How it works: An electric fix for aviation’s air pollution problem

Land vehicles’ contribution to degrading air quality and climate change has been the talk of the town. However, a study led by a team at Massachusetts Institute of Technology (MIT) found that “aviation emissions are an increasingly significant contributor to anthropogenic climate change”.

Upon reaching cruise altitude, aeroplanes spew out steady streams of oxides into the atmosphere. The suspended oxides, mostly nitrogen, linger long enough to spark new chemical reactions with atmospheric oxygen, producing ozone and fine particulate matter. Such highly reactive and toxic nitrogen oxides (NOx) are known to cause asthma, decreased lung function and cardiovascular disorders.

“When you consider the full flight, which includes emissions from take-off, cruise and landing, aircraft emissions are also responsible for around 16,000 premature deaths a year from impaired air quality,” says Dr Sebastian Eastham, a lead research scientist at the MIT Laboratory for Aviation and the Environment.

Fortunately, there now seems to be a fix. With a new concept for aircraft propulsion, MIT engineers claim they can finally solve aviation’s air pollution problem. The prototype aeroplane, which they say is capable of cutting aviation’s hazardous nitrogen oxide emissions by 95 per cent, takes its inspiration from heavy-duty ground vehicles with post-combustion emission control systems.

Post-combustion emissions-control systems work by selectively trapping and eliminating pollutants in exhaust gases. Special catalysts or absorbents are used to achieve the results. MIT researchers have proposed a similar design for aviation, but with a turbo-electric twist.

Until now, it has neither been possible nor feasible to include an emissions-control device under an aircraft’s wing. This is because tucked beneath each wing is a jet engine with a gas turbine. The turbines power propellers or fans to provide the necessary thrust for a plane to move forward. The exhaust from the jet engines flows out radially through special vents, adding to the lift. Such an arrangement is so precise, the researchers say, that placing any additional device or system to change the wing’s planform would mean interfering with the aerodynamics of flight.

However, MIT’s new hybrid-electric design changes the equation by situating two gas turbines and a generator inside the aircraft’s cargo hold. The schema completely isolates thrust-producing propellers from power-generating gas turbines – yet the functions remain the same.

The researchers explained that the gas turbines will work to run the generator. The generator, in turn, will produce enough electricity to power wing-mounted propellers or fans for thrust. Finally, the smoke emanating from the gas turbine feeds into an emissions-control system, comparable to those routinely used in diesel vehicles, to treat hazardous pollutants including NOx.

“This would still be a tremendous engineering challenge, but there aren’t fundamental physics limitations,” says 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.”

Storage would not be an issue either. Considering commercial aircraft’s plentiful cargo hold, Barrett suggests a hybrid-electric system, which includes the gas turbine, an electric generator, and an emissions control system, that can fit right into the belly of the aircraft. The emission control system could be “folded up accordion-style” in the plane’s cargo bay, he adds.

Additionally, Barrett and his team found that if the new hybrid-electric system were to be integrated into a Boeing 737 or Airbus A320, the aircraft would need approximately 0.6 per cent more fuel to fly.

“This would be many times more feasible than what has been proposed for all-electric aircraft,” says Barrett. “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.”

Next in order of precedence for the MIT team is a zero-impact aeroplane that flies without emitting NOx or any other climate-altering greenhouse gases, including the dreaded carbon dioxide.

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