Zero-emissions air travel may be one step closer after scientists at the Massachusetts Institute of Technology unveiled a new type of sodium fuel cell that could one day make electric-powered flight viable.
'Environmental benefits at no cost'
The sodium-air fuel cell – designed by a team led by Yet-Ming Chiang, professor of materials science and engineering at MIT – works by combining liquid sodium metal with oxygen drawn from the air to create a continuous reaction.
The device is "based on well-established electrochemical principles", said The Times. But "unlike conventional batteries, which must be recharged, it is designed to be refuelled, with its energy-rich material being replaced as it is consumed". In this way it is "more similar conceptually to hydrogen fuel cell systems", said MIT Technology Review.
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Liquid sodium metal "doesn't require the super-cold temperatures or high pressures that hydrogen does, making it potentially more practical for transport". There are also "economic factors working in favour of sodium-based systems" as sodium metal can be made from sodium chloride – commonly known as table salt – which is "incredibly cheap".
The sodium-air cell also produces sodium hydroxide as a chemical by-product, which could be used to absorb CO2 from the atmosphere or turned into sodium bicarbonate for ocean-based carbon removal. It is an "essentially free" by-product, "producing environmental benefits at no cost", said Chiang.
'Within the next year'
With a Boeing 747 estimated to burn around 3,800 gallons of fuel per hour, or about one gallon every second, according to Simple Flying, the aviation industry has long sought a way to make air travel more sustainable (and cheaper).
The focus up to now has been on sustainable aviation fuel (SAF), made from non-petroleum-based renewable sources, such as waste oil and biomass. The International Air Transport Association said it expects the amount of SAF produced globally to double in 2025 to reach 2 million tonnes, said Reuters. Yet this still represents just 0.7% of airlines' total fuel consumption worldwide, with questions around the supply of SAF and its cost, which is far more than traditional jet fuel.
When it comes to electric-powered flight, the energy density of a power source – the amount of energy stored per unit of weight – is "critical", said The Times. Aircraft must be "as light as possible to achieve lift and to maximise range, and the lithium-ion batteries used in electric cars have too low an energy density to power most aircraft. Without a large boost in energy density, electric-powered flight remains impractical for all but the smallest aircraft over short distances."
In laboratory tests, the MIT sodium-air fuel cell prototype delivered more than five times as much energy per kilogram as the lithium-ion batteries currently used in cars.
But even so, sodium-air-powered flight is still some way off. The "next step", said MIT, is to continue research to "improve the cells' performance and energy density, and to start designing small-scale systems", with drones a potential option.
"We'd like to make something fly within the next year," Chiang said.
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