Influence of solar radiation reflectance on water evaporation

Abstract

The evaporation of a water reservoir may be reduced by increasing the reflectance of solar energy by the water surface. Such a change in reflectance will require a surface modification such as the application of a special surface film or monolayer with good reflecting properties. Since colored materials reflect at wavelengths at which the solar energy has its peak intensity, colored monolayers and artificially dyed monolayers were studied experimentally. A large number of dyes were investigated, but in no instance did the dyes color the monolayer or form a monolayer themselves. A yellow silicone oil film on a water surface was found experimentally to reflect solar energy about 1.7 times better than a plain water surface and also to reduce the evaporation rate. In addition, the film was extremely difficult to remove from a water surface. Its durability is a real advantage where there is the need to constantly resupply monolayers as in present evaporation suppression techniques. Analytical studies were conducted to determine the evaporation reduction resulting from increased solar reflectance. The transient energy equation for an element of a water surface was solved by numerical methods. The analytical model gave an average evaporation suppression by cetyl alcohol of 22.6%, which is consistent with previous experimental work. Results for monolayers with hypothetical reflectance properties and with the diffusion properties of a cetyl alcohol monolayer showed average evaporation reductions of 25 to 45%, depending on the reflection properties.