January 24, 2020, ©. Leeham News: In the last Corners we tested some of the ideas why electric/hybrid propulsion would be more efficient and found the ideas could be easier and better implemented with existing technology, yet they are not. One wonders why. Perhaps the ideas are not that brilliant after all.
Before we look at alternative technologies that can help us lower air transports CO2 footprint we shall scope the problem and look at what part air transport plays. To what extent is air transport at the center of the problem?
First, does the world have a CO2 problem? Figure 1 shows our concentration of CO2 in the atmosphere is galloping since the 1960s. The effects of this change are becoming more and more visible. Of the contributors, air transport contributes around 2% at present.
The consumption statistics, which are hard to come by, are from the US Department of Energy and is their Worldwide energy consumption by sector from 2012. But as Figure 2 shows, fossil fuels dominate our energy consumption. Figure 1’s right-hand side, therefore, shows the main culprits in our CO2 emission problem.
One source of the phenomenal increase of CO2 concentration is shown in Figure 3. China has in the last 30 years gone from an emission level of Japan to double the level of the US. The major reason for the growth in CO2 emissions in China is the industrial growth, where steel and cement production and the generation of electricity is driven by fossil fuels.
As shown in Figure 1, air transport is a small part of the global CO2 problem. But it is highly visible and by it, an easy target. The airliners are very visible and the emission of exhausts from their engines is easy to spot. Further, air traffic grows by 5% per year, so the emissions will increase unless the fuel consumption of the worldwide aircraft fleet is decreasing with 5% per year.
We have experienced a 15-20% decrease in consumed fuel per passenger-mile for the last generation of aircraft but this is over 15 to 20 years. It means our decrease in fuel consumption per passenger-mile is more like 1% per year. And this is only for the new aircraft added to the existing 20,000 aircraft flying daily.
The above shows the importance of reducing fossil fuel consumption further for our airliners and our efforts to change as much of the consumed fuel from fossil to non-fossil fuel.
But as I have shown in this series the way to drastically changing our propulsion technology for our airliners is difficult. The reason is the efficiency of our modern airliner’s turbofans is very high. Road transport has engines with thermal efficiencies for the latest engine generations of below 30%. The thermal efficiency of our latest turbofans is 55%, double the efficiency of our cars.
On top of the differences in engine core efficiencies, road traffic is inefficient as a system as built-up potential energy is wasted at the next stoplight through our thermal waste brakes. Electric/hybrid cars recover part of this potential energy and can, therefore, compete with combustion cars. But the principle has a hard time to compete with airliner gas turbines. The airline mission has no segment that wastes energy, hence the energy recovery potential is not present.
It’s clear Air transport shall do its part to cut global green gas emissions. But the importance of these emissions must be put in its context, compared with other industries like energy production where non-fossil fuel alternatives are well established and can have a major impact on CO2 emissions.
The messaging on what is reasonable to expect from air transport must improve. It helps no-one if the world’s climate change is blamed on an easy target such as our airliners when it’s nigh impossible to replace hydrocarbons as fuel for anything but extreme short haul over the next decades.
It’s better to communicate what is done already, like the superior efficiency of the airline engine, the non-wasteful flight cycle compared with our road transport, the increased use of renewable fuels and the inefficiency of efforts to change long haul air transport to other propulsion technologies when the same efforts can achieve more elsewhere.
We will not contribute efficiently to solving the world’s CO2 problem if we let the public believe we will fix it with electric airliners tomorrow when the reality is we can’t. Instead, the focus shall be on what part of the problem does air transport amount to and how can it contribute to a fix.
With this said, we will focus the next Corners on what technologies can help us to a sustainable air transport system, including sensible applications of electric/hybrid technologies.