If deployed to their maximum potential, liquid hydrogen-powered aircraft could enable aviation emissions to be capped at 2035 levels, although a 6-12% reduction in CO2e emissions relative to projected 2050 levels is more realistic, finds an ICCT study. However, such aircraft, assuming they enter service by 2035 as targeted by Airbus, are likely to come with performance penalties and much higher fuel costs relative to fossil-fuel aircraft. In 2020, Airbus unveiled three concept hybrid-hydrogen aircraft: two powered by turbofan and turboprop engines and the other a revolutionary blended design with turbofan engines. The new study focuses on the smaller turboprop aircraft targeting the regional market and a narrow-body turbofan aircraft suitable for short and medium haul flights, benchmarked against the ATR 72-600 and Airbus A320neo respectively. Together, the two liquid hydrogen powered aircraft could service 31-38% of all passenger traffic, as measured by RPKs. Another ICCT study just published concludes environmental constraints are likely to severely limit the market for supersonic aircraft.
A significant challenge for hydrogen-powered aircraft design is fuel storage as hydrogen stores 2.8 times the energy on a per unit basis than fossil Jet A. However, its volumetric energy density is significantly lower than that of Jet A and for sufficient hydrogen to be carried in an aircraft, its density needs to be increased. This is achieved by storing gaseous hydrogen (GH2) at high pressure, or by liquefying it and storing the liquid hydrogen (LH2) at very low temperatures. Producing the energy of a unit volume of Jet A requires seven times that volume of compressed GH2 and four times that volume of LH2, so making the latter a superior option from the perspective of improving the payload capacity and range of potential aircraft designs.
The hydrogen study notes that compared to fossil-fuel aircraft, LH2-powered aircraft will still be heavier, less efficient as they have a higher energy requirement per revenue passenger km (RPK) and also have a shorter range. Nevertheless, the LH2-powered narrow-body turbofan could transport 165 passengers up to 3,400 km and the turboprop could transport 70 passengers up to 1,400 km. This represents, estimates ICCT, 57-71% of all RPKs serviced by narrow-body aircraft and 89-97% of all RPKs serviced by turboprops. Aircraft with lighter fuel systems and tighter seating density would provide larger market coverage.
The analysis finds fuel costs for a green LH2 (made using additional renewable electricity) will be higher than fossil Jet A-fuelled aircraft but cheaper than blue LH2 (generated from fossil fuels with carbon sequestration) or synthetic e-kerosene for the missions investigated by the study. The price advantage could be smaller or reversed when accounting for the cost of building hydrogen refuelling infrastructure at airports. The cost of e-kerosene production could also increase if more energy-intensive direct air capture, rather than point source carbon capture, is required. ICCT suggests green LH2 will be cheaper than e-kerosene on routes up to 3,400 km.
To make green LH2 cost-competitive with fossil jet fuel, ICCT says taxes will need to be levied on CO2 emissions, with a carbon price of around $250/tonne CO2e needed in 2025 for cost parity in the United States, falling to $100 in 2050. Europe, where renewable hydrogen is expected to be more expensive, may require a higher carbon price.
Using airline route data for 2019, the analysis projected the CO2e mitigation potential of each LH2-powered design running on green hydrogen from 2035 to 2050. ICCT forecasts RPKs to grow at 3% annually, while historical trends suggest the fleet-averaged fuel burn for narrow-body and turboprop aircraft has decreased at 0.5% annually.
Assuming that fleet renewal and growth is sufficient for LH2 designs to cover between 20% and 40% of the addressable market in 2050, there is a mitigation potential of 126-251 million tonnes (Mt) of CO2e in 2050, representing 6-12% of passenger aviation’s CO2e inventory that year. Deployed to their maximum potential (100%), 628 Mt of CO2e could be mitigated in 2050, representing 31% of the projected inventory.
The study, led by Dr Jayant Mukhopadhaya, calls for supportive government policies if LH2-powered aircraft are to succeed, to include carbon pricing, low-carbon fuel standards or alternative fuel mandates to bridge the cost gap with fossil fuel, together with life-cycle accounting to ensure aviation has access to the cleanest sources of hydrogen. An important next step, it recommends, would be to determine where and how to invest in hydrogen infrastructure to maximise the CO2 mitigation potential while minimising cost.
“Even after considering the performance penalties for carrying LH2 as a fuel source, the aircraft modelled in this work can capture a large section of the aviation market,” concludes Dr Mukhopadhaya. “They can provide significant reductions in carbon emissions of the captured market and potentially, at a maximum, cap global passenger aviation emissions at 2035 levels. The aircraft can fit into existing airline route operations but will require significant investment in infrastructure to make them viable.”
Meanwhile, a joint study by ICCT and MIT’s Laboratory for Aviation and the Environment (LAE) has found that both ‘low-boom’ designs and ultralow cost sustainable aviation fuels will be needed for a sizeable supersonic aircraft market to develop. A small number of US aircraft manufacturers, such as Boom Supersonic, which has backing from United Airlines and Japan Airlines, are seeking to develop and reintroduce supersonic transport aircraft (SST). They are pledging to use alternative jet fuels like e-kerosene to address greenhouse gas emissions and contribute to the industry’s net zero target. The US is pushing ICAO to develop international standards that would enable supersonic flights and a meeting of ICAO’s CAEP committee is due to meet February 7 to discuss whether SSTs should have to comply with the same environmental standards as subsonic designs.
ICCT expects SSTs to burn 7 to 9 times more fuel per seat-km than subsonics and e-fuels, despite their ability to reduce life-cycle emissions by around 90% compared to fossil Jet A if made using renewable energy, would only “modestly” reduce CO2 per seat-km compared to today’s aircraft, it says. Additionally, burning low sulphur e-kerosene at the high altitudes SSTs cruise could lead to stratospheric warming and increase the medium-term climate impact of commercial aviation by two-thirds, despite providing only 0.6% of projected available seat kilometres, estimates the study.
“Supersonics and clean aviation fuels mix like oil and water,” contends Dan Rutherford, ICCT’s Aviation Director and co-author of the study. “Supersonics shouldn’t be given weaker environmental standards on the theory that exotic new fuels will clean up the mess.”
The study predicts a market for 130-240 supersonic aircraft in 2035, assuming no environmental constraints, which include overland noise restrictions as well as control of emissions. However, that market falls by 95% to 100% after taking into account overland flight bans and projected e-kerosene costs. Only by using conventional Jet A do the economics work, suggests the study.
“Our modelling confirms that supersonic aircraft will have larger atmospheric impacts per passenger than today’s aircraft,” said Sebastian Eastham, co-author of the study and research scientist at LAE. “We know that fuel composition matters – for the supersonic fleet we evaluated using low sulphur fuel could increase medium-term climate warming without significantly reducing ozone impacts.”
Image: Airbus ZEROe hybrid-hydrogen concept aircraft