Principles of naval engineering - Historic Naval Ships Association

Chapter 25. -NEW DEVELOPMENTS IN NAVAL ENGINEERINGA^Arthus neutralizing the space charge effect and allowingthe unimpeded flow of electrons. Thermionicconverters in which the space betweenelectrodes is filled with cesium vapor appear tobe promising.Because a thermionic converter must operateat an extremely high temperature in order to developany great efficiency, some thought has beengiven to utilizing the decay heat of radioactiveisotopes as the heat source. One device that hasbeen successful in laboratory tests combinesreactor fuel and the thermionic converter in oneunit, thus in effect producing a "thermionic fuelcell." It is also believed that thermionic conversionmay be suitable for solar energy conversionfor use in space.Magnetohydrodynamic GeneratorsFigure 25-8.— Fuel cell.147.165the two electrodes to a load establishes a circuitand allows the flow of electric current.The thermionic converter, like the thermoelectricconverter, is a static heat engine andis limited by Carnot cycle considerations. It isbelieved, however, that higher efficiencies canbe obtained with the thermionic converter thanwith the thermoelectric converter. One reasonfor the higher efficiencies is that the thermionicconverter typically operates at substantiallyhigher temperatures (and with substantiallygreater temperature differentials) than the thermoelectricconverter. Another reason for thehigher efficiencies is that very little thermalenergy is lost between the hot and cold terminalsbecause of the vacuum that is maintained betweenthe cathode and the anode.In itself, however, the use of a vacuum createssome significant problems in connection with thethermionic converter. As electrons pass througha space under vacuum, there is a tendency for anegative "space charge" to be built up. Thisspace charge is merely a cloud of electronswhich, after escaping the cathode, have insufficientenergy to reach the anode. As the negativespace charge builds up, electron flow is greatlydiminished. The space charge effect can be reducedby putting the electrodes extremely closetogether, but this is difficult to do except in verysmall converters. One recent approach to thisproblem is to fill the space with positive ions.The magnetohydrodynamic (MHD) generatoris still another device for converting heat to direct-current electricity. In the MHD generator,the conversion is accomplished by passing a hightemperature gas through a magnetic field. Heatingthe gas to a very high temperature ionizes thegas and makes it electrically conductive. Passingthe electrically conductive gas through a fixedmagnetic field induces an electrical voltage inaccordance with Faraday's law.©LOAD