August 4, 2023
By Poly Martin

Green hydrogen production will have a negligible impact on global water use, says US think-tank

RMI points out that fossil-fuel-based H2 requires similar amounts of H2O, but that care still needs to be taken to prevent renewable hydrogen projects from straining local water supplies

Water consumption is often raised as a potential future problem for green hydrogen. But analysis from US-based think-tank Rocky Mountain Institute (RMI) suggests that transitioning away from fossil fuels and developing renewable H2 will actually have very little impact on global water use.

An electrolyser needs a minimum of nine litres of water to produce one kilogram of H2.
But RMI calculates that inefficiencies in water purification and process cooling mean that electrolysis actually requires 20-30 litres of water per kilo of hydrogen.

The main highly polluting method of grey hydrogen production, steam methane reforming (SMR), theoretically only needs 4.5 litres of water to produce one kilo of hydrogen. But RMI suggests that similar process inefficiencies for both SMR and upstream natural gas production mean that the actual water use for grey H2 is closer to 21-27 litres per kilo.

Coal gasification, the most emission-intensive pathway for hydrogen production, uses around 25-26 litres of water per kilo of H2.

Blue hydrogen made using SMR with carbon capture and storage (CCS) meanwhile is estimated to have a water footprint of around 32-39 litres per kg of H2, due to the extra water required by CCS.

This means that if green H2 were to displace fossil-fuel-based sources of the molecule, it would have very little impact on overall water consumption.

RMI also notes that an electrolyser’s overall water consumption will depend on where it draws power from, since fossil fuel-fired power plants use more water than wind or solar.

Since hydrogen produces water vapour when burned, some end-uses such as steelmaking could be designed to capture this H2O and feed it back to a co-located electrolyser, the think-tank adds.

And even if green hydrogen production massively ramps up by mid-century as it expands beyond sectors that currently use it, RMI projects that this will still have a minimal impact on water consumption.

“Globally, estimates for future hydrogen demand vary widely, but using the Hydrogen Council’s forecasted 660 [million tonnes] of demand [in 2050] would require 13.2 billion cubic meters of water, or 0.33% of current global freshwater,” the think tank notes, adding that this does not account for potential additional savings if H2 displaces water-intensive fossil-fuel-driven industrial processes.

However, while renewable hydrogen is expected not to be a strain on global water supplies, often water scarcity is a much more localised problem, particularly with rising temperatures increasing the intensity of droughts — which could make siting projects in areas with low freshwater availability for citizens a potential source of conflict.

This has already come to a head in Uruguay. A proposed project in Tambores, co-developed by German company Enertrag and Uruguayan consultancy SEC Ingeniería, currently faces a legal challenge from residents, who fear the plant’s use of around 500 and 700 cubic metres per day of groundwater from the Guaraní aquifer will deplete local water stores.

“Although green hydrogen will not significantly add to global water consumption, diversification of raw water sources, careful consideration of project siting, and review of water pricing schemes will help limit strain on local freshwater access,” RMI notes.

The think tank estimates that desalination and associated water processing — usually required for green hydrogen projects in arid regions — only add around $0.10 to the cost of producing a kilogram of H2 while using 1% of the electricity needed to power electrolysis.

And desalination plants can potentially be upsized in coastal areas to increase freshwater access for the wider community, RMI adds.

However, desalination is controversial from an environmental perspective, since waste brine pumped back into the sea can be harmful to marine ecosystems.

When it comes to pricing, the think-tank recommends moving away from current “decreasing-block” structures that provide discounts for bulk water consumption, and even increasing block rates in order to incentivise the design of green hydrogen plants that prioritise efficiency and the use of non-freshwater feedstock to limit freshwater consumption.

However, the think-tank also notes that there should ultimately be more careful consideration of sites to “avoid adding strain on water systems”, calling for permitting processes to assess local and downstream water availability, competing uses and rights.

RMI suggests that the natural sites for green hydrogen plants would be to replace retired coal plants or other water-intensive industries, “providing continuity of water allocation at a local level”.(Copyright)