Chapter 25. -NEW DEVELOPMENTS IN NAVAL ENGINEERING 



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147.165 



Figure 25-8.— Fuel cell. 



the two electrodes to a load establishes a circuit 

 and allows the flow of electric current. 



The thermionic converter, like the ther- 

 moelectric converter, is a static heat engine and 

 is limited by Carnot cycle considerations. It is 

 believed, however, that higher efficiencies can 

 be obtained with the thermionic converter than 

 with the thermoelectric converter. One reason 

 for the higher efficiencies is that the thermionic 

 converter typically operates at substantially 

 higher temperatures (and with substantially 

 greater temperature differentials) than the ther- 

 moelectric converter. Another reason for the 

 higher efficiencies is that very little thermal 

 energy is lost between the hot and cold terminals 

 because of the vacuum that is maintained between 

 the cathode and the anode. 



In itself, however, the use of a vacuum creates 

 some significant problems in connection with the 

 thermionic converter. As electrons pass through 

 a space under vacuum, there is a tendency for a 

 negative "space charge" to be built up. This 

 space charge is merely a cloud of electrons 

 which, after escaping the cathode, have insuffi- 

 cient energy to reach the anode. As the negative 

 space charge builds up, electron flow is greatly 

 diminished. The space charge effect can be re- 

 duced by putting the electrodes extremely close 

 together, but this is difficult to do except in very 

 small converters. One recent approach to this 

 problem is to fill the space with positive ions. 



thus neutralizing the space charge effect and al- 

 lowing the unimpeded flow of electrons. Ther- 

 mionic converters in which the space between 

 electrodes is filled with cesium vapor appear to 

 be promising. 



Because a thermionic converter must operate 

 at an extremely high temperature in order to de- 

 velop any great efficiency, some thought has been 

 given to utilizing the decay heat of radioactive 

 isotopes as the heat source. One device that has 

 been successful in laboratory tests combines 

 reactor fuel and the thermionic converter in one 

 unit, thus in effect producing a "thermionic fuel 

 cell." It is also believed that thermionic con- 

 version may be suitable for solar energy con- 

 version for use in space. 



Magnetohydrodynamic Generators 



The magnetohydrodynamic (MHD) generator 

 is still another device for converting heat to di- 

 rect- current electricity. In the MHD generator, 

 the conversion is accomplished by passing a high 

 temperature gas through a magnetic field. Heat- 

 ing the gas to a very high temperature ionizes the 

 gas and makes it electrically conductive. Passing 

 the electrically conductive gas through a fixed 

 magnetic field induces an electrical voltage in 

 accordance with Faraday's law. 



LOAD 



HEAT IN (T 



147.166 

 Figure 25-9.— Thermoelectric converter. 



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