Physical-metallurgical Faculty
Speciality: Heat and Power Engineering
Principle of MHD generator. Plasma MHD generators.
Magnetohydrodynamic generator is a device that converts kinetic energy of the conductive flow moving in a transverse magnetic field, into electricity. In the flow of induced electric field with strength ENNd-[VB], where V - flow velocity, B - magnetic induction. If the channel has a width of, then on the channel walls parallel to the direction of the magnetic field (electrode walls), emf E = VBe. As long as the electrodes are not closed to the load, the electromagnetic forces on the flow do not act. With the closure of the chain in the flow of working fluid (liquid or gas) will flow current I = E (lk) / Rr, where Rr - internal resistance of the generator, a = U / E - the load factor; U - the voltage across the load; In accordance with the laws of electrodynamics per unit length of the conductor with current I, located in a transverse magnetic field, a force F = IB, which slows down the stream and converts its kinetic energy into electrical energy flowing current.
Electric power N, is generated in the channel of the generator, can be defined either as the product of the retarding force on the flow rate The working fluid in the MHD system can be electrically conductive gas-plasma, which is a quasi-neutral collection of ions, electrons, neutral atoms or molecules. Gas into a plasma when its ionization. If the ionization is achieved by high temperatures, it is called heat. Thermal ionization obeys the law of mass action, like any chemical reaction. Reaction heat of ionization, expressed in electron-volts, is called the ionization potential.
Advantages and disadvantages of the MHD generator
Main advantage of the MHD generators is that they are increasing at 10-20% efficiency compared with thermal power plants can now produce electricity on a commercial scale. In the MHD generator, as described above, the electric current is the flow of ionized gas (plasma), perpendicular to the magnetic field. Negative and positive charges in a magnetic field are deflected in different directions and sent everyone on their probe. Between the electrodes is formed a potential difference, and for the closure of the external circuit, an electric current. For producing ions fuel is burnt at ZOOOK in a special chamber in which to facilitate the emergence of the ions added to it the salt of potassium or cesium. Since a large fraction of the energy is transformed in this case is still in heat, in the case of the MHD generator is not quite possible to speak of the direct conversion of chemical energy into electrical energy. Gas temperature, exhaust in the MHD generator is 2000K. Using it in the usual way, the turbine generates more electricity about the same as producing an MHD generator. Therefore, relatively high efficiency of the entire installation (50-60%) is achieved using two-step process.
Effect of electromagnetic induction is used in conventional power sources --electric generators, where across the magnetic field moving rigid conductors placed on the rotating rotor. In contrast, in the MHD generator are replaced by hard wires conducting liquid or gas. What benefits arise here? The rotor rotates the rotary generator steam turbine or other heat engine in which thermal energy is converted into mechanical energy. MHD generator is able to directly convert heat into electricity without any intermediate complex devices such as steam turbines or internal combustion engine. Why is it still using conventional generators? The problem lies in creating the necessary "working body" for MHD generators. Liquid metals are inconvenient to use, and it is not so easy to get them to move through the channel with high velocities. Therefore, the MTD-generators for liquid metals are not widespread. Often applied MHD pumps for liquid metals and other conductive liquids, the device where an electric current causes the movement of a conducting liquid, and not vice versa as in the generator. Conductivity of gases is negligible. It grows only when the gas is being heated, begins to become ionized and converted into a plasma. But to obtain sufficient conductivity even when the most easily ionized substances - vapors of alkali metals - requires the temperature to 2000-3000 ° C. Such high temperatures are reached in the jet rocket engine, and based on that, indeed, make good MHD generators, compact and very powerful device. However, the current collection electrodes such generators at such high temperatures can run only a very short period of time - seconds or at most tens of seconds. With the use of rocket engines have already been established MTD generators designed for very high power and short duration work. They are used, for example, for geophysical studies.
In order to use the MHD generator in a large energy, we must find a way to get good electrical conductivity of gas at much lower temperatures (preferably above 1000 °), which can be achieved in today's high nuclear reactors.