Chapter 25. -NEW DEVELOPMENTS IN NAVAL ENGINEERING 



147.162 

 Figure 25-5.— Small craft with waterjet 

 propulsion. 



direct energy conversions that may ultimately 

 have application to the production of power for 

 ship propulsion. This interest arises from two 

 major considerations. First, the Carnot cycle^ 

 which is the thermodynamic basis for our heat 

 engines is inherently inefficient, with the the- 

 oretical maximum efficiency of the cycle being 



Ti - T2 



Tl 



limited to where Ti is the absolute temperature 

 at which heat flows from the source to the 

 working fluid and T2 is the absolute temperature 

 at which heat is rejected to the receiver. Since 

 the temperature of the heat receiver (the ocean) 

 cannot be lowered, the only way to improve the 

 efficiency of an actual cycle based on the Carnot 

 cycle is to increase the temperature of T^. The 

 past few years have seen great advances in the 

 use of higher Tj temperatures (e.g., boilers 

 operating at higher pressures in order to in- 

 crease the difference between Tj and T2), but 

 materials limitations eventually impose a bar- 

 rier to progress in this direction. 



The second reason for current interest in 

 novel energy conversions is that the actual ship- 

 board cycles in which stored energy is converted 

 to thermal energy which is then converted to work 



°The Carnot cycle is discussed in chapter 8 of this 

 text. 



require a great deal of equipment to perform 

 these various energy conversions. Beginning 

 with an inherently inefficient cycle which cannot 

 operate unless a great deal of heat is "wasted" 

 because it must be rejected to a heat receiver, 

 we must accept even greater inefficiency because 

 of the mechanical losses and miscellaneous heat 

 losses that inevitably occur throughout the plant. 

 There are two major approaches to the prob- 

 lem of direct energy conversion. One approach 

 is to find an energy conversion which is not based 

 on the Carnot cycle and is therefore not limited 

 by the requirement that some heat be rejected 

 to a low temperature heat receiver. The other 

 approach is to utilize a "static" heat engine 

 which is based on the Carnot cycle, and therefore 

 subject to its limitations, but which has no mov- 

 ing parts and therefore no mechanical losses. 

 The fuel cell is an example of a device that by- 

 passes the Carnot cycle to make a direct energy 

 conversion. Thermoelectric converters, ther- 

 mionic converters, and magnetohydrodynamic 

 generators are examples of static heat engines 

 which, although operating on the Carnot cycle, 

 come very much closer to the maximum theore- 

 tical efficiency of the cycle by reducing or elimi- 

 nating mechanical losses. 



Fuel Cells 



A fuel cell is a battery-type device in which 

 chemical energy is converted directly into elec- 

 trical energy. The reaction involves a free 

 energy release, without the rejection of heat to a 

 heat sink; hence the process is independent of the 

 Carnot cycle and free of Carnot cycle limitations. 



The major parts of a fuel cell (fig. 25-8) are 

 an anode, a cathode, and an electrolyte. The fuel 

 is fed continuously to the anode, while the oxidant 

 is fed continuously to the cathode. The conversion 

 from chemical energy to electrical energy oc- 

 curs as electrons, released at the anode, flowto 

 the cathode. 



Several types of fuel cells are under investi- 

 gation and development. Some operate at rela- 

 tively low pressures and temperatures, others at 

 high pressures andtemperatures. A wide variety 

 of fuels have been considered for fuel cells; hy- 

 drogen, various hydrocarbons, and methanol ap- 

 pear to offer particular promise for many appli- 

 cations, while a sodium amalgam is being 

 considered for use in certain small fuel cells. 

 The oxidants most commonly used are air and 

 oxygen; however, peroxides, chlorine, and other 

 substances have also been tried. Electrolytes 



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