Feature Report: Ocean Energy

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Salt Water, Driving Osmotic Energy

Using the salinity of the oceans to produce electricity is an idea whose time has come in Norway, which boasts the world’s first osmotic power plant. The technology is still being tested.

At the osmotic power plant in Tofte, Norway, electricity is generated when saltwater and fresh water meet. © J. HECKER / AFP

Energy Based on the Principle of Osmosis

Osmotic energy uses the salt concentrated in seawater to produce electricityForm of energy resulting from the movement of charged particles (electrons) through a conductor.... The key element of this technology is a semi-permeable, double-sided membrane that lets water through but captures mineral salts. The membrane is in contact with freshwater on one side and seawater on the other. The salt molecules attract the freshwater, which migrates to the compartment containing salt water: this phenomenon is called osmosis. Driven by the movement of the water, a turbine generates powerIn physics, power is the amount of energy supplied by a system per unit time. In simpler terms, power can be viewed as energy output...1.

Thousands of square meters of osmotic membrane would be required to use salt water as an energy source.

The success of this technology is based on high-performance membranes, which must have good wear resistance and the capacity to attract enough water to drive the turbine and generate power with optimal efficiency. They also, of course, have to be affordable.

An osmotic powerOsmotic power involves placing a volume of salt water and a volume of fresh water in adjoining chambers... plant cannot be built just anywhere along the coastline. It needs to be close to reservoirs of fresh water and seawater. Consequently, the mouths of rivers are the only locations where such plants can be located. In practice, in an osmotic power plant, a network of pipes conveys freshwater and seawater into different chambers, separated by a membrane. 

A Pioneering Plant in Norway

After a number of experimental facilities, Statkraft, a Norwegian electric utility specialized in renewable energyEnergy sources that are naturally replenished so quickly that they can be considered inexhaustible on a human time scale..., commissioned its first osmotic power plant in Tofte, Norway, in 20092. This is a prototype used to test the resistance of membranes over time and the feasibility of the technology. Two other projects are being developed, in Japan with a pilot plant in Fukuoka and in the United States.

Currently, 1 square meter of membrane provides output of 3 W. Statkraft works with theoretical osmotic pressure of 12 bars, the equivalent of a 120-meter drop. More adjustments need to be made to improve efficiency and reach the target of 5 W per square meter. At this stage, a 1 MW osmotic power plant would require 200,000 square meters of membrane. A power plant of this capacity (1 to 2 MW) is being considered for Sunndalsøra3.

 

Sources:

(1) Ocean Energies - InterMines (french only)

(2) Bulletins electroniques (french only)

(3) Statkraft