Long distance transport over sea and air cannot be electrified in the foreseeable future. The energy density of batteries is too low to sustain movements of more than a few tens of kilometres. Fuel cell airplanes are possible, but are unlikely to operate successfully on journeys of more than an hour. Similarly some short distance sea travel can use batteries. Ferries are good examples. But most marine transport will need an alternative fuel.

This article argues that ammonia will be energy provider for much of global shipping but that aviation needs synthetic fuels made from hydrogen and captured CO2.

Why? Ammonia is not energy dense enough for aviation, and raw hydrogen would use too much space. But for shipping, where space and weight used by the fuel are not important constraints, ammonia will have a more important role. It may well be cheaper than liquid hydrogen and certainly less costly than synthetic fuels. Nevertheless, synthetic methanol may challenge ammonia, partly because the transition away from fossil oil will be easier to manage.

Aviation.

Decarbonisation of long-distance air travel is not yet widely discussed, perhaps because it seems too difficult. Airbus has published some details of ‘concept’ aircraft designed to run on hydrogen.[1] Reaction Engines, a start-up from the space technology cluster around Oxford, recently announced a joint venture to build reactors that turn ammonia back into hydrogen for use on board airplanes and for other applications, including rockets.[2]

Neither manufacturers nor airline have detailed plans for decarbonisation although ‘Sustainable Aviation Fuel’ (SAF) is often mentioned. SAF is assumed to be made from biomass or waste but, as stated in the previous article on this site, nobody contends that more than a small fraction of total aviation fuel can be made in this way.

Broadly speaking, the real options for full decarbonisation of long-distance aviation are liquid hydrogen, ammonia or synthetic fuels. My belief is that although synthetic aviation fuels are likely to be more expensive than fossil energy for decades they are nevertheless the logical way to decarbonise aviation.

Ammonia is disadvantaged by its ratio of energy to weight.

Ammonia 5.2 MWh per tonne

Conventional aviation fuel 11.9 MWh per tonne

So to provide the same amount of power to take off, cruise and then land would take over twice as much weight as today’s aviation fuel (and therefore also more than twice as much weight as an identical synthetic kerosene made from CO2 and H2).

Does this matter? Yes, a lot. Please take a look at the numbers below.

Empty weight Max. fuel weigh

Airbus A380 – 900 277 tonnes 254 tonnes

Boeing 737 -900 45 tonnes 24 tonnes

For the Airbus 380 long distance aircraft, the amount of fuel that can be taken on board is almost the same as the empty weight of the aircraft. If it were ammonia, the energy value of this fuel would be less than half that of aviation kerosene. The maximum distance travelled would therefore be considerably less than half what it is with today’s fuel. (Why ‘considerably less’? Because takeoff consumes much of the fuel for a heavy aircraft, and this is the same whether the journey is 1 hour or 10 hours).

Before the pandemic, the longest scheduled Airbus A380 flight was just over 14,000 km (Dubai – Auckland).[3] With ammonia as the fuel, this aircraft would probably have only been able to travel between New York and London (5,600 km).

Would this disadvantage be outweighed by ripping out a few seats and reducing the number of passengers? No – when the Airbus 380 is full of fuel the weight per passenger of the aviation kerosene is around 400 kg. It doesn’t use that each flight but even a single London-New York flight might use around 150 kg per passenger. The industry assumption is that the typical passenger weighs just under 80 kg. So cutting the number of passengers will not solve the weight problem.

Liquid hydrogen faces a different issue. Although it is more energy dense than aviation fuel in terms of kwh per kg, it requires about four times as much space to store a megawatt hour of energy. The airplanes that used it would have to have a very different shape and be completely re-engineered in other ways. Airbus has shown us photographs of how the planes might look, but the process of designing and producing these aircraft in large numbers will take decades. We also shouldn’t underestimate how expensive it will be to create liquid hydrogen. Not only is the energy cost of liquefaction approximately one third of the energy value of the H2, but the cost of the processing plant is also very high. One recent estimate from the US government is $800m for a 70,000 tonnes a year facility.[4]

We need to start decarbonisation now and synthetic aviation fuels are by far the best option. Be wary of those who push either ammonia or liquid hydrogen.

Shipping

Ammonia will need far more storage space than fuel oil, and it will be difficult to handle and safely store. But it is also likely to be cheaper than synthetic fuel, or liquid hydrogen. This should be enough to ensure its eventual dominance as the core fuel for long distance shipping.

I assume that the cost of the hydrogen necessary to make the ammonia is $1.50 a kg, a level likely to be reached within a decade or so in sun-rich countries. Simple projections of the cost of turning H2 and nitrogen into NH3 (ammonia) suggest an eventual price of around $60 per megawatt hour. At a price for heavy fuel oil or around $730 a tonne, conventional fuel has a cost of around $56 a megawatt hour. So ammonia is eventually likely to be about the same price as fuel oil. But today it is likely to be at least twice the price at most ports around the world.

Synthetic direct substitutes for conventional oils are always to be more expensive per megawatt hour than ammonia, whatever the price of hydrogen. This is because green ammonia doesn’t require the capture of carbon dioxide and its processing into carbon monoxide in the way that zero-carbon synthetic fuels do.

Many shipowners ordering vessels now, and looking for a low-carbon fuel, see ammonia as the logical energy source (although relatively few orders have been made). But is synthetic methanol a viable alternative? Methanol (CH3OH) has a higher ratio of hydrogen to carbon than kerosene and therefore may offer a cheaper route for manufacturing because less CO2 has to be captured to make an equivalent quantity of fuel. However, at whatever price of hydrogen we assume, methanol will probably be more expensive to make than ammonia.

That may not stop shipping businesses purchasing methanol vessels. After the announcement in the late summer that Maersk would buy 8 large dual-fuel methanol container ships, the Singapore shipping company Xpress Feeders announced this week that it would also commission 8 new smaller ships in 2023/24.

The big advantages of methanol include fewer safety concerns and much easier storage and bunkering. It also has a greater energy density per cubic metre than ammonia, although the difference is not huge and methanol will require more space in the fuel tank than oil. Perhaps most importantly, a dual-fuel methanol/fuel oil vessel would be able to switch back to heavy fuel oil if methanol was not available.

Conclusion.

Transport is often described as the core market for hydrogen. But we need to be somewhat nuanced about this conclusion. In the case of the most important market, batteries may be more suitable for heavy long distance road freight than we current assume. Shipping is likely to shift to ammonia and aviation will move to synthetic equivalents to aviation kerosene.

[1] https://www.airbus.com/en/innovation/zero-emission/hydrogen/zeroe

[2] https://www.reactionengines.co.uk/news/news/press-release-joining-forces-deliver-world-leading-decarbonisation-technology

[3] https://simpleflying.com/longest-airbus-a380-flight/

[4] https://www.hydrogen.energy.gov/pdfs/19001_hydrogen_liquefaction_costs.pdf