As the world progressively recognizes the need for “net zero” greenhouse gas emissions by 2050, renewed interest for hydrogen is surging. This need was implicit in the United Nations Framework Climate Change Convention’s ultimate objective, which calls for the stabilization of GHG atmospheric concentrations, though no agenda —and hence no standards— were set. The Paris Accord, by setting the target to limit the global temperature change to 2°C above pre-industrial levels, and striving to limit it to 1.5°C, has filled this void. Various countries, including the U.K., E.U., Japan, New Zealand, and South Africa have adopted a net zero GHG emission (NZE) target by 2050. President Biden had promised the same, and the CLEAN Future Act that has been introduced by the Democrats in the US House in early March places it as one of its goals. Chinese President Xi has also announced a NZE target by 2060.

 

In the last ten years, the costs of variable renewables such as solar photovoltaics and wind power have been divided by about 10 and 3, respectively. They now offer the lowest costs for bulk electricity in many countries when new electricity-generating capacities are compared. In some countries, notably China and India, building a new solar plant is less expensive than burning coal in existing plants.

 

The decisive role of electrification

 Albeit responsible for 37.5% of energy-related CO₂ emissions, electricity only represents 19% of final energy use. However, the electrification of buildings, industries, and transports is key to their decarbonization as electricity itself gets more and more decarbonized thanks to almost unlimited solar and wind resources globally. Thanks to the greater efficiency of heat pumps and electric motorization, electrification will drive large energy efficiency improvements by itself.

 

Renewable heat from geothermal or solar devices as well as from the combustion of biomass will be part of the solution, though their role is more limited by costs, convenience, geographic availability and, with respect to biomass, possible conflicts with food production, biodiversity, and the use of land and water.

Although this may seem paradoxical, the electrification of the broader economy will ease the integration of variable renewables in the energy mix of power systems because it will add new electric loads that can be interrupted at times of peak demand or low sunshine and winds (“dark doldrums”).

 

The various colours of hydrogen

Hydrogen, or rather dihydrogen (H₂) is often presented as “the clean energy of the future”. However, as of now, hydrogen is an industrial gas that serves mostly to transform and clean oil products, and as a feedstock to produce ammonia and methanol, two important inputs for the chemical industry. Its global production absorbs 6% of the natural gas, it accounts for 2% of total coal consumption, and it is responsible for about 830 million tonnes of CO₂ emissions, as much as the entire aviation sector. Only a small fraction of hydrogen – less than 5% - is produced from electrolysis, as a by-product of the chlor-alkali process that provides the industry with chlorine.

 

Green hydrogen produced by water electrolysis that runs on renewables is getting closer to competitivenesss, with “blue” hydrogen from fossils fuels and carbon capture and storage and even with “grey” hydrogen from fossil fuels and large CO₂ emissions (~10 t CO₂/t H₂ from natural gas, 20 t CO₂/t H₂ from coal). Contrary to commonly held belief, electrolysers need not run 24/7 to produce green hydrogen at competitive costs, because the bulk of the cost come from electricity, provided the load factor is at least 30%.

 

The industry, aviation, and maritime transport

Green hydrogen will be key in decarbonizing the chemical and steel making industries, notably ammonia, methanol and naphta production and the reduction of iron ores. As long as oil products are in use, it can also contribute to reduce their GHG footprint. In all these roles, hydrogen acts as an input or a process agent, not as an energy fuel in the usual meaning. Energy is used in the industry sector as either electricity or heat, at various temperature levels, and there is nothing to prevent a breadth of electric technologies to deliver it to all processes. Compact high-temperature heat storage is being developed and will soon turn variable electricity fluxes in constant heat fluxes, at significantly lower costs than electric batteries.

 

Green hydrogen will also produce hydrogen-based fuels for some hard-to-electrify sectors, including maritime transportation and aviation. Green ammonia, a carbon-free fuel that could be used in the current large ships’ Diesel engines after some modifications, appears to be the maritime transport industry’s first choice. Flying requires fuels with the highest possible energy content – both in volume and weight. Biofuels and synfuels are the only realistic options, but the constraints on sustainable biofuels make synfuels the most likely workhorse of sustainable aviation. To be perfectly “carbon-neutral”, however, the carbon atoms they contain must be procured from the air, directly or via the biomass: from any given amount of biomass, one may extract three times more sustainable fuel by combining it with green hydrogen.

 

Ground transports and buildings: electrification prevails

It is less likely that green hydrogen will ever play a major role in ground transports given the greater efficiency of battery electric vehicles and the ongoing improvements of batteries. One needs about two to three times more green electricity to power a vehicle through hydrogen than through a battery. For private cars and light-duty vehicles, it is hard to find a good reason to opt for hydrogen fuel-cell vehicles rather than for battery-electric ones.

 

The odds for hydrogen are slightly better for heavy-duty vehicles, coaches, or trains, possibly with a fuel cell as a “range-extender” coupled to battery. Nonetheless, hydrogen compression, storage, transport, and distribution costs are high. And this is not the only option. Electric road systems, with catenaries or one conductive rail in the ground, are tested in various countries. Synfuels imported from countries with bountiful renewable resources could be imported for road users and could also power these “range-extenders”. Even lower return efficiency would be compensated by lower production, transport, and distribution costs.

 

Using hydrogen for low-temperature heat in buildings – space heating, sanitary waters, and cooking – appears even less justifiable. Electric heat pumps deliver 3 or more kWh of heat for 1 kWh of green electricity used, while the entire hydrogen chain would deliver only 0,5 kWh for the same initial production.

 

Green hydrogen will also play a role in the power sector itself, for long duration storage returned to the grid as electricity through large fuel cells or by combustion in gas turbines. Its low “return efficiency” suggests this role will be limited to guaranteeing electricity security during “dark doldrums”. Large seasonal imbalances will be primarily addressed with the right mix of resources that match the variations of demand – solar in hot countries, wind in temperate countries. Shorter demand – supply mismatches will be addressed with pumped-storage hydropower, compact heat storage and turbines, and batteries.

 

Finally, green hydrogen will support a new trade of energy from countries with large and cheap resources to countries with high demand and more constrained resources. Hydrogen can be stored economically in underground saline cavities and it can be shipped in pipelines when these options are available. Otherwise, it will be produced close to its point of use, including for the production of chemicals feedstocks and fuels that are easy to shore and ships, such as ammonia, methanol, synthetic hydrocarbons or metallic iron.

 

Energy security is not autarky. The European continent today depends heavily on fossil fuel imports. In its effort to decarbonize the economy, improve energy efficiency, electrify buildings, industry, and transports, and develop indigenous renewable energy capacities, it will sharply reduce this dependence. Through important hydrogen-rich feedstocks and fuels from other countries — notably on the south shore of the Mediterranean Sea — and by helping them develop the relevant technologies, the European Union will optimize its investments and diversify its suppliers, while contributing to their sustainable development.