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August 4, 2023
By Poly Martin
Green hydrogen
production will have a negligible impact on global water use, says US
think-tank
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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)
Green Play Ammonia™, Yielder® NFuel Energy.
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