Technical insights and perspectives on hydrogen valleys deployment as enablers of European clean energy transition

The urge to curb pollutant and greenhouse gases emissions under the levels set by international regulations and initiatives like the European Green Deal poses tough challenges to the industry, residential, transportation, and energy sectors [1]. The energy system integration, the inclusion of new, low-carbon, and resilient energy technologies, and sector coupling could ensure the effectiveness of the clean energy transition in the medium-long term. In this context, hydrogen (H2) produced from Renewable Energy Sources (RES) could play a crucial role for the decarbonization of both industry and mobility sectors, as it can be used as a fuel, a chemical feedstock and an energy carrier [2]. Indeed, the EU commission is posing new challenging targets towards 2030 that will accelerate the establishment of a H2 economy. Among these, some actions aim at scaling up electrolyzer capacity, at decarbonizing existing H2 use in industry, at promoting H2 for new use-cases (e.g. for heavy duty transport), while other actions envision the development of a H2 distribution infrastructure that includes storage facilities. The so-called H2 valleys have emerged as potential enablers for the H2 technology rapid development and for energy system integration. Therefore, this study aims to provide technical insights and perspectives on H2 valleys deployment. To this end, a literature review regarding planned and developed H2 valleys is here proposed. Starting from the definition of an H2 valley, H2 valley as envisioned by EU is a regional and industryfocused H2 ecosystem, where H2 production, transportation, and various end uses (such as mobility or industrial feedstock) are linked together, allowing to match environmental sustainability with economic competitiveness [2,3]. These initiatives carried out by EU commission are intended to address the H2 supply to industry sector, such as refineries, ammonia, methanol and steel industries, which, nowadays, requires almost 90 million tons of H2 per year. At present, H2 is produced largely from fossil fuels and is the cause of emissions of 800 million tons of CO2 per year [4]. H2 and H2 carriers could also contribute to the decarbonization of mobility sector and could play a crucial role especially for applications which are hard to electrify, such as trucks, cargo handling equipment and airplanes. The large-scale deployment of H2 technologies for industry and mobility could enable the establishment of green energy hubs and the creation of new clean fuels infrastructures [5–7]. The development of H2 valleys is not only a key priority for EU but it has also gained a relevant interest in literature. For instance, Schrotenboer et al. [8] analyzed optimal strategies for operating integrated energy systems consisting in RES, H2 production, H2 storage, and direct gas-based usecases. The study was applied in the context of the Northern Netherlands where Europe’s first H2 Valley is being established. Results show that an increase of grid renewable share, and the definition of H2 offtake and power purchase agreements will be crucial in the establishment of the H2 valley. Similar results were obtained by Parra et al. [9] who analysed the technical and economic advantages of integrating H2 technologies in heating, electricity, and mobility sectors. Bohm et al. [10] analysed practical synergies of power-to-H2 technologies and district-heating systems, finding that by 2030 up to 12% of all of Austrian district-heating demand could be covered by recovering heat resulting from electrolysis processes. Petrollese et al. [11] proposed a techno-economic analysis for proposing H2 valley in the industrial area of Cagliari. In their study, stationary fuel cells, a H2 refuelling station and an injection system for feeding H2 in the natural gas pipeline were considered as potential applications. Wulf et al. [12] reviewed the Power-to-Gas projects developed in Europe by 2018, which may be included as part of larger projects considering H2 valleys deployment, e.g. HyBalance and REFHYNE. Roboam et al. [13] modelled a H2 hub proposed in the context of the HYPORT project at Toulouse– Blagnac Airport. Authors developed a simplified energy management model for integrating renewable sources, electrolyzers, fuel cells, electric demand and transport load profile for future airport using H2. The first cross-border H2 Valley is under development in the area among Italian Autonomous Region Friuli-Venezia Giulia, Croatia and Slovenia [14]. The imbalance between supply and demand due to the stochasticity of RES and the improvement of techno-economic performance of H2 technologies are well known issues in the deployment of H2 valleys. Other challenges regard the lack of a clear regulatory and policy framework, following by the need of subsidies and incentives for the initial stage of H2 economy. Solutions like reversible solid oxide cells, biological H2 methanation, and underground H2 storage need improvement for a largescale application. The suggestions, improvements and lesson learned from the review of literature and technical studies can be fundamental for the planning and design of new H2 valleys in Europe and worldwide.