Energy Relationships in the Design of Floating Covers for Evaporation Reduction

A theoretical analysis of the energy balance equation for a partially covered body of water indicates that surface reflectance for solar radiation and infrared emittance are important properties to consider in designing covers for maximum efficiency in reducing evaporation. White materials satisfy the requirement that both these parameters be as large as possible. Experiments were conducted on insulated evaporation tanks partially covered with foamed wax, lightweight concrete, white butyl rubber, and styrofoam. Several shapes and sizes were tested. The radiative properties of the cover were again noted to be important, and thin covers proved to be slightly more efficient than thick insulated covers. Evaporation reduction was proportional to the percent of surface area covered, the constant of proportionality depending upon the color and type of materials used. For the white, impermeable materials tested, the constant of proportionality was near unity. The reduction in evaporation was also highly correlated with the reduction in net radiation.