PRINCIPLES OF NAVAL ENGINEERING 



147.163 



Figure 25-6.— Principle of waterjet propulsion. 



COMBUSTION 

 CHAMBER 



DISCHARGE ^ 

 NOZZLE 



FORWARD MOTION OF VESSEL 



147.164 



Figure 25-7. — Underwater ramjet propulsion system. 



that have been used in fuel cells include potas- 

 sium hydroxide, fused salts, and liquid salts. 

 As may be inferred, the fuel cell is simpli- 

 city itself in basic concept, and it offers the 

 promise of enormously greater efficiencies than 

 can ever be obtained through any energy conver- 

 sion that is based on the Carnot cycle. However, 

 there are many problems to be overcome before 

 the fuel cell can be regarded as a major source 

 of power for applications requiring large power 

 outputs. 



Thermoelectric Converters 



A thermoelectric converter is a device for 

 converting heat to direct- current electricity. 

 The general arrangement of a thermoelectric 

 converter is illustrated in figure 25-9. As may 

 be seen, the converter is basically a thermo- 

 couple device in which an emf is developed 

 through the application of heat to dissimilar ma- 

 terials. 



In the thermoelectric converter, one of the 

 dissimilar materials is of the type known as an 

 N material and the other is of the type known as 

 a P material. Asfar as electron behavior is con- 

 cerned, N and P materials react differently when 

 heat is applied to one end. In N materials, the 

 application of heat tends to make negatively 

 charged free electrons move toward the cold end 



of the piece. In P materials, the application of 

 heat tends to produce positive charges toward the 

 cold end of the piece. When heat is applied to the 

 hot junction of a thermoelectric converter (as 

 shown in fig. 25-9) thecoldendof the Immaterial 

 is negative and the cold end of the P^ material is 

 positive. When the cold ends are connected 

 through a load, the electric circuit is complete. 

 The efficiency of the thermoelectric con- 

 verter is limited by Carnot cycle limitations. 

 As with all Carnot cycleenergy conversions, the 

 efficiency of the thermoelectric converter in- 

 creases as the temperature differential in- 

 creases. It is believed that the maximum attain- 

 able efficiency of the thermoelectric converter 

 may be on the order of 25 to 30 percent, although 

 efficiencies thus far achieved are not nearly so 

 high. 



Thermionic Converters 



Another device for converting heat to direct- 

 current electricity is the thermionic converter. 

 In its simplest form, the thermionic converter is 

 similar to a vacuum tube, consisting of two metal 

 electrodes separated by a space under vacuum. 

 The cathode is heated, and the anode is main- 

 tained at a lower temperature. When the cathode 

 is heated, electrons are thermally agitated and 

 driven from the cathode to the anode. Connecting 



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