[ water treatment ]
of electrolysis , which will require a thousand times more , around 5,000 kWh of electricity .
Desalinating seawater for electrolysis – design conditions
In electrolyzer systems , the stack takes up 40 – 60 % of the total cost . The main job of a desalination system is to protect this investment by meeting a series of requirements :
• Product water of ultrapure quality
• Flexible operation
• Robust operation in harsh conditions
• Small footprint
• Cost effectiveness
Electrolyzers function optimally with ultrapure water , the quality of which is often stipulated in ASTM guidelines for type I – IV water . Type I water is commonly employed in the operation of PEM electrolyzers , while alkaline electrolyzers typically utilize type II or type IV water . Table 1 shows ASTM type I and type II water compared to seawater . To meet the requirements for type I quality water , the desalination system must reduce conductivity by a factor of 900,000 , the concentration of sodium with a factor of 11,000,000 and the concentration of chloride by a factor of 19,000,000 . In comparison , silica and organics need to be reduced by a factor of 3,000 and 20 , respectively .
Green hydrogen electrolyzers operate with a high degree of variable load depending on the amount of renewable energy available and experience frequent starts and stops . The water treatment system must be able to handle this variation while maintaining water of high quality .
A portion of the market for seawater-based green hydrogen is expected to be located offshore . The desalination system must be able to operate properly under these harsh conditions without needing frequent service .
For green hydrogen plants in locations with limited space , for instance offshore platforms or containerized systems , compact systems with a small footprint are an advantage .
Finally , the water treatment solution must be cost-effective to contribute to reducing total cost of green hydrogen .
Table 1 . Comparison of ultrapure water and seawater
22 Hydrogen Tech World | Issue 16 | June 2024