Magnetohydrodynamic power generator
Magnetohydrodynamic power generator
A system for the generation of electrical power through the interaction of a flowing, electrically conducting fluid with a magnetic field. As in a conventional electrical generator, the Faraday principle of motional induction is employed, but solid conductors are replaced by an electrically conducting fluid. The interactions between this conducting fluid and the electromagnetic field system through which power is delivered to a circuit are determined by the magnetohydrodynamic (MHD) equations, while the properties of electrically conducting gases or plasmas are established from the appropriate relationships of plasma physics. Major emphasis has been placed on MHD systems utilizing an ionized gas, but an electrically conducting liquid or a two-phase flow can also be employed. See Electromagnetic induction, Generator
Electrical conductivity in an MHD generator can be achieved in a number of ways. At the heat-source operating temperatures of MHD systems (1300–5000°F or 1000–3000 K), the working fluids usually considered are gases derived from combustion, noble gases, and alkali metal vapors. In the case of combustion gases, a seed material such as potassium carbonate is added in small amounts, typically about 1% of the total mass flow. The seed material is thermally ionized and yields the electron number density required for adequate electrical conductivity above about 4000°F (2500 K). With monatomic gases, operation at temperatures down to about 2200°F (1500 K) is possible through the use of cesium as a seed material. In plasmas of this type, the electron temperature can be elevated above that of the gas (nonequilibrium ionization) to provide adequate electrical conductivity at lower temperatures than with thermal ionization. In so-called liquid metal, MHD electrical conductivity is obtained by injecting a liquid metal into a vapor or gas stream to obtain a continuous liquid phase.
The conversion process in the MHD generator itself occurs in a channel or duct in which a plasma flows usually above the speed of sound through a magnetic field. High power densities are one of the attractive features of MHD power generators.
Under the magnetic field strengths required for MHD generators, the plasma displays a pronounced Hall effect. To permit the basic Faraday motional induction interaction and simultaneously support the resulting Hall potential in the flow direction, a linear channel requires segmented walls comprising alternately electrodes (anode or cathode) and insulators. From an electrical machine viewpoint, both individual cells and the complete generator may be regarded as a gyrator. The optimum loading of the MHD channel is achieved by extracting power from both the Faraday and Hall terminals, and this is most readily accomplished through consolidation of the dc outputs of individual electrode pairs using power electronics.
Improvement of the overall thermal efficiency of central station power plants has been the continuing objective of power engineers. Conventional plants based on steam turbine technology are limited to about 40% efficiency, imposed by a combination of working-fluid properties and limits on the operating temperatures of materials. When combined with a steam turbine system to serve as the high-temperature or topping stage of a binary cycle, an MHD generator has the potential for increasing the overall plant thermal efficiency to around 50%, and values higher than 60% have been predicted for advanced systems. See Electric power generation, Steam turbine
MHD power generation also has important potential environmental advantages. These are of special significance when coal is the primary fuel, for it appears that MHD systems can utilize coal directly without the cost and loss of efficiency resulting from the processing of coal into a clean fuel required by competing systems.