The editor muses on the questions and challenges of alternative aircraft propulsion systems, sustainable aviation fuel and decarbonisation
In the process of compiling EVA magazine and writing on subjects across the aviation industry, your editor hears a surprising range of opinion on sustainable aviation fuel (SAF) and new propulsion technologies. The beliefs expressed vary widely, as does the understanding of the practical realities of new or proposed technologies and the challenges still to be overcome in the decarbonisation of aviation. What follows is not another opinion, but rather a snapshot of those conversations.
The data varies depending on the source, but it is reasonable to claim that aviation is responsible for around 2.5% of anthropogenic carbon dioxide emissions. Business aviation generates around 2% of that 2.5%. Overall, aviation’s contribution to global warming, factoring in contrail and other emissions, is around 4%, of which business aviation is a tiny portion.
These are familiar statistics.
Even at 4%, aviation is a relatively minor contributor to global warming compared to other industries; some sources suggest that fashion, for example, is responsible for as much as 10% of global carbon emissions.
By that standard, aviation is unfairly targeted by climate campaigners and governments. So, why should the industry, and the business and VIP aviation niche in particular, bother to clean up its act? Why invest billions in new technologies that will only very slowly and incrementally reduce aviation’s near total reliance on fossil hydrocarbon fuels?
Perhaps because aviation is so difficult to decarbonise. Flying requires a lightweight, powerful energy source, a fuel of high power density. Nothing satisfies that requirement as well as a liquid hydrocarbon, which is why aviation has evolved around fossil hydrocarbon fuel for more than a century. Power density is rarely so critical in other sectors, where decarbonisation is therefore likely to happen faster. As those sectors clean up their acts, aviation (unchanged) becomes responsible for an increasing proportion of overall carbon emissions and an even greater chunk of global warming, especially as the 10% of the population who fly today is expected to expand over the next couple of decades.
Climate change doubters and conspiracy theorists believe climate change is a construct without fact. Perhaps they are correct. On the other hand, I live in a low-lying area, less than 1m above sea level. If the scientific predictions for the longer-term effects of global warning, especially rising sea level, are correct, then I should be worried. Depending on where you live, perhaps you should too?
Electric dreams
Electric aircraft fly with zero emissions. Is electric propulsion then the solution to decarbonisation? Electric power delivers zero emission in flight, but there is always the question of how the electricity used to charge an aircraft’s batteries was generated and what emissions are associated with that process. At least one UK business aircraft operator hooks its Velis Electro up to a photovoltaic array. It’s a noble effort, but the aeroplane seats two people and flies for less than an hour, so while it is a step in the right direction, it is not even close to replacing the same operator’s Citation jets.
The fundamental challenge faced by electric aircraft designers is energy storage. Batteries are heavy and their energy density compared to a tank of avgas or Jet A is much lower. There is a constant trade-off in aviation between aircraft weight, fuel burn and performance. Every drop of additional fuel capacity makes an aircraft heavier, increasing fuel consumption for the same performance. A balance therefore must always be struck. Battery-powered aircraft are further compromised because their fuel weight does not diminish in flight. As a conventional aircraft burns fuel it becomes lighter and therefore performs better, but a battery weighs the same fully charged as it does dead.
Battery power and electric propulsion are already proving themselves in general aviation and flight training, but the challenges of operating a 40-minute training flight with an electric two-seater suggest that the dream of matching a modern business jet’s performance with an electric aeroplane is untenable.
Unlike a turbine or piston engine, an electric motor is agnostic in terms of energy source, which means it can consume stored electricity released from a battery just as well as electricity generated by a hydrogen fuel cell. Google readily provides an overview of hydrogen fuel cell chemistry; suffice to say here that a constant supply of hydrogen is required as fuel. Large tanks are needed to carry sufficient hydrogen gas for useful range, with the alternative being heavy-duty engineering to enable an aircraft to carry liquid hydrogen.
The only emission from using hydrogen as a fuel in this way is water. Ironically, water vapour contributes considerably more atmospheric warming effect than CO2, but in a short-term, self-regulated kind of way and likely to be of little concern at the lower altitudes at which a hydrogen fuel cell-powered aircraft might operate. Again, there is likely a place for this technology in general aviation and small commercial aircraft, but could the dream of a hydrogen fuel cell-equipped aircraft matching the performance of a conventional light jet ever become reality?
Hybrid propulsion, where a conventional engine drives a turbogenerator producing electricity may offer a more realistic solution to achieving commercially useful range and payload using electric motors. It relies upon liquid hydrocarbons of course, but since motors convert electrical power into propulsive force more efficiently than any turbine converts fuel, there are still carbon emissions savings to be made – and even more so if the engine burns SAF. Hybrid propulsion could sit alongside battery and fuel cell power, perhaps even in the space currently occupied by business aviation turboprops, but it does not deliver jet performance.
Electric motors can turn propellers, rotors or fans. Perhaps once the conundrum of energy storage has been tackled then a complete portfolio of fixed- and rotary-wing aircraft will be possible. But it will take decades.
In the meantime, many companies, most of them start-ups, have chased a new dream: the eVTOL dream. Electrically-powered vertical take-off and landing (eVTOL) vehicles could revolutionise urban mobility, taking traffic off our roads, connecting city centres with airport hubs and delivering sustainable, on-demand, high-speed travel, free from the restrictions and congestion of surface movement.
Setting aside the fact that an electrically powered aircraft only operates truly sustainably if the electricity that charges its batteries is generated without greenhouse emissions, one might ask whether the eVTOL concept is really sustainable, or just a case of chasing down a possibility offered by emerging electric propulsion technologies. Has the industry been drawn into a costly developmental cul-de-sac, or is there a real chance that eVTOLs will whisk sports fans across LA for the 2028 Summer Olympics?
SAF
Accepting the limitations of electric propulsion, and assuming there is still a place for business jets and jet airliners in the skies of tomorrow, liquid hydrocarbons will surely remain the primary power source. The key to dramatically reducing carbon emissions therefore must be sustainable aviation fuel. Manufactured from source materials – feedstocks – rather than extracted from ancient underground reservoirs, SAF does not release ‘new’ carbon into the atmosphere when it is burned.
There are many routes to SAF using a variety of feedstocks. Most of will have a role to play in manufacturing the unimaginably large volume of fuel required to meet ICAO’s stated 2050 deadline for commercial aviation to become carbon neutral. Unfortunately, manufacturing SAF is costly. Extracting oil and refining it into jet fuel is cheap by comparison.
Hydrogen is a seldom-discussed factor in SAF production. It is critical to several SAF manufacturing pathways and its extraction requires lots of energy. That comes with a high cost and, unless green electricity or other renewable energy resources are used, carbon emissions. The cheapest and most widely employed method of hydrogen extraction is from natural gas – a fossil fuel. Arguably, if the hydrogen is not green, then neither is the SAF. Making SAF in the quantities required is therefore unavoidably expensive and potentially not truly sustainable.
But if the business plans are good and the incentives right, then we can only hope the investment in SAF will continue. In the meantime, airport operators are seldom backwards in voicing a particularly sad lament: “We have SAF, but uptake is low because it’s typically twice the cost of Jet A.” Basic economics says greater demand leads to increased production, improving supply and lowering costs. It is a classic chicken and egg situation, one perhaps best left to the machinations of governments to solve. Or is it an individual responsibility? Is there a moral obligation to buy SAF? Should a flight department or individual with a fixed fuel budget fly less in order to fly exclusively on SAF?
There could be a hidden truth in all this. Regardless of the scale of its production, most SAF pathways require hydrogen, which must be extracted without CO2 emissions, otherwise there is very little point to the exercise. That process alone is expensive. Is it time to consider that SAF will always cost more than fossil jet? Is it time to accept that flying must, unavoidably, become more expensive?