Effect of Wind Flow and Solar Radiation on Functionality of Water Evaporation Suppression Monolayers

Abstract

The average evaporation in Iran is 3 times higher than the world average. Applying chemical monolayers on water surfaces is one of the promising methods for suppressing the evaporation. Literature studies have shown that the mixture of cetyl and stearyl (ratio of 1 to 9) is the state-of-the-art monolayer to minimize the evaporation. Adding calcium hydroxide increases the spreading rate and self-healing of the monolayer. Despite long study and investigation on monolayers, there are inconsistencies in explaining the mechanism by which monolayers decrease the evaporation. The mechanisms used to explain the evaporation reduction are: (i) increasing the reflected solar radiation, (ii) dampening the waves formed by winds and decreasing the water surface area, consequently, and (iii) limiting the escape of water molecules. In this paper, by design of experiments (DOE), we try to answer the above question. Evaporation rate from a container in absence of wind or low wind (~0.2 m/s) and at moderate temperature and relative humidity (~20 °C and 45%) is ~ 10 mm/day. Utilization of the monolayer can save 41% of the evaporated water. Also, in absence of radiation, a 9 m/s wind caused ~ 15 mm/day evaporation. By increasing the wind speed from 0 to 9 m/s, effectiveness of the monolayer deteriorated from 60 to 13%. Therefore, the main mechanism is neither reflecting the radiation (as in absence of radiation, monolayer was still effective) nor dampening the waves and decreasing the surface area (as in absence of wind, monolayer was effective; also, at higher wind speeds where the surface area increases, monolayer efficiency decreases). Therefore, the main mechanism by which monolayers decrease the evaporation rate is limiting the escape of water molecules. So, monolayers may be effective even during the nights even though radiation is at its lowest.

Appendix

Fig. 6

To show the effect of shrink wrap on absorbing or reflecting the solar radiation, evaporation from two identical containers are compared. Height of the pure water in both containers within 24 h is almost equal which indicates that the applied shrink wrap was transparent to solar radiation

To make sure that the shrink wrap does not affect the solar radiation, two identical containers with pure water are tested. One of the containers was covered by shrink wrap. The level of water in both containers was measured every 4 h and for 24 h. This experiment was repeated 3 times and average results are presented in Fig. 6. It is shown that the solar radiation absorbed or reflected by the shrink wrap is negligible. As it is clear from Fig. 6, by comparing these two containers, the difference between height of water in both containers is 1 mm. This shows us that the shrink-wrap only slightly absorbs the radiation and it can be assumed that the amount of radiation that enters the body of water is the same for both containers.