Hydrogen As Fuel: We Should Be Selective About How We Make And Use It
- Aug 8, 2022 4:23 pm GMT
-- Depending on how it is produced, hydrogen can eliminate carbon dioxide emissions.
-- But new research has found that leaked hydrogen produces indirect "greenhouse gas" effects on a scale far higher than previously thought.
-- Hydrogen leakages are inevitable, and they become more likely as we use hydrogen in smaller applications.
-- This means we should be very selective about how we make and use hydrogen.
In his 2022 State of the Nation address,
The private sector has followed the government, with Anglo-American unveiling a mine truck powered by hydrogen produced by electrolysis from solar power.
The switch to hydrogen will not be simple. Hydrogen industry requires extreme storage conditions, and each step of its production and use introduces energy inefficiencies. It remains to be seen if hydrogen will be more efficient and cost-effective than other ways of generating electricity from renewable sources.
But there is another mounting concern about hydrogen's widespread use: its global heating effect.
Hydrogen's greenhouse effect
Hydrogen acts as an indirect greenhouse gas mainly by slowing down the breakdown of methane, which has powerful direct greenhouse gas effects. This has been known for a while, but the effect has been considered negligible.
Now, in recent research commissioned by the
Global Warming Potential is a ratio of a gas's global warming effects to those of an equivalent mass of carbon dioxide (CO2). Hydrogen's GWP-20 - its effect over 20 years - is now estimated at 33, with a range between 20 and 44. This means that over the course of the next 20 years, each ton of hydrogen that enters the atmosphere will cause approximately 33 times as much global warming as each ton of CO2. Over a time period of 100 years, hydrogen's GWP-100 ranges between 6 and 16. Different greenhouse gases exert their effects over different time periods.
This alarming statistic suggests that we should re-examine some of our plans for using hydrogen as a means of replacing fossil fuels.
Atomic escape artist
Hydrogen leaks are not the same as fossil fuel emissions. Hydrogen is expensive, so producers have an incentive to keep leakage as low as possible. By contrast, every time we burn fossil fuel, CO2 is emitted.
But hydrogen is an escape artist. It is a tiny molecule which can get through all kinds of materials that would be considered airtight. Some leakage is unavoidable in all applications. Hydrogen can pass - though slowly - even through solid steel. It can be leaked through imperfectly sealed or damaged equipment, released as waste gas, released during the shutdown or startup of a plant.
Different industries have different leakage rates, and rates are affected by the scale of the operation, duration of storage, and piping distance. As equipment ages, leakage rates worsen.
The rule of thumb across industry is that leakage tends to be somewhere between 1% and 10%. Currently, hydrogen is seldom stored for long or piped over long distances, and when it is stored it is on a very large scale, in tightly managed or industrial settings at the upper end of technical expertise.
But there are some applications of hydrogen where leakage is more likely. The Hydrogen Society Roadmap proposes that hydrogen should be used for long-term energy storage, exported by boat, piped long-distance across
Many of these uses could result in leakage rates far higher than industry norms, so 10% could be a conservative estimate of the overall leakage in the hydrogen economy as described in the Hydrogen Society Roadmap. Progress towards reducing leaks will likely be gradual - we have been trying to minimise gas leakage in industry for almost two centuries, so there will be no sudden technological leap forward.
So how much global warming would our hydrogen economy be causing, and are there alternatives we should rather prioritise?
How should we use hydrogen?
For applications with excessively high leakage, hydrogen's global warming effects are comparable to those of fossil fuels, while for applications with minimal leakage the effects are a tiny fraction of those of fossil fuels, even if the term 'zero-emissions' is still not applicable.
This means we should be highly selective about where we use hydrogen.
Retail or household scale applications of hydrogen would require complex infrastructure that would leak at distressing rates, exacerbated by the need for storage at several points.
As a result, these applications might heat the planet almost as much as the fossil fuels they are replacing, and should not be considered environmentally-friendly, even if they are carbon free.
Industrial-scale applications for hydrogen that require storage and long-range distribution are more feasible, but will still have global warming effects that are not insignificant. These should be considered as a last resort, to be used when feasible alternatives are not available, and with careful consideration of leakage rates. In cases where hydrogen use is unavoidable, it should be produced on-site and used directly to minimise leakage.
The author is a scientist in
How much global warming might hydrogen cause?
This depends on the specific uses of hydrogen. But we can get an idea of a 20-year global warming effect compared to the tailpipe CO2 emissions associated with standard fossil fuels, namely coal, methane and diesel fuel. The results of a simple calculation for a molecule of each fuel is shown below.
Comparison of estimated GWP-20 for fuels based on tail-pipe CO2 emissions and assumed hydrogen (H2) leakage rates. Source: Author's calculations
The values in this table are a major simplification. For the fossil fuels, the GWP includes only the direct tailpipe CO2 emissions, not the overall emissions for the full supply chain. All three fossil fuels have additional emissions throughout their supply chains as well as some methane leakages at their points of extraction and a swathe of other environmental effects.
On the other side of the equation, the calculation for hydrogen only includes leakage, and omits the source of energy used for manufacturing. So, this number is applicable only for the idealised scenario where hydrogen is produced entirely from renewables, which will not be the case for the foreseeable future.
Currently, virtually all of
Regardless of the simplifications, the above calculations show that the cleanness of hydrogen as a fuel doesn't just depend on how it is made, but also on how it is used.
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