Before the pandemic grounded most flights, commercial aviation accounted for about 2.5% of global emissions of carbon dioxide. Under pressure to reach net-zero greenhouse gas emissions by 2050, airlines are experimenting with alternatives, including so-called sustainable aviation fuels, or SAF, as well as all-electric aircraft and hydrogen.
Hydrogen is a colorless, odorless, and nontoxic gas that won’t produce acid rain, deplete the ozone, or produce harmful emissions. When converted to energy it only has one byproduct: water. This makes hydrogen an incredibly clean fuel source. Plus, hydrogen can produce electricity and electricity can produce hydrogen, creating an energy loop that is renewable and harmless to the environment.
Compressed hydrogen gas can be used in two ways for aviation: for direct combustion in a turbine and in a fuel cell to generate electricity. Whether hydrogen is used to power a fuel cell to generate electricity or directly combusted for motive power, the only waste product is clean water.
A fuel cell is a device that generates electricity by a chemical reaction. It converts hydrogen and oxygen into water, and in the process also creates electricity. Fuel cells provide many advantages, they are environment friendly as they don’t produce pollutants or greenhouse gasses, significantly improving our environment, high energy efficiency ( can be close to 80% where they generate both heat and electricity), scalable providing power from milliwatts to megawatts, and complementary i.e. readily be combined with other energy technologies, such as batteries, wind turbines, solar panels, and super-capacitors.
Professor Tobias Grosche from the University of Worms in Germany sees electric aircraft only to a limited extent as an alternative. In his opinion, hydrogen has a better potential to power future aircraft. During combustion, modified engines could use the hydrogen together with the ambient air for propulsion. Only small amounts of water vapor and other combustion gases are produced, but no harmful CO₂, according to the professor. Importantly in the context of flight, hydrogen packs a lot of energy per unit of mass – three times more than conventional jet fuel, and more than a hundred times that of lithium-ion batteries.
The disadvantages start with physics and chemistry. Hydrogen has higher energy by mass than jet fuel, but it has lower energy by volume. This lower energy density is because it is a gas at typical atmospheric pressure and temperature. The gas needs to be compressed or turned into a liquid by cooling it to extremely low temperatures (-253C) if it is to be stored in sufficient quantities.
The energy density of liquid hydrogen is only about a quarter of that of jet fuel. This means that for the same amount of energy it needs a storage tank four times the size. As a consequence, aircraft may either have to carry fewer passengers to make space for the storage tanks, or become significantly larger. The first option, which applies to Airbus’s first two concept planes, would mean a reduction in ticket revenue, other things being equal. The second option, embodied in Airbus’s third concept, requires a bigger airframe, which is subject to more drag.
Further, an entire new infrastructure would need to be put in place to transport and store hydrogen at airports, and the refueling itself, would have to be converted to hydrogen. “Storage tanks for the compressed gas or liquid are complex and heavy,” says Finlay Asher, a former aircraft engine designer at Rolls-Royce and founder of Green Sky Thinking, a platform exploring sustainable aviation.
In addition, there is the question of whether hydrogen can be produced at scale and at a competitive price without itself having a large carbon footprint. The great majority of hydrogen used in industry today is created using fossil fuel methane, releasing carbon dioxide as a waste product. Hydrogen can be produced from water through a process called electrolysis, driven by renewable power, but this process is currently expensive and requires large amounts of energy. Only about 1% of hydrogen is produced this way at present.
As things stand, liquid hydrogen is more than four times as expensive as conventional jet fuel. Over the coming decades the price is expected to drop as infrastructure is scaled up and becomes more efficient. But according to Britain’s Royal Society and the management consulting group McKinsey, it is likely to remain at least twice as expensive as fossil fuels for the next few decades.
In a transition period, airport operators would most likely have to have two systems, one for kerosene and another for hydrogen — an expensive option. On the other hand, hydrogen requires more volume than kerosene and has to be either heavily compressed in special tanks or cooled to below minus 200 degrees. “In the end it will be a question of space,” supposes Grosche, because the tanks will be spherical or cylindrical due to the higher loads.
Hybrid Hydrogen Initiatives
Governments and companies are investing in this potential. ZeroAvia’s 2020 hydrogen-powered flight, known as HyFlyer I, was supported by the UK Government, whose Jet Zero Council promises “a laser focus on UK production facilities for sustainable aviation fuels and the acceleration of the design, manufacture and commercial operation of zero-emission aircraft.”
The UK government, together with private investors and commercial partners are supporting ZeroAvia in the development of an aircraft with a hydrogen-electric (fuel cell) powertrain capable of carrying up to 20 passengers about 350 nautical miles (648km). ZeroAvia’s founder and chief executive Val Miftakhov, says the company expects to offer commercial flights using such a plane as early as 2023, and that by 2026 it will be able to realise flights over a range of 500 nautical miles (926km) in aircraft with up to 80 seats. For 2030, Miftakhov has even bigger plans: “We will have single-aisle jets, 100-seat category,” he says.
There is ambition in mainland Europe too. Hydrogen “is one of the most promising technology vectors to allow mobility to continue fulfilling the basic human need for mobility in better harmony with our environment”, says Grazia Vitaldini, chief technology officer at Airbus, the world’s largest aircraft manufacturer. In September 2020, Airbus announced that hydrogen-fuelled propulsion systems would be at the heart of a new generation of zero-emissions commercial aircraft. The project, named ZeroE, is a flagship of the European Union’s multibillion-euro stimulus package, aimed at greening the bloc’s economy.
Airbus has presented three concept planes which it says could be ready for deployment by 2035. The first is a turboprop (propeller) driven aircraft capable of carrying around 100 passengers about 1,000 nautical miles (1,850km). The second, a turbofan (jet), could carry 200 passengers twice as far. Both look similar to already existing planes, but ZeroE’s third concept is a futuristic-looking blended-wing design that is a striking departure from commercial models today. Airbus says this third design could be capable of carrying more passengers over longer distances than the other two, but has not released more detail at this stage. All three designs are envisaged as hydrogen hybrids, which means they would be powered by gas-turbine engines that burn liquid hydrogen as fuel, and also generate electricity via hydrogen fuel cells.