Figure 8 THERMIONIC CONVERSION
Thermionic converters may be flat-plate types or cylindrical types. The cylindrical converter (a) is an experimental type for ultimate use in nuclear reactors. Courtesy Los Alamos Scientific Laboratory.
a INSULATOR COOLED COLLECTOR INCANDESCENT URANIUM FUEL ELEMENT CESIUM PLASMA CIRCULATING COOLANT VACUUM INSULATOR CESIUM POOL b WASTE HEAT OUT LOAD ELECTRONS LOW WORK FUNCTION COLLECTOR Tc CESIUM ION PLASMA FILLED GAP BOILED OFF ELECTRONS HIGH WORK FUNCTION EMITTER Tc HEAT IN
Result: A Plasma Thermocouple
Unless a voltage difference exists across the plates, no external work can be done. In the thermocouple the voltage difference was caused by the different electrical properties of the p and n semiconductors. Both the emitter and collector in the thermionic converter are good metallic conductors rather than semiconductors, so a different tack must be taken.
The key is the use of an emitter and a collector with different work functions. If it takes 4.5 electron volts to force an electron from a tungsten surface and if it regains only 3.5 electron volts when it condenses on a collector with a lower work function, then a voltage drop of 1 volt exists between the emitter and collector.
To summarize, then, the thermionic emission of electrons creates the potentiality of current flow. The difference in work functions makes the thermionic converter a power producer.
There is an interesting comparison that helps describe this phenomenon. Consider the emitter to be the ocean surface and the collector a mountain lake. The atmospheric heat engine vaporizes ocean water and carries it to the cooler mountain elevations, where it condenses as rain which collects in lakes. The lake water as it runs back toward sea level then can be made to drive a hydroelectric plant with the gravitational energy it has gained in the transit. The thermionic converter is similar in behavior: hot emitter (corresponding to the sun-heated ocean); cooler collector (lake); electron gas (water); different electrical voltages (gravity). Without gravity the river would not flow, and the production of electricity would be impossible.
Thermionic Power in Outer Space
Thermionic converters for use in outer space may be heated by the sun, by decaying radioisotopes, or by a fission reactor. Thermionic converters can also be made into concentric cylindrical shells ([Figure 8]a) and wrapped around the uranium fuel elements in nuclear reactors. The waste heat in this case would be carried out of the reactor to a separate radiator[9] by a stream of liquid metal. Since thermionic converters can operate at much higher temperatures than thermoelectric couples or dynamic power plants, the radiator temperature, Tc, will also be higher. Consequently, space power plants using thermionic converters will have small radiators. Once thermionic converters are developed which have high reliability and long life, they will provide the basis for a new series of lighter, more efficient space power plants.