Hydrogen unlikely to become fuel for aircraft – it is no magic bullet solution for aviation CO2
Date added: March 4, 2017
Over the past decades many have investigated the possibility of using hydrogen as jet fuel, in the hope of keeping the aviation industry growing without massively increasing carbon emissions. A new paper from the Netherlands is enthusiastic about the use of hydrogen, saying it could be a good fuel as it is light. The professor writes: “It is a defect that kerosene is so irrationally cheap, which triggers much unnecessary air travel. A worldwide tax on kerosene – if at all politically possible – should be something to pursue.” However desirable it might be to fuel planes with hydrogen, the reasons it has been rejected in the past are first that producing hydrogen itself takes a huge amount of energy. Then it must be stored, very cold, in tanks far larger than (maybe 4 times as large) those used now on aircraft, even if stored as slush, not compressed gas. Metal hydride storage is also possible. All the options increase the weight of engines etc, outweighing the fact the hydrogen is lighter than kerosene. There could be challenges to using premixed injection with hydrogen rich fuel, since the reaction rate for hydrogen is faster than for jet fuel – there is a danger of flashback, which would have to be dealt with. The problem with contrails and non-CO2 impacts would be as great as with conventional jet fuel.
‘Light, environmentally friendly’ liquid hydrogen proposed as aircraft fuel
By Josh Loeb (E&T Engineering & Technology)
February 24, 2017
Using hydrogen to fuel passenger aircraft deserves serious consideration as a potential solution to the problem of emissions, according to Dutch physicist Professor Jo Hermans, who compared the energy efficiency of modes of transport ranging from bikes to flights.
In a paper published in the journal MRS Energy and Sustainability, Prof Hermans of Leiden University concludes there are numerous advantages involved in using liquid hydrogen for air transport – most notably that it is so light.
Kerosene is currently used to power jet planes, and Hermans acknowledges that, in terms of cost and sheer convenience, this fuel is at present impossible to beat.
But he writes: “It is a defect that kerosene is so irrationally cheap, which triggers much unnecessary air travel. A worldwide tax on kerosene – if at all politically possible – should be something to pursue.”
In his paper, the academic, who has authored several popular science books with titles like Physics is Fun, adds: “Given the severe weight limitations for fuel in aircraft, liquid hydrogen may be a viable alternative in the long run.”
He discounts the potential of having solar-powered planes as this would be all but hopeless without revolutionary changes in aircraft design, writing, “Direct use of solar power is within reach for cars, provided that customers are willing to accept a lower degree of comfort. By contrast, for aviation purposes the direct solar power option seems to be beyond hope.”
Hydrogen is highly flammable and must be stored and handled with care, but Hermans believes taking necessary precautions would be perfectly feasible within the context of an already tightly regulated airport environment.
He points out that losses through ‘boil-off’ are also much less of an issue when using liquid hydrogen to fuel planes, as opposed to in cars.
For road transport, Hermans argues that liquid hydrogen is not a viable option due to safety issues around handling.
Electric vehicles offer the most promising solution, he believes. However, the challenge is to improve the performance of batteries to prolong the driving time, as well as improving the performance of super-capacitors for more rapid charging of the batteries.
Direct driving using solar power is difficult, Hermans finds, even under a clear sky, but he concludes solar family cars will be feasible in future if consumers are willing to sacrifice on comfort.
Transport makes up around 20 per cent of energy use globally – a figure that appears set to grow over coming decades.
One of the most efficient ways to reduce energy use in future, Hermans writes, is to reduce our mobility – for example, through having shorter distances between the workplace and home.
“In other words, urban planning provides an important key,” he concludes.
I remember reading that because hydrogen is so bulky, hydrogen planes are more likely to have to fly in the stratosphere, where water vapour acts as a very strong greenhouse gas. I believe there are studies that suggest the GHG impacts could be worse than those of current aviation.
This report doesn’t say much more than the idea seems worth exploring. Producing the hydrogen uses energy and probably therefore carbon emissions. Contrails might be more troublesome. Its only benefit would be if it stops the aviation biofuel nonsense.
It is entertaining but not very sensible. If hydrogen was burnt it would be hotter and produce more NOx – which is not very helpful. Hydrogen fuel cell + electric motor is not viable for big aircraft. Hydrocarbon fuels are liquid hydrogen courtesy of carbon atoms (the energy needed to create the hydrogen in the first place) – they would need carbon credits to offset the emissions. That could be done. So holding back ever increasing demand for air travel is a better idea.
Aerospace and Aeronautical Engineering: Why don’t jet airplanes run on hydrogen, instead of jetfuel?
The density of liquid hydrogen is very low, only about 71 g/L at 20K. Slush hydrogen may be a bit better.
So even though the energy of combustion per unit mass is pretty high, at about 120 MJ/kg, which compares very favorably with say Jet A, at 42.8 MJ/kg, there seems to be a problem with storage.
Liquid hydrogen needs to be pressurized somewhat, at least, and cooled to cryogenic temperature if you want to use the liquid or slush form. It probably requires about 4 times the fuel tank volume of jet fuel.
Or else hydrogen must be stored at very high pressure if you want to use the gaseous form.
Metal hydride storage is possible and reversible.
But all of these approaches will add significant weight to an aircraft, due to the weight of the specialized storage equipment and no doubt specialized fuel supply system.
There are some challenges to using premixed injection with hydrogen rich fuel, since the reaction rate for hydrogen is seven times as fast as for jet fuel – there is a danger of flashback or autoignition. But NASA is developing gas turbines that use lean direct injection which seem to solve that problem. That part of the technology can likely be handled.
But still, it’s not clear that the reduction in the mass of the fuel load by a factor of 3 would be enough to win out over the added inert mass.
Also as Bill said in his answer, cooling and acquiring hydrogen are pretty energy intensive.
There are certainly pollution advantages to using hydrogen as fuel … there is of course no CO2 emission, and NOx can be strongly mitigated.
And there is certainly a long history of running and designing gas turbines to run on hydrogen.
There have also been a number of experimental development programs as well as experimental aircraft developed that used liquid hydrogen as fuel. The European Cryoplane program has been restarted. Here’s a nice public relations talk summarizing the ideas and some of the challenges: More at
By Michael Fitzpatrick (BBC,Science and technology reporter)
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 73 years ago.
Now hydrogen is being dropped again by the aviation industry.
But this time the promised “green” fuel for powering flights of the future has been quietly shelved in favour of biofuels and more fossil fuel-sipping aviation.
And while hydrogen as a potential “greener” fuel for foreseeable flights gets dumped worldwide, airlines and aircraft manufacturers are also jettisoning their once radical ideas for such hydrogen-burning, sci-fi-like, 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.
It was hoped that hydrogen – whose volatility so spectacularly ended the hegemony of the airship when last used for flight – would 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.
“The energy costs of making hydrogen are enormous,” Professor Ian Poll, head of technology for the UK government-funded sustainable aviation Omega organisation tells the BBC.
“Currently it has to be created with an awful lot of energy. We need a source of electricity to make hydrogen that does not emit CO2, and there are not many of those around.”
He also points out that as world oil prices have been pegged at $70-85 per barrel, alternative fuels are simply not viable and can 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 liquified gas. The result would have been new-look cryo-jets reminiscent of Thunderbird 2, with short wings and a bulging fuselage containing the liquified gas.
Millions of taxpayers’ money has been funnelled into projects that did not seemingly take on board the the fact that hydrogen power would remain costly and polluting for some time to come.
Starting back in 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 of the future.
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 overall operating costs by around 5%.
Despite the drawbacks, reactions from the air industry were positive, with Airbus and its partners Daimler-Benz Aerospace avowing a goal of replacing kerosene with hydrogen to run their engines by 2020.
But for the aerospace giants, hydrogen’s appeal is now much diminished, and the emphasis seems to be on making fossil fuels go further.
“Kerosene is a very good fuel and very difficult to compete with,” explains Rainer von Wrede who works in Airbus’s research and technology department.
“In principle it is possible to fly with hydrogen and we have a proof of concept but for the moment we can not produce enough hydrogen in an environmentally friendly manner for aviation.”
On your bike
Where Airbus, and the aviation industry as a whole, is devoting its research is into reducing consumption further 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 at the moment is alternative jet fuels. Principally biofuels that come from sustainable sources, and do not compete with food and water, ecetera,” Christopher Surgenor, editor and publisher of GreenAir Online tells the BBC.
“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.”
Aviation is growing at around 9% a year according to the Intergovernmental Panel on Climate Change (IPCC), with greenhouse gas emissions from aviation currently accounting for approximately 3.5% of emissions from developed countries.
In addition, the impact of nitrogen oxides emissions and the impact of contrails are estimated to be “about two to four times greater than those of CO2 alone”.
The greening of the skies, it seems, is going to be as difficult as putting the board of British Airways on bicycles.