Environmental Engineering Reference
In-Depth Information
exceeds the velocity of the products of the chemical reaction in power engines by
an order of magnitude, so ion propulsion is effective for cosmic engines, where
the problem of the fuel weight must be overcome. Ion and Hall thrusters in the
cosmos exhibit the properties of a long lifetime and high reliability.
Along with stationary MHD generators, which now do not have a suitable place
in contemporary energetics, there are pulsed MHD generators, which are analo-
gous to guns with a weakly ionized plasma instead of shells. The plasma is formed
as a result of combustion of gunpowder or some equivalent source of propellant
gas. When this plasma passes through a region with an external magnetic field,
electric power is created. To estimate the possibilities of this system, let us perform
a simple calculation. We assume that the plasma velocity equals to the velocity
which a bullet acquires in the gun. We take the efficiency of energy transformation
to be of the order of 50%, and the length of the region with a magnetic field to
be 1m. Then the specific power of this generator is of the order of 10
8
W/g and
the pulse duration is approximately 7
10
4
s. Thus, the power output of a pulsed
MHD generator, for this brief instant, corresponds to the total power output of all
the electric power plants in the world if the mass of the powder used is of the order
of 10 g. The pulsed MHD generator transforms the chemical energy of the powder
into electric energy. Owing to the simplicity and yield parameters of these systems,
they are convenient for special applications as autonomous pulsed sources of elec-
tric energy.
The other system where a plasma is used for generation of electric energy is the
thermoemission converter. It contains two parallel metal plates with different work
functions (the work function is the binding energy of an electron at a surface). One
of these plates is heated and emits electrons, which reach the other plate, so an
electric current is created. Connection of the plates through a load leads to release
of the electric energy in the load. It is evident that a plasma is not the underly-
ing basis for this device. Nevertheless, the use of a plasma in the gap between the
plates makes it possible to overcome an important difficulty attendant on this sys-
tem. If there is no plasma in the gap, electron charge is accumulated in this region
and creates an electric potential between the plates that increases with the elec-
tron number density in the gap, and is opposite in direction to the potential that
initiated the phenomenon. Beyond a certain level, the counterpotential will stop
generation of the electric energy. For typical energy fluxes in these systems (ap-
proximately 1W/cm
2
) the distance between plates must be less than 10
m. It is
difficult to combine this condition with the high temperature of the heated plate
(approximately 2000 K). Introduction of a plasma into the gap between plates pro-
vides a means of overcoming this difficulty. It is interesting that the principles of
operation of MHD generators, thermoemission converters, and plasmotrones were
suggested as early as the end of the nineteenth century. Now, with the advantage of
contemporary materials and technology, these devices have acquired a new life.
In various applications of a plasma with an intense input of energy, the plasma
is generated in a moving medium. The plasma generator, or plasmotrone, is usu-
ally an arc discharge established in a flowing gas or vapor that is an equilibrium
thermal plasma [9]. Such plasma generators produce plasma torches which have
μ
