Judging from recent conversations I’ve been having, many people soon plan to fly for the first time since covid-19 arrived. Air travel is still expected to take at least another year or two to return to pre-pandemic levels, but as the aviation industry does recover, you can expect its climate change impact to return to the fore.
The issue with aviation is not so much its size today – it’s about 2 percent of global carbon dioxide emissions – but the rapid rate at which its emissions are growing. This year, a new supersonic plane (Boom’s XB-1) is due to have its first flight, raising the prospect of a new wave of fuel-hungry journeys too.
Some people argue the only real answer to aviation emissions is to cut demand and fly less, but as recent flight growth shows, people still want to travel by plane. That’s why today’s Fix the Planet turns to one prospect for a long-term techno-fix: battery-powered planes.
What are the main contenders for delivering guilt-free flights?
In the short term, there’s no such thing as a green flight. The only real options available today are more efficient engines, biofuels, and carbon offsetting. The first is welcome but failed to stop emissions rising pre-pandemic. The other two are problematic and limited.
In the longer run, we are probably looking at either hydrogen, batteries, a combination of the two, or something new. I’ve written before about hydrogen, which is attractive to plane-makers because of the energy it can pack for its mass. Still, it faces a host of issues, not least the challenge of making the stuff in a low-carbon way.
Why don’t we see battery-powered planes?
Replacing jet fuel with batteries is hard, says Venkat Viswanathan, who works with a team at Carnegie Mellon University in Pennsylvania to make better batteries. In a paper, Viswanathan and his colleagues lay out the challenges, which range from the energy density of batteries, their weight, and concerns over mid-flight battery fires to the trade-off of energy density versus how many times you can recharge the battery. To put the problem in perspective, a large airliner takes off with the equivalent energy delivered by 30,000 Tesla cars. “The goal of the paper is to set realism against all the optimism going around,” says Viswanathan. Aviation is uniquely demanding, he explains: “You need safety, you need economics, you need all the reliability and you can’t compromise on any.”
Are battery planes a non-starter?
Progress on battery chemistry and materials means we shouldn’t rule out the prospect of battery-powered flight. Paul Shearing at University College London says even today’s lithium-ion batteries, of the kind used in an electric car, could work for small aircraft such as flying taxis (Boeing invested $450 million in one “vertical take-off and landing” vehicle start-up just this week). “My two cents is we’ll see lithium-ion chipping into bits of the aerospace sector reasonably soon,” he says.
But progress is unlikely to stop there. Using new chemistry and materials could unlock batteries for medium-sized planes capable of carrying tens of passengers a few hundred kilometers – a flight from London to another north-western European capital, for example. The key to that breakthrough will be the use of a new generation of batteries that are rechargeable and lightweight. Shearing thinks that could mean lithium-sulfur batteries and potentially solid-state ones too, where the battery’s electrolyte is solid rather than liquid. He says solid-state batteries now routinely show an energy density of about 400 watt-hours per kilogram, almost twice that of most lithium-ion batteries (although state-of-the-art ones can get to 250Wh/kg).
The US government has been funding efforts to hit 500Wh/kg, and the program last year received $75 million for a second stage. But Viswanathan believes it is possible to reach around 600Wh/kg in the next 10 years, given a targeted and substantial investment in batteries for aviation. Lithium-metal is one material he thinks holds promise. But he says a good approach for finding new lightweight batteries will be to look at so-called primary batteries – single-use ones – and try to develop a rechargeable version. Lithium-carbon-monofluoride batteries are one possible candidate. “There has never been a serious shot by battery scientists looking at aviation as a primary market,” he says. Relying on the sort of incremental improvements seen in electric car batteries won’t produce the breakthrough that planes need, he adds.
And what about a transatlantic flight?
Making batteries work for planes with hundreds of passengers will be crucial to reducing aviation’s climate change footprint. Large aircraft are responsible for more than 95 percent of aviation emissions. Those with 200-plus seats generate more than half of total emissions. At that sort of size, Shearing thinks batteries will probably need to be used in some sort of hybrid system, such as combining them with a hydrogen fuel cell. “Long-haul flights are going to be a tough nut to crack,” he says.
Could battery swaps help?
A few years ago, Israeli entrepreneur Shai Agassi tried to convince the world that electric cars with batteries that could be swapped out were the future. He failed, with his Better Place start-up. But Viswanathan says it could be a “reasonable strategy” for planes because they are organized as centrally managed fleets with ground crews. No one appears to be pursuing this route yet, but it is being discussed.
What are the reasons for optimism?
A start would be shifting the goalposts on energy density targets for batteries, by aiming for 800Wh/kg rather than the US’s 500Wh/kg, says Viswanathan. He thinks there are several reasons to think researchers can get there and begin unlocking bigger planes. The first is that advances in imaging (from-to ) now offer an unprecedented ability to look inside batteries in real-time and learn how to tweak them. The second is robotics. The third is using machine learning to discover new materials, which he likens to the way pharmaceutical firms are using it to develop new drugs and vaccines. “I think batteries are having a similar inflection point,” says Viswanathan.
In the UK, Shearing is working with groups such as the UK government-funded Faraday Institution to develop better batteries. He’s hopeful we will see electrification in some parts of the aerospace sector (such as flying taxis) in the next five to 10 years and thinks those successful demonstrations should spur efforts on larger aircraft. At each different scale of the plane, different battery chemistries will reach a ceiling of what they can deliver before a new one is needed. “I would be optimistic about decarbonizing this sector over the next 50 years,” he says. “I don’t think it’s a next-century problem, it’s something that has to happen quite soon.
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