Key to greening heavy industry

For decades, solar and wind arrays offered a visible indication that the transition towards a green economy was progressing. But if we are to reduce – and, eventually, eliminate – carbon dioxide emissions, we will have to find a "clean” way not only to produce electricity, but also to power hard-to-abate heavy industries, such as steel, cement, and fertiliser production. Solar and wind energy alone cannot do this job, but hydrogen can.Most hydrogen produced today is sourced from fossil fuels. But there is a better way: solar and wind energy can be used to power an electrolyser that splits hydrogen from water molecules, producing zero-emission "green hydrogen.” The only waste product in this process is oxygen, which can safely be vented to the atmosphere. When the hydrogen is used – for example, in steel production – water is generated again, closing the cycle.In other words, with green hydrogen, heavy industry can be powered by a grand water cycle. So, why hasn’t the world – especially its hard-to-abate industrial sectors – embraced it?The technology, as originally developed, requires huge amounts of fresh water – a resource that, in many parts of the world, is in short supply. Seawater has not been viewed as a viable substitute for fresh water, because chlorine gas and other impurities would build up in and corrode the electrolyser. And while one could desalinate the water before splitting the molecules to create hydrogen, this process is too energy-intensive, inefficient, and expensive to be a practical – let alone economically attractive – approach to large-scale production of green hydrogen.Fortunately, researchers in China seem to have found a solution. Last November, Heping Xie and his collaborators from Shenzhen University and the Institute of New Energy and Low-Carbon Technology at Sichuan University reported in the journal Nature that they had achieved direct electrochemical saline water electrolysis. In other words, they had converted seawater directly into hydrogen. Crucially, their method appears to be "efficient, size-flexible, and scalable,” and does not bring a "notable increase” in operating costs compared to splitting freshwater H2O.If the world’s vast ocean resources can be tapped directly to produce hydrogen, there will be no holding back the green transition. After all, while much of the world’s attention has been focused on decarbonising road transport, heavy industries have been contributing 22% of global CO2 emissions, and they are considered one of the most difficult sectors to decarbonise. With green hydrogen powering this sector – including the production of steel, cement, and ammonia (for fertiliser) – "green growth” will become a reality.How much hydrogen would be needed to realise this vision? The International Energy Agency (IEA) estimates that 450mn tonnes would be required by 2050 if the world is to reach net-zero emissions. While this figure is dwarfed by the 12bn metric tonnes (Gt) of oil, gas, and coal we currently consume each year, it is still daunting, as it amounts to about six times more than the amount of hydrogen produced today, over 95% of which still comes from fossil fuels.Moreover, as shown in a recent book, A Solar-Hydrogen Economy: Driving the Green Hydrogen Industrial Revolution, the IEA estimate is far too low. Green hydrogen matters because it can provide a substitute for fossil fuels in virtually every application, including not only heavy industry, but also transport and domestic and industrial heating. Accounting for hydrogen’s energy density – much higher than that of oil, gas, or coal – it would take 4Gt of green hydrogen to provide an energy-equivalent replacement for the 12bn tonnes of oil-equivalent that the fossil-fuel industry uses to produce energy for all applications each year. This estimate is about eight times the IEA target. This means that, if we follow the IEA’s path, by 2050, the world might be living off an energy system comprising one part hydrogen and seven parts fossil fuels. But one-eighth green is not very green.Make no mistake: green hydrogen can completely replace fossil fuels. But vast investment will be needed to make this happen. In order to produce 4Gt of green hydrogen annually by 2050, we must urgently deploy vast arrays of solar and wind devices, which can power vast arrays of electrolysers that convert vast quantities of seawater into the zero-emission fuel of the future.