When thinking about renewable energy, the energy of wind, solar, tides and tides immediately comes to mind, and the devices that convert them are wind power plants, solar photovoltaic converters, hydro turbines that are already familiar today. All this is already massively used all over the world. But the list of renewable energy sources does not end there. There is another type of energy production that has not yet become widespread, but this is a matter of the future - this is osmotic energy.
Recently it became known about the launch in Norway of the world's first power plant, which allows you to extract energy from the difference in salt concentration in fresh water and salt water. The production of electricity is carried out as a result of the phenomenon of osmosis. The station is located near the capital of Norway, Oslo, on the shores of the Oslo Fjord. The construction investor was the Norwegian energy company Statkraft, which is the third largest producer of energy resources in the Scandinavian region, as well as the largest producer of energy based on renewable energy sources in Europe. This news was the reason for writing this article.
So what is osmotic energy?
Osmotic energy is the energy obtained as a result of osmosis, or, as you can say, as a result of the process of diffusion of a solvent from a less concentrated solution to a more concentrated solution.
According to Wikipedia.org, the phenomenon of osmosis is observed in those environments where the mobility of the solvent is greater than the mobility of the solutes. An important special case of osmosis is osmosis through a semipermeable membrane. Semi-permeable membranes are called, which have a sufficiently high permeability not for all, but only for some substances, in particular, for a solvent.
Osmosis plays an important role in biological processes. Thanks to him, nutrients enter the cell, and vice versa - unnecessary ones are removed. Through osmosis, plant leaves absorb moisture.
Osmotic energy refers to a renewable source that, unlike solar or wind energy, produces a predictable and sustainable amount of energy regardless of the weather. And this is one of the main advantages of this technology.
Why was osmosis not used earlier for energy production, but only now?
The main difficulty lies in the efficiency and cost of the membranes used. This is the stumbling block. Electricity is produced in generators fed with salt water from tanks where fresh and salt water are mixed. The faster the mixing process, the faster the water is supplied to the turbines, the more energy can be obtained.
The idea to produce energy using osmosis appeared in the 70s of the last century. But then the membranes were still not effective enough, as they are today.
Osmotic power plant in Norway
The experimental power plant built uses the difference in salt concentration in fresh and salt water. Sea and river water is sent to a chamber separated by a membrane. Due to the phenomenon of osmosis, the molecules tend to move to the region of the chamber where the concentration of dissolved substances, in this case salt, is higher. This process results in an increase in volume in the salt water compartment. As a result, increased pressure is formed, which creates a pressure equivalent to the impact of a water column 120 meters high. This pressure is sent to the turbine that rotates the generator.
The constructed power plant uses a membrane with an efficiency of 2-3 W/m2. Therefore, the main task is to find more efficient membranes. According to the researchers, in order for the use of osmotic energy to be beneficial, it is necessary to achieve a membrane efficiency of more than 5 watts/m2.
Now the station does not generate much energy - 4 kW. In the future, it is planned to constantly increase the capacity. Ststkraft plans to bring the station to a self-sustaining level by 2015.
The disadvantages include the fact that it is not possible to build such a power plant everywhere. After all, this simultaneously requires two sources of water - fresh and salty. Therefore, construction is impossible in the depths of the continent, but only on the coasts near the source of salt water. In the future, it is planned to create membranes that use the difference in salt concentration of only sea water.
Another disadvantage is the efficiency of the station, which is primarily related to the efficiency of the membranes used.
The task of the station is mainly to research and develop technologies for commercial applications in the future. This is definitely a step forward. After all, the world potential of osmotic energy, according to Statkraft, is estimated at 1600-1700 TWh of energy annually, which is equivalent to 50 percent of the total energy production in the European Union.
So far, there is only one operating prototype of an osmotic power plant in the world. But in the future there will be hundreds of them.
The principle of operation of the osmotic power plant
The operation of the power plant is based on the osmotic effect - the property of specially designed membranes to allow only certain particles to pass through. For example, we will install a membrane between two containers and pour distilled water into one of them, and saline solution into the other. Water molecules will freely pass through the membrane, but salt particles will not. And since in such a situation the liquids will tend to balance, soon fresh water will spread by gravity to both containers.
If the difference in the compositions of the solutions is made very large, then the liquid flow through the membrane will be quite strong. By placing a hydro turbine in its path, it is possible to generate electricity. This is the simplest design of an osmotic power plant. At the moment, the optimal raw material for it is salty sea water and fresh river water - renewable energy sources.
An experimental power plant of this type was built in 2009 near the Norwegian city of Oslo. Its performance is low - 4 kW or 1 W from 1 sq.m. membranes. In the near future, this indicator will be increased to 5 W per 1 sq.m. By 2015, the Norwegians intend to build a commercial osmosis power plant with a capacity of about 25 MW.
Prospects for the use of this energy source
The main advantage of the IPS over other types of power plants is its use of extremely cheap raw materials. In fact, it is free, because 92-93% of the planet's surface is covered with salt water, and fresh water is easy to obtain using the same osmotic pressure method in another installation. By installing a power plant at the mouth of a river that flows into the sea, all problems with the supply of raw materials can be solved in one fell swoop. Climatic conditions for the operation of the IPS are not important - as long as the water flows, the installation works.
At the same time, no toxic substances are created - the same salt water is formed at the outlet. The ECO is absolutely environmentally friendly, it can be installed in close proximity to residential areas. The power plant does not harm wildlife, and for its construction there is no need to block rivers with dams, as is the case with hydroelectric power plants. And the low efficiency of the power plant is easily compensated by the mass nature of such installations.
The phenomenon of osmosis has been used on an industrial scale for over 40 years. Only this is not the classic direct osmosis of Abbé Nolle, but the so-called reverse osmosis - an artificial process of the penetration of a solvent from a concentrated into a dilute solution under the influence of a pressure exceeding the natural osmotic pressure. This technology has been used in desalination and purification plants since the early 1970s. Salty sea water is injected onto a special membrane and, passing through its pores, is deprived of a significant proportion of mineral salts, and at the same time bacteria and even viruses. It takes a lot of energy to pump salty or polluted water, but the game is worth the candle - there are many regions on the planet where the shortage of drinking water is an acute problem.
It is hard to believe that the difference in concentration of two solutions alone can create a serious force, but it is true: osmotic pressure can raise the level of sea water by 120 m.
Experiments on converting osmotic pressure into electrical energy have been carried out by various scientific groups and companies since the early 1970s. The principle scheme of this process was obvious: the flow of fresh (river) water, penetrating through the pores of the membrane, increases the pressure in the sea water tank, thereby allowing the turbine to spin. The waste brackish water is then thrown into the sea. The only problem was that classic membranes for PRO (Pressure retarded osmosis) were too expensive, capricious and did not provide the necessary flow power. Things got off the ground in the late 1980s, when the Norwegian chemists Thorleif Holt and Thor Thorsen from the SINTEF institute took up the task.
On schematic images, the osmotic membrane is drawn as a wall. In fact, it is a roll enclosed in a cylindrical body. In its multilayer structure, layers of fresh and salt water alternate.
Loeb membranes required clinical grade to maintain peak performance. The design of the membrane module of the desalination station provided for the obligatory presence of a primary coarse filter and a powerful pump that knocked debris from the working surface of the membrane.
Holt and Thorsen, having analyzed the characteristics of most promising materials, opted for inexpensive modified polyethylene. Their publications in scientific journals caught the attention of Statcraft and the Norwegian chemists were invited to continue their work under the auspices of the energy company. In 2001, the Statcraft membrane program received a government grant. The funds received were used to build an experimental osmotic unit in Sunndalsior to test membrane samples and test the technology as a whole. The active surface area in it was slightly over 200 m2.
The difference between the salinity (in scientific terms, the salinity gradient) of fresh and sea water is the basic principle of operation of an osmotic power plant. The larger it is, the higher the volume and flow rate on the membrane, and hence the amount of energy generated by the hydroturbine. In Toft, fresh water flows by gravity to the membrane, as a result of osmosis, the pressure of sea water on the other side increases dramatically. The power of osmosis is colossal - the pressure can raise the level of sea water by 120 m.
Further, the resulting diluted sea water rushes through the pressure distributor to the turbine blades and, having given them all its energy, is thrown into the sea. The pressure distributor takes part of the flow energy, spinning the pumps pumping sea water. Thus, it is possible to significantly increase the efficiency of the station. Rick Stover, chief technologist at Energy Recovery, which manufactures such devices for desalination plants, estimates that the energy transfer efficiency of the distributors is close to 98%. Exactly the same devices during desalination help to deliver drinking water to residential buildings.
As Skillhagen notes, ideally, osmotic power plants should be combined with desalination plants - the salinity of the residual seawater in the latter is 10 times higher than the natural level. In such a tandem, the efficiency of energy generation will increase by at least twice.
Construction work in Toft began in autumn 2008. A vacant warehouse was rented on the premises of the Sódra Cell pulp mill. On the first floor, a cascade of mesh and quartz filters was arranged to purify river and sea water, and on the second floor, a machine room. In December of the same year, the lifting and installation of the membrane modules and the pressure distributor was carried out. In February 2009, a group of divers laid two parallel pipelines along the bottom of the bay - for fresh and sea water.
The intake of sea water is carried out in Toft from depths of 35 to 50 m - in this layer its salinity is optimal. In addition, there it is much cleaner than at the surface. But, despite this, the membranes of the station require regular cleaning from organic residues that clog micropores.
Since April 2009, the power plant has been operated in a trial mode, and in November, with the light hand of Princess Mette-Marit, it was launched to its fullest. Skillhagen assures that after Tofte, Statcraft will have other similar, but more advanced projects. And not only in Norway. According to him, an underground complex the size of a football field is able to continuously supply electricity to an entire city with 15,000 individual homes. Moreover, unlike windmills, such an osmotic installation is practically silent, does not change the usual landscape and does not affect human health. And nature itself will take care of replenishing the reserves of salt and fresh water in it.
There is no error in the title, not from "space", but from "osmosis"
Every day we are convinced that we are surrounded by a mass of the most unexpected sources of renewable energy. In addition to the Sun, wind, currents and tides, generators that run on salt can be used to generate electricity - or rather, on the difference that it creates between fresh and sea water. This difference is called the salinity gradient, and thanks to the phenomenon of osmosis, it can be used to obtain excess liquid pressure, which is converted into electricity by conventional turbines.
There are several ways to convert the energy of the salinity gradient into electricity. The most promising for today is osmosis-assisted conversion, so the energy of the salinity gradient is often referred to as the energy of osmosis. But other ways of converting the energy of the salinity gradient are also fundamentally possible.
The phenomenon of osmosis is as follows. If you take a semi-permeable membrane (membrane) and place it as a partition in a vessel between fresh and salt water, 
then the osmotic forces will begin, as it were, to pump fresh water into salt water. Fresh water molecules will pass through the separating membrane into the second half of the vessel filled with salt water, and the membrane will not let salt molecules into the first half with fresh water. For this property, the membrane is called semi-permeable. The energy released during this process is manifested in the form of increased pressure that occurs in the part of the vessel with salt water. This is the osmotic pressure (sometimes called the osmotic waterfall). The maximum value of osmotic pressure is the pressure difference between the solution (i.e. salt water) and the solvent (i.e. fresh water) at which osmosis stops, which occurs due to the formation of pressure equality on both sides of a semipermeable membrane. The resulting increased pressure in half of the vessel with salt water balances the osmotic forces that forced fresh water molecules through a semipermeable membrane into salt water.
The phenomenon of osmosis has been known for a long time. It was first observed by A. Podlo in 1748, but a detailed study began more than a century later. In 1877, W. Pfeffer measured the osmotic pressure for the first time when studying aqueous solutions of cane sugar. In 1887, van't Hoff, on the basis of Pfeffer's experiments, established a law that determines the osmotic pressure depending on the concentration of the solute and temperature. He showed that the osmotic pressure of a solution is numerically equal to the pressure that the molecules of the solute would exert if they were in a gaseous state at the same values of temperature and concentration.
To obtain osmotic energy, it is necessary to have a source with a low salt concentration near a more or less concentrated solution. In the conditions of the World Ocean, such sources are the mouths of the rivers flowing into it.
The salinity gradient energy calculated from the osmotic pressure is not subject to efficiency limitations associated with the Carnot cycle; this is one of the positive features of this type of energy. The question is how best to convert it into electricity.
The world's first power plant using osmosis to generate electricity opened recently in Norway. Using only salt and fresh water in its work, the current prototype of the power plant will generate 2-4 kilowatts, but in the future this figure will increase significantly. To produce energy, the station, built by the Norwegian company Statkraft, uses the phenomenon of osmosis, that is, the movement of solutions through the membrane to the side higher salt concentration. Since the concentration of salts in ordinary sea water is higher than in fresh water, the phenomenon of osmosis develops between the fresh and salt water separated by a membrane, and the movement of the water flow causes the turbine that generates energy to work. The power of the already launched prototype is small and amounts to two to four kilowatt-hours. As Stein project manager Eric Skilhagen explained, the company did not have a goal to immediately build an industrial-scale power plant, it was more important to show that this technology could in principle be used in the energy sector. , notes the Statkraft website. According to the calculations of engineers, today it is possible to build an osmotic power plant with a capacity of 1700 kilowatts per hour. At the same time, unlike other stations on alternative energy sources - solar or wind - the weather will not have any effect on the operation of the station. The power of the existing prototype is enough to provide electricity to just a coffee maker, but by 2015 Statkraft hopes to build a power plant that supplies electricity to a village of 10,000 private homes.
Among the challenges ahead is the search for more energy efficient membranes. For those used at the station in Hurum, which is 60 km south of Oslo, this figure is 1 W / m2. After some time, Statkraft will increase the power to 2-3 watts, but to reach a cost-effective level, you need to achieve 5 watts.
Osmosis (from the Greek word Osmos - push, pressure), diffusion of a substance, usually a solvent, through a semipermeable membrane that separates a solution and a pure solvent or two solutions of different concentrations. Semi-permeable membrane - a partition that allows small molecules of the solvent to pass through, but is impermeable to large molecules of the solute. The phenomenon of osmosis (leveling the concentrations of solutions separated by a semipermeable membrane) underlies the metabolism of all living organisms. For example, the cell walls of plants, animals, and humans are a natural membrane that is partially permeable because it freely allows water molecules to pass through, but not molecules of other substances. When plant roots absorb water, their cell walls form a natural osmotic membrane that allows water molecules to pass through and most impurities are rejected. Herbs and flowers stand upright only due to the so-called osmotic pressure. Therefore, with a lack of water, they look withered and lethargic. The filtering ability of the natural membrane is unique, it separates substances from water at the molecular level and this is what allows any living organism to exist.
The use of membranes for separating one component of a solution from another has been known for a very long time. In the first, Aristotle discovered that sea water becomes desalinated when it is passed through the walls of a wax vessel. The study of this phenomenon and other membrane processes began much later, at the beginning of the 18th century, when Réaumur used semipermeable membranes of natural origin for scientific purposes. But by the mid-20s of the last century, all these processes were of purely theoretical interest, not going beyond laboratories. In 1927, the German company "Sartorius" received the first samples of artificial membranes. And only in the middle of the last century, American developers launched the production of cellulose acetate and nitrocellulose membranes. In the late 1950s and early 1960s, with the start of the widespread production of synthetic polymeric materials, the first scientific works appeared, which formed the basis for the industrial application of reverse osmosis.
The first industrial reverse osmosis systems appeared only at the beginning of the 1970s, so this is a relatively young technology compared to the same ion exchange or adsorption on activated carbons. However, in Western countries, reverse osmosis has become one of the most economical, versatile and reliable methods of water purification, which allows you to reduce the concentration of components in the water by 96-99% and get rid of microorganisms and viruses by almost 100%. The mechanism for the transfer of water molecules through an osmotic membrane is most often a conventional filtration, in which particles larger than the diameter of the porosmotic membrane are retained. Equalization of concentrations on both sides of such a membrane is possible only with one-way diffusion of the solvent. Therefore, osmosis always goes from a pure solvent to a solution, or from a dilute solution to a concentrated solution. In particular, the phenomenon of osmosis is observed when two salt solutions with different concentrations are separated by a semi-permeable membrane. This membrane allows molecules and ions of a certain size to pass through, but serves as a barrier to substances with larger molecules. Thus, water molecules are able to penetrate the membrane, but salt molecules dissolved in water are not. If on opposite sides of a semi-permeable membrane there are saline solutions of water with different salt concentrations, water molecules will mix through the membrane from a weakly concentrated solution to a more concentrated one, causing an increase in the liquid level in the latter. Through the phenomenon of osmosis, the process of water penetration through the membrane is observed even when both solutions are under the same external pressure. The difference in the height of the levels of two solutions of different concentrations is proportional to the force under which water passes through the membrane. This force is called "osmotic pressure". On the Rice. 23.1. A diagram illustrating the phenomenon of osmosis is given.
Rice. 23.1.
The principle of operation of an osmotic power plant is based on the formation of osmotic pressure. In places where the river flows into the sea, fresh river water simply mixes with salty sea water, and there is no pressure that could serve as a source of energy. However, if, before mixing, sea water and fresh water are separated by a filter - a special membrane that allows water to pass through, but does not pass salt through, then the desire of solutions for thermodynamic equilibrium and equalization of concentrations can be realized only due to the fact that water will penetrate into the salt solution, and salt into fresh water will not enter. A special membrane that allows water to pass through but impermeable to salt molecules is placed between the two tanks. One of them is filled with fresh water, the other is filled with salt water. As such a system tends to balance, the saltier water pulls the fresh water out of the reservoir. If this happens in a closed tank, then excess hydrostatic pressure arises from the side of sea water. At the same time, pressure appears, creates a water flow. If we now install a turbine with a generator, the excess pressure will rotate the turbine blades and produce electricity. Rice. 23.2. A simplified diagram of the osmotic station is shown. On this Fig.: 1 - sea water; 2 river water; 3 - filters; 4 - membrane; 5 - working chamber; 6 - output of waste river water; 7 - turbine with electric generator; 8 - output.
Rice. 23.2.
Theoretical developments in this area appeared as early as the beginning of the 20th century, but the main thing that was lacking for their implementation was a suitable osmotic membrane. Such a membrane had to withstand a pressure of 20 times the pressure of a conventional domestic water supply, and have a very high porosity. The creation of materials with similar properties has become possible with the development of technologies for the production of synthetic polymers. Indeed, the thickness of the effective membrane is about 0.1 micrometer. For comparison: a human hair has a diameter of 50 to 100 micrometers. It is this thinnest film that ultimately separates sea water from fresh water. It is clear that such a thin membrane cannot by itself withstand high osmotic pressure. Therefore, it is applied on a porous sponge-like but extremely durable base. By the way, a membrane for direct osmosis is not a thin wall, which is drawn on simplified diagrams, but a long roll enclosed in a cylindrical body. The connection with the hull is made in such a way that in all layers of the roll there is always fresh water on one side of the membrane, and sea water on the other side, as shown in Rice. 23.3. On this Fig.: 1 - fresh water; 2 - sea water; 3 - membrane. On the Rice. 23.4. The device of the membrane placed in a metal case is shown, cylindrical in shape. On this Fig.: 1 - fresh water; 2 - sea water; 3 - membrane; 4 - metal case. Currently used composite membranes can significantly reduce hydrodynamic resistance. In them, a thin selective layer is deposited chemically on a porous base (substrate). The thickness of the selective layer is 0.1-1.0 µm, and the thickness of the porous base is 50-150 µm. The substrate creates practically no resistance to flow due to the wide pores, and the resistance of the selective layer is significantly reduced due to a significant reduction in its thickness. In general, the composite structure of the membrane provides mechanical strength due to
Rice. 23.3.
Rice. 23.4.
thickness of the porous substrate, and in addition, it allows to reduce the overall resistance of the membrane due to the thinness of the selective layer. The selective layer of reverse osmosis membranes is made of polyamide material.
On Fig. 23.S. the device of an osmotic station is shown, it uses rolled membranes.
On this Fig.: 1 - the introduction of sea water; 2 - introduction of river water; 3 - filters; 4 - roll membranes; 5 - sealed chamber with high osmotic pressure; 6- turbine with electric generator.
In 2009, in Toft, Norway, the world's first power plant began operating, using the difference in salinity between sea and fresh water to generate electricity. In the constructed osmotic power plant, in the compartment with sea water, pressure is created that is equivalent to the pressure of a water column 120 meters high. This pressure drives the turbine shaft which is connected to an electric generator. Fresh water flows by gravity to the membrane. The intake of sea water is carried out in Toft from depths of 35 to 50 meters - in this layer its salinity is optimal. In addition, there it is much cleaner than at the surface. But, despite this, the station membranes require regular cleaning from organic residues, clog its micropores. To date, this osmotic station produces about 1 kW of energy. In the near future, this figure may increase to 2-4 kW. In order to be able to talk about the profitability of production, it is necessary
Rice. 23.5. Osmotic station with rolled membranes
obtain an output of about 5 kW. However, this is a very real challenge. By 2015, it is planned to build a large plant that will generate 25 MW, which will supply electricity to 10,000 average households. In the future, it is assumed that osmotic power plants will become so powerful that they will be able to produce 1700 TW per year, as much as half of Europe currently produces.
Advantages of osmotic stations. Firstly, salt water (ordinary sea water is suitable for the operation of the station) is an inexhaustible natural resource. The surface of the Earth is 94% covered with water, 97% of which is salty, so there will always be fuel for such stations. Secondly, the construction of osmotic power plants does not require the construction of special hydraulic structures. The environmental friendliness of this method of generating electricity. No waste, oxidized tank materials, harmful fumes. Osmotic power plants can be installed even within the city without causing any damage to its inhabitants.
Recently, Japan announced that it plans to produce energy using osmosis stations. Japan is surrounded on all sides by the ocean, into which numerous rivers flow. Because they are constantly flowing, the process of generating electricity will become continuous. Among the advantages of the osmotic method of obtaining energy is independence from the terrain, the station will be able to work on the plain. The main ones are the geographical conditions under which the mixing of fresh and salt water occurs. Thus, osmotic power plants can be installed in any areas of Japan where rivers flow into the ocean. The osmosis plant will be able to produce 5-6 million kW of energy, compared to 5-6 nuclear power plants, according to Akihiko Tanioka, a professor at Tokyo Technical University. In addition, Japan is one of the main manufacturers of osmotic membranes. Now Japanese companies account for 70% of global membrane imports.
