Prepare to Fly the ENVIRONMENTALLY Friendly Skies

Through innovations in hydrogen, electrification, modeling, and design optimization, sustainable aircraft are approaching takeoff. Grainger Engineers are helping to usher in this new era of aviation.

Spring 2023

Across the globe, more than 100,000 airplane flights occur each day. While each one may have a relatively small impact on the environment, the net effect is substantial: the aviation industry produces roughly 2% of global greenhouse gas emissions and 3% of all carbon dioxide (CO2) emissions.

Members of the International Civil Aviation Organization, acknowledging the problem, recently agreed to aim for a goal of net-zero carbon emissions by 2050. Federal agencies such as NASA and the FAA have increased funding in basic research to support these efforts, and major players in the aerospace industry, such as Raytheon and Boeing, are also on board. 

“It would be really difficult to solve the climate change problem without addressing aviation,” said Phil Ansell, an associate professor of aerospace engineering who is leading the new Center for Sustainable Aviation in The Grainger College of Engineering. “At UIUC, we have been working on various aspects of green aviation engineering for years, but more recently, we have made a pivot to efforts that are even more comprehensive.” 

For some years, so-called “green engineering” has aimed to improve energy efficiency, emissions, and more through the development of new technology. Today, the focus is on far more ambitious “sustainability” efforts, which aim for nothing less than net-zero greenhouse gas emissions. The Center for Sustainable Aviation, which was established with support from The Grainger College of Engineering and the Department of Aerospace Engineering, seeks to collaboratively leverage resident expertise across UIUC in order to tackle the ambitious challenge of sustainable solutions in the aviation ecosystem. 

To achieve sustainability, efforts must focus on jet aircraft, which produce 93–95% of all CO2 and other greenhouse gas emissions that come from aviation. Commercial airplanes typically use kerosene, a fossil fuel that is a major culprit in the high emissions. 

In the lab of Don Wuebbles, who was a White House advisor to President Barack Obama on climate science, researchers use complex 3D modeling to evaluate the effects that the emissions produced by each flight have on the climate. The team created a digital twin of the Earth, which enables them to study the physics and chemistry of the atmosphere to see how emissions impact the environment. 

Wuebbles found that while CO2 was, of course, a major pollutant, it was far from being the only one. For example, nitrogen oxides (NOx) produced by the heat of the engines can react with ozone in the atmosphere. In the troposphere, that results in production of “bad” ozone, which can affect human and animal lungs and impact agricultural crops. In the stratosphere, NOx can destroy the “good” ozone that protects us from harmful levels of ultraviolet solar radiation. 

Even contrails – the water-vapor-induced ice particles that often trail airplanes and can be visible to the human eye – can negatively impact the climate. Unlike greenhouse gas emissions, which contribute to climate warming, contrails can actually have a cooling effect, by shading us from the sun’s rays, in addition to a warming effect, by trapping heat radiating from the Earth’s surface. 

“We really need to get away from the emissions associated with fossil fuels because of their effects on the climate,” said Wuebbles, a professor emeritus of atmospheric sciences and an alumnus of UIUC’s Department of Electrical and Computer Engineering. Wuebbles will provide climate science guidance to the Center for Sustainable Aviation. “At the same time, we need to make sure that alternatives are truly more beneficial to the environment and better for human health. 

Our 3D modeling will enable us to provide the analyses needed to do just that.”

Alex Solecki, a graduate student in mechanical science and engineering who works with professor Tonghun Lee’s research group, is using the FAA’s Alternative Jet Fuels Test Database to compile, track and analyze trends in sustainable fuels, with the goal of accelerating their certification, scaling and adoption. 

Fuel producers aim to develop fuels that look and act much like conventional kerosene and can be used as “drop-in” alternatives. Solecki is part of a team that is analyzing the chemical and physical property profiles of the alternatives to see how they stack up against kerosene in terms of safety and performance. 

“The aviation sector is highly safety-oriented. Large-scale changes to something as influential as fuel type will not be seen until an overwhelming amount of scientific support is presented for its suitability,” Solecki said. 

She noted that sustainable fuels present significant potential for supplementing the use of conventional petroleum-derived jet fuel in the commercial aviation sector and, in fact, are already doing so in small volumes. While these alternative fuels are chemically diverse, they can achieve the same standards as conventional Jet A fuel (a kerosene civil aviation fuel). However, it will take more research and analysis before these synthetic fuels – which are currently certified at blending ratios of 50% or less with conventional kerosene – are accepted by governing bodies, aviation companies and consumers. 

“This work requires as much fuel test data as possible to develop a more resource-efficient certification process, support advancements in engine manufacturing and design, solidify understanding of the behaviors and potential for variability among these fuels, and ultimately to inform the best way to proceed with large-scale integration in industry,” Solecki said.

Kiruba Haran, a professor of electrical and computer engineering at UIUC and the Grainger Endowed Director’s Chair in Electric Machinery and Electromechanics, has been heavily involved in the Grainger Center for Electric Machinery and Electromechanics, an endowed center that works on moonshot technologies. Around 2014, he and other engineers began to explore what large electrified aircraft might look like, and it was soon clear that electric propulsion systems could be an enabler for reduced emissions. 

“If we’re going to go the route of electric aircraft, we need better electric propulsion systems,” said Haran, who also directs Grainger Engineering’s Center for Power Optimization of Electro-Thermal Systems (POETS). “Large aircraft are difficult to electrify, because they become too heavy for flight. We also have some work to do to create EVs for aviation that match the safety and reliability that the public has come to expect. Aviation is one of the safest forms of transportation, and we’re working on electric airplanes that keep it that way.” 

Haran and colleagues are working on research that would increase the power-to-weight or power-to-volume ratio for electric powertrain components such as motors, electronics and associated systems by 4x or greater. They are doing this with electrothermal co-design and new physical architectures, working to increase the technology readiness and integrating those designs in electric vehicle systems; developing advanced controls to operate these systems in an optimal manner; and continuing to feed the pipeline with new enabling technologies such as materials whose properties change as a function of temperature. 

They are also considering hybrid/turbo-electric systems that lead to reduced emissions while still employing some type of fuel to produce energy. 

“This would enable the aviation industry to move more quickly to electrification, without having to wait for batteries to become perfect,” Haran said. “This will also give us the opportunity to gain experience and mature technologies that can be utilized in future zero-emissions aircraft.”

Illinois researchers such as Paul Braun are addressing this issue by designing more advanced, energy-dense batteries. 

Braun and other scientists are working to lighten the load by reducing the weight of anodes and cathodes in batteries, which are responsible for driving the flow of electrical current during discharge. A separator is positioned between the two to prevent electrical short-circuiting. 

Currently, anodes are primarily carbon, while cathodes are made of other materials, including nickel, cobalt, and manganese, which are relatively heavy. Replacing the carbon-based anode with lithium, the world’s lightest metal, would be a first step to making batteries more energy-dense, as lithium anodes store about 10 times more energy than carbon-based anodes of the same weight. However, lithium has a number of intrinsic challenges. 

“Lithium batteries have substantial benefits when it comes to energy density, but we don’t have any right now that are good enough for the task,” said Braun, the director of the Materials Research Laboratory, a Grainger Distinguished Chair in Engineering, and a professor of materials science and engineering. “One major problem is that they aren’t easy to recharge; during the recharging process, the anode can form needles, causing the battery to explode. Exploding batteries are obviously not acceptable on an airplane.” 

Braun’s research group is addressing these limitations by developing new anode chemistries that are high-energy and do not form needles, as well as by creating advanced high-energy cathodes that can operate in both cold and hot temperatures. That’s a requirement for flight, during which temperatures can sometimes be high and sometimes drop to −60 degrees Fahrenheit. The ultimate goal is a safe and cost-effective energy storage technology. Even with the fundamental challenges, Braun believes batteries have a lot to offer aviation. 

“Electrification offers distinct advantages during both takeoff and cruise,” he said. “Jet engines cannot be designed to be efficient across all phases of flight, while batteries offer the potential for high-efficiency propulsion at more phases of flight.”

Ansell has already begun exploring the use of hydrogen fuel cells through the NASA-funded Center for High-Efficiency Electrical Technologies for Aircraft (CHEETA), which he leads. The goal of CHEETA is to generate sustainable power through fuel cells, which create electricity by mixing oxygen in the air with hydrogen. The process generates only water emissions, creating the potential for very clean energy. 

The challenges of creating such a system are numerous. In order to generate electricity, hydrogen must be stored in its liquid state at −423°F or below. These extreme temperatures necessitate heavier fuel tanks, which may not be a big problem for cars and trains, but present a unique challenge for airplanes. The obstacles require novel materials and advancements in cryogenics to allow super-conductivity to be included in the design of power transmission systems and motor coils, as well as the development of lightweight, efficient energy storage. 

In addition, the design of hydrogen-powered aircraft matters greatly. For example, on a conventionally designed aircraft, if the landing gear system malfunctions and punctures the cargo bay, the aircraft could likely make a safe landing. If a hydrogen tank were punctured, it would almost immediately start a fire. 

Also, while hydrogen has about three times the specific energy of jet fuel at one-third of the weight, it takes four times the volume to go the same distance. 

Ansell and the CHEETA team are working on optimizing designs for these aircraft, designing engine systems that connect with both room-temperature and superconducting machines, and developing a new paradigm for the design of modern aircraft systems. 

“Hydrogen-powered aircraft are definitely years, even decades, away from taking flight,” Ansell said. “There’s a lot of foundational work that has to be done to make hydrogen a reality for aviation, but we’ve laid out a roadmap for this technology in the future.”