[ water treatment ]
Why not just operate directly on seawater ?
A popular thought is that we avoid desalination completely and perform electrolysis directly on seawater . Schemes for direct electrolysis of seawater typically fall into two categories :
• Systems where seawater and the electrolyte solution are separated by a contactor membrane
• Electrolyzers with modified electrodes
In the first category , a few studies have claimed to be operating directly on seawater , while in reality they rely on water treatment using a contactor membrane . This could for instance be a forward osmosis membrane or a membrane distillation setup . These schemes do not avoid water treatment but suggest an alternative to traditional SWRO and thermal desalination . So far , no commercial breakthrough has been made for either of these technologies in desalination .
Electrolyzers equipped with modified electrodes to function directly with seawater are frequently presented as the most promising solution . Theoretically , it is possible to manufacture electrodes that can operate directly on seawater . However , such electrodes will be costly and will not bring added value . More energy is required to perform electrolysis on seawater compared to ultrapure water , and optimizing electrodes for these applications foregoes the opportunity for other optimizations , such as energy efficiency .
In conclusion , direct seawater utilization for electrolyzers is not a viable approach . Desalination must remain the initial step in the water treatment process .
In addition to these , the size of the water treatment system should be matched against relevant electrolyzer sizes . A 10 MW system requires 2 m ³/ h of ultrapure water , while a 100 MW electrolyzer requires 20 m ³/ h . Larger electrolyzer installations will often consist of trains of these systems to obtain safety through redundancy and to achieve flexibility . A relevant range of size of water treatment systems for electrolyzers is therefore 2 – 100 m ³/ h .
Desalination technologies – SWRO and thermal
To desalinate seawater , there are generally two options : 1 . Seawater reverse osmosis ( SWRO ) 2 . Thermal desalination
SWRO operates by using high pressure to drive seawater through a semi-permeable membrane . The membrane retains most of the ions and molecules while allowing water to pass . Depending on parameters such as seawater salinity and temperature , a pressure of 60 – 70 bar is typically required . The high pressure in the membrane system is necessary to overcome the osmotic pressure of seawater .
Thermal desalination relies on thermal energy to evaporate water and then condense it back to distilled water . While there is only one type of SWRO process , there are several alternative thermal processes . For integration with green hydrogen electrolyzers , which generate waste heat between 50 – 70 ° C , the two most suitable thermal technologies are Vapor Compression ( VC ) and marine freshwater generators relying on Vacuum Distillation ( VD ).
VC utilizes a mechanical compressor to raise the temperature of water vapors that are used to drive the evaporation process . In contrast , VD relies on an ejector pump to create a vacuum , enabling evaporation at lower temperatures . Compared to other thermal methods like Multi- Stage Flash ( MSF ) desalination and Multi- Effect Distillation ( MED ), VC and VD are usually simpler , have a smaller footprint , and are more
Hydrogen Tech World | Issue 16 | June 2024 23