Stirling cycle engines are currently being developed and are 

 theoretically capable of very high conversion efficiencies. Actual engine 

 efficiencies in earlier engine tests were about 80% of diesel engines up to 

 several hundred horsepower. However, improved performance is considered 

 feasible. An advantage of the Stirling cycle is its inherent low noise level. In 

 size and weight, it compares with the diesel engine. The use of the Stirling 

 cycle as a power source for deep ocean applications was not considered 

 advantageous because of its developmental status and its relatively high cost. 



The Rankine, or vapor-turbine, cycle is most commonly used with 

 steam as the working fluid. This cycle has moderate efficiencies over the 

 power range of interest and is adaptable to a wide range of thermal energy 

 input temperatures. For temperatures below 427°C, the Rankine cycle is 

 more efficient than the Stirling cycle, and an increasing power level favors 

 the Rankine cycle. Rankine cycle equipment is generally available, although 

 considerable engineering and design work is required for maximum economy 

 of weight and space as well as improved efficiencies at low power levels. 



Stored Thermal Energy. Stored thermal energy may be used as an 

 energy source for an underwater power source. The storage material is 

 charged with thermal energy at the surface and must be periodically recharged. 

 The weight of energy storage materials is relatively high, the lightest being 

 lithium hydride. An estimate of the weight for a lithium hydride storage 

 system, including hardware, is 13 Ib/kw-hr. The overall plant efficiency 

 would be about 27%. Since lithium hydride has a low density, this storage 

 material requires a large volume. The material burns and may react violently 

 with moisture in the presence of air. Hydrogen contamination at high 

 temperature is also a problem. The developmental status, the volume and 

 weight units, and the requirement to frequently replenish the thermal energy 

 at the surface do not make the thermal energy storage system attractive for 

 underwater use. 



Chemical Energy. Electrical power may be produced by utilizing 

 chemical energy. The two most predominant methods of converting chemical 

 to electrical energy are by direct conversion and by the generation of thermal 

 energy, which is then converted to electrical energy. Methods currently 

 available for direct conversion employ storage batteries and fuel cells. There 

 are a large number of battery and fuel cell systems in existence; however, only 

 a few are practical at this time. Virtually all chemical systems, including 

 batteries, either have oxygen as an initial component or derive oxygen from 

 chemical compounds. 



A considerable number of chemical systems using fuel oil and oxygen 

 have been proposed and developed. These include closed-cycle diesel, gas 

 turbine, and steam-gas turbine, known as the Walter cycle. These cycles have 



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