Coupling green hydrogen and poly-generation

The current main issues for humanity are related to pollution and global warming. To face these issues, a shift towards renewable energy technologies is currently running in the world. Renewable electricity share is growing. Nowadays it represents 28% of the world's electricity production from just 15% in 2016, and reached the 13.8% of the global energy consumption. The progressive increase of the power share of renewable energy sources (RES) in the electric grid introduces problems related to the natural discontinuity and fluctuations of RES, that are the object of an increasing number of investigations, and require energy storage systems to mitigate them. Although there are different possible technologies for energy storage, hydrogen is considered the most suitable choice for massive and long-term storage of RES power. This is because hydrogen is an energy vector (fuel) and also a commodity gas and a feedstock for many industrial applications, so that it can also strongly contribute to decarbonisation of a number of sectors defined "hard to abate". Nowadays, almost all hydrogen is produced by steam methane reforming (SMR) and gasification of fossils fuels. According to the IEA Global Hydrogen Review 2022, about 82% of the hydrogen produced is directly derived from methane, oil and carbon. while, hydrogen produced by low-emission technologies is about 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. Consequently, the main challenge is how to produce hydrogen at costs that are competitive with these traditional production methods, but without emitting CO2 into the atmosphere. The two approaches currently able to meet this challenge generate the so-called "blue hydrogen" and "green hydrogen". But the most important game is played on green hydrogen, the production of which 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 shaped the policies of Europe and other developed countries, concentrating investments in this direction and forgetting other production pathways of green hydrogen. However, focusing the attention on electrolysis, some new issues involving geopolitical and social aspects are coming up like, for example, the water issue. This is due to the necessity of having huge amounts of high purity water for running electrolysis to produce large amounts of hydrogen. Electrolysis is not the only green path for hydrogen production, but also the other available and foreseen technologies present a number of limitations. In recent years, the authors conducted a number of studies demonstrating that by coupling green hydrogen production with other goods or energy productions it is possible to strongly reduce the green hydrogen production cost. Moreover, coupling the green hydrogen production with poly-generation approach opens a number of new opportunities that introduce new economic and/or social value to the green hydrogen. In this work, an overview of the applicable technologies, with a focus on electrolysis, and possible advantages of coupling green hydrogen production and poly-generation is carried out. This, with the purpose of supplying to hydrogen value chain actors an overview of possible solutions and of suggesting them a holistic perspective looking at all the possible technologic options instead of an approach focused on a single technology and product.