126 Electrical Power Systems Technology
calculated that several large-diameter windmills, with a fairly constant
wind, could produce many kilowatts of electrical power. They could play
an important role in reducing the use of our natural resources for electrical
power production, particularly on an individual basis.
MAGNETO HYDRODYNAMIC (MHD) SYSTEMS
MHD stands for magnetohydrodynamic, a process of generating elec-
tricity by moving a conductor of small particles suspended in a superheat-
ed gas through a magnetic field. The process is illustrated in Figure 5-5.
The metallic conductors are made of metals such as potassium or cesium,
and can be recovered and used again. The gas is heated to a temperature
much hotter than the temperature to which steam is heated in convention-
al power plants. This superheated gas is in what is called a plasma state at
these high temperatures. This means that the electrons of many of the gas
atoms have been stripped away, thus making the gas a good electrical con-
ductor. The combination of metal and gas is forced through an electrode-
lined channel which is under the influence of a superconducting magnet
that has a tremendous field strength. The magnet must be of the supercon-
ducting type, since a regular electromagnet of that strength would require
too much power. A superconducting magnet, therefore, is one of the key
parts to this type of generation system.
With high operating temperatures and a high-speed gas flow, it be-
comes a difficult task to keep the conductive channel from becoming de-
stroyed. Cooling is very important and is accomplished by circulating a
suitable coolant throughout jackets built into the channel. Also, due to the
high temperatures and the metal particles moving at high speed, erosion
of the channel becomes a critical problem. This problem has been eased by
using a coal slag injected into the hot gas and metal stream. The coal slag
acts to replace the eroded material as it is lost.
Pollution problems are very few. The main one is the high levels of ni-
trogen oxides that are produced as a direct result of the high combustion
temperatures inherent to the system. Sulfur oxides and ash are also a prob-
lem for any plants using coal or oil. An afterburner system has been pro-
posed to eliminate the nitrogen oxides, while the sulfur oxides and the ash
would be collected, separated chemically, and then recycled.
At this time, MHD generators are primarily experimental. There have
been several large units made, but they have not operated for any signifi-