Thermally driven ultrapure water production for water electrolysis - A techno-economic analysis of membrane distillation

The transition to hydrogen production with renewable energies necessitates ultrapure water (UPW) as a raw material for water electrolysis, which imposes stringent quality and economic demands on water treatment technologies. This study evaluates the techno-economic feasibility of membrane distillation (MD) as an alternative to reverse osmosis (RO) for UPW production. Utilizing waste heat from a 5 MW proton exchange membrane (PEM) electrolyzer, which would otherwise require active cooling to sustain operation of the electrolyzer, the proposed MD system employs permeate gap membrane distillation to treat feedwaters with brackish and seawater salinities (i.e., 5 g NaCl /kg and 34.3 g NaCl/kg, respectively). The study incorporates numerical simulations to analyse system design parameters and energy consumption of MD and RO systems designed to produce 1t/hr of distillate, as well as experimental data on distillate water quality. Results demonstrate that MD systems achieve high-quality distillate (<3 µS/cm) under the tested salinities and at competitive costs. Estimated unit costs per ton of produced distillate for MD range from €2.33 to €2.85 for either brackish or seawater feed at electricity prices of €0.10-€0.40/kWh, while RO costs range from €2.80 to €5.51 under comparable conditions. The presented annual costs also reflect the cost advantage, especially for seawater desalination with MD costs, approx. 40% lower than RO. MD shows advantages in energy efficiency with thermal energy provided as low-grade heat. This work shows that MD can be a cost- efficient and versatile solution for UPW production powered by PEM electrolyzer waste heat.