From a chemical perspective it’s about as near to perfect as one could wish for, but not quite… Here’s some chemistry that you might not have heard of:

Why hydrogen is an „indirect“ greenhouse gas…

Is hydrogen a greehouse gas?! Sounds like a ridiculous question, right? But in 2023, researchers published the latest paper in a series showing that hydrogen actually has significant indirect global warming potential over 100 years (GWP100): it’s around 12 times that of CO2. This research built on many previous papers from others. The atmospheric chemistry of escaped hydrogen also influences ozone concentrations, and this chemistry is as yet little researched: it may or may not be significant, but it presents one of many facets of the complicated chemistry of hydrogen — another atmospheric system with dynamics and ultimate consequences that are hard to predict.

Once in the atmosphere, escaped hydrogen quickly destroys a rather useful chemical, „hydroxyl radicals“ (.OH) that would otherwise destroy methane. Methane is a strong greenhouse gas, and so, escaped hydrogen increases methane concentrations in the atmosphere, causing an indirect global warming effect.

The challenge of hydrogen leakage…

Hydrogen is such a small molecule that it leaks very easy from distribution and storage infrastructures. From liquid hydrogen storage vessels it’s estimated at around 1% per day, but may be as high as 5%; in piplines it’s likely more. The largest leakages are anticipated in the transport sector, where supply to the distribution infrastructure, temporary storage in economical vessels, and the filling-up of individual vehicles present greater vulnerability compared with high-volume constantly-running industrial processes.

So-called „fugitive“ hydrogen is very hard to quantify. A recent and comprehensive review in this area presents liquid hydrogen loss ranges of 0.15 – 10% during liquefaction; 2 – 20% during transport and handling; 2 – 15% during re-fuelling. Considering these three stages as a sequence, the losses become multiplicative, hence giving the following potential losses:

Minimum across all stages: 4.1%
Maximum across all stages: 39%
Average across all stages: 23%

Completely new infrastructures for storage/distribution are needed…

Existing infrastructures used for storing/distributing natural gas (largely consisting of methane) are not adequate for pure hydrogen. The difference in size of the molecules is significant:

Normal gas pipelines can carry a modest percentage of hydrogen, because the methane basically competes with the hydrogen for the leak locations, preventing much of the hydrogen leakage. Pure hydrogen is an altogether different matter.

The way in which we need to store and distribute hydrogen creates large challenges of its own. And what if, despite all efforts, 20% of the hydrogen escapes into the atmosphere (an estimate that is not fantastical by any means) during storage, distribution and use? can we predict the indirect global warming effect that that would have in a full-size global pure hydrogen economy?

Quantifying the potential global warming impact of escaped hydrogen…

We can do a back-of-an-envelope calculation based on converting all of the current global primary energy demand into hydrogen-powered technology. This suggests to me that the GWP100 of „fugitive“ hydrogen could be equivalent to at least 25% of the current CO2 emissions. In effect it would be like failing to reach zero CO2 emissions by a large margin. The minimum value could be equivalent to just over 5% current emissions; but the maximum could be as high as 49%. Clearly, there are many, many uncertainties in such estimates, so take them or leave them… it might be something that we just need to keep a close eye on. The final reasoning is this: we will never be able to stop methane emissions as long as we have agriculture, so we must reduce — not exacerbate — the contribution of methane on global warming.

Factoring in H2 losses into the total efficiency of an H2 economy…

Finally, we can have a stab at comparing the overall — i.e. full-chain — efficiencies of pure hydrogen economies with those of e-fuels. To do this, we would work out the energetic efficiencies of production, and then also factor in fuel losses that occur in storage/distribution. All of these lower the final fuel energy available to us, so we can, indeed, consider them efficiency losses. We would, at current technology, get something like this:

Hence, a final thought in this section: it seems that in a pure hydrogen economy, compared with an e-fuel economy, we would have 1. lower energy efficiency on average; 2. a more costly infrastructure; and 3. a larger global warming potential. Hydrogen is, despite initial impressions, far from the perfect fuel for human economies…

Further reading:

> A multi-model assessment of the Global Warming Potential of hydrogen
> Perspectives and opinions on the possible climate penalty of hydrogen leakage
> On the percentage ranges of likely leakage from a pure-hydrogen infrastructure of storage, distribution and utilization
> Academic review of literature on percentage leakage of hydrogen in different scenarios and calculated by different researchers

Copyright Andrew Moore 2024