To date, solar cells have been successfully used in many navigational aids at remote 

 coastal locations, where the periodic recharging or exchange of discharged batteries would 

 present a severe problem in logistics. 



SUBMERGED APPLICATIONS 



Overview of Problem Areas 



Submerged applications for solar cells pose a real technical challenge. Their power 

 output is significantly less than in above-water applications, and the solar cells and associated 

 electrical connectors must remain dry, withstand hydrostatic pressure, and remain free of 

 biofouling so that their marginal power output is not completely eliminated. Because of 

 these problems, solar cells have not been utilized as power supplies for submerged navigational 

 or oceanographic devices, although the above mentioned technical problems can be solved 

 with existing technology. Because of its interest in ocean technology, NOSC sponsored an 

 exploratory study into the area of submerged solar cells. These findings, summarized in this 

 report, should help provide much of the technological basis and encouragement for design 

 of submerged marine electronic devices powered by solar cells. 



Underwater Insolation 



Sunlight striking the water surface is either partially or totally reflected from the 

 surface, depending on the angle that the rays make with a vertical to the water surface. At 

 normal incidence (0-deg angle with vertical) on a calm water surface the reflectance is only 

 2 percent, while at incidence angles in excess of 53 deg (Brewster's angle for seawater) the 

 reflectance rapidly increases until at 90 deg all light is reflected and none transmitted into 

 the water (figure 5). Based on these data, it can be deduced that in the early morning and 

 late afternoon the oblique incidence angle of the sun's rays causes a significant portion of the 

 sunlight to be reflected from a smooth water surface rather than transmitted. The presence 

 of large waves, however, tends to negate this effect to some degree, as the slopes of waves 

 facing the sun decrease the angle of incidence thus decreasing the percentage of light that 

 would otherwise be reflected from a smooth water surface. The reflectance of the smooth 

 water surface in the early morning and late evening hours is also somewhat decreased if the 

 sky is completely overcast. This phenomenon can be explained by the fact that an overcast 

 sky diffuses the light and thus all rays do not strike the surface at an oblique angle. The 

 transmission of sunlight through a calm water surface under overcast skies or through rough 

 water surface under clear skies is approximately 92 percent regardless of the sun's elevation. 



The rays of the sun which are not reflected by the water surface refract towards the 

 vertical, since the seawater's index of refraction ( 1.34) is higher than that of air. The rays 

 which enter the water encounter absorption and backscatter, which adversely affect propa- 

 gation. Since water acts like a selective filter, the spectral composition of refracted sunlight 

 will change with depth and quantity of dissolved organic matter in the water. Measurements 



2. Schenck, H., Jr., and Kendall, H. W., "Underwater Photography," Cornell Maritime Press, Cambridge, 

 MD 1957. 



