6/12/2003 California Institute of
Technology
Hydrogen economy might impact Earth's
stratosphere, study shows
According to conventional wisdom,
hydrogen-fueled cars are environmentally friendly because they emit only water
vapor -- a naturally abundant atmospheric gas. But leakage of the hydrogen gas
that can fuel such cars could cause problems for the upper atmosphere, new
research shows.
In an article appearing this week in the
journal Science, researchers from the California Institute of Technology report
that the leaked hydrogen gas that would inevitably result from a hydrogen
economy, if it accumulates, could indirectly cause as much as a 10-percent
decrease in atmospheric ozone. The researchers are physics research scientist
Tracey Tromp, Assistant Professor of Geochemistry John Eiler, planetary science
professor Yuk Yung, planetary science research scientist Run-Lie Shia, and Jet
Propulsion Laboratory scientist Mark Allen.
If hydrogen were to replace fossil fuel
entirely, the researchers estimate that 60 to 120 trillion grams of hydrogen
would be released each year into the atmosphere, assuming a 10-to-20-percent
loss rate due to leakage. This is four to eight times as much hydrogen as is
currently released into the atmosphere by human activity, and would result in
doubling or tripling of inputs to the atmosphere from all sources, natural or
human.
Because molecular hydrogen freely moves up and
mixes with stratospheric air, the result would be the creation of additional
water at high altitudes and, consequently, an increased dampening of the
stratosphere. This in turn would result in cooling of the lower stratosphere and
disturbance of ozone chemistry, which depends on a chain of chemical reactions
involving hydrochloric acid and chlorine nitrate on water ice.
The estimates of potential damage to
stratospheric ozone levels are based on an atmospheric modeling program that
tests the various scenarios that might result, depending on how much hydrogen
ends up in the stratosphere from all sources, both natural and anthropogenic.
Ideally, a hydrogen fuel-cell vehicle has no
environmental impact. Energy is produced by combining hydrogen with oxygen
pulled from the atmosphere, and the tailpipe emission is water. The hydrogen
fuel could come from a number of sources (Iceland recently started pulling it
out of the ground). Nuclear power could be used to generate the electricity
needed to split water, and in principle, the electricity needed could also be
derived from renewable sources such as solar of wind power.
By comparison, the internal combustion engine
uses fossil fuels and produces many pollutants, including soot, noxious nitrogen
and sulfur gases, and the "greenhouse gas" carbon dioxide. While a
hydrogen fuel-cell economy would almost certainly improve urban air quality, it
has the potential unexpected consequences due to the inevitable leakage of
hydrogen from cars, hydrogen production facilities, the transportation of the
fuel.
Uncertainty remains about the effects on the
atmosphere because scientists still have a limited understanding of the hydrogen
cycle. At present, it seems likely such emissions could accumulate in the air.
Such a build-up would have several consequences, chief of which would be a
moistening and cooling of the upper atmosphere and, indirectly, destruction of
ozone.
In this respect, hydrogen would be similar to
the chlorofluorocarbons (once the standard substance used for air conditioning
and refrigeration), which were intended to be contained within their devices,
but which in practice leaked into the atmosphere and attacked the stratospheric
ozone layer.
The authors of the Science article say that
the current situation is unique in that society has the opportunity to
understand the potential environmental impact well ahead of the growth of a
hydrogen economy. This contrasts with the cases of atmospheric carbon dioxide,
methyl bromide, CFCs, and lead, all of which were released into the environment
by humans long before their consequences were understood.
"We have an unprecedented opportunity
this time to understand what we're getting into before we even switch to the new
technology," says Tromp, the lead author. "It won't be like the case
with the internal-combustion engine, when we started learning the effects of
carbon dioxide decades later."
The question of whether or not hydrogen is bad
for the environment hinges on whether the planet has the ability to consume
excess anthropogenic hydrogen, explains Eiler. "This man-made hydrogen will
either be absorbed in the soil -- a process that is still poorly understood but
likely free of environmental consequences -- or react with other compounds in
the atmosphere.
"The balance of these two processes will
be key to the outcome," says Eiler. "If soils dominate, a hydrogen
economy might have little effect on the environment. But if the atmosphere is
the big player, the stratospheric cooling and destruction of ozone modeled in
this Science paper are more likely to occur.
"Determining which of these two processes
dominates should be a solvable problem," states Eiler, whose research group
is currently exploring the natural budget of hydrogen using new isotopic
techniques.
"Understanding the effects of hydrogen on
the environment now should help direct the technologies that will be the basis
of a hydrogen economy," Tromp adds. "If hydrogen emissions present an
environmental hazard, then recognizing that hazard now can help guide
investments in technologies to favor designs that minimize leakage.
"On the other hand, if hydrogen is shown
to be environmentally friendly in every respect, then designers could pursue the
most cost-effective technologies and potentially save billions in needless
safeguards."
"Either way, it's good for society that
we have an emission scenario at this stage," says Eiler. "In past
cases -- with chlorofluorocarbons, nitrogen oxides, methane, methyl bromide,
carbon dioxide, and carbon monoxide -- we always found out that there were
problems long after they were in common use. But this time, we have a unique
opportunity to study the anthropogenic implications of a new technology before
it's even a problem."
If hydrogen indeed turns out to be bad for the
ozone layer, should the transition to hydrogen-fueled cars be abandoned? Not
necessarily, Tromp and Eiler claim.
"If it's the best way to provide a new
energy source for our needs, then we can, and probably should, do it,"
Tromp says.
Eiler adds, "If we had had perfect
foreknowledge of the effects of carbon dioxide a hundred years ago, would we
have abandoned the internal combustion engine? Probably not. But we might have
begun the process of controlling CO2 emissions earlier."
Contact: Robert Tindol (626) 395-3631
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