Greta Thunberg crossed the Atlantic on a zero-emission yacht, but how realistic is it to de-carbonize air travel? James McKenzie thinks he has the answer
A friend of mine recently took his boat across the Atlantic. It was great fun and a real adventure – I particularly loved the photos of dolphins he posted online. But as a practical mode of transport, going by boat just doesn’t cut it in the modern age. Unless, of course, you’re Greta Thunberg, who sailed to New York to make a serious point about the impact of climate change before delivering a powerful speech to the United Nations on the matter.
But when Thunberg was named as Time magazine’s person of the year for 2019, it got me thinking. For all the amazing advances in aircraft technology – flying from Europe to New York is now nearly twice as efficient as going by ship – aviation still has a big environmental problem. Planes spew out carbon dioxide and nitrogen oxides, which form ozone in the upper troposphere. They also emit particulates and leave water-vapour trails, both of which trap heat.
Indeed, the downsides of air travel have led to a rapidly growing “flight-shame” movement, particularly in Europe. In Sweden, for example, passenger numbers are down year-on-year by 11%. A similar fall has occurred in Germany, where the federal government has responded by cutting tax on train travel.
Going green
Thankfully, airlines are seeking to improve their environmental credentials. Last November low-cost carrier easyJet announced it would, from this year, offset carbon emissions from the fuel used for every flight in its network. Other airlines have taken similar steps although none has promised as much as easyJet. The company is also supporting the US start-up Wright Electric, which is producing a range of all-electric planes for short-haul flights.
Airbus, meanwhile, is one of 50 firms to join the Air Transport Action Group (ATAG) – a not-for-profit association that wants to halve the aviation industry’s CO2 emissions by 2050 (compared to 2005 levels). However, these ambitious targets (though I doubt Thunberg would see them as such) cannot be achieved using existing technologies. Members of the ATAG therefore believe that alternative propulsion technologies – including electric and hybrid-electric systems – will be required.
Airbus, along with Rolls-Royce and Siemens, has already developed the E-Fan X – a demonstrator craft in which one of the four jet engines is replaced by a 2 MW electric motor, which has roughly the power of 10 medium-sized cars. When high power is required – at take-off, for example – the system’s generator and battery supply energy together. Unfortunately, today’s batteries are so heavy and bulky that it’s unlikely this plane could be used for long-haul flights, which make up four-fifths of aviation emissions.
The clear winner is hydrogen, which has an energy density of over 140 MJ/kg.
So what’s to be done? Well, we can forget nuclear fuel as a solution: uranium-235 has a huge energy density of 8 x 107 MJ/kg but no-one’s going to want fission-powered planes landing at their local airport and who’d want to get on board in the first place? As for lithium-ion batteries, they have a storage capacity of 0.95 MJ/kg at best – nowhere near the 43 MJ/kg of kerosene. The clear winner is hydrogen, which has an energy density of over 140 MJ/kg. Burning it emits almost no CO2 and few nitrogen oxides. It leaves just a bit of water vapour, which I suspect we can live with.
But hydrogen has problems. It’s highly volatile so you can’t store it in a plane’s wings. Most aircraft designs that use liquid or pressurized hydrogen therefore store it in the fuselage. That in turn means you’d need a larger fuselage (for the same number of passengers) than a conventional kerosene fuelled aircraft, leading to a bigger friction drag and wave drag – and hence higher energy costs.
On the plus side, 1 kg of hydrogen can provide the same energy as 3 kg of kerosene, cutting the gross take-off gross mass of a Boeing 747-400 aircraft from 360 to 270 tonnes. Given that hydrogen is likely to be cost-competitive with kerosene by 2037, I think existing aircraft designs and jet engines could be adapted to run on hydrogen without too much difficulty. Problem is, that date is so far off that, even though kerosene supplies are dwindling, no-one’s in a rush to make the switch fast.
Money matters
What will drive the change to low-carbon flight is economics. Every passenger leaving the UK currently has to pay £26 in Air Passenger Duty (APD) tax for every short-haul flight and £150 for long-haul flights. Introduced in 1993 to offset the environmental impacts of air travel, APD currently brings in £3–4bn to the UK Treasury’s coffers. Ticket prices would only rise further if planes were fitted with hydrogen fuel tanks as these swallow up about a third of the craft’s available passenger space.
To me, shifting the cost of greener air travel to customers is the wrong way of going about things.
To me, shifting the cost of greener air travel to customers is the wrong way of going about things. The EU and UK currently put no tax on aircraft fuel (zero VAT), but if kerosene were taxed it would encourage aircraft manufacturers to develop even more efficient planes and be a catalyst for faster change in the industry. Aircraft and engine makers would also have new revenue streams in the form of refurbishing planes to run on more efficient kerosene engines or even converting existing planes to hydrogen.
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According to the UN’s International Civil Aviation, the global air-transport network is expected to double in size by 2030. With 23,000 commercial aircraft in service in 2017, Boeing says we’ll need almost 40,000 new planes over the next 20 years. By 2037 there should therefore be more than 63,000 aircraft in the world. Greener air travel is a problem we need to solve now – not in 20 years’ time.
The challenge is to make hydrogen fuel cleanly and economically and, for me, the only solution is to do so by electrolysing water rather than extracting it from fossil fuels. And if the power used to create, compress or liquefy hydrogen can itself be from carbon-free sources such as renewables or nuclear, then surely hydrogen is the future of carbon-free aviation.
