Atmospheric water, relative humidity, drop-wise condensation, dew point, water harvesting
Many atmospheric water harvesting technologies have been produced to mitigate water scarcity as an auxiliary source of water. These technologies may be described as direct and indirect water collection techniques. The direct method is to induce a phase-change turning from vapor to liquid on a cooling surface and generate condensate without interim processes. On the other hand, the indirect method has absorption or adsorption processes to take water from humid air before producing water. This paper focuses on the direct methods and discusses the effects of humidity and surface temperature on water generation rates and condensate droplet formation patterns in a macro-and micro-view with previous experimental data. In the view of water harvesting, the generation rate of condensate showed a dependency on the temperature difference between a dew point and a surface temperature. As a result of analyzing droplet formation behaviors considering the importance of the subcooling effect in the macro-view, it was investigated that droplet formation rates and the growth regimes of the condensate also had strong relationships with the humidity of air and the surface temperature. This review would be useful for further research on the modeling of condensate droplet formation and condensation enhancement for thermally driven water generation systems.