Big Word, Simple Concept
Magnetohydrodynamic (MHD) conversion is very unlike thermoelectric or thermionic conversion. The MHD generators use high-velocity electrically conducting gases to produce power and are generically closer to dynamic conversion concepts. The only concept they carry forward from the preceding conversion ideas is that of the plasma, the electrically conducting gas. Yet they are commonly classified as direct because they replace the rotating turbogenerator of the dynamic systems with a stationary pipe or duct.
Figure 9 In the MHD duct (a), the electrons in the hot plasma move to the right under influence of force F in the magnetic field B. The electrons collected by the right-hand side of the duct are carried to the load. In a wire in the armature of a conventional generator (b) the electrons are forced to the right by the magnetic field.
a MHD Duct HOT PLASMA IN COOL GAS OUT TO RADIATOR Magnetic Field LOAD ELECTRONS b CONVENTIONAL GENERATOR SHAFT LOAD Magnetic Field ARMATURE WIRE ELECTRONS
In the conventional dynamic generator, an electromotive force is created in a wire that cuts through magnetic lines of force, as shown in [Figure 9]b. It may be helpful to visualize the conduction electrons as leaving one end of the wire and moving to the other under the influence of the magnetic field.
The force on the electrons in the wire is given by
F = qvB
where
F = the force (in newtons[10])
q = the charge on the electron (1.6 × 10⁻¹⁹ coulomb)
v = the wire’s velocity (in meters per second)
B = the magnetic field strength (in webers per square meter[10])
The surge of electrons along the length of the wire sets up a voltage difference across the ends of the wire. A generator uses this difference to convert the kinetic energy of the moving wire or armature into electrical energy. The wire is kept spinning by the shaft which is connected to a turbine driven by steam or water.
Let us try to eliminate the moving part, the generator armature. What we need is a moving conductor that has no shaft, no bearings, no wearing parts. The substance that meets these requirements is the plasma. Examine [Figure 9]a. The MHD generator substitutes a moving, conducting gas for the wires. Under the influence of an external magnetic field, the conduction electrons move through the plasma to one side of the duct which carries electrical power away to the load.
The MHD generator gets its energy from an expanding, hot gas; but, unlike the turbogenerator, the heat engine and generator are united in the static duct. The gradual widening of the duct shown in [Figure 9]a reflects the lower pressure, cooler plasma at the duct’s end. Some of the plasma’s thermal energy content has been tapped off by the duct’s electrodes as electrical power.