Polyethylene Sheeting as a Water Surface Cover in Sub-zero Temperatures

Abstract

The occurrence of temperatures below -20°C in central Alaska produces a situation conducive to the formation of ice fog. By far the largest source of ice fog in the Fairbanks area is the evaporation of water in the cooling ponds of power plants. In an attempt to find methods to reduce this evaporation and subsequent fogging, a study was conducted during the winter of 1973 in order to examine the feasibility of using po1yethylene sheeting as a water surface cover. An uncovered insulated tank of water was placed on the roof of the Engineering Building of the University of Alaska. The water was circulated to prevent stratification and kept from freezing by a thermostatically controlled heater. From January 23 through February 2, the water surface was 1eft uncovered. Evaporation rates were measured daily by maintaining the water surface at a constant level. During the period of February 2 through 11, the water surface was covered with a sheet of clear polyethylene, thereby eliminating evaporation. Throughout the period of study, daily readings were made of the power consumption of the heater and pump. Temperatures within and above the tank were also frequently measured with copper-constantine thermocouples. From the data co11ected, a daily energy balance for the tank was calculated. Taken into consideration were the net short-wave and long-wave energy exchange, heat loss due to evaporation and sensible heat transfer, heat loss through the sides of the tank, change in stored energy, and energy input from heater and pump. Results indicate that polyethylene is an effective water surface cover that could be used to virtually eliminate evaporation from cooling ponds.