Green Hydrogen, not just electrolysis

To face the pollution and global warming issues a shift towards renewable energy sources (RES) is currently running in the world energy mix. The progressive increase of the RES power share in the electric grid introduces issues related to their natural discontinuity and fluctuations requiring energy storage systems to mitigate them. Although there are different possible approaches for energy storage, hydrogen is considered the most suitable choice for massive and long term storage of RES power [1]. This is because hydrogen is an energy vector, a commodity gas and a feedstock for many applications, so that it can also strongly contribute to decarbonisation of a number of sectors defined "hard to abate" [2]. Nowadays, almost all hydrogen is produced by Steam Methane Reforming (SMR). Hydrogen produced by low-emission technologies is less than 0.7%, with the majority of this coming from fossil fuels with CCUS and only 0.04% coming from renewable electricity via water electrolysis. In 2021, the emissions associated with hydrogen production were more than 900 Mt CO2. Hydrogen production costs by these processes are low, $1-2/kg H2 for SMR and $1-1.5/kg H2 for gasification, respectively. The main challenge is how to produce hydrogen for today's and future uses at costs that are close to current ones, but without emitting CO2 into the atmosphere. The two approaches currently able to meet this challenge are "blue hydrogen" and "green hydrogen". But the most important game is played on green hydrogen, whose production does not entail CO2 emissions as only renewable energy is used. Green hydrogen can be produced by various methods, but the most suitable and well developed technology is based on electrochemical water splitting (electrolysis), then "Green Hydrogen" is usually defined as "Hydrogen made via electrolysis using renewable electricity". This concept has focused the policies of Europe and other developed countries, concentrating investments in this direction forgetting other production paths of green hydrogen. Indeed, there are also other technologies that could be used, although not intrinsically CO2 neutral, these could have a net zero CO2 emission if the whole cycle is considered. This because if residual biomass are used as raw materials for hydrogen production, the full process cycle is carbon neutral also when the specific technology is not carbon free in principle [3]. In this way the concept of green hydrogen technologies could be enlarged offering new opportunities. Here, an analysis of advantages and shortcomings of different green hydrogen technologies (biomass pyrolysis and gasification, water electrolysis, etc.) is carried out, with a focus on the electrolysis as the most promising method for large scale and distributed generation of hydrogen [4]. The purpose is to supply to policy makers and energy economist an overview of possible solutions and of suggesting them a holistic approach looking at all the possible technologic approaches in front of an approach focused on a single technology. Moreover the distributed hydrogen generation could allow a more efficient local resources utilisation. This work is linked to the work entitled "Green hydrogen's role in energy transition and water issues", by A. Nicita, G. Maggio, G.Squadrito and presented in this conference where the hydrogen trade and related water issues are analysed.