Decarbonising aviation is perhaps the most difficult challenge facing ‘net zero’. Earlier this week the UK’s Royal Society, its premier scientific research institution, produced a very useful summary of just how demanding the transition away from Jet A fuel is going to be. I thought a precis might be helpful, along with some comments about what I see as potential errors in the report.

The Royal Society looked at five leading options

· The use of biofuels, which resemble jet fuel but are made from organic wastes or foodstuffs

· Using hydrogen as the source of energy for aircraft engines

· Employing ammonia as the fuel, which requires hydrogen

· Making synthetic fuels from hydrogen and captured CO2.

· Continuing to use fossil fuels but mandating sufficient carbon capture from air to balance the emissions.

Perhaps surprisingly, the report dismisses battery-powered aviation on the basis that ‘battery technologies are unlikely to have been developed to give the energy density required for most commercial flights in the timescale available to reach net zero by 2050’. Given the potential speed of improvement in the ratio of battery power to weight, this looks just a little too pessimistic.

The central conclusion of the Royal Society’s work is that all the prospective alternatives will demand either a very large share of the UK’s land area or a multiple of today’s production of renewable electricity. If hydrogen is involved, large amounts of electricity are demanded, if biofuels are used, impossibly large amounts of land are needed.

This is uncontroversial. The UK has – I think – the highest percentage of aviation fuel use to total energy need of any large country. The Royal Society tells us that fuel for flying supplied in the UK has an energy content of 145 TWh, which is almost 8% of total energy use of around 1650 TWh for the country as a whole. This is partly because Britons fly more than almost any other nationality but also because some of the UK airports act as hubs for passengers flying from one country to another.

The context of the report is important. Most energy-using activities such as heating or transport will cause reductions in total energy demand in the UK. For example, a heat pump typically uses much less energy (in the form of electricity) than a boiler burning gas. An electric car is roughly 80% efficient at using energy, or over three times as effective as a petrol vehicle.

Aviation is different. Total energy demand to carry a person a thousand kilometres will certainly rise under all of the Royal Society’s options. In the case of synthetic alternatives to aviation fuel, this is because of the energy losses converting electricity to hydrogen, capturing CO2 and then converting the mixture into a chemically identical replacement for aviation fuel. By the way, the same will be true of long-distance shipping, as with any other energy using activity that transfers from a fossil fuel to a gaseous or liquid alternative.

So as decarbonisation proceeds, total energy demand for heating purpose and surface transport will fall but air and sea energy requirements will sharply rise. That is, of course, assuming that the world and its trade continues to move around as much via aircraft and ship. It is possible to imagine a scenario in which air and sea transport might use a half or more of all energy demand in the UK.

This helps us scale the Royal Society’s numbers. At first sight, their figures for the eventual energy demand from aviation look shockingly high. It suggests a maximum of 660 TWh if all fuel is made synthetically, or over four times the current energy demand. For further comparison, this figure is well over twice today’s total UK electricity consumption or around 300 TWh. But if almost all other demands are falling, the requirements for energy look slightly less intimidatingly large.

Energy requirements (mostly electricity) to make alternative fuels

Total energy required

Current aviation fuels Circa 145 TWh

Hydrogen 207-290 TWh

Ammonia 217-332 TWh

Synthetic fuel 468-660 TWh

Jet A plus equivalent DAC 61-148 TWh

In the case of hydrogen, for example, the Royal Society is suggesting that the amount of electricity required to make sufficient quantities of the gas will be between 207 and 290 TWh, or up to double the energy quantity in the fuels of today. This is because the report assumes that electrolysers to make hydrogen may only offer 50% efficiency. One unit of electricity into the electrolyser will only make half a unit of hydrogen expressed in its energy value.

Other alternative fuels would have even lower efficiency, meaning that the amount of energy inputted would have to be even higher. The exception is the interesting alternative of simply collecting CO2 on the ground using direct air capture to balance the emissions in the atmosphere. According to the Royal Society, this has the lowest additional energy requirement.

Of course many will want to know exactly how this balancing is checked and enforced. However in theory all that would have to happen is that each airlines would report its use of fuel bought in the UK and then buy an equivalent amount of CO2. (Each tonne of aviation fuel results in about 2.5 tonnes of CO2 in the atmosphere).

In addition the analysis looks at how much land would be required to meet aviation needs from a variety of different crops and waste materials. In the case, for example, of rapeseeds, the Royal Society estimates that about 68% of all the UK’s agricultural land would be required to produce sufficient to manufacture 12 million tonnes of jet fuel. Other waste materials could supplement the rapeseed but the conclusion has to be that large parts of the entire country would be needed to satisfy fuel demand.

The Royal Society also compares the potential energy requirements for each of the five options to the current (2020) level of renewables output. Excluding biofuels such as biomass burning at Drax and other power stations and electric generation at anaerobic digestion plants, the report says that the UK generated 86 TWh of renewable electricity in 2020. If the country moved entirely to using hydrogen as the fuel for aircraft propulsion, this number would have to approximately quadruple just to meet airline and existing needs.

The implication which we are supposed to draw is that it will prove extremely difficult to replace fossil aviation fuel with either substitutes based on electricity and hydrogen or on biological materials, or both in combination.

Some queries about the numbers and the analysis used.

I certainly don’t want to question the main conclusion of the Royal Society report. Creating an industry which cost-effectively produces a zero carbon alternative to aviation fuel is clearly difficult.

Nevertheless, many of the numbers and analyses in the document do need to be questioned. In general, I believe the Royal Society has chosen numbers that make the transition seem more demanding than it actually is. I also want politely to suggest that the report is cavalier in using unsourced or incorrect figures.

1, Some of the core data is incorrect. For example, the document asserts that the UK’s total production of renewable electricity in 2020 was 123 TWh. The actual number, according to DUKES Energy, the central source for government estimates, was just under 135 TWh.[1] (Please see table 6.2). The Royal Society also suggests throughout its document that the amount of electricity generated by sources not using biomass was 86 TWh. DUKES Energy says that the number was 95.4 GWh.

2, Of much greater importance are the key assumptions about the efficiency of hydrogen electrolysis. The Royal Society uses a range from 50% now to 70% in 2050, giving a government reference for these figures.[2] Unfortunately I could not find these numbers in the government document. However these numbers are far too low. The current estimate of NEL, probably the world’s largest electrolyser manufacturer is over 78% for its most efficient unit.[3] Of course this is a manufacturer’s claim, and needs to be checked, but the percentage is far greater than today’s estimate by the Royal Society. At the lower bound of the report’s estimates, the electricity needed to produce aviation fuel replacements is overstated by at least one third. This affects all the Royal Society’s calculations. And electrolysis can be even more efficient than this if a reliable source of heat is available so that Solid Oxide Electrolysis (SOEC) can be used. In the case of synthetic fuels, using Fischer Tropsch in the manufacturing process produces a high level of waste heat that could be used to give energy to a SOEC.

3, The problem is reversed with green ammonia. Here the Royal Society assumes a manufacturing energy efficiency of greater than hydrogen (71%). But since ammonia is made from hydrogen this is inconsistent with its assumptions about electrolyser efficiency.

4, The report gives figures for the expected energy requirements for making synthetic fuels but it gives no source or other rationale for these numbers. I think the estimates they have used are substantially too pessimistic but one of the tens of companies now in the synthetic fuels industry could have provided robust estimates. LanzaTech, for example, is referred to other places in the document and their figures would have provided some rationale for the low efficiency estimates provided.

In my view there are several other problems with the text and its sources. They range from typos, such as calling the world’s pre-eminent catalyst company Topside instead of Topsøe, through to unsupported assertions, such as saying electrolysers are costly in terms of GHG emissions (they are not).

However the central point is true. The UK, with its high needs for aviation fuel and small land area, is going to struggle to make its own substitutes for Jet A. (Of course, it may be much cheaper to make the hydrogen elsewhere and then import it). But the projected expansion to over 50 GW of offshore wind by 2030 will probably on its own provide almost 200 TWh of extra power. Other energy requirements will also have claims on this new electricity but there is no reason why the UK’s ample wind and solar resources could not provide the energy for fossil fuel substitutes for aviation fuel.

It would be even better if aviation demand fell, not least because the non-CO2 global heating impacts of aircraft are probably as great as the direct effect of burning fuel. We cannot get rid of these problems by substituting hydrogen for Jet A.

[1] https://www.gov.uk/government/statistics/renewable-sources-of-energy-chapter-6-digest-of-united-kingdom-energy-statistics-dukes. Please see table 6.2

[2] https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/1024173/Options_ for_a_UK_low_carbon_hydrogen_standard_report.pdf (accessed 31 August 2022).

[3] https://nelhydrogen.com/product/atmospheric-alkaline-electrolyser-a-series/ and https://www.idealhy.eu/index.php?page=lh2_outline for the energy value of a m3 of H2