LBST Analysis

New study exhibits the environmental harmlessness of a global hydrogen economy

The Science magazin published a study ("Potential environmental impact of a hydrogen economy on the stratosphere" by T.K. Tromp, R.-L. Shia, M. Allen, J.M. Eiler, Y. L. Yung, Science, vol. 300, 13. June 2003, p. 1740-1742) which investigates the potential environmental impact of a future hydrogen economy. To be at the safe site, the authors assumed that hydrogen emissions from a global hydrogen economy would amount to 120 Tg/yr, at worst, however, also pointing out that "it is likely that such emissions could be limited or even made negligible, although at some cost". For comparison's sake, 120 Tg/yr are more than the entire European road and air transport would consume if it were fully converted to hydrogen. By the way, the two references on articles which seem to justify their assumption of 10 - 20 % ermission losses are incorrectly quoted.

With these worst case assumptions the authors conclude further that anthropogenic emissions would rise by a factor of four, and at the same time they assumed the hydrogen concentration at the surface to increase by a factor of four, to 2.3 ppmv. However, as well known in the scientific community, usually the decomposition rates also increase with increasing concentration, limiting the final figure to a lower level. Therefore the authors admit "Second, a large, possible dominant, sink of H2 from the atmosphere is uptake in soil. ...It is possible that this process could entirely compensate for new anthropogenic emissions, although a study will be needed whether this is the case."

In addition, not mentioned in the article, at least part of present H2 emissions will be omitted in a renewable hydrogen economy, these are emissions from industrial fossil burning process (which are estimated in the range of between 10 - 15 Tg/yr in the study but according to other sources could be as high as 57 Tg/yr), and atmospheric hydrogen production by the decomposition of hydrocarbons (CH4 and higher) which, at least partly, are due to fossil energy extraction and burning. It has been estimated by other authors, that 2 -3 % percent emissions of a world wide hydrogen economy (This would be about two to threetimes as much as from todays natural gas infrastructure) would outweight the ceased losses of fossil fuel extraction and burning and not lead to additional emissions. (see also here)

A direct result of these crude assumptions is that stratospheric water content would rise by about 30 percent - again neglecting that today a large source for stratospheric water vapor is methane decomposition in high altitudes, which would be reduced once fossil fuel extraction and burning are ceased. Based on these assumptions the stratospheric ozone decomposition could be enhanced by about 1 percent. However, according to the authors, indirect effects might be more severe: Colder temperatures (the study indicates that temperatures could decrease by as much as 0.5 K!) would create more polar stratospheric clouds, delay the break up of the polar vortex, and thereby make the ozone hole deeper, larger (in area), and more persistent (in spring). With these assumptions, at worst the ozone depletion is about 5 to 8 % enhanced in the boreal sping. This leads the authors to the conclusion that "anthropogenic emissions of H2 could substantially delay the recovery of the ozone layer that is expected to result from the regulation of chlorofluorocarbons." But the authors also admit that beyond 2020 ozone levels will have recovered to a status where these additional H2 emissions will have even much less influence.

Consequently, the authors conclusion is not to stop a hydrogen economy but to delay the introduction of fuel cells and hydrogen economy beyond the year 2020, not realizing that large amounts of hydrogen anyhow will be handled only beyond 2020, due to the long lead times of its introduction. Keeping in mind the crude assumption are taken in this study, it can be concluded that this study admits that no severe consequences on ozone depletion are to be expected. Finally, other effects were mentioned but not studied in detail: These are a possible influence of H2 decomposition on OH concentration, potential impacts of increased mesospheric H2O levels on atmosphere chemistry, and the influence of higher H2 concentrations on microbial nutrients. But at least the consumption of OH radicals might be more than outweighed by the reduction of other emissions (CH4, CO...) which dominate the hydroxyl consumption (OH) for their decomposition.

However, keeping in mind that automotive exhaust emissions have increased their share of hydrogen emissions during the last two decades due to the introduction of catalysts, and that atmospheric hydrogen concentrations are increasing since at least 15 years (between 1985 and 1989 with an average increase of 0.6 percent per year [see Khalil M.A., Rasmussen R.A. "Global increase of atmospheric molecular hydrogen", Nature 347 (1990), 743-745] it is very likely, that in a global hydrogen economy anthropogenic hydrogen emissions are rather reduced compared with today's values.

Werner Zittel, L-B-Systemtechnik, 18 June 2003