How hydrogen can be harnessed to help in the decarbonisation effort

Hydrogen is expected to play a significant role in the deep decarbonisation of transport and industry, especially for energy services that will be difficult or expensive to electrify.

The utility value of hydrogen stems from its versatility as an energy carrier, storage medium, and chemical feedstock. Hydrogen has potential to intermediate the provision of electricity, mobility, heat, and work.

If hydrogen is to contribute to decarbonisation efforts, attention needs to be paid to several aspects of green hydrogen production:

• The energy, materials and emission flows embedded in the hydrogen supply chain

• The scale of green hydrogen deployment necessary to contribute to global decarbonised energy supply

• The net energy of hydrogen production

• The challenges of powering electrolysers with a variable power source.

Life cycle assessment (LCA) and net energy analysis (NEA) are tools for addressing these aspects. The dilemma for LCA practitioners is that there are no large-scale renewable-electrolysis plants in operation from which to collect operating data and plant specifications.

To address this gap, we conducted an environmental and net energy assessment of a hypothetical large-scale solar-electrolysis plant.

Hydrogen production

Hydrogen can be produced from a diverse range of primary energy resources, including renewable and nuclear energy, biomass, natural gas, coal, and oil.

The “clean” and “low-carbon” labels can be applied to several production pathways; however, the “green” label is generally reserved for hydrogen produced almost entirely with renewables.

Much of the international policy discussion has focused on green hydrogen deployment, with an increasing number of jurisdictions announcing policies to accelerate deployment. Australia has announced a National Hydrogen Strategy to develop a “clean hydrogen” industry.

Water electrolysis powered by wind power and solar photovoltaics (PV) are promising candidates for large-scale production of green hydrogen in Australia.

Emissions embodied in hydrogen

In contrast to hydrogen produced via fossil fuels, electrolysis powered by renewable energy produces almost no direct emissions during operation. However, the infrastructure is energy and materials-intensive.

To the extent that fossil fuels are used to manufacture, construct, and operate the infrastructure, the impacts of extracting and burning those fossil fuels will be indirectly embedded in the hydrogen. A global transition from fossil fuels will, of course, reduce those embedded impacts.

Hydrogen certification schemes typically recognise “low carbon” as at least a 60% reduction in greenhouse gas emissions compared to steam methane reforming (currently the dominant hydrogen production method).

However, the measured reduction in emissions depends on the system boundary used for the environmental assessment. The European CertifHy scheme applies the “well-to-gate” system boundary, which is relatively narrow, but captures most of the emissions of fossil fuel-based hydrogen production.

This boundary includes the direct emissions emitted from within a hydrogen production facility, as well as the immediate upstream emissions associated with supply of feedstocks and fossil fuel-based electricity generation.

The proposed Australian certification scheme, or Guarantee of Origin (GO), hasn’t yet determined the system boundary, but stakeholder feedback is generally supportive of the “well-to-gate” boundary adopted for CertifHy.

The broadest system boundary used for environmental assessment, the “cradle-to-grave” boundary, includes the full life-cycle impacts during the manufacture and construction, and subsequent decommissioning, of hydrogen production facilities.

Read more: Mapping Australia’s hydrogen future for large-scale production and delivery

Irrespective of system boundary, estimation of emissions for fossil fuel-based hydrogen is straightforward, because most emissions occur as result of combustion within clearly identified industrial plants and electricity generators.

In contrast, nearly all the emissions associated with renewable-electrolysis are embedded in global supply chains. Since environmental impacts are diffused in a web of energy and material flows, it’s much harder to identify and measure impacts.

Nonetheless, the supply chain of the plant and equipment is critical to the sustainability of green hydrogen.

Energy return on investment

A further factor is that there’s been a marked decline in the energy return on investment (EROI) of the global energy system in recent decades, leading to concerns that a further decline may impede energy transition.

EROI is the ratio of the energy returned from an energy supply process, compared with the energy invested in that process over the full life cycle. It’s conceptually an energy profit ratio, and based on the fundamental principle that any supply system must return more energy than has been diverted from other uses to get that energy.

Resource depletion tends to lower EROI, while technological improvement tends to increase it. To date, technological improvements in oil and gas extraction, significant technological developments in renewables, and smart grids, haven’t been able to reverse the global EROI decline.

As a physical metric, EROI can be useful for identifying physical constraints that may not be obvious from techno-economic analyses alone.

The magnitude of the upscaling required

An important consideration in the context of climate mitigation is the enormity of upscaling required – both at the global scale with respect to the investment, land area, materials, and embodied energy; and at the project scale with respect to the potential localised impacts of gigawatt-scale plants.

Assuming that hydrogen was to supply 5% of final energy in 2050, roughly 5000GW of wind and solar would need to be dedicated to hydrogen production. This implies that perhaps hundreds of gigawatt-scale wind and solar plants will need to be in operation by 2050.

Targets for hydrogen demand beyond 2050 are much greater. Along with hydrogen production and use, energy transition will involve the synchronous upscaling of renewable electricity, batteries, and energy-use technologies. The materials and metals demanded by a low-carbon economy are projected to be immense, creating challenges along the full supply pathway.

Water electrolysis powered by a variable power source

A related consideration is understanding how renewable electrolysis facilities will operate as integrated facilities.

Water electrolysis, hydrogen compression, liquefaction, and ammonia production are subject to physical and thermodynamic operating constraints. Chemical plants are ordinarily optimised for steady-state operation at medium to high operating capacity.

Read more: A breakthrough in green production and storage of hydrogen gas

The challenges of powering electrolysers with a variable power source are known, but the principal strategy for managing renewable variability of pilot-scale plants has been grid buffering. However, grid buffering may significantly increase the GHG emissions embedded in hydrogen, even where grid imports comprise a minor share of energy.

Another strategy is electricity buffering via batteries or other electricity-storage technology.

All these factors tend to increase environmental impacts and reduce net-energy.

An integrated approach needed

Design for sustainability implies that environmental assessment parameters need to be treated as objective functions in plant optimisation.

We recommend that life cycle assessment and net energy analyses are integrated with project planning, along with conventional techno-economic analysis, to inform decision-making to ensure hydrogen meets the goals of sustainable production.