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Hydrogen is a clean and efficient fuel. When burned, it does not emit carbon dioxide or other air pollutants. It can also be used in fuel cells to create electricity, and its only waste is water. It can be produced from renewable sources, such as solar or wind power, making it an attractive option for reducing greenhouse gas emissions.
It presents some challenges, such as the large amount of energy required for its production and the difficulty in storing and transporting it, given its low energy density and its tendency to leak from containers. It also requires advanced technologies to extract its full energy potential.
Despite these challenges, there are several sectors where hydrogen is already being used as a fuel, such as in public and private transportation, industry, and power generation. As technology improves and costs fall, hydrogen is likely to become an increasingly important part of the global energy mix.
And this is a Problem?
“Hydrogen is theoretically the fuel of the future,” says Matteo Bertagni, a postdoctoral researcher at the High Meadows Environmental Institute. “However, in practice, it raises many environmental and technological concerns that need to be addressed.”
Hydrogen readily reacts in the atmosphere with the same molecule responsible for breaking down methane, which is a potent greenhouse gas. If the level of hydrogen emissions exceeds a specific threshold, this reaction is likely to cause a buildup of methane in the atmosphere, which will have long-term climate consequences.
Bertagni is the lead author of a research paper published in Nature Communications explaining the effect of hydrogen emissions on atmospheric methane. Above a certain threshold, in the theoretical case that it replaces the use of fossil fuels, a hydrogen-based economy could cause short-term environmental damage by increasing the amount of methane in the atmosphere.
The risk is given by the leaks that its storage can produce. The damage is compounded by hydrogen production methods that sometimes require the use of methane. To obtain pure hydrogen, it is necessary to extract it from the compounds in which it is part of or combined, mainly water, fossil fuels, and organic matter. This conclusion highlights the critical need to manage and minimize emissions during hydrogen production.
“We have a lot to learn about the consequences of using hydrogen so that the switch to an apparently clean fuel does not create new environmental challenges,” warns Amilcare Porporato, a professor of civil and environmental engineering at the High Meadows Environmental Institute.
The Chemistry of Hydrogen
The problem boils down to a small, difficult-to-measure molecule known as a hydroxyl (OH) radical. It has often been referred to as “the detergent of the troposphere” because it plays a key role in removing greenhouse gases such as methane and ozone from the atmosphere.
The hydroxyl radical also reacts with gaseous hydrogen in the atmosphere. Since a limited amount of OH is generated each day, any increase in hydrogen emissions means that it would be used to break down hydrogen, leaving less available to break down methane. As a consequence, methane would stay longer in the atmosphere, extending its impact on warming.
According to Bertagni, the effects of a peak in hydrogen production could have climate consequences for the planet for decades. “If some hydrogen is emitted into the atmosphere now, it will lead to a progressive buildup of methane in the following years,” explains Bertagni. “Although hydrogen only has a lifetime of around two years in the atmosphere, the methane feedback from that hydrogen would last for another 30 years.”
In the study, the researchers identified the tipping point at which hydrogen emissions would lead to an increase in atmospheric methane, calling into question some of the short-term benefits of hydrogen as a clean fuel. By identifying that threshold, the researchers were able to set targets to help manage hydrogen emissions. “It is imperative that we be proactive in setting thresholds for hydrogen emissions to use in the design and implementation of future hydrogen infrastructure,” says Porporato.
Green and Blue Hydrogen
In the case of green hydrogen, produced by the electrolysis of water, dividing it into hydrogen and oxygen, and using renewable energy sources, Bertagni sets the critical threshold for hydrogen emissions at around 9%. This means that if more than 9% of the produced hydrogen leaks into the atmosphere, either at the point of production, at some point during transport, or at any other time along its value chain, atmospheric methane would increase over the next few decades.
In the case of blue hydrogen, produced from natural gas by capturing and storing carbon dioxide (CO2) instead of releasing it into the atmosphere, the emissions threshold is even lower. Because methane itself is a primary feedstock, blue hydrogen producers must consider direct methane leakage in addition to hydrogen leakage. For example, the researchers found that even with a methane leak rate as low as 0.5%, hydrogen would have to be kept below 4.5% to avoid rising methane concentrations in the pipeline and atmosphere.
Therefore, managing hydrogen and methane leak rates will be critical. According to Bertagni, “A small methane and hydrogen leak would make using blue hydrogen no better than using fossil fuels, at least for the next 20 to 30 years.”
The research emphasizes the importance of the time scale on which the effect of hydrogen on atmospheric methane is considered. In the long term, in a century, for example, the shift to a hydrogen economy would generate net benefits for the climate, even exceeding the levels of methane and hydrogen leakage. Bertagni explains that “eventually, atmospheric gas concentrations would reach a new equilibrium, and the shift to a hydrogen economy would demonstrate its climate benefits.” However, before that happens, the possible consequences will be seen “in the short term since hydrogen emissions could cause irreparable environmental and socioeconomic damage.”
Therefore, if institutions are to meet mid-century climate goals, hydrogen and methane leaks into the atmosphere must be controlled as hydrogen infrastructure begins to roll out. Because hydrogen is a small molecule that is difficult to control and measure, managing emissions will require researchers to develop better methods to track hydrogen losses throughout the value chain, Bertagni adds.
“If companies and governments are serious about investing money to develop hydrogen as a resource, they need to make sure they do it correctly and efficiently,” concludes Bertagni. “Ultimately, the hydrogen economy must be built in a way that does not offset the efforts of other sectors to mitigate carbon emissions.”