University of Alaska, Institute of Water Resources
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.The authors wish to acknowledge the assistance of the Institute of
Water Resources, University of Alaska, who provided the funds for the
construction of the tank and the instrumentation used in the study, and
M. Gavin and B. Strickler, who helped in its construction