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_.
[Illustration: 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
9b. 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.