At school we learned about osmosis and the principle of equilibrium, which evens out differences in the concentration of a solute: if freshwater and saltwater are put into a pressurized container, separated only by a thin membrane, the freshwater will move through the semi-permeable membrane and dilute the saltwater, thus lowering its concentration.
As the freshwater flows through the membrane, the pressure of the saltwater in the container can rise by as much as 25 bar, equivalent to a water column or imaginary waterfall of 250 metres. The power plant can exploit around 100 metres of this pressure difference by allowing the pressurized water to flow through a turbine, a process known as “pressure-retarded osmosis”.
The idea of a Norwegian saline power plant was launched at SINTEF in 1980 by a couple of enthusiasts, and Statkraft took up the project in 1997.
The membrane is the vital component of saline power plant technology, since this is where the pressure and energy are generated. A research team at SINTEF has specified the requirements and objectives for a good membrane, and in the course of the past ten years, tested hundreds of membranes from Europe and the USA.
One of the major challenges is the supporting membrane, which needs to provide strength but offer little resistance to diffusion. The membranes are packed into modules in order to obtain the greatest possible surface area per unit volume. In the protoype at Tofte near Oslo, they cover an area of 2000 square metres. The most important aim of the Norwegian prototype was not to put an optimal membrane into place, but to try out all the components at the same time.
The Tofte prototype uses 20 litres of seawater and 10 litres of freshwater a second to generate 2 kW.