. They have a hard, inorganic shell to protect them from the environment. This cell wall is made out of silica, the same material as glass, and contains many nanometre-sized pores. Inspired by the pill-box shape of these shells ( see figure ), it was Santiago Garcia, associate professor in the same group and the supervisor of Denissen, who came up with the idea to use them for active corrosion protection in coatings. Garcia explains that “my idea was to fill these shells with alternative corrosion-inhibitors, and then add these loaded shells to the primer coating. I envisioned the pill-box structure to prevent the unwanted chemical reaction between inhibitors and coating.” He also imagined the pores to allow the immediate and sustained release of these inhibitors when the protective layers are damaged, and the metal surface is exposed. “And these algae shells are easily available at low-cost,” Denissen adds. Rapid development Denissen explains that his 2015 master’s thesis was merely a feasibility study, to see if this approach could be successful. “Now, we are three years into my PhD and despite limited resources we have just shown corrosion protection potentially equalling that of chromium-6. We still use our first pick of algae shells, but we have substantially increased their filling with inhibitors as well as their release efficiency, leading to a much-improved protection.” 30-day protection by algae coating with corrosion-inhibitors Testing in Paris After intensive work in Delft to proof the concept, the researchers travelled to Paris for a challenging experiment. “We were curious as to the long-term protective power of our coating for large damages, as required by several companies,” Denissen says. Together with their collaborators from the group of Polina Volovich at Chimie ParisTech, they applied a 1 mm wide scratch to samples of aluminium used for airplanes, covered in a variety of their test ‘algae-coatings’. These samples were subsequently immersed in large volumes of a highly corrosive environment. The researchers got what they bargained for ( see figure ). “We were astounded,” continues Denissen, “what we saw was full protection against corrosion, even after thirty days of immersion. Only a couple of alternative solutions come this close to the results obtained with chromium-6. It’s an amazing result after only such a short period of development.” Visualising corrosion protection Denissen and Garcia have also developed a novel method to study the onset and development of corrosion. It allowed them to gain a detailed understanding of the results they obtain with their algae shells, guiding further optimisation. “It is relatively simple technology, using a basic optical camera,” Garcia explains. “Optical techniques have traditionally been used to obtain qualitative information or to make beautiful pictures. What we have shown is that optics can be used to monitor and quantify local corrosion processes at a very high resolution, in real time. It is mature technology, allowing us to analyse any coating, commercially available or still in development.” Optimal protection “We use our experimental findings to build a computer model for further optimization of our coatings,” Denissen says. This can prove very beneficial as these algae shells come in more than 100.000 sizes and shapes. And there are more variables to tune, such as the type of corrosion-inhibitor used, whether or not to add an outside layer to the algae shell to even better regulate inhibitor release, or the optimal concentration of shells in the coating. “We may for example want to use disc-shaped shells to reduce our protective layer to the thickness currently used by the industry,” Denissen explains. “We are also looking into using combinations of inhibitors and shells in our coatings, further improving corrosion protection.” A small revolution It is not an easy task to replace chromium-6. “There are many barriers, resulting in a lack of good alternatives,” Denissen says. “For example, the Dutch Ministry of Defence wants proof that alternatives will provide twenty-year protection of their military equipment. But there are no good methods to accelerate this evaluation, to validate it in only a limited time-span.” More importantly, he explains, many of the tests used to validate the efficiency of new coating materials are designed specifically for chromium-6. “It is not a level playing field. It means that you have to prove your alternative coating to behave similar to chromium-6, rather than prove that it provides adequate protection.” Nevertheless, a small revolution has recently taken place. Rather than waiting for coating manufacturers to replace chromium-6, airplane manufacturers are now actively developing their own solutions as well. “At the moment, we are already talking to both.” Future perspective Despite very promising results, Denissen stresses that “we need a few more years to develop and demonstrate our algae-based coating before it can be used on planes, bridges or any metal surface that needs protection against corrosion. Does our coating protect sufficiently against scraping and scratching? Can it withstand frequent variations in outside temperature? Will it bond well with the other protective layers?” Garcia adds that “our main commitment is to find solutions to societal problems. We are currently talking to several industry partners about collaboration. Together we can speed up the development and launch of our technology and we expect to be ready for operational experiments by 2022, on an airplane.” Until completion of those experiments and passing the required certifications, the airplane industry may require the European Commission to again extend its leniency, tolerating the use of chromium-6 for the time being. You can find more information about real-time corrosion measurements here (in Dutch). You can find scientific publications, related to this research, here in Corrosion Science and in Electrochimica Acta .

A wealth of information on noise

In aircraft, noise is an unwanted side-effect of propulsion. In sonar, it is a useful tool for conducting accurate depth measurements. But irrespective of whether it involves a disruptive noise or the useful application of sound, it always tells you something about the source and the surrounding environment. As Associate Professor in the Aircraft Noise and Climate Effects (ANCE) group, Mirjam Snellen specialises in techniques for imaging sound and using sound for imaging. She is only happy when all of the information has been extracted from the measured noise, often using detectors developed by the group. With its noise research, the small and close-knit ANCE research group, led by Prof. Dick Simons, is deliberately positioned at the interface between metrics and modelling. “If all you do is measure and take readings, you will not have the knowledge to design quieter aircraft,” says Mirjam Snellen. “Other groups are specialised in calculation. They build models that use the shape and material of an aircraft fuselage to predict the noise that it will produce at different speeds. Of course, this then needs to be validated using measurements.” As far as the ANCE group is concerned, even an accurate noise reading on a scale model in a wind tunnel is not sufficient. They prefer to measure it in practice, in the most realistic conditions possible. A key factor in this process is to ensure that the source of the noise, in this case the fuselage, can be properly separated from other noise sources, such as the engines. Mirjam Snellen: “We can achieve that in ANCE, using our arrays”. Separating out sound A single microphone only enables you to measure the volume and pitch of the sound. This changes if you use an array of several microphones, spread across a surface. For example, the noise that comes from the aircraft's nose cone reaches the closest microphone first, followed shortly by the microphones slightly further away. Software can be used to reconstruct the different sources of noise. “This is called beamforming,” explains Mirjam Snellen. “We can use it to separate the measured noise over time according to source location, pitch (frequency) and volume. We then make corrections for background noises, noise reflections on the ground and the Doppler effect, if necessary.” Certain preconditions are required for an array of this kind. To achieve a good resolution of the low pitches, which have a long wavelength, the microphones need to be slightly further apart. For the higher pitches, they need to be slightly closer together, to prevent distortion. For this reason, an array generally has a mixture of greater and smaller distances between microphones. An array is also limited in size in order to enable transportation. A large white surface The researchers in this group have mastered the beamforming techniques down to the finest details. In the summer, they took their first readings using the newly-designed array of 64 microphones, over a surface of 4x4 square metres. With it, they can also distinguish between the different sources of noise on small aircraft. Before setting off to work, the researchers always call the airport. “After all, you're going to be covering a large white surface there. Even now, someone from the airport always comes along, but that's just out of interest.” Array readings of this kind produce impressive pictures that clearly show which part of an aircraft produces which noise. The airflow around the wings typically results in slightly lower sound frequencies whereas the engines and the blast behind them produce higher frequencies. “Our measurements test the predictions made by models. If we are not expecting much noise from the undercarriage and our measurements show something different, the model may need to be revised.” In other words, different types of aircraft have different ‘fingerprints’ and this calls for a different way of reducing noise nuisance. Quieter aircraft Schiphol and Lelystad airports are reaching their permitted noise limits and are committed to quieter aircraft for the future. In 2015, Prof. Symons joined forces with the German Aerospace Center (DLR) in organising a workshop with experts from across the world. The key question was to what extent can aircraft become quieter, and how can this be achieved. “It's not easy,” says Mirjam Snellen. “For example, one option is ultra-high bypass engines. They are much quieter, but also extremely large and therefore heavy. The additional propulsion required to take off and fly could potentially end up increasing noise nuisance. Using alternative materials can alleviate the problem slightly.” The group is working on this together with the wind energy group in the same faculty, which is exploring the use of porous materials for wind turbines. “They could be used to make the nose and wing flaps or the landing gear of an aircraft slightly less noisy.” New developments like this are first tested in a wind tunnel. For this research, the group placed a microphone array in the relatively quiet vertical wind tunnel. The walls were covered with absorbent material to minimise sound reflection. Mirjam Snellen: “The challenge is to only measure the object. The array itself cannot be allowed to cause sound reflections and the microphones need to be positioned to enable precise measurements and beamforming.” Virtual noise It is easy to forget that the listener also plays an important role in noise nuisance and this also points the way towards new developments. If a group of test subjects are not disturbed by the hum from the fuselage, nothing needs to be adapted. Dick Simons was closely involved in the development of the Virtual Community Noise Simulator by the Netherlands Aerospace Centre (NLR). In it, you can experience the noise of a landing aircraft in a completely simulated natural environment. This requires carefully measured or modelled sound, something that the ANCE group can provide. Mirjam Snellen: “In a simulator, you can switch off the noise from the wings and engines only or change the pitch, for example.” Although the latter may seem futuristic, groups are already trying to achieve it using smart modifications in shape and material. Safety in shipping Beamforming also plays an important role in using echo sounding to determine the depth of water, which can improve safety in shipping. For example, precise information about the current water depths provides useful input for the frequency of dredging in and around the port of Rotterdam, ensuring it remains accessible even to the largest ships. Identifying water depths in this way can be done efficiently using multi-beam echo sounders (MBES). An MBES is an echo sounder that emits several narrow beams of sound simultaneously (a ‘ping’). Each of the beams propagates through the water, reflecting on the bottom before being recorded by the MBES again. Combined with the noise profile as a function of the depth, half of the time between sending and receiving a ping gives you the distance from the MBES to the bottom. Each ping identifies the bottom across a wide line, perpendicular to the direction of travel of the ship being studied. The entire bottom of the channel of water can be charted by using several parallel movements. Mirjam Snellen: “in the Netherlands, we are one of very few groups with a good knowledge of all the technical details of MBES; the system, its impact on the environment and the algorithms.” The MBES systems are complicated and expensive but achieve the level of precision required by government of just a few centimetres at depths of up to several tens of metres. Any new systems and cheaper alternatives will also need to prove they can meet these criteria before they will be permitted for use by a dredging company, for example. Mirjam Snellen: “We have developed a model that determines whether measurements meet these standards, given a set of measurement systems.” In addition, it is possible to obtain more information from the current systems, for example by also looking at the intensity of the echo signal alongside its two-way travel time. Mirjam Snellen: “In recent years, we have developed several methods to not only determine the water depths, but also the composition of the seabed using MBES measurements. For example, would it be a suitable site for a wind farm?” Using all of the data It was her determination to obtain all of the information available from sound that resulted in a method for improving the accuracy of MBES depth measurements. The raw data from an MBES system has to be corrected for many variables, including the underwater sound speed profile. This depends on the pressure (depth), temperature and salt content. In estuaries or tidal areas in particular, local variations in the sound speed can result in inaccurate readings. Mirjam Snellen: “This lack of precision increases towards the edges of the strip being measured. In the past, this was simply accepted or the measurements were taken again. But that is not necessary if you realise that the strips measured partly overlap.” She developed an algorithm that uses the extra data from this overlap to calculate the sound speed profiles. “It's not an estimate, it's physics,” says Mirjam Snellen. “With the occasional re-measurement, of course,” she adds with a laugh. Measurement is the key to knowledge Her algorithm for the underwater sound speed profile was recently implemented in commercial hydrographic software, but actually originates from 2009. “As in other fields, it can take time for solutions to be applied in practice,” says Mirjam Snellen. Another current topic to which this applies is the calculation of aircraft noise impact at airports. For this, so-called noise power distance tables are used. “No one knows how accurate the use of these tables is. This is 2018, why not position some arrays at the airport and actually measure this noise experience?” Find more information about the ANCE group here.

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