The aircraft industry is expecting a seven-fold increase in air traffic by 2050, and a four-fold increase in greenhouse gas emissions unless fundamental changes are made.
But just how “fundamental” will those changes need to be and what will be their effect on the aircraft we use?
The crucial next step towards ensuring the aircraft industry becomes greener is the full electrification of commercial aircraft. That’s zero CO2 and NOx emissions, with energy sourced from power stations that are themselves sustainably fuelled.
The main technological barrier that must be overcome is the energy density of batteries, a measure of how much power can be generated from a battery of a certain weight.
Tesla CEO Elon Musk has said that once batteries are capable of producing 400 Watt-hours per kilogram, with a ratio of power cell to overall mass of between 0.7-0.8, an electrical transcontinental aircraft becomes “compelling”.
Given that practical lithium-ion batteries were capable of achieving energy-densities of 113Wh/kg in 1994, 202Wh/kg in 2004, and are now capable of approximately 300Wh/kg, it’s reasonable to assume that they will hit 400Wh/kg in the coming decade.
Another aspect is the exponential fall in the cost of solar panels, which have already become the cheapest form of power in most US states.
The expected 70% reduction in cost of lithium-ion batteries by 2025, and the rapid rise seen in the cost of kerosene-based jet fuel means that there will be a large and growing disparity in the costs of running aircraft that will greatly favour electrification.
As is often the case, the reasons that will slow transition are not technological, but are rooted in the economic and political inertia against overturning the status-quo.
Biofuels while we wait
Considering the average service-life of passenger and freight aircraft are around 21 and 33 years respectively, even if all new aircraft manufactured from tomorrow were fully electric, the transition away from fossil-fuelled aircraft would take two to three decades.
In the meantime, biofuel offers carbon emissions reductions of between 36-85%, with the variability depending on the type of land used to grow the fuel crops.
As switching from one fuel to another is relatively straightforward, this is a low-hanging fruit worth pursuing before completely phasing out combustion engines.
Even though a biofuel-kerosene jet fuel blend was certified in 2009, the aircraft industry is in no hurry to implement change. There are minor technological hurdles and issues around scaling up biofuel production to industrial levels, but the main constraint is price – parity with fossil fuels is still ten years away.
The adoption of any new aircraft technology – from research, to design sketches, to testing and full integration – is typically a decade-long process.
Given that the combustion engine will be phased out by mid-century, it would seem to make more economic and environmental sense to innovate in other areas: airframe design, materials research, electric propulsion design and air traffic control.
Chopping the tail
Once electric aircraft are established, the next step will be to integrate a gimballed propulsion system, one that can provide thrust in any direction. This will remove the need for the elevators, rudders, and tailplane control surfaces that current designs require, but which add significant mass and drag.
Ash Dove-Jay, University of BristolThe skeleton of a new generation wing, making the best of wind-tunnel experiments.
The wings we are already designing are near their peak in terms of aerodynamic efficiency, but they still do no justice to what nature has achieved in birds.
Aircraft design templates are a century old – constrained by the limitations of the day then, but technology has since moved on. We no longer need to build wings as rigid structures with discrete control surfaces, but can turn to the natural world for inspiration. As Richard Feynman said:
I think nature’s imagination is so much greater than man’s, she’s never going to let us relax.