Plane taking off

Carbon-neutral aviation possible, says study

Carbon-neutral aviation is possible, but in future aircraft are likely to continue to be powered by fossil fuels, according to a new study. To aid with this, MIT researchers have demonstrated a concept hybrid-electric plane to help the aviation industry curb its pollution excesses.

According to researchers from ETH Zurich, while future aircraft are most likely to still be powered by fossil fuels, the CO2 they emit must be systematically stored underground. This is the most economical approach for dealing with aviation's carbon footprint.

ETH Professor Marco Mazzotti and his team compared various options that appear to be the easiest to implement in the short and medium term and evaluated them according to factors such as cost-effectiveness. It should be technically feasible for aviation to become climate-neutral.

The researchers concluded that the most favourable option is to continue powering aircraft with fossil fuels in future, but then remove the associated CO2 emissions from the atmosphere using CO2 capture plants and store that CO2 permanently underground (carbon capture and storage, CCS).

"The necessary technology already exists and underground storage facilities have been operating for years in the North Sea and elsewhere," said Viola Becattini, a postdoc researcher in Mazzotti's group and the study's first author.

Mazzotti added: "The approach may become a cost-competitive mitigation solution for air travel in case, for example, a carbon tax or a cap-and-trade system were imposed on emissions from fossil jet fuels, or if governments were to provide financial incentives for deploying CCS technologies and achieving climate goals".

CO2 can be captured either directly from the air or indirectly at a site where organic material is burned, for example in a waste incineration plant. "Roughly speaking, half of the carbon in the waste burned in municipal incinerators comes from fossil sources, such as plastic that has been produced from petroleum. The other half is organic material, such as wood or wood products like paper and cardboard," Mazzotti said.

From a climate action perspective, capturing and storing the share of carbon that has fossil origin is a zero-sum game: it simply sends carbon that originated underground back to where it came from. As to the share of carbon from organic sources, this was originally absorbed from the air as CO2 by plants, so capturing and storing this carbon is an indirect way to remove CO2 from the air. This means CCS is a suitable method for putting carbon from fossil aviation fuels back underground, effectively making air travel carbon-neutral.

In their study, the ETH scientists were able to show that indirect carbon capture from waste incineration gases costs significantly less than direct carbon capture from the air, which is also already technically feasible.

The scientists also investigated producing synthetic aviation fuel from CO2 captured directly or indirectly from the air (carbon capture and utilisation, CCU). However, because the chemical synthesis of fuel from CO2 is energy-intensive and expensive, this approach is considered less economical than using fossil fuel and CCS. The research suggests that CCU is approximately three times more expensive than CCS.

The ETH researchers calculated the costs of the various options for carbon-neutral aviation, from today to 2050. They expect both CCS and CCU technologies to become less expensive as technology advances and through economies of scale, while the price of CO2 emissions levied as carbon taxes is likely to rise.

The researchers emphasised that there are other ways to make air travel carbon-neutral, such as aircraft that run on either electricity or hydrogen. Mazzotti noted drawbacks with both approaches, namely that electrically powered aircraft are unlikely to be suitable for long-haul flights due to the weight of the batteries required, while using hydrogen as a fuel will require the neccessary supply infrastructure to be developed and built from scratch.

The team's findings were reported in the journal Industrial & Engineering Chemistry Research.

Meanwhile, a team of MIT engineers have shown a concept for a hybrid-electric plane that could help reduce aviation's air pollution problem. The proposed design could reduce nitrogen oxide emissions by 95 per cent, according to their study.

At cruising altitude, airplanes emit a steady stream of nitrogen oxides into the atmosphere, where the chemicals can linger to produce ozone and fine particulates. Nitrogen oxides (NOx) are a major source of air pollution and have been associated with asthma, respiratory disease and cardiovascular disorders. Previous research has shown that the generation of these chemicals due to global aviation results in 16,000 premature deaths each year.

The MIT team – all members of MIT's Laboratory for Aviation and the Environment – came up with a concept inspired by emissions-control systems used in ground transportation vehicles. Many heavy-duty diesel trucks house post-combustion emissions-control systems to reduce the NOx generated by their engines. The researchers propose a similar design for aviation, with an electric twist.

Today's planes are propelled by jet engines anchored beneath each wing. Each engine houses a gas turbine that powers a propeller to move the plane through the air as exhaust from the turbine flows out the back. Due to this configuration, it has not been possible to use emissions-control devices, as they would interfere with the thrust produced by the engines.

In the new hybrid-electric design – described as "turbo-electric" – a plane's source of power would still be a conventional gas turbine, but it would be integrated within the plane's cargo hold. Rather than directly powering propellers or fans, the gas turbine would drive a generator, also in the hold, to produce electricity, which would then electrically power the plane's wing-mounted, electrically driven propellers or fans. The emissions produced by the gas turbine would be fed into an emissions-control system, broadly similar to those in diesel vehicles, which would clean the exhaust before ejecting it into the atmosphere.

"This would still be a tremendous engineering challenge, but there aren't fundamental physics limitations," said Steven Barrett, professor of aeronautics and astronautics at MIT. "If you want to get to a net-zero aviation sector, this is a potential way of solving the air pollution part of it, which is significant, and in a way that's technologically quite viable.

"The research that's been done in the last few years shows you could probably electrify smaller aircraft, but for big aircraft, it won't happen anytime soon without pretty major breakthroughs in battery technology.

"I thought, maybe we can take the electric propulsion part from electric aircraft, and the gas turbines that have been around for a long time and are super reliable and very efficient, and combine that with the emissions-control technology that's used in automotive and ground power to at least enable semi-electrified planes."

Barrett envisions the bulk of the hybrid-electric system – gas turbine, electric generator, and emissions control system – would fit within the belly of a plane, where there can be ample space in many commercial aircraft.

In their research paper, the team calculate that if such a hybrid-electric system were implemented on a Boeing 737 or Airbus A320-like aircraft, the extra weight would require about 0.6 per cent more fuel to fly the plane.

"This would be many, many times more feasible than what has been proposed for all-electric aircraft," Barrett said. "This design would add some hundreds of kilograms to a plane, as opposed to adding many tonnes of batteries, which would be over a magnitude of extra weight."

The hybrid-electric design would also eliminate an estimated 95 per cent of NOx emissions. If the system were fitted to all aircraft around the world, approximately 92 per cent of pollution-related deaths due to aviation could be avoided.

The team is now working on designs for a "zero-impact" airplane that flies without emitting NOx and other chemicals such as climate-altering carbon dioxide.

"We need to get to essentially zero net-climate impacts and zero deaths from air pollution," Barrett said. "This current design would effectively eliminate aviation's air pollution problem. We're now working on the climate impact part of it."

Full details of the concept were published in the journal Energy and Environmental Science.

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