March 12, 2009 In 1748 the monk/physicist Jean-Antoine Nollet exploded a wine-filled pig's bladder when he submerged it in a trough of water (What did he think would happen?). The resulting discovery was of course, osmosis, and two teams of clever Europeans think they can use this natural process to give the world clean, green and perpetual electricity by exploiting the chemical difference between salt-water and fresh water.

  • A quick science lesson: Osmosis is, according to Donald Haynie, author of Biological Thermodynamics (1) "a physical process in which a solvent moves, without input of energy, across a semi-permeable membrane (permeable to the solvent, but not the solute) separating two solutions of different concentrations". In this case, the low-concentrate solvent is fresh water, the solute is sodium chloride (salt) and the high concentration solution is salt-water.

The two teams, Wetsus (The Centre for Sustainable Water Technology) in the Netherlands and Statkraft in Norway, are racing against the clock to be the first to make salinity power a viable alternative to traditional methods. And while they've based their energy solutions (pun intended) on similar science, their approaches to generating electricity are quite different.

Statkraft: Pressure retarded osmosis (PRO)

The Norwegians at Statkraft are putting their hopes in
Pressure Retarded Osmosis or PRO for short. Invented by American/Israeli researcher
Sidney Loeb in 1973, the science is fairly straight forward: two chambers, one with salt-water and one with fresh water are divided by a semi-permeable membrane.

This membrane is like a one-way valve, which draws the 'dilute' fresh water through it into the 'concentrate' of salt-water. This increases the pressure in the salt-water chamber, and this resulting pressure can be used to drive a turbine, thereby generating electricity. The only waste product is 'brackish' (slightly salty) water, which flows back into the sea. Statkraft have estimated that salinity power could eventually provide around 10 per cent of the Norway's electricity needs, or in other words, around 12 terawatt-hours of electricity per year.

The company is building the world’s first complete facility for osmotic power generation and believes a full-scale commercial plant could be up and running as early as 2015.

Wetsus: Reverse electrodialysis (RED)

Wetsus is batting on the
Reverse Electrodialysis (RED) team. Wetsus believes it can use salt-water from the North Sea and fresh water from the Rhine (where the waters meet and form an estuary) to make a kind of battery using osmotic principles they've dubbed
"Blue Energy". With enough "Blue Energy" batteries, Wetsus feels the estuary could generate more than a gigawatt of electricity - or to put it another way - enough to supply around 650,000 homes.

The Blue Energy method works much like a car battery and employs two types of membranes - in this case, both are impermeable to water, but are permeable to ions. One for sodium ions, the other for chloride ions (both of which are abundant in salt-water - of course). So in flows the sea-water, where the positively charged sodium ions move through one membrane into a fresh water channel, and the negatively charged chloride ions move through the other membrane - and in the opposite direction. This separation of charged particles results in an electrical difference between two electrodes at either side of the device and this gives us our chemical battery.

The pros:

This is as green as energy gets, the only waste product is brackish water, which flows into the sea and mixes harmlessly with the sea water. It's also a weather-proof technology (barring the odd tsunami) as it's not reliant on erratic forces such as sunshine or wind. Wind-farms average about 3,500 operational hours per year, whereas salinity power plants could arguably churn out power for 7,000+ hours per year - and at a fairly constant rate. Statkraft estimate that the global potential for salinity power is about 1,600 to 1,700 terrawatt-hours each year, which works out as roughly 1 per cent of the planet's energy needs.

The plants could be easily be combined with existing power-plants, built underground, in basements etc. reducing cost, and visual pollution. Basically, anywhere salt-water and fresh water coincide is a potential green power station just waiting to happen.

The cons:

The biggest obstacle for both teams is that membrane development isn't up to the level they'd both like. The technology needs to advance, and soon. Bio-fouling of the membrane - with silt and algae - is also a big issue. The ionic membranes used by Wetsus for their Blue Energy technology are less prone to fouling, but efficiency and durability are still issues to resolve. Statkraft are looking at anti-fouling coatings and considering option like occasionally reversing water flow to flush the system.

Of course, salinity power isn't as egalitarian as say, wind power. While it can be implemented in any situation where there's an abundant supply of salt-water and fresh water it clearly suits countries with extensive coastline and a lot of rivers - which means plenty estuaries where the power plants can be established.

By their own estimates, Salinity power only has the potential to meet around 1 per cent of global energy. That's actually a lot of juice, but it's no great white (salty) hope. Still, if they can improve efficiency, salinity power will be a welcome addition to the growing green energy family.

Matt H Kennedy

Further reading at New Scientist.

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