Demand for cooling water reshapes global water-sustainable hydrogen production

Green hydrogen is central to many decarbonization strategies, yet its water footprint is often reduced to the water consumed by electrolysis itself. As electrolyzer plants scale up, cooling can become a hidden water demand, especially in hot and water-stressed regions where many renewable hydrogen projects are planned. Here we combine a thermodynamic cooling model with climate reanalysis, global water-stress data and renewable capacity-factor maps to quantify evaporative-cooling water demand for electrolysis across regions and seasons. We show that cooling can dominate total water use and that high solar-resource regions frequently coincide with high water stress and high cooling demand. Wind-rich regions, in contrast, are more often located in cooler or more water-abundant settings. A composite Water Risk Index identifies where freshwater-based evaporative cooling is likely to require alternatives such as dry or hybrid cooling, desalination or reclaimed-water supply. Our results show that cooling technology and water sourcing are central to water-sustainable hydrogen planning.