Figures 23 and 24 display the results of varying the air flow In 
1-inch-ID piping. Although low efficiencies were the result of using 1-inch- 
ID piping, the small diameter must be used because larger pipes are subject 
to crushing at low pressures. The problem of crushing arises because air has 
a much lower density than the seawater and thus develops only a slight static 
head. To prevent collapse, the system air pressure would have to be raised. 
This would result in (1) lower efficiencies if this excess pressure was throttled 
to the atmosphere or (2) a more complicated system if the circuit was com- 
pletely closed and thus maintaining an initial pressure head. Also, large pipes 
have relatively low bursting strength, and thus thick-walled pipe would be 
required near the surface, where there is very little external pressure on the 
piping. 
The closed-circuit pneumatic power transmission system was found 
to be more efficient than the open circuit, since the energy lost by line flow 
was less than the energy lost be exhausting into a pressure of 9,000 psi. How- 
ever, both systems would have to be operated with rigid pipes because of the 
high pressure and both systems require large input horsepowers because of 
their low efficiencies. The reason for the low efficiencies can be seen in a 
pressure volume diagram (Figure 25). When the system has a high exhaust 
pressure, very little of the work expended in compressing the air is used to 
drive the pneumatic motor. However, with a low exhaust pressure as in shal- 
low depths, most of the energy input for compression is used to drive the 
pneumatic motor. 
Conclusions 
Of the four basic power transmission systems investigated, only one, 
the pneumatic system, does not warrant further study. It would require 
excessive line pressure and rigid pipes and would have low efficiency. 
The open-circuit seawater hydraulic system and the three-phase AC 
system were comparable in efficiency. While both schemes have high effi- 
ciencies, they require high potentials near the surface, 4,330 volts AC for 
the electrical system and 6,160 psi for the hydraulic system. Operating at 
57 gpm through a 2-inch-I|D hose, the open-circuit seawater system has an 
efficiency of 85%. The available 2-inch-|D hose has a 4.75-inch outside 
diameter and a weight of 17.5 pounds per foot. This weight would have 
to be supported by a tension cable or a series of buoys. The three-phase 
electrical system with three number 6 wires in a 1.97-inch diameter cable 
has an efficiency of 95%. This electrical system is not as bulky and heavy 
as the hydraulic systems and thus is more applicable to greater depths. 
LD) 
