Future of flight: Fuel for thought
The future of flight was supposed to come from hydrogen-powered planes, but kerosene still fuels the world's jets. So how will we fly when it runs out? Michael Fitzpatrick investigates
Tuesday 17 May 2011
It took just 32 seconds to extinguish faith in the airship and the hydrogen that once buoyed the Hindenburg, which erupted in a fatal inferno 70 years ago. Now hydrogen has been ditched again by the flight industry. But this time the promised "green" fuel for powering flights of the future has been quietly shelved in favour of biofuels and more fuel-sipping aviation.
"Hydrogen isn't getting a serious look in now by aircraft and engine manufacturers. It is unlikely to meet anything near to standards required for safe operation of aircraft," says Christopher Surgenor, editor and publisher of GreenAir, an online publication focused on aviation and the environment.
"There are very stringent conditions, naturally, attached to jet fuels – freeze points, flash points, etc. Then there is also the question of infrastructure: how would the hydrogen fuel be stored, transported, etc; it would require a plant the size of a power station by every airport." And while hydrogen as a potential "greener" fuel gets dumped worldwide, airlines and manufacturers are also jettisoning their once radical ideas for hydrogen-burning "cryoplanes".
Should we be concerned? The aviation industry clearly is. Because whatever fuel becomes the de facto power for all tomorrow's flights, the future of the passenger jet as we know it is doomed. Facing a fate shared by other fossil-fuel guzzlers, the jet will have to find alternatives to burning kerosene if it is to survive beyond the middle of the next century, which is when, according to the most optimistic figures, the Earth gives up its final barrel of oil. Hydrogen was hoped to provide the fuel for the next generation of passenger jets, or cryoplanes. Now those hopes are dashed.
Three times more efficient than oil but four times bulkier – even in its liquid state – hydrogen already powers several prototype cryoplanes around the world. But, despite the billions poured into research, their promised commercialisation has come to nothing as hydrogen failed to prove itself any greener then other energy sources. "The energy costs of making hydrogen are enormous," says Professor Ian Poll, head of technology for the UK Government-funded sustainable aviation Omega organisation. "It has to be created with an awful lot of energy. We need a source of electricity that does not emit CO2, and there are not many of those around." He points out that when world oil prices were pegged at $70-85 a barrel, alternative fuels were, until recently, simply not viable and could not compete economically.
But just 12 years ago, experts and much of the aircraft industry seemed bullish about hydrogen's chances as the new super-fuel. Generated from hydropower, liquid hydrogen, they thought, would be the ultimate non-polluting fuel source that, with some modification, be readily used by today's aircraft. Radical redesign of the world's airline fleet was planned to carry the bulky liquefied gas. The result was to be new-look cryojets reminiscent of Thunderbird 2 – with short wings and a bulging fuselage. Millions of pounds of taxpayers' money has been funnelled into projects that seemingly did not take on board the fact that hydrogen power would remain costly and polluting.
From 2000, Airbus was involved with the 26-month EC-funded Cryoplane Project to assess the feasibility of hydrogen, in its bid to develop a zero carbon-emissions aircraft. Researchers found that aircraft would require fuel tanks four times larger than today's. Models showed that the larger exterior surface areas would increase energy consumption by well over a tenth, and operating costs by around 5 per cent.
The greatest problem has been one of storing the cumbersome hydrogen while keeping a plane's aerodynamic properties. The answer, according to engineers at Daimler-Benz Aerospace, who built a small demonstration jet, is to set the fuel tank above the passenger compartment. Which might make passengers, nursed under tonnes of deep-frozen and highly inflammable hydrogen, slightly nervous. They needn't be, said the experts
"The arrangement of the tanks permits both cabin and cargo compartment to remain undisturbed, and in the event of an accident or leak, the hydrogen could escape upwards. Several years ago, tests by Nasa demonstrated that hydrogen presents only little danger to passengers, even in crash cases," says Daimler-Benz Aerospace spokesman Rolf Brandt.
Being burnt to death, it would appear, is not a looming fate. Hydrogen burns at a low temperature so a plane's aluminium fuselage would shield those inside. Freezing to death, however, might be another matter. Hydrogen used for cryoplanes is super-cooled to -253°C.
Despite the drawbacks, reactions from the air industry were positive, with Airbus and partners Daimler-Benz Aerospace vowing to replace kerosene with hydrogen by 2020. But, for the aerospace giants, hydrogen's appeal has diminished. The emphasis is on making fossil fuels go further.
"Kerosene is a very good fuel and very difficult to compete with," explains Rainer von Wrede, of Airbus's research and technology department. "In principle, it is possible to fly with hydrogen and we have a proof of concept, but now we can not produce enough hydrogen in an environmentally friendly manner for aviation."
Airbus, and the aviation industry, is devoting its research to reducing consumption and committing to developing what it calls greener synthetic kerosene and leaner planes and engines. Hydrogen, nuclear-powered planes, solar and electric powered commercial aircraft have all been shelved for the short- to mid-term.
"The big deal is alternative jet fuels. Principally biofuels that come from sustainable sources, and do not compete with food and water," says Mr Surgenor. "They must be 'drop-in' – in other words no major, if any, changes to aircraft engines and no changes to existing fuel transportation systems. Alternative fuels include coal-to-liquid (CTL) and gas-to-liquid jet fuels that are now fully certified in 50-50 blends, although CTL jet fuels have been in use at South African airports for many years."
Given the long product cycles in aviation, if the airlines don't want to see themselves grounded in the next millennium, they might be wise to convert to alternatives sooner than later.
But with aviation growing at around 9 per cent a year, according to the Intergovernmental Panel on Climate Change, greenhouse gas emissions from aviation accounting for around 3.5 per cent of emissions from developed countries, plus the impact of nitrogen oxide emissions and contra trails estimated to be "about two to four times greater than those of CO2 alone", the greening of the skies is going to be as difficult as putting the board of British Airways on bicycles.
The other fuels that could power planes
1) Bio fuels: biomass-to-liquid (BTL)
A study produced by a US government research centre shows that 17 per cent of the United States' imported oil for transportation could be replaced by biofuel made from algae.
Ongoing experiments to make fuel are using everything from algae to logging waste to garbage as a source.
Until five years ago, biofuels had not been thought technically viable as a replacement for jet kerosene. Now considered the best way to ensure a sustainable green future for the aviation industry, they are expected to be certified for commercial airline use in 50-50 blends within the next six months.
Growing plants to make fuel may create more CO2 than burning kerosene. A report by ActionAid and RSPB found that the development of jatropha oil plan-tations would produce 2.5 to 6 times more emissions than fossil fuels.
2) Coal-to-liquid (CTL)
Converting coal to a liquid fuel (CTL) – a process referred to by experts as coal liquefaction – allows coal to be utilised as an alternative to oil. Now fully certified in 50-50 blends, CTL jet fuels have been in use at South African airports for many years. The country has the only commercial coal to liquids industry in operation today.
Even if there were an unremitting rise in the oil price, increasing coal-to-liquids production to significant levels will take decades. Very carbon heavy.
3) Gas-to-liquid (GTL)
A refinery process converts natural gas or other gaseous hydrocarbons into gasoline or diesel fuel. Methane-rich gases are converted into liquid synthetic fuels, either via direct conversion or into a synthetic fuel using something called the Fischer Tropsch process.
GTL has the same CO2 lifecycle as conventional jet fuel, although it scores high in local air-quality terms, owing to no sulphur and reduced particulate emissions and it can utlilise the usually burnt-off methane from oil drilling platforms.
Releases greenhouse gases and refining is energy hungry.
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