The climate impacts of hydrogen emission

The increasing importance of hydrogen as a "clean" fuel in facilitating the shift towards a decarbonized energy system is being widely recognized. To address the pressing need for decarbonization, governments and industries are rapidly pushing for the expansion of hydrogen technologies, infrastructure, and applications, with massive national incentives and direct investments being allocated for this purpose. Numerous uncertainties still exist in adopting hydrogen as a sustainable clean solution to future energy needs. One of them is the environmental effects of hydrogen leakage and more specifically the atmospheric warming effects of hydrogen leakage. This article gives you a brief explanation of the climate consequences of hydrogen leakage.

The indirect warming

While hydrogen is recognized as crucial for the shift towards a low-carbon economy and meeting net zero greenhouse gas emissions goals, its potential atmospheric warming effects from leakage should not be overlooked. Though hydrogen itself has a short lifespan, its indirect warming impacts result from its interaction with other chemical elements in the atmosphere, such as methane, tropospheric ozone, and stratospheric water vapor, thereby increasing the lifespan of these greenhouse gases. Since the global warming potential (GWP) is calculated based on emissions impact on climate over 100 years, the impact of hydrogen is not typically considered as a long-term contributor.

The extent of the indirect warming effects resulting from hydrogen leakage into the atmosphere is highly contingent on the quantity of the gas leaked throughout its entire value chain, spanning from production to storage and use. Given hydrogen's diminutive molecular size, it can potentially permeate through these stages, with limited sensor technology currently available to detect smaller leakages. In addition, for safety reasons, hydrogen is often deliberately vented into the atmosphere.

When hydrogen is released or leaked into the atmosphere, approximately 70% to 80% of it is absorbed by the soil through diffusion and bacteria uptake. The remaining hydrogen ascends into the atmosphere where it reacts with naturally occurring hydroxyl radicals (OH), leading to an increase in greenhouse gas concentrations in the troposphere and stratosphere. This oxidation process in the troposphere reduces the availability of OH radicals for oxidizing existing methane, thereby prolonging the lifespan of methane and making hydrogen an indirect greenhouse gas.

H₂ + OH —> H + H₂O

Oxidation of hydrogen

Oxidation of hydrogen in the troposphere generates atomic hydrogen that initiates a chain of reactions resulting in the formation of tropospheric ozone that contributes to approximately 20% of hydrogen's radiative impacts.

H + O2 —> HO₂

HO₂ + NO —> NO₂ + OH

NO₂ + hv —> NO + O

O + O₂ + M —> O₃ + M

Oxidation of hydrogen in the stratosphere increases the water vapor levels which causes an increase in the infrared radioactive capacity of the stratosphere resulting in stratospheric cooling. This cooling of the stratosphere has a warming effect on the climate and accounts for 30% of hydrogen’s climate impact. The stratosphere has the ability to absorb and emit infrared radiation which is a type of electromagnetic radiation that is responsible for heat transfer in the atmosphere. The increase in the amount of water vapor resulting from the oxidation of hydrogen enhances this ability leading to stratospheric cooling. When the stratosphere cools it emits less energy out to space since the energy escaping into space from the atmosphere is from a cooler temperature. This causes a radioactive imbalance where more energy enters the earth’s atmosphere than is leaving it. In order to balance this, the Earth’s lower atmosphere and the surface warms up leading to catastrophic climate impacts like melting of the polar ice caps and rising sea levels. In addition, stratospheric cooling can also lead to an increase in stratospheric polar clouds which in turn triggers more ozone-destroying reactions.

Considering both the tropospheric and stratospheric effects, hydrogen’s indirect warming potency is 200 times greater than that of carbon dioxide and several times more than that of methane but is short-lived i.e., occurring within a decade after emission. However, the magnitude of this indirect warming potency is still unknown because it is uncertain how much hydrogen gets leaked over its entire value chain. Hydrogen leak sensors available today are only able to detect larger leaks that impact the safety of the system rather than small leakage that impact the climate.

Actions and Solutions

1. Several research must be performed on hydrogen’s indirect warming effect and compared with other greenhouse gases in order to create a better understanding of the effect of hydrogen on global temperature under different leakage scenarios.
2. Accurate measurement of very small hydrogen concentrations in order to determine the actual leakage rates. This requires specialized sensors that could measure hydrogen concentrations at the parts per billion level.
3. Developing new metrics that reflect the climate impact of hydrogen on a short-term scale rather than the present 100-year measurement term. This is because hydrogen’s climate impacts are mostly short-lived i.e., for one or two decades, and hence is often overlooked in the current climate metrics.
4. Efforts must be made to reduce hydrogen transport. Hydrogen needs to be processed or conditioned prior to and post transport which requires additional energy and acts as a medium of hydrogen leakage. Hence, hydrogen must be produced and used in close proximity.
5. Reducing the retrofitting of existing natural gas infrastructure for hydrogen. Though natural gas infrastructure can be retrofitted or used directly for hydrogen it is better to newly build the infrastructure for hydrogen as it can be better optimized to prevent hydrogen leakage.

In conclusion, while hydrogen holds great promise as a clean fuel for the transition to a decarbonized energy system, it is crucial to consider the potential atmospheric warming effects resulting from hydrogen leakage. These indirect impacts stem from hydrogen's interaction with other greenhouse gases, prolonging their lifespan and contributing to climate change. The extent of these effects depends on the amount of hydrogen leaked throughout its value chain, and detecting smaller leakages remains a technological challenge. Furthermore, the deliberate venting of hydrogen for safety reasons adds to its atmospheric presence. Recognizing and addressing these concerns is vital for ensuring that hydrogen deployment aligns with long-term climate goals and delivers the intended environmental benefits.

References:

1. Ocko, I. B., & Hamburg, S. P. (2022). Climate consequences of hydrogen emissions. Atmospheric Chemistry and Physics, 22(14), 9349–9368. https://doi.org/10.5194/acp-22-9349-2022
2. Menon, S. (2022, July 20). Everyone’s excited about this new climate solution, but it could create a new climate problem. Environmental Defense Fund. https://www.edf.org/article/we-need-talk-about-hydrogen?addl_info=Hydrogen%3A Climate friend or foe%3F
3. Kurmayer, N. J., & Kurmayer, N. J. (2021). Scientists warn against global warming effect of hydrogen leaks. www.euractiv.com. https://www.euractiv.com/section/energy/news/scientists-warn-against-global-warming-effect-of-hydrogen-leaks/
4. For hydrogen to be a climate solution, leaks must be tackled. (2022, March 7). Environmental Defense Fund. https://www.edf.org/blog/2022/03/07/hydrogen-climate-solution-leaks-must-be-tackled
5. Hydrogen leakage “could reduce climate benefits of green H2 in half”: EDF study. (2022, August 2). Recharge | Latest Renewable Energy News. https://www.rechargenews.com/energy-transition/hydrogen-leakage-could-reduce-climate-benefits-of-green-h2-in-half-edf-study/2-1-1270605