The phenomenon of osmosis has been used commercially for over 40 years. Only this is not the classic direct osmosis of Abbot Nolle, but the so-called reverse osmosis - an artificial process of penetration of a solvent from a concentrated solution into a diluted solution under the action 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 pumped onto a special membrane and, passing through its pores, loses a significant proportion of mineral salts, and at the same time bacteria and even viruses. Pumping salty or polluted water requires large amounts of energy, 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 this is true: osmotic pressure can raise the sea water level 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 basic scheme of this process was obvious: the flow of fresh (river) water, penetrating through the pores of the membrane, builds up the pressure in the seawater reservoir, thereby allowing the turbine to spin up. The spent brackish water is then discharged into the sea. The only problem was that the classic membranes for PRO (Pressure retarded osmosis) were too expensive, capricious and did not provide the required flow power. Things got off the ground in the late 1980s, when Norwegian chemists Thorleif Holt and Thor Thorsen from the SINTEF Institute took on the task.
In schematic images, the osmotic membrane is drawn in the form of 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's membranes required clinical cleanliness to maintain peak performance. The design of the membrane module of the desalination station provided for the mandatory presence of a primary coarse filter and a powerful pump that knocked debris from the working surface of the membrane.
Holt and Thorsen, after analyzing the characteristics of most promising materials, opted for inexpensive modified polyethylene. Their publications in scientific journals attracted the attention of experts from Statcraft, and the Norwegian chemists were invited to continue working under the auspices of the energy company. In 2001, the Statcraft membrane program received state grant... The funds received were used to build an experimental osmotic facility in Sundalsior for testing membrane samples and running the technology in general. The active surface area in it was slightly above 200 m2.
The difference between the salinity (scientifically - 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, consequently, the amount of energy generated by the turbine. In Toft, fresh water flows by gravity onto the membrane; as a result of osmosis, the seawater pressure on the other side increases sharply. Osmosis has a colossal power - the pressure can raise the sea water level by 120 m.
Further, the resulting diluted sea water rushes through the pressure distributor onto the turbine blades and, having given them all its energy, is thrown into the sea. The pressure distributor takes a part of the flow energy, spinning up the pumps that pump seawater. Thus, it is possible to significantly increase the efficiency of the station. Rick Stover, chief technologist at Energy Recovery, which makes such devices for desalination plants, estimates that the power transmission efficiency in the distributors is close to 98%. Exactly the same apparatus for desalination helps 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 production will increase at least twice.
Construction work in Toft began in autumn 2008. An empty warehouse was rented at the Sódra Cell pulp mill. On the first floor, a cascade of mesh and quartz filters was set up to purify river and sea water, and on the second, a machine room. In December of the same year, the membrane modules and the pressure distributor were lifted and installed. In February 2009, a group of divers laid two parallel pipelines along the bottom of the bay - for fresh and sea water.
Sea water is taken in Toft from depths of 35 to 50 m - in this layer its salinity is optimal. In addition, it is much cleaner there than at the surface. But, despite this, the station membranes require regular cleaning from organic residues clogging micropores.
Since April 2009, the power plant has been operating in a trial mode, and in November, with the light hand of Princess Mette-Marit, it was launched to its fullest. Skillhagen assures that following Tofte, Statcraft will have other similar, but more advanced projects. And not only in Norway. The underground complex, the size of a football field, is able to supply uninterrupted electricity to an entire city of 15,000 individual houses, he said. Moreover, unlike wind turbines, 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 the replenishment of salt and fresh water in it.
A special membrane that allows water to pass through, but does not allow salt molecules to pass through, is placed between the two reservoirs. Fresh water is poured into one of them, salty water into the other. Since such a system strives for equilibrium, more salty water as it draws fresh water from the reservoir. If a generator is placed in front of the membrane, the excess pressure will rotate its blades and generate electricity.
The idea, as often happens, was suggested by live nature: according to the same principle, substances are transferred in cells - the same partially permeable membranes ensure the elasticity of cells. Osmotic pressure has long been successfully used by humans in the desalination of sea water, but it has been used for the first time to generate electricity.
At the moment, the prototype generates 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 to obtain an output of about 5 kW. However, this is a very real task. By 2015, it is planned to build a large station that will generate 25 MW, which will supply 10,000 medium-sized households with electricity. In the future, it is assumed that ECOs will become so powerful that they will be able to generate 1,700 TWh per year, as much as half of Europe now generates. the main task at the moment - to find more efficient membranes.
The game is definitely worth the candle. The advantages of osmosis stations are obvious. First, salt water (normal sea water is suitable for the operation of the station) is inexhaustible natural resource... The surface of the Earth is 94% covered with water, 97% of which is saline, so there will always be fuel for such stations. Secondly, the organization of an ECO does not require the construction of special sites: any unused premises of already existing enterprises or other office buildings will do. In addition, ECOs can be delivered at river estuaries where fresh water flows into a salty sea or ocean - in which case it is not even necessary to specially fill the reservoirs with water.
Fresh water + seawater \u003d energy source
Usually, where the river flows into the sea, fresh water is simply mixed with salt, and there is no pressure that could serve as a source of energy. Professor Klaus-Viktor Peinemann from the Institute for Polymer Research at the GKSS Research Center in the town of Geesthacht in northern Germany, calls the conditions necessary for osmotic pressure to develop: "If, before mixing, sea water and fresh water are separated by a filter - with a special membrane that allows water to pass through, but impermeable to salt, then the aspiration of solutions to thermodynamic equilibrium and equalization of concentrations can be realized only due to the fact that water will penetrate into the salt solution, and salt will not enter fresh water. "
If this happens in a closed reservoir, then an excess hydrostatic pressure, called osmotic pressure, arises from the seawater. To use it for energy production, at the place where the river flows into the sea, you need to install a large reservoir with two chambers separated from each other by a semi-permeable membrane that allows water to pass through and does not allow salt to pass through. One chamber is filled with salt water, the other with fresh water. "The resulting osmotic pressure can be very high," emphasizes Professor Paynemann. "It reaches about 25 bar, which corresponds to the water pressure at the foot of a waterfall that rushes down from a height of 100 meters."
Under such a high osmotic pressure, water is fed to the turbine of a generator that generates electricity.
The main thing is the right membrane
It would seem that everything is simple. Therefore, it is not surprising that the idea of \u200b\u200busing osmosis as a source of energy originated almost half a century ago. But ... “One of the main obstacles at that time was the lack of membranes of the proper quality, - says Professor Payneman. - The membranes were extremely slow, so the efficiency of an osmotic electric generator would be very low. But in the next 20-30 years there were several technological breakthroughs. We learned today to produce extremely thin membranes, which means that their throughput has become significantly higher. "
Specialists of the GKSS Research Center made a significant contribution to the development of the very membrane, which has now made it possible to implement osmotic energy production in practice, albeit for the time being purely experimental. One of the developers, Carsten Blicke, explains: “The membrane is about 0.1 micrometers thick. In comparison, a human hair is 50 to 100 micrometers in diameter. It is this thinnest film that ultimately separates the seawater. from fresh ".
It is clear that such a thin membrane cannot by itself withstand high osmotic pressure. Therefore, it is applied to a porous, sponge-like, but extremely durable base. In general, such a partition looks like glossy paper, and the fact that there is a film on it cannot be seen with the naked eye.
Iridescent perspectives
The construction of the pilot plant required an investment of several million euros. Investors willing to take risks, although not immediately, were found. Statkraft, one of the largest energy companies in Norway and the European leader in the use of renewable energy resources, volunteered to finance the innovative project. Professor Peinemann recalls: "They heard about this technology, were delighted and signed a cooperation agreement with us. The European Union allocated 2 million euros for the implementation of this project, the rest was contributed by Statkraft and a number of other companies, including our Institute."
"Several other companies" are research centers in Finland and Portugal, as well as one of the Norwegian research firms. A pilot plant with a capacity of 2 to 4 kilowatts, erected in the Oslofjord near the town of Tofte and inaugurated today, is designed to test and improve the innovative technology. But the management of Statkraft is confident that in a few years time it will come to commercial use of osmosis. And the total world potential of osmotic energy production is estimated at no less than 1600-1700 terawatt-hours per year - this is about half of the energy consumption of the entire European Union. The most important advantage of such installations is their environmental friendliness - they do not make noise and do not pollute the atmosphere with greenhouse gas emissions. In addition, they are easy to integrate into existing infrastructure.
Sustainability
Separately, I would like to note the absolute environmental friendliness of this method of generating electricity. No waste, no oxidizing materials for tanks, no harmful fumes. ECO can be installed even within the city, without causing any damage to its inhabitants.
Also, the operation of the ECO does not require other sources of energy to start and does not depend on climatic conditions... All this makes ECO an almost ideal way to generate electricity.
Seas and rivers, inexhaustible sources of energy, not only drive the turbines of tidal, wave power plants and hydroelectric power plants. Sea and fresh waters can work in tandem - and then such a factor as a change in water salinity acts as an energy generator. Despite the fact that salt energy is only at the beginning of its technological development, it already has obvious prospects.
Working principle and potential of salt stations
Salt generation is based on a natural process called osmosis. It is widely represented in nature, both living and inanimate. In particular, due to osmotic pressure, sap in trees in the course of metabolism overcome a considerable distance from the roots to the top, rising to an impressive height - for example, for a sequoia, it is about hundreds of meters. A similar phenomenon - osmosis - is inherent in water bodies and manifests itself in the movement of molecules. The movement of particles is carried out from a zone with a large number of water molecules into a medium with salt impurities.
Salinity changes are possible in a number of cases, including when the sea or lakes come into contact with fresher waters - rivers, estuaries and lagoons off the coast. In addition, the proximity of salty and fresh waters is possible in regions with an arid climate, in areas where underground salt deposits, salt domes, and also under the seabed are located. The difference in the salinity of the communicating water masses can arise artificially - in evaporation reservoirs, solar stratified ponds, in solutions of discharges from the chemical industry and in water reservoirs of power facilities, including nuclear power plants.
The movement of ions, like any natural force, can be used to generate energy. The classical principle of salt generation provides for the arrangement of a membrane permeable to ions between fresh and salt solutions. In this case, the particles of the fresh solution will pass through the membrane, the pressure of the salt liquid rises and compensates for the osmotic forces. Since in nature the supply of fresh water to rivers is constant, the movement of ions will be stable, since the pressure difference will not change. The latter drives the turbines of the generators and thus produces energy.
The possibilities for energy production depend primarily on the salinity of the water, as well as on the level of its consumption in the river flow. The average salinity of the World Ocean is 35 kilograms per cubic meter of water. Osmotic pressure at this rate reaches 24 atmospheres, which is equivalent to the force of falling water from a height of 240 meters. The total discharge of water from fresh water bodies into the sea is 3.7 thousand cubic kilometers per year. If we use 10% of the potential of the largest rivers of the European Union - the Vistula, Rhine and Danube - to generate, then the generated volume of energy will exceed the average consumption in Europe three times.
A few more impressive figures: when arranging power plants in the area where the Volga flows into the Caspian, it will be possible to produce 15 TWh of energy in a year. Generation of 10 TWh and 12 TWh of energy is quite possible in the areas of the confluence of the Dnieper-Black Sea and Amur-Tatar Strait, respectively. According to experts from the Norwegian company Statkraft, the total potential of the salt industry reaches 0.7-1.7 thousand TWh, or 10% of the world's needs. According to the most optimistic estimates of experts, the maximum use of the possibilities of using the salinity of water will allow getting more electricity than humanity currently consumes.
Europe: completed projects
The first attempts by scientists to achieve electricity generation by creating osmotic pressure, which would be able to drive turbines of generators, date back to the seventies of the twentieth century. Even then, it was proposed to use a semi-permeable membrane as the main component of a generating installation of a new type, inaccessible for the return flow of salts, but quite freely passing water molecules.
The first developments could hardly be called successful - the membranes did not provide a sufficiently powerful flow. Materials were required that would withstand a pressure two dozen times greater than in water supply networks, and at the same time would have a porous structure. Progress in development began in the mid-eighties, after the Norwegian company SINTEF created a cheap modified polyethylene based on ceramics.
After receiving new technology the Norwegians have actually opened the way to the practical implementation of salt generation projects. In 2001, the government awarded Statkraft a grant to build an experimental osmotic plant with a total membrane area of \u200b\u200b200 square meters. The construction of the station took about $ 20 million. The facility was built in the city of Toft (located in the commune of Hurum). The construction was based on the infrastructure of the Södra Cell Tofte paper mill.
Södra Cell Tofte paper mill with pilot plant
The power of the generator turned out to be more than modest - the station produces a maximum of 4 kW of energy, which is enough only for the operation of two electric kettles. In the future, it is planned to increase the capacity indicator up to 10 kW. However, it should be remembered that the pilot project was launched as an experiment and was intended primarily to test technologies and test theoretical calculations in practice. It is assumed that the station can be transferred to commercial operation if the experiment is found to be successful. In this case, the profitable power of the generator should be increased to 5 W per square meter of membrane area, but now this figure for a Norwegian station is no more than 1 W per square meter.
Experimental Osmosis Unit
The next stage in the development of salt generation based on membrane technologies was the launch in 2014 of a power plant in Afsluitdijk, Netherlands. The initial capacity of the facility was 50 kW, according to unverified data, it can be increased to tens of megawatts. The station, built off the coast of the North Sea, if the project develops, will be able to meet the energy needs of 200 thousand households, calculated in the company Fudji, which acted as a membrane supplier.
Russia and Japan as promising territories
If we talk about in which regions of the world the next stations will appear, then most of all prospects for this type of energy are in Japan. This is primarily due to the well-established production of the necessary components - the country's companies produce 70% of the world volume of osmotic membranes. The geographical factor is likely to work as well - the specialists of Tokyo technical institute came to the conclusion that Japan has great potential for the development of salt energy. The country's islands are surrounded on all sides by ocean waters, into which a large number of rivers flow. The use of osmotic stations will provide an opportunity to receive 5 GW of energy, which is equivalent to the generation of several nuclear power plants, most of which in the Japanese region were closed after the Fukushima disaster.
Osmotic membranes
The Russian territory is no less attractive for the development of this segment. According to domestic experts, the construction of an osmotic station in the area where the Volga flows into the Caspian Sea can be a fully realizable project. The level of water consumption at the river mouth is 7.71 thousand cubic meters per second, while the potential capacity of salt generation will fluctuate within 2.83 GW. The capacity of the station, using 10% of the river flow, will be 290 MW. However, the developed economic activity in the region, the abundance of fauna and flora in the Volga delta will to some extent complicate the project for the construction of the station - it will be necessary to erect a number of engineering structures, channels for the passage of fish and watersheds.
In addition, Crimea is one of the promising sites for the introduction of osmosis generation. Although the total potential of the rivers of the peninsula is not high, it could still satisfy the energy needs of individual objects, for example, hotels. Experts, purely hypothetically, even consider the possibility of using sewage in Crimea as a fresh source for osmotic stations. The volume of wastewater that is now discharged into the sea area in the summer period in the region may exceed the flow rate of individual rivers. Nevertheless, in this case, the issue of the technology of effective cleaning of equipment from contamination becomes especially acute.
On the other hand, despite the favorable geographic conditions and the possibility of a wide choice for the location of generating facilities, systemic developments on these issues in Russia have not yet been carried out. Although, according to some reports, in 1990, on the basis of the scientific group of the Far Eastern Scientific Center of the Academy of Sciences of the USSR, a study was carried out of the possibility of developing salt power engineering up to the laboratory experiments, however, the results of this work remained unknown. For comparison, in the same Europe, research in the field of creating osmotic stations has sharply intensified under the pressure of environmental organizations since the beginning of the nineties. All kinds of startups are actively involved in this work in the EU, government subsidies and grants are practiced.
Ways of further development of technologies
The most promising research in the field of salt energy is mainly aimed at increasing the efficiency of energy production using the above-mentioned membrane technology. French researchers, in particular, managed to increase the power generation rate to the level of 4 kW per square meter of membrane, which has already brought the likelihood of transferring stations to a commercial basis very close to reality. Scientists from the USA and Japan went even further - they were able to apply the technology of graphene films in the membrane structure. A high degree of permeability is achieved due to the ultra-thin membrane thickness, which does not exceed the size of an atom. It is assumed that using graphene membranes, the generation of energy per square meter from the surface can be increased to 10 kW.
A group of specialists from the Federal Polytechnic School of Lausanne (Switzerland) began to study the possibility of efficiently capturing an energy charge by a third-party way - without the use of generator turbines, but directly in the process of ions passing through membranes. To do this, they used three-atom thick molybdenum disulfide plates in test facilities. This material is relatively cheap, and the amount of its reserves in nature is quite large.
Micro-holes are made in the plates for the passage of charged salt particles, which generate energy during movement. One such pore of the membrane can produce up to 20 nanowatts. According to the Swiss Federal institute of Technology in Zurich, membranes of this type with an area of \u200b\u200b0.3 square meters generate about a megawatt of energy. Obviously, such an indicator in the case of successful experiments can be considered a real breakthrough in the industry. To date, research is being carried out on initial stage, scientists have already faced the first problem - they are not yet able to make a large number of evenly spaced nano-holes in the membranes.
In the United States, Israel and Sweden, meanwhile, methods are being developed to obtain energy through reverse electrodialysis - one of the varieties of membrane technology. This technique, which involves the use of ion-selective membranes, makes it possible to implement a scheme for the direct conversion of water salinity into electricity. The role of the nominal generation element is an electrodialysis battery, consisting of electrodes and several membranes placed between them, designed separately to ensure the exchange of cations and anions.
Reverse electrodialysis circuit
The membranes form several chambers, which receive solutions with varying degrees of salt saturation. When ions pass between the plates in a certain direction, electricity accumulates on the electrodes. Perhaps, with the use of the latest membrane technologies, the efficiency of such installations will be high. So far, experiments with the creation of installations of a similar design - with dialytic batteries - have not shown impressive results. In particular, the use of cationic and anionic membranes provides only 0.33 watts per square meter of membranes. The latter are quite expensive and short-lived.
In general, membrane technologies are not mastered from scratch - in principle, such designs are similar to plates used in water desalination plants, but at the same time they are much thinner and more difficult to manufacture. The leading companies in the production of desalination membranes, including General Electric, have not yet taken up the supply of plates for osmotic stations. According to the press service of the corporation, it will start production of membranes for power engineering no earlier than in five or ten years.
Against the background of the difficulties with the development of traditional membrane technologies, a number of researchers have devoted their activities to the search for alternative methods of salt generation. For example, physicist Doriano Brogioli from Italy proposed using the salinity of water to extract energy using an ionistor - a capacitor with a large capacity. The accumulation of energy occurs on the activated carbon electrodes in the process of sequential supply of fresh and salt water to the same chamber. During a practical experiment, the scientist managed to generate 5 microjoules of energy in one cycle of filling the reservoir. He estimated the potential of his installation much higher - up to 1.6 kilojoules per one liter of fresh water, provided that ionistors of a higher capacity are used, which is quite comparable to membrane generators.
American specialists from Stanford University followed a similar path. The design of their batteries provides for filling the battery chamber with fresh water with further small recharging from an external source. After changing from fresh water to seawater, due to an increase in the number of ions dozens of times, the electric potential between the electrodes increases, which leads to the generation of more energy than that spent on recharging the battery.
A completely different principle of using the salinity of water is quite difficult to implement, but it has already been tested on mock-ups of generating plants. It provides for the use of the difference in saturated vapor pressures over water bodies with salt and fresh water. The fact is that with an increase in the degree of salinity of water, the vapor pressure above its surface decreases. The pressure difference can be used to generate energy.
When using microturbines, it is possible to obtain up to 10 watts of energy from each square meter heat exchanger, however, this requires only water bodies with high degree salinity - for example, the Red Sea or the Dead Sea. In addition, the technology provides for the need to maintain a low, close to vacuum, atmospheric pressure inside the unit, which is problematic when the generator is located in an open water area.
Energy from salt: more pluses
In the field of salt generation, as in other energy sectors, the priority stimulus for development is the economic factor. In this regard, salt energy looks more than attractive. So, according to experts, provided that existing technologies for energy production using membranes are improved, the production cost will be € 0.08 per 1 kW - even in the absence of subsidies for generating companies.
For comparison, the cost of energy production at wind farms in European countries ranges from € 0.1 to € 0.2 per kilowatt. Coal generation is cheaper - € 0.06–0.08, gas-fired - € 0.08–0.1, but it should be taken into account that thermal power plants pollute the air. Thus, in the price segment, osmotic stations have a clear advantage over other types of alternative energy. Unlike wind and solar power plants, salt generators are also more efficient technically - their operation does not depend on the time of day and season, and the salinity level of the water is practically constant.
Construction of osmotic stations, as opposed to hydroelectric power plants and other types of stations on water bodies, does not require the cost of building special hydraulic structures. In other types of marine energy, the situation is worse. Pronedra wrote earlier that the construction of tidal stations requires the construction of a large-scale and complex infrastructure. Recall that similar problems relate to energy facilities operating on power ocean currents and sea waves.
As one of the areas of alternative energy, salt generation is characterized by an "ecological plus" - the operation of osmotic stations is absolutely safe for environment, it does not disturb the natural balance of living nature. The process of generating energy from the salinity of water is not accompanied by noise effects. You don't have to change the landscape to run stations. They do not have emissions, waste or any kind of fumes, and therefore such stations can be installed, including directly in cities. The stations only use the usual natural processes of desalination of salt water at river mouths to generate energy and do not in any way affect their course.
Despite a number of obvious advantages, salt energy also has certain disadvantages associated primarily with the imperfection of existing technologies. In addition to the above-mentioned problems with the creation of highly productive, reliable and at the same time inexpensive membranes, there is an acute question of the development of effective filters, since the water entering the osmotic power plant must be thoroughly purified from organic matter that clogs the channels intended for the passage of ions.
The disadvantages of the stations include the geographical limitation of the possibility of their use - such generators are installed only at the borders of fresh and salt water bodies, that is, at river mouths, or on salt lakes. Nevertheless, even with the existing shortcomings and against the background of its huge advantages, and subject to overcoming the problems of the technological plan, the salt energy, undoubtedly, gets great chances to occupy one of the key positions in the global generation market.
Osmosis (from the Greek word Osmos - push, pressure), the diffusion of a substance, usually a solvent, through a semipermeable membrane separating a solution and a pure solvent or two solutions of different concentrations. A semi-permeable membrane is a septum that allows small solvent molecules to pass through, but impermeable to large solute molecules. The phenomenon of osmosis (equalization of 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, since it freely allows water molecules, but not molecules of other substances. When the roots of plants have absorbed water, their cell walls form a natural osmotic membrane that allows water molecules to pass through and most of the 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 sluggish. The filtering ability of a natural membrane is unique, it separates substances from water by molecular level and this is what allows any living organism to exist.
The use of membranes for separating some components of a solution from others has been known for a very long time. In the first, Aristotle discovered that sea water is desalinated if 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 Reaumur used semi-permeable membranes of natural origin for scientific purposes. But by the mid-20s of the last century, all these processes were of purely theoretical interest, without leaving the 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 set up the production of cellulose acetate and nitrocellulose membranes. In the late 50s - early 60s, with the beginning of the widespread production of synthetic polymer materials, the first scientific workwhich formed the basis for the industrial application of reverse osmosis.
The first industrial osmotic systems appeared only in the early 70s, 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 water by 96-99% and get rid of microorganisms and viruses by almost 100%. The mechanism of transfer of water molecules through the osmotic membrane is most often 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-sided 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 saline 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 molecules of salts dissolved in water are not. If on opposite sides of the 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 the latter to increase the liquid level. 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 height between the levels of two solutions of different concentration is proportional to the force by which the water passes through the membrane. This force is called "osmotic pressure". On Fig. 23.1. A diagram illustrating the phenomenon of osmosis is given.
Fig. 23.1.
The principle of operation of an osmotic power plant is based on the formation of osmotic pressure. In the 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, seawater and fresh water are separated by a filter - a special membrane that allows water to pass through, but does not allow salt to pass through, then the tendency of solutions to 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 is impermeable to salt molecules is placed between the two reservoirs. One of them is filled with fresh water, the other is filled with salt water. As such a system strives for equilibrium, the saltier water draws fresh water out of the reservoir, as it were. If this happens in a closed tank, then an excess hydrostatic pressure arises from the seawater. 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. Fig. 23.2. A simplified diagram of an osmotic station is shown. In this Fig .: 1 - sea water; 2 river water; 3 - filters; 4 - membrane; 5 - working chamber; 6 - outlet of waste river water; 7 - turbine with an electric generator; 8 - conclusion.
Fig. 23.2.
Theoretical developments in this area appeared at the beginning of the 20th century, but the main thing was lacking for their implementation - a suitable osmotic membrane. Such a membrane had to withstand a pressure 20 times higher than the pressure of a conventional domestic water supply, and have a very high porosity. The creation of materials with similar properties became possible with the development of technologies for the production of synthetic polymers. Indeed, the effective membrane thickness is about 0.1 micrometer. For comparison, a human hair is 50 to 100 micrometers in diameter. It is this thinnest film that ultimately separates seawater from fresh water. It is clear that such a thin membrane cannot by itself withstand high osmotic pressure. Therefore, it is applied to 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 to the body is made in such a way that in all layers of the roll, on one side of the membrane, there is always fresh water, and on the other side, the sea side, as shown in Fig. 23.3. In this Fig .: 1 - fresh water; 2 - sea water; 3 - membrane. On Fig. 23.4. Shown is the device of a cylindrical membrane placed in a metal case. In this Fig .: 1 - fresh water; 2 - sea water; 3 - membrane; 4 - metal case. The currently used composite membranes can significantly reduce the hydrodynamic resistance. In them, a thin selective layer is chemically applied to a porous base (substrate). The thickness of the selective layer is 0.1-1.0 microns, and the thickness of the porous base is 50-150 microns. The substrate creates practically no resistance to flow due to its 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
Fig. 23.3.
Fig. 23.4.
the thickness of the porous substrate, and, in addition, allows to reduce the overall resistance of the membrane due to the thinness of the selective layer. The selective layer of reverse osmotic membranes is made of polyamide material.
In Fig. 23.S. the device of an osmotic station is shown, it uses roll membranes.
In this Fig .: 1 - introduction of sea water; 2 - introduction of river water; 3 - filters; 4 - roll membranes; 5 - sealed chamber with high osmotic pressure; 6- turbine with an electric generator.
In 2009, the first in the world power plant in the city of Toft, Norway, began operation, using the difference in salinity of sea and fresh water to generate electricity. In the constructed osmotic power plant, a pressure is created in the compartment with sea water, equivalent to the pressure of a column of water 120 meters high. This pressure drives the shaft of a turbine which is connected to an electric generator. Fresh water flows by gravity onto the membrane. Sea water is taken in Toft from depths of 35 to 50 meters - in this layer its salinity is optimal. In addition, it is much cleaner there than at the surface. But, despite this, the station membranes require regular cleaning from organic residues, clog its micropores. Today 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
Fig. 23.5. Osmotic station with roll membranes
get an output of about 5 kW. However, this is a very real task. By 2015, it is planned to build a large station that will generate 25 MW, which will allow 10,000 medium-sized households to be supplied with electricity. In the future, it is assumed that osmotic power plants will become so powerful that they can produce 1,700 TWh per year, as much as half of Europe now produces.
Benefits of osmotic stations. First, 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 saline, 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. Environmental friendliness of this method of generating electricity. No waste, oxidized materials for tanks, harmful fumes. Osmotic power plants can be installed even within the city without causing any damage to its inhabitants.
Japan recently announced that it plans to generate energy through osmotic 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 generating energy is independence from the terrain, the station will be able to operate on the plain. The main ones are the geographical conditions under which there is a mixing of fresh and salt water. Thus, osmotic power plants can be installed anywhere in Japan where rivers flow into the ocean. An osmotic station will be able to produce 5-6 million kW of energy, for comparison, 5-6 nuclear power plants produce the same amount, according to Akihiko Tanioka, professor of Tokyo technical university... In addition, Japan is one of the main producers of osmotic membranes. Now the share of Japanese companies accounts for 70% of world imports of membranes.