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How green is hydrogen?

Last updated: 03/08/2022

How much carbon footprint is associated with the usage of hydrogen depends on its source. While hydrogen is emission-free at the point of use, carbon can be released to the atmosphere during its production. For the purpose of this question, let us focus on three types of hydrogen production techniques:

  • Green hydrogen produced by electrolysis of water using electricity from renewable sources, and can be considered "no-carbon"
  • Blue hydrogen produced from natural gas via a process called steam reforming, and the residual CO2 is captured via some form of carbon capture and storage (CCS), and can be considered "low-carbon"
  • Grey hydrogen produced from natural gas without carbon capture and storage (CSS), and can be considered "with-carbon"

While the majority of the world's hydrogen right now is grey, the amount of green hydrogen produced is increasing fast as the costs of both renewable electricity and electrolysis equipment come down.

To fully understand how much carbon saving can be achieved by using different fuels, one must also consider the application the hydrogen is being used in, the how, where and for what. We believe that hydrogen is THE logical answer to diesel for off-grid electricity. When considering the emissions impact of using a hydrogen fuel cell powered generator, all forms of hydrogen provide a reduction in CO2e over using a diesel generator. Below is a comparison of the "well to site" CO2e footprint for diesel and different types of hydrogen:

DieselGrey hydrogenGreen hydrogen
CO2e to produce 1kg of fuel (kg)0.7 [1]10.89 [2]1 [3]
CO2e to transport 1kg of fuel (kg)0.04 [1]0.8 [4]0.8 [4]
CO2e at the point of use (kg)3.0700
Total CO2e (kg)3.8111.691.8
Energy per kg of fuel (kWh)11.833.633.6
Typical generator efficiency10% - 30%45% - 55%45% - 55%
Useful energy produced (kWh)1.18 - 3.5415.12 - 18.4815.12 - 18.48
CO2e : useful energy ratio (kg/kWh)3.23 - 1.080.77 - 0.630.12 - 0.10
CO2e reduction compared with dieseln/a54% (range: 28% - 80% *)93% (range: 89% - 97% *)

The last row in the above table illustrates that the CO2 footprint reduction associated with the use of hydrogen for off-grid electricity generation is typically 54% for grey or up to 97% for green, when switching from diesel.

[1] https://innovationorigins.com/en/producing-gasoline-and-diesel-emits-more-co2-than-we-thought/

[2] Hydrologiq's own calculations based on data from https://doi.org/10.1016/j.biombioe.2015.08.019 (table 5). https://www.forbes.com/sites/rrapier/2020/06/06/estimating-the-carbon-footprint-of-hydrogen-production/?sh=1dad905024bd gives an even lower estimate of 9.3kg.

[3] https://gh2.org/our-initiatives/gh2-green-hydrogen-standard

[4] Based on the scenario of a 20ft tube trailer being carried by an articulated HGV over 250km in the UK. Freight emission data from https://www.gov.uk/government/publications/greenhouse-gas-reporting-conversion-factors-2023.

* the min value is based on the "high efficiency diesel generator vs low efficiency hydrogen generator" scenario, and the max value is based on the "low efficiency diesel generator vs high efficiency hydrogen generator" scenario

What about HVO?

HVO, Hydrotreated Vegetable Oil, is a fossil-free drop-in replacement for diesel. It is estimated to have a 90% lifetime carbon footprint reduction compared with diesel, thanks to the CO2 absorbed during the growth of the feedstock. Based on this estimation and the figures above, the CO2e : useful energy ratio for HVO is around 0.32 - 0.1 kg/kWh. This is a lower emission factor compared with grey hydrogen. However, green hydrogen, still presents an average of 70% CO2e reduction compared with HVO.

There is currently much debate around the 90% reduction claim as, depending on the feedstock and the supply chain, the production of HVO can lead directly or indirectly to deforestation, which has significant CO2 implications. Also, unlike hydrogen which is truly emission-free, HVO still emits CO2, NOx and other pollutants at the point of use (tail-pipe emissions) - significant amount of CO2 is still going into the atmosphere contributing to global warming, and various gases and particles can still cause major health issues for humans.

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