One technology, threefold profit: from seawater to drinking water, salt and energy

To produce hydrogen, an electrolyser is needed to split water into hydrogen and oxygen. Currently, this process consumes large amounts of freshwater: around 11 million cubic metres per year to reach the eight-gigawatt target, equal to about one percent of the Netherlands’ total drinking water capacity. “That puts considerable pressure on available freshwater resources, especially during dry periods,” explains Irma Steemers-Rijkse, Programme Manager for Circular Water Technology. “Electrolysis also generates significant residual heat, requiring even more water for cooling. That’s not truly sustainable.”

An alternative is to use seawater. Through a process called ‘reverse osmosis’, salt water can be transformed into fresh water. However, one litre of seawater yields only half a litre of fresh water. “Reverse osmosis also requires substantial amounts of energy, which is far from ideal in the context of sustainable energy production,” says Steemers-Rijkse. “In addition, it produces residual heat and a highly saline by-product called brine. On land, this brine causes soil salinisation, affecting crop growth, while at the coast, high salt concentrations can harm marine life. So using seawater for hydrogen production brings its own challenges.” 

Efficient conversion from salt to fresh water 
Steemers-Rijkse and her colleagues therefore sought a more efficient way to produce fresh water. They found it in ‘membrane distillation’, a process that uses residual heat from electrolysis to warm the water. The resulting vapour passes through a membrane that separates it from the remaining liquid. The salts stay behind, while the vapour condenses into pure water.

“Membrane distillation produces enough water for hydrogen production….and even more,” she explains. “It can also supply drinking water and process water for agriculture and industry.”

The amount of brine that remains is much smaller: a concentrated salt solution. “You can see brine as waste, or you can explore how to use it as a resource,” says Steemers-Rijkse. “That’s where the idea arose to combine multiple technologies.” 

Extracting table salt from brine 
The researchers already had a proof of concept for recovering salt from brine: membrane distillation crystallisation. This process also uses residual heat from the electrolyser. It combines membrane distillation with salt crystallisation by adding another, more soluble salt. This triggers the formation of sodium chloride (table salt) crystals, which can then be harvested.

Over the next two years, the team will further develop this salt recovery method. “We want to find out, for instance, whether salt accumulates in unwanted places in the system, and whether we can recover pure table salt rather than a mixture of salts,” Steemers-Rijkse explains.

If successful, the next step will be to extract other salts and minerals from brine. “The same principle can be applied to other salts, but that’s only feasible if we can make sodium chloride recovery work efficiently and economically.” Valuable minerals on the team’s wish list include phosphorus for horticulture, magnesium for pharmaceuticals, and even lithium for mobile devices. 

“The beauty of this approach is that you can fine-tune it for optimal performance,” says Steemers-Rijkse. “If you want to maximise fresh water production, you can use more of the electrolyser’s residual heat for that purpose. If your goal is salt recovery, you can stop after extracting table salt, or continue to recover a range of salts. Each additional step uses more of the available heat, so less energy is wasted. The remaining heat can even be used to generate more electricity. By combining processes, you can make the entire system more efficient and profitable.”

The SeaHydrogen concept is not only relevant for green hydrogen production. “It’s also highly interesting for factories that generate low-grade waste heat, or for data centers,” says Steemers-Rijkse. “Other companies are exploring the reuse of their wastewater.”

Collaborations have already been initiated with Aviko, which aims to reuse all its wastewater; Circle Infra Partners, which treats wastewater from factories at the Chemelot industrial site and wants to stop discharging it into the River Meuse; and Nobian, a salt producer seeking to make its salt production more sustainable. “In two years, we hope our brine utilisation technologies within the SeaHydrogen concept will be ready for real-world application,” she concludes.