Hydrogen is perceived as the „holy grail“ of clean, CO2-free energy. But H2 is very hard to contain, because it is such a small molecule. Also, to get high energy:volume, it must be highly cooled/compressed. Storage/distribution require very special technology and materials. Even then, H2 leaks easily, producing an indirect greenhouse effect: 12 times that of CO2 over 100 years!

The consequences of hydrogen’s small size…

Hydrogen burns in air to produce water and small amounts of NOx, but no CO2! However, a hydrogen molecule is around 2/3 the size of a methane molecule, and this enables it to leak through joints in containers and pipes: it can even burrow its way into metal, making it brittle. The longer the „chain“ from hydrogen production to hydrogen use, and the longer it is stored, the more it leaks. In the atmosphere H2 reacts with molecules that would otherwise destroy methane. Methane is a strong greenhouse gas, and the more escaped hydrogen, the longer the lifetime of methane in the atmosphere.

Challenges of hydrogen storage and distribution…

Hydrogen is typically produced by splitting water with electricity: electrolysis. That produces H2 gas, and O2 (oxygen) gas. To store hydrogen in large quantities, we must compress and cool it: how cool? Minus 253 oC (-423 oF). For these processes we need large amounts of energy, essentially accounting for 30% of the energy within the stored hydrogen. Distribution is via highly specialized multi-layer pipes, often with a vacuum shell that continuously needs pumping out. For shorter-term storage, high-tech multi-layer containers are used. The lifespan of this infrastructure ranges from 10 to 50 years maximum. By contrast, most of the infrastructure for carbon-based fuels has a lifespan of 70 years or more, and is much less costly to make/maintain/mend.

Using a „hydrogen-carrier“ instead…

Hydrogen certainly has desirable properties, but it also sets enormous challenges. Biology does, in fact, use the chemical energy of hydrogen, but not by burning it directly; rather biology uses carbon as a hydrogen-carrier, thus making the energy much more convenient to store and distribute. The human equivalent would be e-fuels (synthetic carbon/hydrogen fuels). A frequent argument against e-fuels is their inefficiency of production: a lot of energy is „wasted“. However, if we calculate the energy losses and leakages in a pure hydrogen economy, they don’t turn out to be less than those from an e-fuel economy. Additionally, hydrogen necessitates a very expensive, and relatively short-lived fuel infrastructure.

Click here for more details of the „non-ideal“ aspects of hydrogen…

Tempted by perfection, but is hydrogen really perfect?…

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:

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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:

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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…

Click here for more details of the hydrogen storage and distribution infrastructure…

The challenges of a pure hydrogen economy…

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A very expensive new infrastructure requiring a great deal of energy to build, run and maintain, and with a relatively short lifespan.

From refrigerating hydrogen to minus 253 oC, through large-scale storage in multi-layered vacuum-insulated tanks, via highly-specialized pipelines, to a variety of short-term storage vessels that are also highly specialized… This is the most complicated energy infrastructure yet devised.

Key industries need highly-compressed hydrogen…

The steel and cement industries need highly-concentrated energy because they use energy at an extremely high rate (i.e. power in terms of megawatts). For that reason, they need highly compressed hydrogen: hydrogen at normal temperature and pressure (20 oC and 1 atmosphere) has an energy per liter of 0.013 MJ. That is less than half the energy per liter of methane at normal temperature and pressure (0.036 MJ).

Liquid fuels have the advantage that much more can be pumped into a process per second than in the gas phase: that’s what we need in industry. Cooled and compressed to liquid, hydrogen can reach almost 10 MJ per liter, but liquid methane is more than three times higher, at 35 MJ per liter. So in terms of the volume needed to supply the same amount of energy to thermal processes in industry: we need more than three times the hydrogen volume compared with methane.

Liquefied hydrogen (at minus 253 oC) leaks at much higher percentages than gaseous hydrogen, which, for example, can be used directly to make e-fuels. Estimates for leakage of liquid H2 vary greatly. Taking ranges from a recent review, a best-case scenario could be 4.1% loss in the chain from storage to use; the worst-case could be as high as 39%. Could the truth be somewhere in between? i.e. 23% loss?

Liquid hydrogen is particularly challenging…

An additional disadvantage is the service lifespan of the hydrogen infrastructure. This is considerably lower than that of the carbon-based fuels infrastructure:

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Hydrogen squeezes through the smallest cracks and escapes from the storage/distribution network; it even pushes its way into metal! Yes, hydrogen-induced brittleness (embrittlement) of steel is a known phenomenon. And it leads to the faster „ageing“ of vessels and pipes that contain hydrogen.

Harnessing hydrogen’s power in more convenient ways…

So, can we harness hydrogen’s massive potential without these problems? Yes, and that is what biology has done for 3.5 billion years, but not by storing and distributing it as a gas or liquid… We can produce very high-energy-density energy carriers in ways analogous (not identical) to biology. That means using similar net-zero material cycles with carbon compounds. To find out more, click here.

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
> Academic review presenting ranges of hydrogen leakage from the main parts of the storage and distribution infrastructure
> Hydrogen embrittlement: list of summaries of review and research papers
> Life cycle assessment of hydrogen storage tanks, giving anticipated lifespans in years
> Life expectancy of long-term large-scale liquid hydrogen storage tanks

Copyright Andrew Moore 2024