Can hydrogen-fuelled engines be the next big leap in aviation?

In the test on a hydrogen-driven aero engine, Rolls-Royce and easyJet appear to have overcome a crucial challenge that is often mentioned when hydrogen engines are discussed — although hydrogen is meant to be a green fuel, it is produced using electricity, and generating that electricity can involve burning of fossil fuels. The ground test used green energy to produce green fuel created by wind and tidal power, the two companies said in a press release on Monday.

There are still other challenges to address before a hydrogen-driven aircraft can become a mass reality, including storage and transportation, but there is also research underway to enable the large-scale use of hydrogen as a fuel for internal combustion engines. And if the challenges can be met, there is a lot in favour of using hydrogen as fuel, chief of them being the promise of zero emission of greenhouse gases.

According to a 2021 study published in the journal of Atmospheric Environment, researchers from UK estimated that in 2018, a pre-Covid year, global aviation contributed 1.04 billion tonnes of carbon dioxide, or 2.5% of the total CO2 emissions that year. While other transport sectors are switching to renewable energy, such as in the case of electric cars, the challenge is unique for aviation since jet engines are energy intensive and do not yet have a similar alternative.

How it works

As far as the principle is concerned, a hydrogen internal combustion engine (ICE) works on the same principles as a traditional ICE, except that it uses hydrogen in place of a petroleum-based fuel. Burning the hydrogen fuel produces the energy needed to drive the engine. Liquid or gaseous hydrogen is burned in a modified gas-turbine engine to generate thrust, according to an article on the subject by Airbus.

It is important to note here that a hydrogen ICE is not the same as a hydrogen fuel cell, which has been used in some electric vehicles. In those, hydrogen powers the cells leading to creation of electricity that will drive the vehicle’s engine.

While the primary objective behind promoting it as an internal combustion fuel is that it will not involve greenhouse gas emissions, hydrogen also has other useful properties. A wider flammability range (the temperature band within which hydrogen can be combusted) allows for a wider range of fuel-air mixtures. Hydrogen engines also have greater thermal efficiency than hydrocarbon fuel engines.

History of its use

It was hydrogen that powered what is today credited as being the world’s first combustion engine. In 1804, the French-Swiss inventor Isaac de Rivaz used a mixture of hydrogen and oxygen gases as the fuel for his prototype.

In 1988, the Soviet Union’s Tupolev Tu-155 became the world’s first experimental commercial aircraft operating on hydrogen (liquefied), and later on liquefied natural gas. Tupolev Tu-55 was discontinued after around 100 flights.

In their press release, Rolls-Royce and easyJet, too, described their experiment as a first: the world’s first run of a “modern aero engine” on hydrogen.

Why it’s not widespread

Hydrogen is quite easily produced by running an electricity through water, which splits into hydrogen and oxygen. But if fossil fuels were used to generate that electricity, it would defeat the very purpose of reducing the emissions.

Other challenges include the risk of engine backfire and premature ignition, where the fuel can be stored safely aboard the vehicle, and transportation.

There is no large network of pipelines in place anywhere for transportation of hydrogen. To be carried across long distances in the absence of pipelines, hydrogen would need to be liquefied, which would make transportation easier but would be a very expensive process.

One challenge met

Rolls Royce and easyJet said they used green hydrogen created by wind and tidal power; the hydrogen was supplied by the European Marine Energy Centre (EMEC), generated using renewable energy at a facility in the Orkney Islands, UK.

EMEC has a 500-kW electrolyser for splitting water into hydrogen and oxygen. According to EMEC’s website, the electricity is supplied by tidal energy converters (which are machines that extract energy from moving masses of water) at a company test site, and a community-owned wind turbine.

A novel approach

Just four days before the announcement by easyJet and Rolls Royce, Science journal published a study describing a novel, inexpensive method for producing hydrogen. Researchers at Princeton University and Rice University combined iron, copper, and a simple LED light.

They used nanotechnology to split hydrogen from liquid ammonia. This chemical principle itself is known, but the processes so far had been expensive. Cracking ammonia into its elemental components requires high temperatures (above 400°C), a process that is not only energy-intensive but also requires special equipment.

The new study used light from a standard LED to crack the ammonia without the need for high temperatures. The process is scalable, Princeton University quoted the researchers as saying.