Green hydrogen explained: the promise, the physics and the catch
Green hydrogen is often billed as a clean fuel for everything, but the energy losses involved in making and using it mean it will likely end up reserved for the jobs electricity cannot easily do.
Why it matters
As countries try to cut emissions from industries and transport that cannot simply be plugged into a battery, hydrogen keeps coming up as a solution. Steelmaking, ammonia production, long-haul shipping and aviation, and heavy machinery all need dense, storable energy, and hydrogen can in principle provide that without the carbon dioxide produced by burning fossil fuels. Green hydrogen, made using renewable electricity, is the version that gets the most attention because it promises a fuel with no direct emissions at the point of production. Understanding how it is made, and why it is not a simple drop-in replacement for oil and gas, helps explain why it features in UK and international energy strategy but has been slower to scale than early enthusiasm suggested.
What makes hydrogen ‘green’
Hydrogen itself has no colour. It is a gas that must be separated from other elements, because on Earth it almost always exists bound up with something else, usually oxygen in water or carbon in fossil fuels. The colour labels refer to how it is produced. ‘Grey’ hydrogen comes from natural gas through a process called steam methane reforming, which releases carbon dioxide. ‘Blue’ hydrogen uses the same process but captures and stores much of that carbon dioxide. ‘Green’ hydrogen is produced by electrolysis, using electricity to split water into hydrogen and oxygen, with that electricity coming from renewable sources such as wind or solar. Because no fossil fuel is burned and the electricity input is low-carbon, the resulting hydrogen carries a much smaller carbon footprint across its production, though not necessarily zero once manufacturing and infrastructure are accounted for.
The physics problem: efficiency losses stack up
The fundamental catch with green hydrogen is thermodynamic. Electrolysis is not perfectly efficient, so some of the input electricity is lost as heat rather than converted into stored chemical energy in the hydrogen. Further losses occur if that hydrogen then has to be compressed or cooled into a liquid for storage and transport, both of which require additional energy. If the hydrogen is later converted back into electricity, through a fuel cell or by burning it in a turbine, there are more losses at that stage too. The result is that using green hydrogen as an intermediate step between renewable electricity and a final energy service is significantly less efficient than using that electricity directly, for example to charge a battery or run a heat pump. This is why energy analysts generally argue that hydrogen should be prioritised for uses where direct electrification is impractical, rather than treated as a universal substitute for electricity.
The infrastructure catch
Hydrogen is also physically awkward to handle. It is the lightest element, which means that even as a gas it takes up a large volume relative to the energy it contains, so it must be compressed to high pressures or liquefied at very low temperatures to be transported economically. Its small molecules can leak through seals and pipework that would contain natural gas without issue, and hydrogen can make some metals brittle over time, a problem for existing gas pipelines and storage tanks. Building dedicated hydrogen pipelines, storage caverns, refuelling stations and industrial equipment represents a substantial and long-term infrastructure undertaking, separate from the cost of the electrolysers and renewable power needed to produce the hydrogen in the first place. Electrolyser technology itself, whether alkaline, proton exchange membrane, or solid oxide systems, is well understood in principle, but scaling manufacturing while driving down costs remains an ongoing industrial challenge.
Where it is likely to matter most
Given these constraints, most energy planners, including those setting UK industrial and energy strategy, see green hydrogen as valuable chiefly for sectors that are genuinely hard to decarbonise by other means. This includes chemical feedstocks such as ammonia and fertiliser production, high-temperature industrial heat, and potentially some shipping and aviation applications where energy-dense liquid fuels are hard to avoid. It may also offer a way to store surplus renewable electricity over longer periods than batteries can manage economically. What it is unlikely to become is a wholesale replacement for electricity in homes, cars or most everyday uses, where the efficiency losses make it a costlier and more wasteful option than staying electric throughout.