Making blue from RED – the potential of salinity gradient power

Harnessing the power of the oceans for energy has long been a possibility, albeit on a smaller scale than we’d have hoped for so far. The next revolution might be at hand though – salinity gradient power. Andrew Tunnicliffe learns about what it is, how it might be used, and if it can overcome the challenges it faces.

Through the use of advanced smart grids, the Swedish district of Hyllie is blazing a trail for “prosumer” culture. Andrew Tunnicliffe delves into the future of the smart city and how Sweden could prove to be a testbed for cities around the world. 


s certain as we are of the uninterrupted flow of tidal waters, we know that for humanity to sustain life, a power source is needed. Sylvia Earle, marine biologist and oceanographer, once labelled the seas “our planet's blue heart”. There is sense in that likeness; the seas facilitate trade, are a source of food, and have allowed humankind to broaden its horizons.

Today, we are on the threshold of a new frontier of waterpower. Tidal energy is well-understood and heavily invested in. However, it is yet to make any significant impact in the search for renewable power that is sustainable – at least in a financial sense. Despite its potential, the sector is still in its infancy.

Globally, the market is expected to be worth about $11bn by the middle of this decade, according to some experts. For tidal power, though, there is still work to be done if it is to become a major tool in the future renewable energy mix.

The power of the oceans isn’t limited to tidal though. In its recent flagship report, Ocean Energy Europe (OEE) said: “Seawater typically has 200 times more salt than fresh river water. When the two mix, the resulting chemical pressure can generate non-stop renewable power.”

In ‘Powering Homes’, the not-for-profit organisation, which brings together some of Europe’s leading minds on ocean power, said that although research into this “high-potential technology” was at an early stage, there had already been success.

What is salinity gradient power and ‘blue energy’? 

“Blue energy is harvested from salinity gradients. The ions diffuse from high salinity to lower salinity, creating an electrical potential across an ion-selective membrane, which is similar to the generation of potential across live cell membranes, explains Nicholas Kotov, Irving Langmuir Distinguished Professor of Chemical Sciences and Engineering at the University of Michigan. “This potential can be used to generate external current.“

River and seawater mix through a ‘stack’ of membranes producing usable electricity, known as ‘blue energy’ because of the way it is produced. The combined flow of brackish water goes back into the ocean – in the same way the river would have been flowing – whilst the power generated is fed directly into the grid.

Energy can be created and collected from the difference in the salt concentration between seawater and freshwater through what it calls stacks of alternating anion and cation exchange membranes. The OEE adds that, based on current estimations, there is a realistic annual potential of 5,177TWh of salinity gradient power globally.

Today, the OEE says that the most advanced technology in this space is reverse electrodialysis (RED). It believes that where saltwater meets fresh in a significant way – such as at the mouth of major rivers – the potential to generate power is staggering. Kotov, though, says there are issues still to be overcome.

/ Ion selective membranes need to be much better than what is available now. /

The professor, who is lead researcher in a joint US and Australian project to develop an osmotic system to generate electricity, says: “RED is a great concept technology… but ion selective membranes need to be much better than what is available now; maximising their efficiency, mechanical properties, and chemical resilience.”

He, along with professor Weiwei Lei of Deakin University, have developed a system that replicated biomembranes.

“We designed biomimetic membranes based on aramid nanofibers that maximised ion selectivity and mechanical toughness, while minimising the cost,” Kotov explains.

Aramid nanofibers, taken from Kevlar, are widely available and relatively cheap to produce, a merit that Kotov hopes will see the commercialisation of his technology, and others like it, gain traction.

The Afsluitdijk project

Speaking with Power Technology in 2020, Kotov said that the discovery of composite membranes had the potential to fundamentally change the way water is used to generate power. He spoke of a project at Afsluitdijk dam in the Netherlands, suggesting it might be a blueprint for the future use of RED.

Afsluitdijk is situated in the Wadden Sea, north of the Netherlands, giving it almost perfect proximity to both sea and fresh water. This makes it a challenging but ideal site for a pilot RED plant.

“The water from the Wadden Sea is very rich in biological terms. In addition, it contains a lot of suspended silt particles. Therefore, the water has to be purified before it can be led through the stacks,” REDstack, the company operating the plant, has said.

The water from the Wadden Sea is very rich in biological terms. /

Those challenges have provided significant insight though. The main objective for REDstack was to prove the concept works.

“The pilot plant is working fine,” says Pieter Hack, chairman of the board. “It is meeting the design parameters, operating the whole process flow – from intake, pre-treatment, stacks – and feeding power into the grid. It is performing on real sea and freshwater. Now it’s ready for upscaling to a megawatt.”

The Afsluitdijk plant is proving the case for the wider use of such technologies. It is hoped that the experience gained will help confirm the far wider potential of this as a source of renewable power. The OEE says that energy released from 1m3 of freshwater is comparable to that released by the same 1m3 falling from a height of 260m.

Cutting membrane costs

One of the biggest challenges remains: cost.

“The main barrier now is money. That’s needed both to help us build the upscaled systems and to convince end users, such as power companies and large industries, that it really works and is reliable on a larger scale,” says Hack.

Much of that cost comes from the membranes. The OEE estimates that they cost anywhere between 50%–80% of the total cost for any salinity gradient project and are currently up to three times the cost of commercially available membranes. Hack disagrees though, adding that RED membrane costs are similar to others and continue to fall.

Kotov, however, sees an opportunity to reduce these costs by making state-of-the-art membranes from recycled Kevlar fabric, which will also reduce the pollution.

The capital investment is potentially a problem that can be addressed by combining blue energy generation with geotechnical solutions. /

Furthermore, he says: “The capital investment is potentially a problem that can be addressed by combining blue energy generation with geotechnical solutions for other purposes, such as flooding.”

It’s a view shared by Hack, who says: “It can be implemented in flood protection systems, in deltas where people and industries are.”

Whilst the cost of membranes might be an issue right now, it’s hoped that work like that of Hack and Kotov, combined with innovations by other stakeholders, will soon help drive costs down.

Earlier this year, Sweetch Energy announced that it had secured €5.2m in funding to develop a nanotechnology and eco-material science prototype next-generation membrane, coupled with specifically engineered electrodes and innovative cell designs.

Calling osmotic energy “a breakthrough in renewable energy”, the French company said that its work would herald “a level of cost-efficiency never achieved before, opening the door to large-scale deployment of osmotic energy as a competitive market solution”.

The cure for anything is salt water: sweat, tears, or the sea. /

It is clear there is much hope for this new and potentially revolutionary renewable energy source.

“Blue energy does not require fuels and is non-intermittent, unlike solar and wind. It can consistently provide energy day and night. It can also be used in industrial settings, in conjunction with water treatment,” says Kotov.

Hack adds that because it is continuous, it reduces the need for large-scale, inefficient, and expensive conversion and storage systems.

Baroness Karen Christenze von Blixen-Finecke once said: “The cure for anything is salt water: sweat, tears, or the sea.” Perhaps the writer had a vision of what might be. Those involved in this field certainly have reason for hope.

Credit (inline images): REDstack