Aviation futures

The aviation industry accounts for nearly one billion tonnes of carbon dioxide emissions annually against a global total in excess of 40 billion tonnes, which means it is around 2.5% of total emissions. However it is a high growth sector without obvious alternative energy carriers, implying that as other emissions fall and aviation continues to rise, its fraction of the total increases rapidly. Should global emissions fall by 50% in the same time that aviation doubles, then that fraction becomes ten percent.

Perhaps for the reasons given above, there is a particular focus on aviation emissions, far more so than a sector such as steel where emissions are currently much higher and will likely continue to be for many years. Additionally and unlike sectors such as steel and cement which are seen as essential elements for the provision of basic services such as homes and clean water, some people view aviation as a luxury which isn’t entirely necessary for modern society, hence the calls for limiting air travel as a means of limiting emissions from the sector. I don’t see rationing as a realistic pathway forward; the ability to traverse the globe in under a day has become too valuable.

The aviation sector will need to find another way forward. The current focus is on sustainable aviation fuels, such as those made from waste and sustainably harvested biomass, or perhaps even synthetic hydrocarbon fuels derived from processes that chemically combine hydrogen (from electrolysis of water using renewable energy) and carbon dioxide (from direct air capture). This latter route is currently very costly given the state of the variety of technologies required  and large scale use of biomass always raises questions around its origin, the overlap with agriculture for food and the emissions that might arise from land use to create the biomass. Nevertheless, both these routes are worth pursuing.

But what other options might exist for the industry in the decades ahead?

Apart from balancing the emissions from aviation with some form of atmospheric carbon dioxide removal (from growing trees to direct air capture and geological storage), other routes would have to involve a shift away from hydrocarbon based fuels.

One possibility that is currently gaining favour is the development of electric propulsion, with the plane using battery storage for the electricity. This is also problematic due to the very low energy density of batteries when compared to hydrocarbons, a subject I discussed in an earlier posting. Battery electric planes might emerge for very short haul flights (e.g. London City Airport to Rotterdam, a flight I take quite a few times each year), but it is improbable for longer haul travel due to weight constraints. Today, 80% of the aviation industry’s emissions come from passenger flights longer than 1,500km, which means that a long-haul alternative technology needs to emerge.

One possibility is to use hydrogen directly as a fuel, either in a fuel cell configuration which essentially means an electric plane, or in a jet turbine engine in the same way planes use fuel today. The latter route means that planes could still attain the speeds and altitude of current jets. Hydrogen is well suited to a gas turbine engine.

Hydrogen isn’t weight constrained as an energy carrier, but volume considerations may prove challenging. A new airframe shape would likely emerge as a result.

	Jet A-1	Current Li-Ion Battery	Emerging Li-Metal Batteries	Liquid Hydrogen
Energy MJ/kg	42.8	~0.7	~2	120
Energy MJ/litre	37.4	~2	~4	8

Today, the development of hydrogen for aviation appears limited, with the BBC reporting a decade ago that there was no future for the fuel, despite proof of concept. Cost seemed to be the major issue, but today the cost of renewable electricity has fallen sharply and electrolysis to produce hydrogen is seeing a resurgence of interest.

A trawl through various online resources reveals that hydrogen jet turbines have been demonstrated for over 60 years. The first such flight was a modified B-57 Canberra Bomber, with one jet engine powered by liquid hydrogen from a cryogenic wing suspended tank (shown below)

In the 1980s Tupolev modified a Tu-155 to act as a test vehicle for a variety of alternate fuels, which included liquid hydrogen. The Tu-155 first flew on 15 April 1988. It used hydrogen fuel and later liquified natural gas (LNG). It flew until the fall of the Soviet Union and is currently stored at Ramenskoye Airport near Zhukovskiy. The cryogenic tank was stored in the main fuselage of the plane, in part due to the volume constraint noted above.

In the mid to late 1950s Lockheed proposed a concept reconnaissance aircraft that operated on liquid hydrogen and a design was also proposed for a Lockheed L-1011 passenger plane some years later. This was in response to the oil shocks of the 1970s.

Despite over sixty years of on-off development of hydrogen powered flight, hydrocarbon fuels remain the only energy carrier for commercial aviation. Today, the industry faces a long term challenge that may require it to revisit shelved technologies, applying new concepts and materials to the variety of issues that couldn’t be fully solved in the decades past. In the Sky Scenario, we imagine that the first hydrogen intercontinental flight takes place in the mid to late 2040s, with a new generation of planes entering service from about 2050. Even then, it takes decades more for this to become a mainstay within aviation, but then could well be the only technology of the 22^nd century. In the interim in Sky, biofuels and removal technologies (bioenergy with CCS in Sky or BECCS) are used to deliver a net-zero emissions aviation system.