How getting a handle on hydrogen could slash aviation’s CO2 emissions
The challenge with most transport is that you usually need to carry your energy source with you.
Very few forms of transport are directly connected to the grid, with the exception of electrified trains and trams, so finding an efficient way of storing fuel on-board is crucial.
In the 19th and 20th century most forms of transport used coal, followed by petroleum oils. So, in order to decarbonise transport, we need to look for alternative energy carriers other than these petroleum-based products such as kerosene, petrol and diesel which emit CO2 when combusted in an engine. This is a particularly acute challenge for aerospace, where so much fuel is consumed.
Any energy carrier for transport must have a high energy density. There are two types of energy density measurement that designers, engineers and scientists use. Energy density per unit mass (EDM) tells you how much it will weigh and energy density per unit volume (EDV) tells you how much space it will take up. Both are of critical concern in aviation.
Hydrogen, which is seen as the most viable alternative to the kerosene currently used, is actually three times more energy dense per unit mass (EDM) than petroleum-based products. So, theoretically, we could reduce the weight of fuel by a third using hydrogen.
The problem comes when you look at the other measure – EDV. Because hydrogen is a gas at room temperature it takes up lots of space. Even if you compress it to 500 times atmospheric pressure it would take up approximately 10 times more space than petroleum based liquid fuels.
"Because hydrogen is a gas at room temperature it takes up lots of space. Even if you compress it to 500 times atmospheric pressure it would take up approximately 10 times more space than petroleum based liquid fuels."
In addition, containing the gas at such high pressure means you need a very strong vessel to contain it and, even using the latest composite materials, this would add significant weight. Typically, only 5 % of the mass is hydrogen while the rest is the tank and infrastructure. This is a solution for buses, HGV and fuel cell electric vehicles where a large tank and increase in mass can be accommodated, but clearly poses a problem for aviation.
For medium and long-haul aircraft the low energy density (EDV) means compressed hydrogen is not practical. You could liquify it, but it would need to be stored at very low temperatures, -253˚C, and it would still take up four times more space (EDV) than kerosene requiring significant redesign or extension of aircraft fuselage.
The University is leading research into several alternative solutions to address these problems. For aviation we know we need to use materials that can match the benefits of liquid hydrogen, but without the need to cool to such low temperatures.
The University’s hydrogen group is researching complex hydrides – compounds that combine hydrogen with metal - such as borohydrides and more recently metal boron ammoniates. The challenge is to be able to release the hydrogen from these compounds but at modest temperatures, below 100˚C, without losing capacity or stability. This remains the holy grail for research groups across the world.
Metal hydrides ideal for storing hydrogen at low pressure and room temperatures have lower EDM, about half that of compressed gas. However, since they operate at low pressure they have potential for off-road vehicles, airport tugs and power units. The hydrogen group is looking at machine Learning and modelling to develop new alloys to increase the EDM closer to compressed gas.
The H2COOL project is developing dual-use energy storage technology based on such metal hydrides, capable of delivering hydrogen to a fuel cell and at the same time generating direct cooling for refrigeration - when a metal hydride releases hydrogen it cools itself (endothermic reaction), which is ideal for providing hydrogen for power and cooling at the same time.
There are many such hydrogen-related projects ongoing across the university, all of which have the potential to replace CO2 emitting fuels and help us move to greener forms of energy.
"This is just one of many hydrogen-related projects ongoing across the university, all of which have the potential to replace CO2 emitting fuels and help us move to greener forms of energy."
My colleagues and I are also part of the Ocean REFuel project, a £6.5m five-year project, with the universities of Strathclyde, Cardiff, Newcastle University and Imperial College London and funded by the Engineering and Physical Sciences Research Council, investigating the potential for off-shore wind energy to be converted into hydrogen and zero-carbon fuel for aviation and other industries.
Meanwhile, the University’s Sustainable Hydrogen Centre for Doctoral Training and the Centre for Doctoral Training in Sustainable Chemistry are exploring many other hydrogen-related research areas, including sustainable aviation fuels (SAFs).
The challenge for SAFs is reliability of fuel composition, as small changes can have significant influence on optimum engine performance. Research includes developing fuels from biomass (plant material), using a supercritical water approach combined with hydrogen. Other projects are exploring the use of ammonia (NH3) in internal combustion engine, such as the £1.1m SMaRT project on Sustainable Heavy Duty Truck, Marine and Rail Transport, and addressing the challenges of ammonia’s flame speed by injecting with hydrogen.
Hydrogen can also be fed into a fuel cell, where it combines with oxygen to form water and generate an electric current, and we work closely with the world-leading Power Electronics, Machines and Control Research Group to investigate hydrogen’s potential to support electrification of aircraft.
Another example of our holistic, multidisciplinary approach is life cycle analysis of sustainable fuels and their part the circular bioeconomy, including research projects alongside our social scientists studying acceptance of new fuels and adoption by industry of these new technologies. This is critical if our innovations are to be translated and have an impact in securing net zero aviation.
David Grant is Professor of Materials Science and Head of Advanced Materials Research Group, Faculty of Engineering.