Numerical Modelling of Wastewater Dispersion from Offshore Hydrogen Electrolysis in the Dutch North Sea - implications for the marine environment.

Abstract

The Dutch government has recently unveiled plans to establish the North Sea's first offshore green hydrogen electrolyser in the 'Ten Noorden van de Waddeneilanden' region, situated 84 kilometres off the northern Dutch coast, by 2031. However, hydrogen electrolysis generates a hypersaline brine waste product as a result of desalination, as well as a large amount of thermal effluent after cooling the electrolyser stack, both potentially harmful for the marine environment. This study evaluates the hydrodynamic and ecological impacts of these wastewater discharges using a numerical modelling approach. A high-resolution Delft3D-Flow model is nested within a large-scale, pre-validated outer model named the General Estuarine Transport Model (GETM). The investigation includes 14 scenarios, altering the electrolyser capacity and discharge characteristics during summer and winter conditions. The thermal plume is largely confined to the top 5 meters of the water column but extends into the far-field of the domain. The brine plume shows sinking towards the seabed during minimum current velocity at the change of the tide, as well as limited dispersal in the surface domain. The brine was found to be dispersing at the seabed, though diluted within one tidal cycle. The brine plume shows very marginal salinity increases for all simulated scenarios, whereas the thermal plume shows temperature increases of several degrees Celsius. The winter scenarios show a respective 17.7% and 11.3% reduction in affected seawater volume for coolant and brine simulations compared to summer simulations. Simultaneous release of both coolant and brine showed no effect on the dispersion of heat in both the surface and the vertical of the domain, while the distribution of salinity was profoundly altered compared to the discharge of brine alone. Combined discharge induced positive buoyancy of the brine plume resulting in significantly increased surface dispersion, while the sinking patterns were largely negated. The model is not able to accurately represent extremely slight salinity changes when invoking combined discharge due to the substantially higher flowrate compared to discharging brine alone. The ecological effects of the thermal effluent possibly include the induction of heat stress near the discharge point, while the direct effects of increased salinity due to the brine discharge are far lower than the natural variation within the domain. The results of this study offer a valuable pilot and baseline for this topic of research, underscoring the need for further investigation and suggesting potential areas for future research.