Hydrogen’s role in enabling countries to reach net-zero targets may be complicated by its indirect, short-term global warming potential, suggests a recent study by environmental non-profit the Environmental Defense Fund (EDF).

The study, titled ‘Climate consequences of hydrogen emissions’ and published in peer-reviewed journal Atmospheric Chemistry and Physics, notes that hydrogen’s warming impact is “overlooked and underestimated”, as the indirect warming effects of the gas may last only a couple of decades—whereas standard methods for measuring climate impacts tend to consider warming potential in the long term.

“When we started looking into [hydrogen], we realised that, even though there was a history of literature pointing out these warming effects of hydrogen, it was almost always communicated using the standard global warming potential with [a] 100-year time horizon,” Ilissa Ocko, climate scientist at EDF and co-author of the paper, tells Hydrogen Economist.

1pc – Hydrogen leak rate in best-case scenario

Hydrogen reacts with hydroxyl radicals—a naturally occurring sink for potent short-term greenhouse gas methane—in the atmosphere, which increases the amount of methane as well as other greenhouse gases such as tropospheric ozone and stratospheric water vapour. As such, while switching to hydrogen avoids emissions of long-lived greenhouse gas CO₂, hydrogen leaks can both contribute to and deplete the capacity to remove short-lived greenhouse gases.

Best case, worst case

Assessing what impact scaling up the hydrogen economy would have on the climate in the short term is further complicated by a lack of data on small-scale leaks throughout the value chain.

“We realised it was a very similar issue to what we found ten years ago with natural gas, that there was no data on how much was actually leaking in total,” Ocko says, noting that the scale of small but important hydrogen leaks can be in the parts per billion—too small for most available sensors to measure.

EDF is working with industrial sensor and software developer Aerodyne Research to advance its ability to measure hydrogen leaks to assess the scale of leakage in the existing supply chain.

“We want to start adding it up: how much is leaking from the electrolyser? How much is leaking when you compress it? How much is leaking when you store it? How much is leaking when you move it? When you add it all up, that is when it can become significant,” she says, adding that literature to-date estimates leakage may range from 1-10pc per unit of hydrogen consumed.

While 10pc is on the higher end of estimates and would represent a major loss of valuable product for the industry, the true scale of potential leakage throughout the global supply chain remains unknown.

In the study’s best-case scenario, with a hydrogen and methane leak rate of 1pc, green and blue hydrogen would respectively realise a greater than 95pc and 85pc decrease in warming effect 100 years after switching from fossil fuels, and green would realise this decrease within ten years.

“If you were to minimise your methane leaks… then blue hydrogen should be better for the climate than fossil fuels” Ocko, EDF

But in the worst-case scenario, with a hydrogen leak rate of 10pc and methane leak rate of 3pc, while both forms of hydrogen would still decrease warming 100 years after the switch, blue hydrogen could have a worse climate impact than fossil fuels ten and 20 years on from switching.

“It is that extra methane that really pushes blue hydrogen over the top in terms of being worse [in the short term] than the fossil fuel applications you are replacing,” says Ocko. “If you were to minimise your methane leaks, which we are much further along in terms of having available actions and measures to do that and to monitor that, then blue hydrogen should be better for the climate than fossil fuels. How much better will depend on your hydrogen leakage.”

While reducing long-term warming impacts via avoided CO₂ emissions is important for preventing catastrophic climate change, the consequences of emitting greenhouse gases with short-term global warming potential—including the higher probability that extreme weather events are more frequent and more severe—should not be overlooked, Ocko warns.

“That is why we focus so much on methane, because it is a quarter of today’s warming, but that warming is all from recent emissions in the past 10-20 years. The moment we stop replenishing the atmosphere with methane, we can almost immediately reduce that warming impact,” she adds.

Ammonia

One way to reduce leaks could be to limit the volume of hydrogen stored and transported as a gas. The industry is exploring shipping hydrogen as ammonia, followed by either reconversion or direct consumption. A recent report by consultancy Wood Mackenzie estimates ammonia to be the second-most cost-effective way to ship hydrogen, just behind the liquid form, with existing infrastructure a key advantage.

“First principle is, if you are converting hydrogen into ammonia, and then shipping it as ammonia, you are going to cut out a big part of that value-chain pathway where the hydrogen could be leaking. But ammonia leakage has its own concerns, so it is not perfect, even though it would minimise the hydrogen leakage risk,” says Ocko.

Further research will be needed to quantify how much hydrogen leakage occurs during transportation, conversion to other products and reconversion, she adds.

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Author: Polly Martin