Osmosis power

About osmosis power
The cells in our bodies use osmosis to regulate the fluid balance and the uptake of nutrients. Trees use osmosis to draw water up through their roots. And osmosis is the reason food can be conserved by salting and drying, e.g. salted meat or fish. Wherever fresh water from rivers enters the sea, osmosis can be used to produce electricity in osmosis power plants. It is estimated that osmosis power can contribute about 2000TWh per year.

A question of balance
A power plant based on osmosis is frequently referred to as a salt power plant. The principle behind it is to use the osmotic effect that occur between salty and fresh water to generate pressure that in turn can power a turbine to produce electricity.

When fresh and salty water is kept apart by a suitable membrane, osmosis will cause the fresh water to diffuse over to the salty side. A suitable membrane lets the water molecules pass, but salt molecules and ions are prevented from passing. The simplified explanation of osmosis is that the fresh water will attempt to pass through the membrane to the salty side, in order to dilute the salty water. Thus the water level on the salty side rises, and we get a pressure difference between the two reservoirs. This pressure difference can be used to power an electrical turbine, just like in ordinary hydroelectrical plants.

High pressure and constant power production
A major advantage of osmosis power is the ability to produce power with nearly constant load. In contrast to wave or tidal power, it is not dependent on weather conditions or tides to produce electricity. As long as you have access to enough fresh water, you can produce electricity 24/7 all year around.

With the right membrane and access to high salinity sea water, osmosis can produce surprisingly high pressure. Statkraft’s pilot plant at Hurumlandet in the Oslo fjord operates at 12 bar pressure. That corresponds to a hydraulic head (water column height) of 120m in a traditional hydropower plant.

Some challenges are developing membranes with the right properties and long lifetime, and to increase the effect per unit area of the membrane.

Statkraft is leading the development
The Norwegian company Statkraft is among those who have come closest to commercialize the osmosis power technology. At Hurumlandet in the Oslo fjord, Statkraft have since 2009 been operating the world’s first osmosis power pilot plant. It is a 10kW prototype, used for testing and research into membranes and other technologies in need of improvements.

Osmosis power in other countries
Norway have been favored with plentiful natural resources, and that applies to osmosis power resources too. We have very good conditions for the production of osmosis power. Huge amounts of fresh water runs into the sea, and we already have a power generation industry with great expertise on operating hydroelectrical plants with Pelton turbines and other kinds of high pressure turbines. In total, the estimated production potential in Norway is about 12 TWh per year, about 10% of yearly consumption.

Conditions are also favorable in many other countries, but in very many cases the river water that could be used as a fresh water source simply isn’t clean enough. If the water contains large amount of clay or humus, the membranes will clog up. Where rivers flow into sea areas with lower salinity than the ocean, the effect and efficiency of the plant will go down.