By Loz
Blain
April 11, 2022
Facebook
Twitter
Flipboard
LinkedIn
Over a 100-year time period, a tonne of hydrogen in the
atmosphere will warm the Earth some 11 times more than a tonne of CO2, with an
uncertainty of ± 5 Photo credit: DepositphotosVIEW 1 IMAGES
Hydrogen can be used as a clean energy carrier, and running
it through a fuel cell to produce electricity produces nothing but water as a
by-product. It carries far more energy for a given weight than lithium
batteries, and it's faster to refill a tank than to charge a battery, so
hydrogen is viewed as a very promising green option in several
hard-to-decarbonize applications where batteries won't cut the mustard – for
example, aviation, shipping and long-haul trucking.
But when it's released directly into the atmosphere,
hydrogen itself can interact with other gases and vapors in the air to produce
powerful warming effects. Indeed, a new UK Government study has put these interactions
under the microscope and determined that hydrogen's Global Warming Potential
(GWP) is about twice as bad as previously understood; over a 100-year time
period, a tonne of hydrogen in the atmosphere will warm the Earth some 11 times
more than a tonne of CO2, with an uncertainty of ± 5.
How does hydrogen act like a greenhouse gas?
One way is by extending the lifetime of atmospheric
methane. Hydrogen reacts with the same tropospheric oxidants that "clean
up" methane emissions. Methane is an incredibly potent greenhouse gas,
causing some 80 times more warming than an equivalent weight of CO2 over the
first 20 years. But hydroxyl radicals in the atmosphere clean it up relatively
quickly, while CO2 remains in the air for thousands of years, so CO2 is worse
in the long run.
When hydrogen is present, however, those hydroxyl radicals
react with the hydrogen instead. There are fewer cleanup agents to go around,
so there's a direct rise in methane concentrations, and the methane stays in
the atmosphere longer.
What's more, the presence of hydrogen increases the
concentration of both tropospheric ozone and stratospheric water vapor,
boosting a "radiative forcing" effect that also pushes temperatures
higher.
How does hydrogen escape into the atmosphere?
A lot of it is leakage, according to a second report from Frazer-Nash Consultancy. Store
hydrogen in a compressed gas cylinder, and you can assume you'll lose between
0.12 percent and 0.24 percent of it every day. It'll leak out of pipes and
valves if you distribute it that way, losing some 20 percent more volume than
the methane gas that's now running through municipal pipelines – although since
hydrogen is so much lighter than methane, this larger volume equates to just 15
percent of the weight.
Where hydrogen is transported as a cryogenic liquid,
boil-off is unavoidable, and you can expect to lose an average of about 1
percent of it per day. Currently, this is vented to the atmosphere.
Indeed, venting and purging operations are currently common
across the hydrogen life cycle. They occur during electrolysis, during
compression, during refueling, and during the process of conversion back into
electricity through a fuel cell.
Where there is venting or purging, the percentages tend to
dwarf what's lost through simple leakage – for example, current electrolysis
procedures using venting and purging are assumed to lose between 3.3-9.2
percent of all hydrogen produced, depending largely on how often the process
starts up and shuts down – this is a bit of a worry in situations where
hydrogen production is seen as a way to store excess renewable energy that's
not being snapped up by immediate demand.
Purging and venting emissions can be cleaned up
significantly by adding systems to recombine the vented or purged hydrogen back
into water and feed it back into the process – but it'll be a while before
these kinds of operations are economically viable.
In all, the Frazer-Nash report expects that between 1-1.5
percent of all hydrogen in its central modeling scenario will be emitted into
the atmosphere, with transport emissions responsible for around half of that,
and emissions at the production and consumption ends taking up roughly a
quarter each.
Meanwhile, operating under different assumptions, the first
report linked expects somewhere between 1 percent and 10 percent of all
hydrogen in its global scenario will be emitted into the atmosphere,
Does this mean "green hydrogen" should be avoided
in the race to zero emissions?
No. The UK Government report explains that "the
increase in equivalent CO2 emissions based on 1 percent and 10 percent H2
leakage rate offsets approximately 0.4 and 4 percent of the total equivalent
CO2 emission reductions, respectively," so even assuming the worst leakage
scenario, it's still an enormous improvement.
"Whilst the benefits from equivalent CO2 emission
reductions significantly outweigh the disbenefits arising from H2
leakage," it continues, "they clearly demonstrate the importance of
controlling H2 leakage within a hydrogen economy."
Sources: Atmospheric Implications of Increased Hydrogen Use, Fugitive Hydrogen Emissions in a Future Hydrogen Economy via Recharge News
Source: New Atlas