132 



5.6 POWER 



Virtually all electrical power for tethered ROVs is supplied from a surface 

 generator through an umbilical. Untethered vehicles presently rely upon lead 

 acid or silver zinc batteries for power, and have an operating duration of 

 4 to 5 hours, the primary user of this power is the propulsion system. In 

 order to deploy an untethered vehicle which will provide capabilities comparable 

 with tethered vehicles, a higher energy density power source must be made 

 available. To this end the Continental Group Inc. of New York has developed 

 a lithium battery or power cell which can be packaged for ROV application M 

 or from a manned submersible. The first application will be in a manned vehicle, ■ 

 the AUGUSTS PICCARD, where the power cell will be installed in late 1979 and 

 will provide 1.2 mWH of electrical ei^ergy. A subsequent phase will see instal- 

 lation of a lithium power cell supplying 36 mWH. 



The lithium power cell is an electrochemical system which combines high energy 

 lithium anodes with simple iron cathodes in a highly ionized aqueous electrolyte. 

 The anodes are solid lithium, rolled, cast or extruded from basic ingots. The 

 cathodes are commercial iron or steel screening. Adding aqueous electrolyte 

 to this lithium-iron combination makes a highly efficient cell. Lithium ions 

 go into solution, releasing electrons which travel through the external circuit 

 to release hydroxyl ions and hydrogen at the cathode. The lithium hydroxide 

 solution which results from this activity forms the aqueous electrolyte. The 

 electrolyte, produced by normal cell activity, is circuited through the cells 

 to remove heat and the polarizing products of the electrochemical reaction, and 

 to bring water into the reaction zone to maintain and control cell output. 

 It can achieve high energy and power densities over a wide range of operating 

 and environmental conditions. Lithium and water are consumed at room temperature 

 in a cell in which voltage and power output can be controlled independent of 

 the electrical load, within the limits of cell capacity. The power and voltage 

 capability does not degrade during any given discharge cycle, or from cycle 

 to cycle throughout the life of the system. Replacement of the lithium anodes 

 readies the battery for reuse, which is limited only by the life of supporting 

 equipment such as pumps, motors, and valves. The system also consumes substantial 

 quantities of water, but in a marine system this is neither a cost nor logistic 

 penalty because the water is drawn from the ambient environment. -■ 



Of the 3820 Whr/lb of lithium released (theoretical energy density of the basic 

 lithium-water couple) during the electrochemical reaction up to 2500 Whr/lb 

 of lithium can be delivered from the cell. (The remainder appears as heat in 

 the circulating electrolyte from which it is subsequently removed. ) High values 

 of specific energy are achieved by operating at high cell voltage (and electrolyte 

 molarity) with a corresponding low power density. As with most energy/power 

 systems, there is a tradeoff between maximum energy and maximum power density, 

 and the two characteristics achieve maximum values at different operating points. J 

 The lithium-water cell is typically operated at a cell voltage of 1.25 V which 

 yields moderate values of both specific energy and power. However, cell operating 

 characteristics can be tailored to fit specific requirements. Total system 

 energy and power densities depend greatly upon the specific application. 



