Osmotic power plant: pure salt water energy

It is necessary to warn right away: there is no mistake in the title, there will be no story about cosmic energy consonant with the name. We will leave it to esotericists and science fiction writers. And we will talk about the usual phenomenon with which we coexist alongside throughout life.

How many people know due to what processes the juices in trees rise to a considerable height? For sequoia, it is more than 100 meters. This transportation of juices to the photosynthesis zone occurs due to the work of the physical effect - osmosis. It consists in a simple phenomenon: in two solutions of different concentrations, placed in a vessel with a semipermeable (permeable only for solvent molecules) membrane, a level difference appears after some time. In the literal translation from the Greek language osmosis is a push, pressure.

And now from wildlife we ​​will return to technology. If sea and fresh water are placed in a vessel with a septum, then due to different concentrations of dissolved salts appears osmotic pressure and the sea level rises. Water molecules move from a zone of high concentration to a solution zone, where there are more impurities and fewer water molecules.

The difference in water levels is further used in the usual way: this is the familiar work of hydroelectric power plants. The only question is How suitable is the osmosis effect for industrial use? Calculations show that when the salinity of sea water is 35 g / liter, a pressure drop of 2 389 464 Pascal or about 24 atmospheres is created due to the phenomenon of osmosis. In practice, this is equivalent to a dam with a height of 240 meters.

But besides pressure, the selectivity of the membranes and their permeability are also a very important characteristic. After all, turbines do not generate energy from a differential pressure, but due to the flow of water. Here, until recently, there were very serious difficulties. A suitable osmotic membrane must withstand pressure that is 20 times the pressure in the usual water supply. At the same time, have high porosity, but retain salt molecules. The combination of conflicting requirements for a long time did not allow the use of osmosis for industrial purposes.

In solving the problems of desalination, water was invented Loeb membranewhich withstood tremendous pressure and retained mineral salts and particles up to 5 microns. For a long time, it was not possible to apply Loeb membranes for direct osmosis (power generation), because they were extremely expensive, capricious in operation and had low permeability.

A breakthrough in the use of osmotic membranes came in the late 80s, when the Norwegian scientists Holt and Thorsen suggested using modified ceramic-based plastic film. Improving the structure of cheap polyethylene allowed us to create the design of spiral membranes suitable for use in the production of osmotic energy. To test the technology for generating energy from the osmosis effect, in 2009 the world's first experimental osmotic power station.

Having received a state grant and spending more than $ 20 million, the Norwegian energy company Statkraft has become a pioneer in a new type of energy. The constructed osmotic power plant produces about 4 kW of power, which is enough to work ... two electric kettles. But the goals of building the station are much more serious: after all, testing the technology and testing in real conditions the materials for the membranes open the way to the creation of much more powerful structures.

The commercial appeal of the stations begins with a power removal efficiency of more than 5 watts per square meter of membranes.At the Norwegian station in Toft, this value barely exceeds 1 W / m2. But already today membranes with an efficiency of 2.4 W / m2 are being tested, and by 2015 a cost-effective value of 5 W / m2 is expected.

Osmotic Power Station in Toft

But there is encouraging information from a research center in France. Working with materials based on carbon nanotubes, scientists obtained on the samples the efficiency of osmosis energy extraction of about 4000 W / m2. And this is not just cost-effective, but exceeds the efficiency of almost all traditional energy sources.

Even more impressive prospects promise application graphene films. A membrane with a thickness of one atomic layer becomes completely permeable to water molecules, while retaining any other impurities. The efficiency of such a material can exceed 10 kW / m2. Leading corporations in Japan and America joined the race to create high-performance membranes.

If in the next decade it will be possible to solve the problem of membranes for osmotic stations, then a new source of energy will take a leading place in providing humanity with environmentally friendly energy sources. Unlike wind and solar energy, direct osmosis plants can work around the clock and are not affected by weather conditions.

The global reserve of osmosis energy is huge - the annual discharge of fresh river water is more than 3,700 cubic kilometers. If only 10% of this volume can be used, then more than 1.5TW / h of electric energy can be generated, i.e. about 50% of European consumption.

But not only this source can help solve the energy problem. With highly efficient membranes, the energy of the depths of the ocean can be used. The fact is that the salinity of the water depends on the temperature, and it is different at different depths.

Using temperature gradients of salinity, you can not be attached to the estuaries in the construction of stations, but simply place them in the oceans. But this is the task of the distant future. Although practice shows that making predictions in technology is a thankless task. And tomorrow the future may knock on our reality.