Solar-powered hydrogen production fails to meet clean fuel emission limits

Hydrogen fuel generated from solar struggles to stay below emissions thresholds set for clean hydrogen, though this mode of production is set to dominate. By comparison, hydrogen produced from wind, hydropower and nuclear energy is meeting clean hydrogen standards.

Plane and fuel truck
Hydrogen is a versatile molecule and the most abundant in the universe. The fuel can be used directly or converted into more suitable derivatives, such as sustainable aviation fuels (SAF). Image: Arno SenonerUnsplash

A recent study to quantify the lifecycle emissions of green hydrogen production and transportation found that not all green hydrogen was actually “clean”, based on established guidelines.

The study assessed cradle-to-production greenhouse gas (GHG) emissions of green hydrogen facilities, and found that only hydrogen produced through wind, hydro, nuclear energy and excess renewables – when renewable energy supply temporarily exceeds demand – had emissions below thresholds.

These thresholds refer to clean hydrogen’s emissions standards, measured in kilogrammes of carbon dioxide-equivalent per kilogramme of hydrogen produced (kg CO2e/kg H2). These are set at 4.0 kg CO2e/kg H2 for the United States (US), 3.0 kg CO2e/kg H2 for the European Union (EU), and 2.4 kg CO2e/kg H2 for the United Kingdom (UK).

To be considered “clean”, GHG emissions throughout its production lifecycle cannot exceed these limits. Presently, the UK standards are the most ambitious globally, as per a report by big-four consultancy Deloitte.

A nascent fuel option, green hydrogen – hydrogen fuel produced solely through electrolysis using renewable energy sources – has been gaining traction as a critical contributor in decarbonising hard-to-abate sectors.

A World Economic Forum report highlighted this was important for industrialising Asian countries which faced the dilemma of meeting growing energy demands while transitioning to cleaner, more sustainable sources.

Published in Nature journal, the study looked at over 1,000 planned hydrogen production facilities across 72 countries. Researchers also factored in national grid mixes in 2030, modelled for a policy scenario to limit warming to 2°C.

Under the most optimistic production configuration, the median GHG emissions of all projects amounted to 2.9 kg CO2e/kgH2, falling just below the EU standard but remaining above the UK’s.

The number did not include transport-related emissions, which would add a further 1.5-1.8 kg CO2e/H2, depending on whether the fuel was transported as liquid hydrogen, or piped to its destination.

Emissions from solar-powered production stayed only partly below the US limit due to the higher lifecycle emissions of photovoltaic arrays.

For comparison, wind energy emitted about 34 grammes of CO2 equivalent per kilowatt-hour (g CO2e/kWh) of electricity, whereas solar panels generated nearly 50g CO2e/kWh.

However, solar is expected to power the bulk of hydrogen production by 2050, its share predicted to grow to over 60 per cent, from a projected 40 per cent in 2030.

Optimal power supply configuration

Another issue concerned production models, where the study’s researchers found the “grid-connected: power export” production configuration to emit the lowest amount of GHG, regardless of the source of power used.

In the above configuration, the excess power generated by renewable sources was exported to the grid for use by other consumers, and thus not included in the emissions calculations for hydrogen production.

Nature publication green hydrogen emissions chart based on varying configurations

When different power supply configurations were considered, renewable energy use in the “grid-connected: power export” configuration led to the lowest greenhouse gas emissions. Using nuclear, excess renewables, and hydropower also generate significantly lower emissions, as compared to only using electricity from the grid. Image: Nature Energy

By comparison, the additional power generated in the “off-grid: curtailment” model was not exported, and their emissions were attributed entirely to hydrogen production.

The worst-performing configuration with the highest GHG emissions was the “grid-connected: power import” model, which relied on largely fossil-fueled grid electricity to top off intermittent renewable energy generation. 

China, the world’s largest hydrogen producer in 2023 and set to remain so for the next two decades, currently relies on fossil fuels for production. The nation has committed to generating about 70 per cent of hydrogen with renewable energy by 2050.

However, this means that a third of the electricity required will still come from carbon-intensive sources.

“As long as electricity grids are not fully decarbonised, using electricity from the grid, even if only to supplement intermittent renewables, can result in emissions exceeding those of grey hydrogen production, undermining the climate mitigation potential of [green hydrogen] projects,” the paper highlighted.

Green hydrogen is considered the more sustainable alternative to its grey counterpart, which relies on the highly polluting coal or natural gas for production. 

Clean hydrogen is anticipated to be a key element in the global transition to net zero. But at present, it cannot compete economically with fossil fuels. Scaling up production and patching the renewable energy supply gap of some 100 gigawatts (GW) will be necessary to help meet projected demand by 2030.

Amid increasing investments in green hydrogen and its associated infrastructure, the study’s authors have called for more clarity on sustainability standards.

They added that under current regulations, which assign zero emissions from renewable electricity use and equipment manufacturing, there is a risk of obscuring the true environmental impacts of green hydrogen production.

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