What is green hydrogen?
Green hydrogen is hydrogen produced by a process that does not emit any greenhouse gases (such as carbon dioxide or methane). The best example of green hydrogen is the hydrogen produced by splitting water using electricity from solar plants or wind turbines.
What is not green hydrogen?
Hydrogen produced by a process that leaves some carbon footprint is not green hydrogen. Most of the hydrogen today is produced by steam reforming of methane, which produces some carbon dioxide. While there is no official definition of what any 'color' of hydrogen means, it is generally accepted that when you say brown hydrogen, you are referring to hydrogen produced from coal. That produced from natural gas or petroleum is grey hydrogen, and if you produce grey or brown hydrogen but you capture the carbon dioxide spewed and store it safely away, such hydrogen might be called 'blue hydrogen.
Why is it in the news these days?
Green hydrogen is in the news today because the Government of India (like most other countries in the world) is pitching for green hydrogen in the manufacture of fertilisers and refining of petroleum. That, of course, is for starters. Eventually, any industry would be made to turn to hydrogen for all its energy requirements. For example, the process of steel making is essentially to kick-out the oxygen in the iron oxide (ore). Conventionally, carbon has been used to pick up the oxygen, resulting in carbon dioxide emissions, but even hydrogen can do the job.
The Government wants to make it mandatory for industries (first fertilisers and oil refining) to use green hydrogen for a certain specified percentage of its overall energy requirements. Such a requirement is called 'green purchase obligation' or GPO -- somewhat similar to the 'renewable purchase obligation' (RPO).
Is water splitting using renewable electricity the only way of producing green hydrogen?
It is the most promising technology, but not the only one available. A few other pathways exist and more are being discovered. For example, you can make hydrogen by feeding biomass to microbes such as bacteria, either directly or with the help of enzymes. With emerging technologies one could split water directly using sunshine, bypassing electricity.
What are the challenges?
If you assume that currently the low hanging fruit is the electrolysis of water using renewable energy, the major challenge is 'cost'. To bring down costs, the cost of the electrolyser (the device that splits water) should come down, which is a function of scale--if more and more hydrogen plants are set up, the cheaper will be the cost. Another challenge is the efficiency of the electrolysers--basically, how much electricity it consumes to produce a kg of hydrogen. Today, it is 55 kWhr per kg of hydrogen.
Will it help India become net-zero nation?
Of course, it will. Hydrogen is the cleanest fuel and it should play an important role in India's net-zero ambitions.