Decarbonising aviation: is hydrogen in it for the long haul?

It is well known that aviation, which currently makes up 2% of global CO2 emissions, is difficult to decarbonise. Solutions which can work for some road vehicles, such as the use of batteries, are unlikely to be effective for all but relatively small aircraft due to their weight (similar problems are likely to affect heavy road vehicles and ocean-going shipping). Given the planned decarbonisation of other sectors, combined with the expected growth in demand for flying, the proportion of global emissions represented by aviation is likely to grow unless strong measures are taken to reduce the industry’s reliance on fossil fuels.

One approach which is being pursued widely across the world (including in the UK, the EU, and the US) is the adoption of “sustainable aviation fuel” (SAF). This is chemically similar to the fossil kerosene currently used by aircraft but is mainly derived from biological sources, the idea being that SAFs produce lower emissions over their whole lifecycle. However, SAFs are currently in scarce supply and expensive, and there are concerns about how sustainable they really are. Therefore, there is also broad interest in developing true zero-emission aircraft, either using battery technology for smaller aircraft or, for larger aircraft equivalent to those used by airlines today, using hydrogen as fuel. Such aircraft would generate no CO2 emissions in flight.

In this context, in 2021, Steer was asked by the European Commission to develop a credible rollout plan for zero-emission aviation in collaboration with the German Aerospace Centre (DLR - Deutsches Zentrum für Luft- und Raumfahrt) in order to identify the likely barriers to market uptake of such aircraft. While electrically powered aircraft were in the scope of the study, given their limitations, the main focus was on the rollout of hydrogen-powered aircraft.

Based on current aircraft designs, hydrogen planes likely to be developed in the near future will be suitable for short- to medium-range flights up to 2,000 nautical miles. Such aircraft could cover almost all routes within Europe (and equivalently within other regions such as North America or Southeast Asia), but long-haul, intercontinental aircraft powered by hydrogen will require revolutionary designs to accommodate the larger volume of fuel required and are therefore probably several decades away.

Our research showed that the development and use of hydrogen-powered aircraft on short-haul routes (e.g., within Europe) is technically feasible but that there are some significant barriers to overcome before they could be rolled out.

First is the obvious fact that hydrogen-fuelled aircraft do not currently exist. However, technology such as hydrogen fuel cells and turbofan engines already exist and, with some significant but foreseeable development, can be adapted to power aircraft using hydrogen. Airbus is actively pursuing the development of such aircraft.

A more fundamental issue is the current lack of available hydrogen fuel. In order to be fully sustainable, hydrogen needs to be produced without generating CO2 emissions, which can be done through the electrolysis of water using electricity from non-emitting sources. The technology for this exists already but will require enormous investment both in renewable energy production (e.g., wind turbines or solar) and in electrolysis equipment. There is significant interest in both of these technologies in order to support a “hydrogen economy” for many industries, of which aviation is only a relatively minor element. However, the level of investment required will be extremely high and is likely to need public sector support.

In Europe, hydrogen produced in this way is likely to be located around the continent’s periphery to benefit from the most abundant forms of renewable power and water, whether wind or solar. The hydrogen then needs to be transported to airports if it is to be used in aircraft. For large volumes to be handled, infrastructure to pipe gaseous hydrogen must therefore be created, with piping put in place to airports handling hydrogen-powered aircraft. It is believed that existing pipelines for methane gas can be utilised, with estimates that up to 75% of the existing gas pipe infrastructure can be repurposed. Nevertheless, there is likely to be a very large price tag, which is unlikely to be covered by the aviation industry alone.

All but the smallest hydrogen-powered aircraft require the hydrogen to be stored in liquid form to minimise its volume (gaseous hydrogen is not only more voluminous but also requires heavy pressurised tanks for storage, hence not feasible for commercial aircraft). However, hydrogen only becomes liquid at -253°C (i.e. only 20 degrees above absolute zero), and the process of liquefaction is highly energy-intensive. Even then, for the same energy content of the fuel, liquid hydrogen requires a volume four times greater than kerosene (or SAF) fuel. Therefore, in addition to pipelines to bring hydrogen on-site, airports will require cryogenic fuel storage facilities, a very large power supply, and equipment to liquefy the hydrogen (which will arrive in gaseous form via the pipeline). These facilities are expensive and will need to be funded, ultimately either by air passengers through airports’ charges to airlines, or through public sector support.

Physical delivery of liquid hydrogen onto aircraft also presents challenges. Firstly, the issue of safety needs to be considered, given the potentially explosive nature of hydrogen. Equipment does exist to allow for safe refuelling, but apart from the fire risk, there are potential dangers to personnel from handling the cryogenically cooled equipment, e.g., frostbite. For an airport with significant hydrogen-powered aircraft, refuelling by cryogenically cooled tanker lorry may lead to vehicle congestion on the airport apron, especially as the volumes of fuel involved are four times greater than would be the case for refuelling conventional aircraft. The alternative is to install cryogenically cooled fuel pipes under the apron, which would be very expensive and would stretch pumping and cooling technologies beyond current capabilities. This is likely to be technically feasible but will require further development and additional cost.

There is then the issue of market competition. The United States is committed to the ambitious development of sustainable aviation fuel (SAF) production through tax incentives, while both the EU and the UK have programmes to mandate the use of SAF. The extent to which SAF from biological origins is truly sustainable is a matter of debate, but its ability to use current fuelling infrastructure and aircraft types gives it a financial edge over hydrogen. Currently, Europe is the part of the world which has shown the most interest and enthusiasm for developing hydrogen-powered aircraft and supporting infrastructure. However, if such aircraft are restricted to use in Europe, the market for them would be many times smaller than in a global rollout, making it difficult to develop them competitively.

Any rollout of hydrogen aircraft will require significant policy intervention and, almost certainly, public sector support. Given the likely level of costs involved, it can also be expected to raise ticket prices while remaining in competition with conventional aircraft technologies. Therefore, measures would need to be taken to level the playing field (e.g., through higher carbon prices or taxation on conventional fuel), making aviation overall more expensive. Clearly, there are political challenges associated with such policies.

Nevertheless, it is only through identifying such challenges that the means to overcome them can be found. Steer is proud to have contributed towards helping identify the means to decarbonise aviation through its involvement in this study.

Read the full report here.