IT WAS never going to be that easy. Cars and power plants running on hydrogen have been touted as the answer to all our environmental problems, from global warming to pollution and smog. But a more problematic vision of hydrogen is now emerging, in which spiralling leakage rates contribute to ozone depletion and even to global warming. If we are to avoid these downsides we had better start planning now.
That’s the warning from researchers modelling the effects of hydrogen in the atmosphere, such as Tracy Tromp and John Eiler from the California Institute of Technology in Pasadena. In June, they pointed out that leaked hydrogen could end up in the stratosphere. There it could react with hydroxyl (OH) radicals to form water vapour, helping to form colder and longer-lasting clouds over the poles. This, they argued, would provide a reaction site for halogens such as chlorine to deplete stratospheric ozone, delaying the repair of the ozone layer (Science, vol 300, p 1740).
Now other scientists are warning of other potentially damaging effects. At the American Geophysical Union meeting in San Francisco next month, Larry Horowitz of the National Oceanic and Atmospheric Administration and his colleagues will report that leaked hydrogen could increase the concentration of greenhouse gases in the atmosphere. Again, the main problem is destruction of OH radicals, but this time in the troposphere (see Graphic). OH is an environmental scrubber, reacting with and removing all manner of pollutants, including the potent greenhouse gas methane. Lower levels of OH would allow methane to stick around in the atmosphere longer.
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Most experts agree that these processes could happen. The big question is how how serious their effect will be, and that depends on how much hydrogen leaks into the atmosphere. At present, about 80 million tonnes of hydrogen goes into the atmosphere every year (see “Hydrogen emissions”). Some of that is natural and some is man-made, with 15 million tonnes coming from industrial uses and the burning of fossil fuels. Tromp and her colleagues caused a stir with their estimate that 10 to 20 per cent of all hydrogen for use in fuel cells would leak. On that basis, converting the world’s 1994 fleet of automobiles to hydrogen would increase industrial emissions to between 60 and 120 million tonnes per year. They based their conclusion on earlier work by others which showed that around 10 per cent of liquid hydrogen transported in Germany is lost through leaks.
The estimate was roundly criticised. “It is probably too high by two orders of magnitude,” says Amory Lovins, co-founder of the Rocky Mountain Institute, an energy think tank based in Snowmass, Colorado. He points out that only 0.1 per cent of the hydrogen gas transported by pipeline in Germany leaks out and that burning less fossil fuel would in itself cut hydrogen emissions. “At the moment there’s lots of molecular hydrogen released, chiefly from incomplete combustion of fossil fuels,” Lovins says. “If you change your energy system to a predominantly hydrogen one, then depending on how you get the hydrogen, the existing release of molecular hydrogen should go down, and it may disappear altogether.”
Leakage rates will also depend on whether cars carry their hydrogen fuel in liquid or gaseous form. Tromp believes that most vehicles will store the hydrogen as a cold liquid under high pressure, to maximise the amount of fuel they can carry. Liquid hydrogen tanks will leak more than gas tanks, she says, because heat seeping into even heavily insulated tanks will steadily turn the liquid hydrogen into gas, which will eventually have to be vented.
Tromp’s and Lovins’s estimates of leakage rates are the extremes; other researchers say rates of between 1 and 3 per cent are more realistic. But Tromp warns that even if leakage rates are lower than she fears, the quantity of hydrogen released could still be huge. The US Department of Energy estimates that to supply 1 terawatt of power from fuel cells would need 1060 million tonnes of hydrogen per year. Given that global fossil fuel use is predicted to supply 12 to 15 terawatts by 2020, it would take around 5 billion tonnes of hydrogen per year to replace just a third of fossil fuel usage. Leakage rates of around 1 to 3 per cent would lead to emissions of approximately 50 to 150 million tonnes per year.
Horowitz’s study on the effects of hydrogen on OH radicals modelled what would happen if all the world’s automobiles were converted to hydrogen, with a leakage rate of less than 3 per cent. This, he calculates, would increase the lifetime of methane in the troposphere by 3.5 per cent, contributing to global warming. “That could turn out to be one of the bigger effects of switching to a hydrogen economy,” Horowitz says.
Another potential source of problems is how the hydrogen is produced. A team led by Werner Zittel of Ludwig Bolkow Systemtechnik, a consultancy in Ottobrunn, Germany, specialising in renewable energy sources, reported last month how different ways of producing hydrogen would affect its environmental impact (Science, vol 302, p 624). If it were produced solely from coal gasification, worldwide carbon dioxide emissions would actually increase by about 5 per cent, or 1 billion tonnes per year, Zittel estimates. But make it from natural gas, and CO2 emissions would fall while methane emissions would double. “Hydrogen is not necessarily more benign,” says Zittel. “It depends on how you produce it.”
Horowitz’s colleague Manvendra Dubey of the Los Alamos National Laboratory in New Mexico argues that using fossil fuels to produce hydrogen is still better than running cars on petrol because it opens up the possibility of preventing the resulting CO2 from entering the atmosphere. “If you have one site that makes hydrogen from fossil fuels, then you can capture the CO2 there,” he says. However, no one has yet figured how to do this effectively: researchers are currently looking at methods such as injecting the CO2 into the deep ocean, or turning it into carbonate rocks.
In the long term, Zittel says, the answer will be to produce hydrogen from renewable energy sources such as wind and solar power while minimising leakage. There are already ways to do this, such as storing and transporting the gas at low pressure, or using tanks that can store liquid hydrogen at high pressure and burn off any hydrogen that evaporates as it is vented.
One final uncertainty concerns microbes in the soil that consume hydrogen from the atmosphere. No one is quite sure how they will react to increased hydrogen levels, but based on an analysis of hydrogen isotopes in the environment Eiler and his colleagues calculate that 77 per cent of hydrogen from the troposphere is consumed by the soil (Nature, vol 424, p 918). In their global warming models, Horowitz and his team assumed that uptake by the soil would remain proportional to the concentration of hydrogen in the atmosphere, but that will not necessarily be the case.
“If for some reason [the uptake] did not keep up with the concentration, the changes could be even more dramatic than what we are showing in our model now,” Horowitz says. For example, a wetter climate could make the soil less able to absorb hydrogen. “If you have water over the soil, then hydrogen uptake shuts off,” Dubey says. “The wild card in all this analysis is how, in the future, will the climate and the hydrogen sinks change? We don’t know that yet.”
Horowitz and his colleagues say we need to start monitoring hydrogen production and losses in the atmosphere as soon as possible. That would establish a baseline for the hydrogen cycle today – something that was never done for CO2. This will help scientists to monitor changes in emissions in the future. The researchers also plan to work with energy experts to figure out the best way to introduce hydrogen in various sectors of the economy.
Tromp emphasises that she is not opposed to a changeover to hydrogen either, if the transition is thought through. “We have a chance to do it right,” she says. “If we don’t talk about it, maybe some very poor decisions will be made, when we could have made some very intelligent ones.”