A Review of the Life Cycle Assessment of the Carbon–Water–Energy Nexus of Hydrogen Production Pathways

The hydrogen (H2) economy is seen as a crucial pathway for decarbonizing the energy system, with green H2—i.e., obtained from water electrolysis supplied by renewable energy—playing a key role as an energy carrier in this transition. The growing interest in H2 comes from its versatility, which means that H2 can serve as a raw material or energy source, and various technologies allow it to be produced from a wide range of resources. Environmental impacts of H2 production have primarily focused on greenhouse gas (GHG) emissions, despite other environmental aspects being equally relevant in the context of a sustainable energy transition. In this context, Life Cycle Assessment (LCA) studies of H2 supply chains have become more common. This paper aims to compile and analyze discrepancies and convergences among recent reported values from 42 scientific studies related to different H2 production pathways. Technologies related to H2 transportation, storage and use were not investigated in this study. Three environmental indicators were considered: Global Warming Potential (GWP), Energy Performance (EP), and Water Consumption (WF), from an LCA perspective. The review showed that H2 based on wind, photovoltaic and biomass energy sources are a promising option since it provides lower GWP, and higher EP compared to conventional fossil H2 pathways. However, WF can be higher for H2 derived from biomass. LCA boundaries and methodological choices have a great influence on the environmental indicators assessed in this paper which leads to great variability in WF results as well as GWP variation due credits given to avoid GHG emissions in upstream process. In the case of EI, the inclusion of energy embodied in renewable energy systems demonstrates great influence of upstream phase for electrolytic H2 based on wind and photovoltaic electricity.