Comment: Hydrogen and cryogenics hold keys to net-zero aviation

Commercial aviation, powered by fossil fuels, contributes to around 2.5 per cent of global CO2 emissions, with the UK being one of the top five countries for passenger aviation-related emissions. These are likely to increase, with the International Energy Agency estimating that the demand for passenger (and freight) aviation will triple between now and 2070.

The need to cut carbon emissions in the aviation industry is huge, and the government is making similarly large commitments.  Last year the UK published its Jet Zero strategy, setting out how it would achieve net-zero aviation by 2050.

There is much to do to hit this ambitious target; rapid, disruptive technological innovations are required. Hydrogen-based aviation is one such innovation, and superconducting electrical machines are another. To stand a chance of achieving these net-zero objectives, researchers will need to accelerate the deployment of these technologies in future aircraft.

Hydrogen-based aviation

Hydrogen-based solutions are a leading candidate in the move towards net-zero aviation, pivoting away from conventional jet fuels such as aviation kerosene, to hydrogen combustion (H2C) using cryogenic liquid hydrogen (LH2). Currently, no achievable alternatives for 100 per cent carbon-free long-haul flights exist other than H2C.

LH2 is uniquely suitable as an alternative to kerosene as it offers almost three times its energy density and when burned produces only water vapour, rather than CO2. LH2 can also be produced by electrolysis using electricity generated from renewable sources to generate green hydrogen, offering a 100 per cent carbon-free energy source.

However, hydrogen has a much lower volumetric energy density (around eight times) than jet fuel, so despite carrying more energy per kilogram, liquid hydrogen requires significantly more storage volume. Combined with the fact that hydrogen requires storing in –253°C pressurised containers, this poses significant challenges for traditional aircraft design. As conditions inside these pressurised tanks change and liquid is pumped out, the rate of evaporation of fuel increases. Greater levels of gas inside the tank increase the pressure within, requiring additional safety precautions to avoid the tank becoming compromised mid-flight.

A move to H2C will require redesigning not only combustion engines - an ongoing effort by the likes of Airbus, Rolls-Royce and others - but of the whole aircraft to be able to safely store and manage LH2 onboard. To meet these challenges, significant research is required to better understand the fundamental behaviour of liquid hydrogen.

Superconducting electric machines: the key is cryogenics

All-electric aircraft are another promising alternative to fossil fuels. Yet despite expected efficiencies around performance and emission output, the strict requirements for power-dense, lightweight machines mean applying conventional technology to electrify aircraft poses significant challenges. The incorporation of superconductors at sub-zero cryogenic temperatures promises a path forward.

The ability of superconducting materials to carry large current densities with little resistance makes them attractive in scenarios where high-power density is a necessity, and electrical output must be measured against weight.

The holy grail of superconductivity is a material that superconducts at room temperature, under ambient pressure, and can be fabricated for use in coil windings. It’s unlikely that the latest claim about room-temperature superconductors does this, and until then, superconducting technology will need to be underpinned by cryogenic and vacuum systems. Even “high-temperature” superconductors require cooling to liquid nitrogen temperatures, around –200°C or lower.

For cooling, LH2 is being explored as a viable option for –250°C temperatures since superconductors perform significantly better the cooler they are. Researchers are even looking at dual-purpose LH2 systems for fuelling and cooling all-electric aircraft, like the NASA-backed CHEETA project.

These systems need to be low-weight and inexpensive to see widespread use and must also tackle the issue of ‘AC loss’. When an alternating current (AC) creates a moving magnetic field, this has a dissipative effect on the superconductive materials carrying the current, causing a small resistance where energy is lost – in contrast to direct current (DC) where the resistance is zero.

Because this loss – and the subsequent heat generated – occurs in a cryogenic environment, it takes a lot of energy to extract this heat and maintain the temperatures required for superconductors to operate. For this reason, superconducting machine designs have traditionally only been partially superconducting.

Designing a low-AC loss, fully superconducting machine is therefore of utmost importance, as one would provide unprecedented power densities and pave the way for efficient electric aircraft.

The scale of the issue faced demands a strong response and significant investment is needed to empower the research like that being undertaken in our own Department of Engineering at King’s College London. Industry needs to embrace models which accurately simulate LH2 and superconductors’ combined electromagnetic, thermal and mechanical behaviours to avoid the need for costly physical iterations, so they can be deployed in a tightly regulated industry.

(L-R) Prof David Moxey, Dr Mark Ainslie and Dr Mashy Green from King's College London's Department of Engineering