A carrier, not a source
Hydrogen is the lightest and most abundant element in the universe, but on earth it is almost always bound up in other molecules, such as water (H2O) and hydrocarbons like methane. There are no significant natural reserves of pure hydrogen that people tap routinely today (the possibility of naturally occurring underground hydrogen is being explored, but it is not a mainstream supply). So hydrogen has to be made, and making it takes energy.
That is why it is described as an energy carrier, like electricity, rather than a primary source like coal or sunlight. You spend energy to produce it, then release energy when you use it. Whether this is worthwhile depends on how it is produced, what it replaces, and the losses along the way.
When hydrogen is used in a fuel cell or burned, the main by-product at the point of use is water. This is the basis of its appeal. The emissions question is shifted upstream, to how the hydrogen was made.
How hydrogen is produced
Several routes exist. They are often described with color labels, which are informal and not used consistently everywhere, so check what a source means by them.
- From natural gas (often called gray hydrogen). In steam methane reforming, natural gas reacts with steam to produce hydrogen and carbon dioxide. This is the most common method today, and the carbon dioxide is typically released unless it is captured.
- From natural gas with carbon capture (often called blue hydrogen). The same process, but with much of the carbon dioxide captured and stored. How much is actually captured, and how much methane leaks from the gas supply chain, strongly affects the real climate benefit, and published estimates differ.
- From water by electrolysis (often called green hydrogen when the electricity is renewable). An electrolyzer uses electricity to split water into hydrogen and oxygen. If the electricity comes from wind, solar or other low-carbon sources, the resulting hydrogen has low emissions. If the electricity comes from a fossil-heavy grid, it does not.
- Other routes. Hydrogen can also be made from biomass or waste, from nuclear heat or electricity, and as a by-product of some industrial processes. Some of these are at an earlier stage.
A life cycle assessment is the usual tool for comparing these routes, since the figures depend on electricity mix, equipment and leaks.
Where hydrogen is used now
Most hydrogen today is used in industry rather than for fuel. Oil refining uses a lot, and so does making ammonia for fertilizers and certain chemicals. Much of that supply comes from natural gas. One of the clearest proposed benefits of low-carbon hydrogen is replacing this existing use, because the demand is already there.
Where it might fit
Discussions of future roles tend to focus on applications where direct electrification or batteries are harder:
- Heavy industry. Steelmaking can in principle use hydrogen instead of coal to remove oxygen from iron ore, and some chemical processes need hydrogen as an ingredient. Pilot projects exist, but costs and scale are open questions.
- Long-distance transport. Shipping and aviation are exploring hydrogen or fuels made from it, such as ammonia or synthetic fuels, because batteries are heavy for long routes. Heavy trucks and some trains are being trialed too, though battery electric vehicles are competing strongly in many segments.
- Seasonal energy storage. Surplus renewable electricity could be turned into hydrogen and stored in tanks or underground caverns for later use. This could, in principle, supplement shorter-duration lithium-ion batteries, but round-trip losses are large and the idea is still developing.
- Heating buildings. Blending hydrogen into gas networks or using it for heating has been tested, but many analysts see heat pumps as more efficient for most homes. This is among the more contested applications.
- Cars and light vehicles. Fuel-cell cars exist, but battery electric cars have so far been more widely adopted.
Open questions
Several challenges explain why enthusiasm is often matched with caution.
- Efficiency losses. Making hydrogen from electricity, compressing or liquefying it, moving it, and converting it back to electricity loses a significant portion of the original energy. Direct use of electricity is usually more efficient where it is possible.
- Cost. Low-carbon hydrogen is generally more expensive than hydrogen from unabated natural gas, though forecasts vary and depend on electricity and equipment costs.
- Infrastructure. Hydrogen molecules are tiny and can embrittle some metals, and the gas has a low energy density by volume, so it needs high-pressure tanks, cryogenic liquefaction, or conversion into other carriers. Pipelines, storage and fueling stations are limited.
- Safety. Hydrogen is flammable across a wide range of concentrations and burns with a nearly invisible flame. Handling is done by trained people following codes and standards, never by improvised methods.
- Leakage. Hydrogen is not a direct greenhouse gas in the usual sense, but research suggests that leaked hydrogen can indirectly extend the lifetime of other greenhouse gases such as methane. The size of the effect is still being studied.
- Competing uses for clean electricity. If renewable power is limited, using it for hydrogen may mean not using it elsewhere.
A measured view
Hydrogen is unlikely to be a universal solution, and it is also more than hype. Its likeliest roles are in specific sectors with few alternatives, rather than as a broad replacement for existing fuels. How big those roles become will depend on costs, policy, infrastructure and progress in competing technologies, none of which can be predicted with confidence.
This article is general information, not engineering, safety or investment advice.