Energy balance considerations in the design of floating covers for evaporation suppression.

Abstract

This study consists of a theoretical analysis of the energy balance equation for a partially covered body of water, and experimental analyses of the energy balances of partially covered insulated evaporation tanks. The theoretical analysis indicates that surface reflectance for solar radiation and infrared emittance are the most important cover properties. White colored materials were found to satisfy the requirement that both these parameters be as large as possible. Experiments were conducted using covers of foamed wax, lightweight concrete, white butyl rubber, and styrofoam. A variety of shapes and sizes were tested. Cover radiative properties were again noted to be most important, and thin covers proved to be slightly more efficient than thick insulated covers of the same size. Evaporation reduction was found to be proportional to the percent of surface area covered, the constant of proportionality depending upon the color and type of material used. For the white, impermeable materials tested, the constant of proportionality was near unity. It was also noted that reduction in evaporation and reduction in net radiation, as compared to an open tank, were highly correlated. Evaluation of two Dalton-type expressions, the Bowen ratio method and the combination method, for predicting evaporation from an open water surface, showed the combination method to be better under conditions of this experiment. Based on this finding, a modified combination method was derived. This modified equation proved valid for predicting evaporation from a partially covered body of water. The use of insulated evaporation tanks also provided an easy and accurate method of predicting net radiation over other surfaces, and long-wave atmospheric radiation.