Discussion of Test Data. The decrease of power output from solar cells subjected to 

 splash is negligible, but with depth the power output decreases to the point that it becomes 

 practically useless. Insufficient data have been generated to formulate an empirical relation- 

 ship between the optical properties of water and the power output of horizontally oriented, 

 upward-facing photocells submerged to different depths. 



It appears, however, that there is a discernible pattern in the experimental data which 

 allows formulation of an approximate empirical rule for estimating the power output of a 

 horizontally positioned, upward-facing, solar cell submerged to the visual contrast limit depth. 

 The power output of such cells at that depth amounts to roughly 5 percent of the power out- 

 put which the same cells would provide on the water's surface. With this empirical guide it is 

 feasible to estimate the power output of solar cells in waters with excellent or poor visibility, 

 provided the visual contrast limit depth (underwater visibility) has been experimentally estab- 

 lished with a Secchi disc for the body of water in that particular location. This, of course, can 

 be easily accomplished by lowering a 1 2-in Secchi disc from any vessel and noting the length 

 of line payed-out at the moment that the white Secchi disc becomes indistinguishable from the 

 background. 



This empirical rule does not at first appear to be of great help to the designer, until it 

 is realized that this is the maximum desirable depth for submergence of a device to insure 

 escape from visual detection and thus vandalism from surface vessels. The designer may, of 

 course, want to place the buoy at some depth which is below or above the visual contrast limit 

 depth; however, the rule will still state whether the power output will be less or more than 5 

 percent of the rated potential power output at the water surface. 



UTILIZATION OF SUBMERGED SOLAR CELLS 



POWER OUTPUT 



As field tests have shown, immersion of solar cells in water decreases their power out- 

 put significantly; at depths equal to the visual contrast limit, the available power output is 

 conservatively estimated to be only approximately 5 percent of the potential power output 

 at the surface. If a high-density horizontal solar array is used,* the 5-percent power output 

 translates to approximately 0.6-W/ft" (6.45 W/m 2 ) power density at the visual contrast limit 

 depth when the ocean surface is under normal 100-mW/cm 2 insolation. Although this is a 

 very low power density, it is still high enough to merit interest from the designer of submerged 

 marine electronic devices. 



However, before this power density value can be used for sizing underwater solar cell 

 arrays, the fact that the horizontal ocean surface receives a normal 100-mW/cm" insolation 

 (standard 1.0 sun radiation) only around noon in the equatorial zone from an overhead sun 

 in a cloudless sky must be considered. At higher latitudes during cloudy weather at late and 

 early hours of the day, the intensity, as well as the inclination of solar insolation striking 

 the ocean surface, will be significantly less than that under standard conditions. To compen- 

 sate for the variables of insolation intensity and inclination in the calculation of total energy 



* Similar to the ones utilized in this study, i.e., Solarex arrays 1017 and 1018 rated at 12 W/ft 2 under normal 

 100-mW/cm 2 insolation. 



34 



