Alkaline electrolysis at sea: water purification strategies to reduce the levelized cost of hydrogen

Resumen

With a growing offshore wind industry worldwide that consistently evolves towards lower levelized costs of electricity [1], green hydrogen produced at sea will eventually become cost-effective and a competitive energy vector to send bulk power to distant locations [2]. There are different ways to perform the electrochemical water splitting of water at sea, but the most promising ones are the low temperature electrolysis technologies: Alkaline Electrolysis (AE) of water, and Proton Exchange Membrane Electrolysis (PEME) of water [3]. Both electrolysis technologies require water of high-purity to operate. PEME devices are more sensitive to impurities as they are prone to suffer irreversible damages in the Membrane Electrode Assemblies (MEAs), in contrast AE devices can recover from the damage as the deteriorated electrolyte can be easily replaced [3]. Seawater contains different salts in a concentration that typically pivots around 35000 ppm, which varies with time and location [4]. These salts can be removed almost entirely by different mechanisms, being the most popular Reverse Osmosis (RO) and distillation [5]. The cost of the produced water by RO is typically lower, but the water quality is often worse than that obtained through distillation. In any case, the water produced with a single stage of each methods will yield impurity concentrations in water that will lead to short lifespans of the alkaline electrolyte in an AE device [6,7]. Therefore, more stages of RO and distillation must be added to increase water quality, and thus the lifespan of the electrolyte, at the expense of raising the cost. Producing a water with higher quality will imply a higher cost, but at the same time it will result in economic savings in the periodic replacements of the alkaline electrolyte in AE [7]. The overall effects of water purification strategies in the economic parameters of AE have not been published yet in the scientific literature. It is key to find the water purification strategies that lead to lowest levelized costs of hydrogen (LCOH) to make green hydrogen produced at sea competitive. This work will assess the economic parameters of varying numbers of distillation stages in an AE device, to determine which configuration leads to the lowest LCOH.