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Offline Andrey

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« on: February 26, 2007, 11:58:20 AM »

Modern power engineering is mostly thermal and based on nonrenewable sources, hydrocarbons and uranium. The part of energy based on renewable sources is little and the increase of the part can?t be foreseen for many reasons which we are not going to consider in this article. Promising thermonuclear sources of thermal energy are still at the development stage and their cost will be quite high.
   Existing systems of conversion of thermal energy into electric one are based on vapour-water thermodynamic cycles and their efficiency is limited to 30 % for atomic, 52 % for supercritical coal and 63 % for combined turbo-boiler kettle cycles working on natural gas.
   However there exist real promising ways of considerable efficiency increase of energy conversion cycles  along with operating  temperature reduction and determinative factor here will belong to chemistry of solutions and  membrane technologies.
   Relatively small heat efficiency is the peculiarity of solution of crystalline materials and other substances in solvents. In addition osmotic pressure is frequently large and can be calculated with the help of Van?t Hoff law. Besides osmotic pressure depends on solution temperature. At that it?s necessary to mention that in process of dissolving values of heat efficiency are greatly less than thermal costs required for vaporization of solute. Heat efficiency of solute release out of solutions are the same as we have at dissolving but with reversed sign. So the use of osmotic pressure in energy generation cycles is much more profitable owing to small loss of heat which should be drop into the environment on the low-temperature part of the cycle  in order to get  a parent soluble substance and  a solvent out of solution.
   Let?s briefly explain the principle of working of such a cycle even for a layman to understand it.  For the purposes of illustration we?ll take usual sodium chloride NaCl. A fair amount of heat is necessary to evaporate a mole of its crystal but in water it will evaporate with extremely low heat consumption: water breaks off combinations in  a crystal  and the total heat efficiency of dissolution is rather  low. According to Van?t Hoff law the osmotic pressure of the solution will be as though vaporous salt is placed in empty volume which is equivalent to the volume of water in which the salt was dissolved. We?ll need two selective membranes which let in only water but not salt. So let?s take two capacities coupled by tubes; the capacities are separated by membranes. On the one side of the membranes there is salt in water, on the other side there is pure water; compartments with salt in capacities are coupled by tubes, compartments with water are coupled by tubes too. There is no stir at all as each of the saturated solutions tries to suck in water through the membrane on its side with equal forces (the osmotic pressures are equal). Now let?s place one of the capacities on ice and the other one we?ll heat. We?ll see that the solution and water shift along the assembled contour and the salt will gradually shift from the capacity with a high temperature to the capacity with a low one.
   The reason for this is that the osmotic pressure of the solution is higher at the high temperature than at the low one and water will be pressed out through the membrane out of the solution in the low-temperature capacity as local osmotic pressure of the saturated solution is lower here. As a result a flow of solution is formed from the high-temperature capacity to the low-temperature one. There?s one more thing to be done ? to organize the conveying of salt from the low-temperature capacity to the high-temperature one (or simply change their roles), to put hydraulic motor   into the tube with water, the tubes themselves must be placed into oncoming heat exchanger where water will become warm and the solution  will get cool in the approaching movement. We?ll pay our readers? attention to the fact that steam is not used for work of such conversion energy cycle and consequently we do not need expensive and complicated in manufacturing steam turbine; hydraulic motor or water turbine will be quite enough.
   However there is a good possibility to improve essentially even this scheme. The matter is that different substances dissolve with different thermal effects and generated osmotic pressures are often quite different from theoretically predicted by Van?t Hoff law. There are substances which require fairly large expenses of thermal energy for their dissolving and generate lower osmotic pressure; and there are substances which require small expenses of thermal energy for their dissolving but they generate higher osmotic pressure. It depends on the peculiarities of dissociation of substances in a concrete solvent. There is one more possibility: at one and the same thermal effect of dissolving the solutions can generate different osmotic pressures. So if we use such two substances for one solvent and bring them together into one common contour then we can get a system converting thermal energy into electric one with efficiency exceeding 85%.  It?s necessary to say that nothing prevents us from making an assembling on different contours with different solvents connected by heat exchangers and mechanic energy transmission.
   Let?s briefly explain work of such a scheme. So we have two substances with different thermal effects and similar osmotic pressures. If we combine such two solutions with the help of the selective membrane, letting in only solvent, the solvent will not shift in these solutions. Now we couple such two capacities with membranes by tubes in this way: capacities with the solution of one type will be joined with the help of such a tube, capacities themselves will be at different temperatures. There won?t be any shift of the solvent and we?ll have to use a pump. But let?s pay attention to the fact that the energy consumed by this pump is quite small in comparison with the energy which this pump will be able to lift at a high level. The matter is that this pump will simply have to overcome membrane resistance and drop in several atmospheres is necessary for this. It means that heating factor will be very large and temperature drop may account to 100 -150? C or even more. Such drop will help to organize the thermodynamic cycle with the application of generative osmotic contour as was mentioned above. But it?s not compulsory for capacities with solutions to be joined to each other with the help of one membrane; they may be joined with the help of two membranes and tubes passing the solvent and this will allow differentiate capacities according to their temperatures both at the high and low temperature level. This in its turn will compensate small differences in osmotic pressures or get rid of additional generative contour, if we put hydraulic motor directly into the contour.
   It?s quite evident that great variety is possible in realization of such thermodynamic cycles.
   All advantages in application of such energy conversion cycles based on solutions are quite obvious. For example, 80 % energy in France is provided by nuclear power stations. This country will be able to double its energy potential without building new electric power-stations by increasing efficiency of thermodynamic cycles.

September 12th                                            Pelipenko Andrey, an engineer
2006                                                           Kolisnichenko Nikolay, an engineer