Hydrogen is often held up as a potential clean fuel of the future, because it can be burned like oil or gas but releases no climate-warming carbon dioxide (CO2)—only water. But while hydrogen is the most abundant element on earth, there isn’t a large store of pure hydrogen available. To use it, society must manufacture it.
That manufacturing process can release climate pollution, so how "clean" hydrogen is depends on how it’s produced.
The best option for the climate, says Emre Gençer, a principal research scientist at the MIT Energy Initiative, is so-called "green" hydrogen. (Which, like all hydrogen, is actually colorless.) To make green hydrogen, producers use electricity from a renewable source like wind or solar to split water molecules, removing hydrogen from oxygen and taking the H out of H2O.
This process can emit 1 kilogram or less of CO2 per kilogram of hydrogen produced, depending on the supply chain of the renewable electricity and the overall efficiency of the process (1). Currently, for instance, producing green hydrogen using wind energy is a bit cleaner than using solar energy, says Gençer. That’s because manufacturing solar equipment takes more energy, and wind energy installations produce electricity at their maximum output more often than solar projects of the same size.
This is important, because the CO2 emitted by green hydrogen production is nearly all “embedded emissions,” produced while manufacturing the equipment. The more consistently and efficiently you can make hydrogen with that equipment, the cleaner that hydrogen will be.
“The embedded emissions are divided by a much larger power generation value,” says Gençer. “This translates into a lower carbon footprint for generated power and green hydrogen.”
Today, green hydrogen accounts for less than one percent of hydrogen production in the United States (2). Gençer says about 95 percent of projects in the U.S. are “gray” hydrogen, which is produced from natural gas. Gray hydrogen is usually made by using high-temperature steam to break apart methane (CH4), the main component of natural gas. The reaction produces hydrogen, carbon monoxide, and—crucially—CO2. Around 12 kilograms of CO2 are emitted into the atmosphere for every kilogram of hydrogen produced. “Blue” hydrogen, which combines this process with carbon capture, emits three to five kilograms of CO2 per kilogram of hydrogen (3). That’s compared, again, to potentially less than 1 kilogram for green hydrogen.
“The difference is quite substantial,” says Gençer.
There are two big reasons why green hydrogen, despite its impressively low emissions, is so rare today. First, the “electrolyzers” that split hydrogen from water are costly. And second, solar and wind can only run during certain times of day, which means those electrolyzers are not being used to their full capacity. And while producers can turn to electricity from the grid when sun and wind are not available, that usually means relying on CO2-producing coal and natural gas: the hydrogen will no longer be “green” or quite so clean.
Luckily, the same technological advances that could make green hydrogen cleaner would also generally make it cheaper. “If we get cheaper electrolyzers, you will definitely see more green hydrogen coming online,” says Gençer. And cheaper energy storage would also help produce green hydrogen 24/7.
With advances like these, green hydrogen could play a key role in cleaning up industries, like high-heat manufacturing and air travel, that are very hard to run on clean electricity directly. But the success of hydrogen, Gençer believes, rests on whether it can establish itself as a genuinely clean resource.
“If the carbon intensity of the hydrogen is not low enough, its role in decarbonization is zero,” he says. “The reason we are talking about hydrogen today [is] because there are hard to abate sectors with electrification or other decarbonization options, and that's why we see hydrogen as a solution. But that completely depends on how clean our hydrogen production is.”
Published November 29, 2023
FOOTNOTES
1 Valente, Antonio, Diego Iribarren, and Javier Dufour, "Harmonised life-cycle global warming impact of renewable hydrogen." Journal of Cleaner Production, Volume 149, 2017, doi:10.1016/j.jclepro.2017.02.163.
2 Environmental and Energy Study Institute: "Green Hydrogen | Briefing Series: Scaling Up Innovation to Drive Down Emissions." April 27, 2022.
3 Bauer, Christian, et al., "On the climate impacts of blue hydrogen production." Sustainable Energy & Fuels, Issue 1, 2022. doi:10.1039/D1SE01508G.