Fig.5. (a) Operating principle of a thermionic generator. (b) Potential energy diagram of electrons of the thermionic system
Date: 2015-10-07; view: 402.
Efficiency is not the only parameter of a power converter, but is certainly among the most important, for it establishes the areas of application. To be of much practical interest, the efficiency of a power converter must be at least 10 percent. To determine the efficiency of a thermionic converter, the calculated electric power output that can be delivered to a load can be compared with the total heat input. Some of this heat goes into the useful work; some is transferred to the anode by electron motion; some leaks back through the electrical connection; and most important of all, some is transferred directly to the cold end of the machine by radiation.
The efficiency of conversion depends then on such material properties as the work function, electron emission constants and radiant emissivity, and the operating temperature. The operating temperature is, in turn, limited by the melting point or evaporation rate of the cathode. Thus material properties of the anode and cathode are important in deciding whether an efficient arrangement is practical.
The available combinations of material properties that will result in the optimum device cannot be described in a simple manner. However, Fig.6 shows some calculated efficiencies for a variety of possible cathode materials as a function of cathode temperature. These calculations, meant to be illustrative only, assume an anode reflection that gives an effective emissivity of 0.5, and an anode work function of 1.8 volts. Each curve terminates at a point where cathode evaporation becomes high enough to evaporate a millimeter of material from the cathode in 1,000 hours, a condition assumed to represent end of life.
