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