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Articles - The power of salt: Salinity gradient energy

This article describes the basics of two types of salinity gradient energy generation:

• Using the difference in salt concentration between two flows, such as river water and seawater (also called blue energy);
• Using the difference in salt concentration in layers of stratified salt ponds known as solar ponds.

Blue energy

Two techniques are currently known for harvesting the energy contained in the combination of two water flows with different salt concentrations and a third one may be in the works (but has not been published about yet):

• Reversed electro dialysis or RED;
• Pressure-retarded osmosis or PRO;

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.

Very similar processes as used in blue-energy generation are used in for example desalinization plants (to make freshwater), but these plants consume energy and use only salt water. They are often not environmentally friendly as they use harsh chemicals in cleaning steps.

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.

The membrane technology group at the Institute of Mechanics, Processes and Control Twente (IMPACT) conducts a great deal of research on the membranes used in blue energy. Especially membrane resistance - more specifically the resistance of the diffusion boundary layer at the membrane surface and the electrical double layer - plays a large role.

Ion groups within these membranes, such as tertiary ammonium groups and sulphonic acid groups, along with pore size lend what is called permselectivity to these membranes: They are selectively permeable.

The solution’s salt concentration plays a role as well; a solution’s electrical resistance is greater at lower salt concentrations.

Mixing of the solution helps to minimize the diffusion boundary later and hence its resistance, as it is on the micrometer scale. The electrical double layer, however, is much thinner (several nanometers) and mixing does not affect it.

Ion exchange membranes resemble ion exchange resins in that they both are capable of removing ions from a solution. Ion exchange resins, however, are granular and usually packed into columns through which fluids are passed (either by gravity or by pressure). The ion exchange resin either has to be replaced when its maximum adsorption capacity is reached or a different fluid has to be applied to release and elute the ions from the resin. Membranes can become chemically or physically damaged, although many membranes are stable in a wide range of fluids.

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.

RED	PRO
generates a voltage, which can be used to produce an electric current	generates a pressure, which can be used to drive turbines
Ions migrate	Water molecules migrate
Cation exchange membrane and anion exchange membrane, both impermeable to water	Membrane permeable to water, impermeable to ions
Dutch pilot in Harlingen not linked to grid; next pilot at Afsluitdijk (between IJssel Lake into which IJssel River flows and North Sea)	Norwegian 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).

• Biofouling on membranes;
• Silting up;
• High costs, notably of membranes (although that already appears to be changing rapidly).

Solar ponds

Several countries, notably Israel and Australia, operate solar ponds. Solar ponds harvest heat added to water by the sun. Salt features prominently in solar ponds as well, but here it is a construction material.

Solar ponds are ponds with at least three separate layers of clear water with different salt concentrations. Salt provides the factor that stabilizes the layers: Density.

Normally, when sunlight heats the bottom of a pond and the lower layers, this water becomes lighter and starts to rise. This starts up convection, which attempts to achieve a stable density distribution. Water with a high salt concentration is much denser - heavier - and remains much denser when it heats up.

Natural water bodies display these phenomena too. The denser water is at the bottom; the lighter water is at the top and the middle layer has a stable gradual salinity increase (or decrease, upward), which is called halocline. Ocean water can normally move horizontally, though and that offers other possibilities. The Black Sea, on the other hand, is a stratified basin where water of a higher salinity flows over a threshold (a sill) and is kept behind that threshold. A large number of rivers, including the Danube, flow into the Black Sea, where the lighter freshwater flows over the heavier saltier water.

The top and bottom layers of a solar pond can have convection, but the middle layer is non-convective and acts as a transparent insulator. The top layer has to be topped up regularly to make up for evaporation. The bottom of the pond - often provided with a dark layer (high optical absorption) - heats up and the bottom layer stores the heat. This hot brine can then be passed along or through heat exchangers. Temperatures of 65 to 80 degrees C can easily be achieved; flow must be high enough to avoid boiling.

India had the Bhuj pond. It was completed in 1993 and provided hot water to a dairy plant until 2000. It supplied 80.000 liters of hot water daily and covered 6000 m2. Financial troubles at the company that took over the pond operation, followed by an earthquake that put the dairy plant out of action put a stop to this solar pond.

The US, Australia and Israel are examples of other countries that have solar ponds. The University of Texas has a solar pond in El Paso and the company Enersalt is currently commercializing solar ponds in Australia.

• Low efficiency;
• Requires large areas of land;
• Biofouling in upper layer;
• Brine leakages to be avoided; freon used in a closed circuit;
• Water has to be kept clean (transparent).

• Clean;
• Easy.

Some solar ponds use membranes to help separate the brine layers, but most do not.

Conclusion

It's a safe bet that salt is bound to feature in our future.

Note: Some articles on blue energy refer to ions as "salt particles", which can be confusing. These are not salt grains. Ions are electrically charged particles that cannot be seen with the unaided eye or under an ordinary microscope. A salt is a combination of a positively charged ion and a negatively charged ion. In solution, a salt fall apart into its ions.

Reuters reported on these new developments in March 2008. NewScientist did too, about a year later. See also this blog post and this Ecofys report (pdf), largely in Dutch.

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© Angelina Souren and SmarterScience
http://www.smarterscience.com

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February 4, 2010