Hydrogen - is the lightest and most abundant element in the universe. On earth, it is most commonly found in water, but it exists in every living thing. It burns with an invisible flame. That’s probably all most of us remember about hydrogen from school, which doesn’t do justice to its versatility. After all, it’s not for nothing that there’s this buzz surrounding hydrogen and that it’s often considered the energy storage of the future.
Though it rarely exists on its own, it’s extremely common in compounds (e.g. H2O), which is what makes its production possible. Also, despite being a colorless gas, it comes in different colors - sort of. Depending on the method used to produce it, it’s often referred to as green, blue, grey, or brown hydrogen. (There are other colors too - turquoise, pink, yellow - but they’re not widely used just yet.)
The first in our two-part series about hydrogen will take a closer look at what those colors signify and some of the basics of hydrogen as an energy carrier.
The main reasons why hydrogen is seen as a promising energy carrier are that it can be used in a wide range of industries and has great potential as a type of energy storage. In addition, its supply sources are really diverse. Fossil fuels, as well as low carbon and renewable resources, can be converted or harnessed for their production: gas, coal, biomass, wind, and solar are all reliable sources. Even certain microbes have the ability to produce hydrogen or extract it from biomass. Currently, however, hydrogen is produced almost exclusively from fossil fuels - 99.6% in 2020, according to Wood Mackenzie.
Most commonly, hydrogen is generated from natural gas through a process called steam-methane reforming. The result is grey hydrogen, named as such since the process is environmentally damaging but less so than the production of brown/black hydrogen - produced from lignite or bituminous coal - which causes larger amounts of CO2 emissions. Carbon, the byproduct, is not captured in either process; the production of one kilogram of grey hydrogen emits about 9.3 kg of CO2.
When carbon is captured during the steam-methane reforming or the auto-thermal reforming process and is subsequently stored underground (via carbon capture and storage), we talk about blue hydrogen. It’s generally described as a carbon-neutral energy source, but calling it “low carbon” is more accurate: ca. 10-20% of the CO2 that is generated can’t be captured. Still, it’s a much greener option than either grey or brown/black hydrogen.
Speaking of green, the most environmentally friendly option is, unsurprisingly, green hydrogen, also called “clean hydrogen.” Produced through electrolysis - the process of splitting hydrogen from water molecules - by using energy from renewable sources such as solar or wind power, green hydrogen currently amounts to less than 1% of global hydrogen production, but that number is expected to grow as the cost of renewable energy continues to decrease. Since renewables can’t generate energy at all hours of the day, green hydrogen is seen as the best way of storing the excess produced during peak cycles or at times of low demand and feeding it back into the grid when demand rises. Additionally, green hydrogen can contribute to the decarbonization of several industries, such as the chemical and transportation sectors.
When discussing blue hydrogen, we mentioned that after the steam-methane reforming process, carbon capture and storage (CCS) takes place. “Capture” refers to the process during which carbon is separated from other gases that are produced during industrial processes (e.g. at a steel factory) while “storage” means that the captured CO2 is injected into the ground.
Ideal geological storage sites are saline rock formations and depleted natural gas or oil reservoirs. At depths below 800 meters - the average depth of reservoirs is about 1,500 meters - CO2 is trapped in the subsurface as a supercritical fluid, i.e. a highly-compressed fluid that has the properties of both liquids and gases. But at such depths, temperatures and pressures are so high that CO2 changes its characteristics in a way that it allows for much greater volumes to be stored there than it would be possible at the surface.
Whenever hydrogen is transported - from the point of production to a storage site or to the point of use - it “travels” via pipelines or, in their absence, in super-insulated cryogenic tanker trucks that are designed to carry liquid hydrogen. So, gaseous hydrogen is first liquefied (i.e., cooled to below -253°C, only twenty degrees above absolute zero, the coldest temperature theoretically possible) and then stored in large tanks at the liquefaction plant until it is time for it to be transported. Shipping liquid hydrogen, however, is still in its infancy. Compared to the transport of liquefied natural gas (LNG), it’s an enormous technological challenge that requires a new generation of vessels to keep the hydrogen chilled at -253°C. Susio Frontier, the world’s first ship to transport hydrogen, has been built by Japan’s Kawasaki Heavy Industries and is set to make its maiden voyage early next year. Pilot projects are also being carried out elsewhere in the world (e.g. in Norway) but it should take a few more years for large-scale sea transportation of liquid hydrogen to become a reality.
Hydrogen of all types, especially the green variety, promises to become a significant contributor to energy systems of the future. In part two of our series on hydrogen, however, we will focus on the present and discuss some use cases, power-to-gas technology, sector coupling, and more.