Salinity gradient energy. A solution for Britain?

The key to “sustainable” electricity generation is to limit it to essentials and eliminate as many intermediary steps as possible. (Including transporting trees from the US and Canada to fuel biomass plants in the UK, indeed.)

What is electricity? The flow of electric charges. Burning anything whether or not it contains carbon is, by definition, not the most elegant or efficient way to create power. Using gravity or wind or the flow of water or another form of pressure, or solar does not include chemical conversions but is almost a form of “tai chi”.

Stacking up water behind a dam as a form of gravity-powered energy affects people and other species and also affects the river downstream of the dam.

Wind turbines affect other species but they can be optimised to limit the damage they do to for example birds.

Tidal energy has pluses and minuses too. Off the top of my head, I recall that there is a plant along the coast of France, at the mouth of a tidal river. But tidal energy is not used very much yet and there are reasons for that.

Salinity gradient energy is generated from the osmotic potential between saltwater and freshwater. You let the two streams flow past each other – for example where a river flows into the sea – and you separate them by a sophisticated membrane that either allows water molecules to flow to the saline side or – and that is in my view the more elegant way allows salt ions to migrate to the freshwater side.

The “waste product”? Brackish water.

Of course, this too requires a little power to operate and the production of materials to be used, not to mention fouling etc to be controlled. We also may have to find ways to build these salinity gradient power stations underground rather than on land. Still, it is a solution worth exploring.

The latest on that? (You’ll find papers etc by clicking on the blue pluses when you scroll down.)

The Norwegian pilot plant – opened in 2009 – has meanwhile stopped operating, but the research continues and the Dutch Redstack plant – opened in 2016 – is still running.

Years ago (2010), I wrote about this topic on one of my websites. The following information is old, but perhaps still a good start:

An article in an issue of the Dutch science and technology magazine C2W (September 15, 2007) quotes Cees Buisman, affiliated with the Department of Environmental Technology at Wageningen University and also scientific director of Dutch research institute Wetsus (Centre for Sustainable Water Technology) as follows:

  • 3,000 MW of electricity can be produced this way in the Netherlands. (This number has since been adjusted upward several times and appears to be several teraWatts now.)
  • 20% of the energy requirements can be met globally.

The article goes on to explain that the RED concept is more than twenty years old. Osmosis is the driving force behind it: Ions wanting to migrate from a location with a high concentration to a location with a low concentration. Stacks of membranes are used to allow the salt ions to migrate but are impermeable to water.

When Wetsus started, the C2W article states, the energy yield was only 0.2 W per m2 of membrane. At the time of writing of that article, the production at Wetsus was 2 W per m2.

RED versus PRO

Whereas the Dutch are developing the RED concept, the Norwegians apply the PRO technology. In PRO, it is the water molecules that migrate, not the ions.

Generates a voltage, which can be used to produce an electric currentGenerates a pressure, which can be used to drive turbines
Ions migrateWater molecules migrate
Cation exchange membrane and anion exchange membrane, both impermeable to waterMembrane permeable to water, impermeable to ions
Dutch pilot in Harlingen not linked to grid; next pilot will be at Afsluitdijk (between IJssel Lake into which IJssel River flows and North Sea), permission granted in July 2010Norwegian Statkraft pilot plant in Tofte, at 60 km from Oslo, linked to grid, generates about 4kW of which about 20% is used to pump the water (can be avoided by making use of gravity)

Some advantages of blue energy

  • Can often make use of existing natural features or infrastructure, such as Afsluitdijk in the Netherlands;
  • Can operate 24/7; although river flow is not a constant, it is more dependable than solar and wind energy;
  • Only waste is brackish water (mix of freshwater and saltwater).

Very similar processes as used in blue-energy generation are used in for example desalinisation plants (to make freshwater), but these plants consume energy, use only salt water and are often not environmentally friendly. Do not confuse these two. One is for freshwater generation, the other for energy generation.

One thought on “Salinity gradient energy. A solution for Britain?

  1. Did a quick search, trying to find out whether it had ever been explored in Britain and found to my surprise that it is being investigated at a place in the US that I know pretty well. They, coming from a different angle, might be able to give this a big push; who knows. A closer look into that makes me suspect that it was merely one person’s fleeting interest in the past, but it’s hard to tell from a distance. I’ve messaged a few people (also to see if one of them knows something about relevant work in the UK).

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