Abstract
Hydrogen is currently enjoying a renewed and widespread momentum in many national and international climate strategies. This review paper is focused on analysing the challenges and opportunities that are related to green and blue hydrogen, which are at the basis of different perspectives of a potential hydrogen society. While many governments and private companies are putting significant resources on the development of hydrogen technologies, there still remains a high number of unsolved issues, including technical challenges, economic and geopolitical implications. The hydrogen supply chain includes a large number of steps, resulting in additional energy losses, and while much focus is put on hydrogen generation costs, its transport and storage should not be neglected. A low-carbon hydrogen economy offers promising opportunities not only to fight climate change, but also to enhance energy security and develop local industries in many countries. However, to face the huge challenges of a transition towards a zero-carbon energy system, all available technologies should be allowed to contribute based on measurable indicators, which require a strong international consensus based on transparent standards and targets.
Highlights
Energy systems are facing a transition towards technologies that allow to decrease greenhouse gases (GHG) emissions, to face the huge challenge of climate change
Grey hydrogen, produced by fossil fuels, and causing the emission of carbon dioxide in the process; blue hydrogen, through the combination of grey hydrogen and carbon capture and storage (CCS), to avoid most of the GHG emissions of the process; turquoise hydrogen, via the pyrolysis of a fossil fuel, where the by-product is solid carbon; green hydrogen, when produced by electrolyzers supplied by renewable electricity; yellow hydrogen, when produced by electrolyzers supplied by electricity from nuclear power plants
This paper focuses on green and blue hydrogen pathways, which are the two approaches that are mostly being considered by world countries to support a low-carbon hydrogen economy
Summary
Energy systems are facing a transition towards technologies that allow to decrease greenhouse gases (GHG) emissions, to face the huge challenge of climate change. Grey (or brown/black) hydrogen, produced by fossil fuels (mostly natural gas and coal), and causing the emission of carbon dioxide in the process; blue hydrogen, through the combination of grey hydrogen and carbon capture and storage (CCS), to avoid most of the GHG emissions of the process; turquoise hydrogen, via the pyrolysis of a fossil fuel, where the by-product is solid carbon; green hydrogen, when produced by electrolyzers supplied by renewable electricity (and in some cases through other pathways based on bioenergy, such as biomethane reforming or solid biomass gasification); yellow (or purple) hydrogen, when produced by electrolyzers supplied by electricity from nuclear power plants In addition to these colours, different nomenclatures are often in use when referring to groups of hydrogen pathways, including “clean hydrogen”, “low-carbon hydrogen”,. While each technological pathway presents opportunities and limitations, it is important to remember that the choice of a specific solution is often related to additional aspects, including geopolitical choices based on national strategies driven by the availability of resources, energy security concerns or the support to specific industrial sectors [11].
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