The design of an optimal fog water collector: A theoretical analysis

We investigate the collection efficiency of different fog water catchers assemblies (mainly flat and cylindrical structures equipped with several screens of staggered filaments) by means of parametric equations which take into consideration both impaction and aerodynamic effects. We introduce different models that vary in complexity and range of applicability, and may be used to analyze the effect that geometry, number of screens, spacing and inclination of the filament strands have on the fog water yield of the collector. Increasing the number of impacting screens, n_R, is shown to improve the collection efficiency up to an optimum for n_R = 3-5; beyond n_R > 5 impermeability to the airflow makes the fog catcher less efficient. Geometry of the collector is shown to be relatively important: unless wind direction varies widely, the rectangular flat design is preferred over the cylindrical one, because of its larger drag, i.e. increased aerodynamic efficiency, η_a. In fact η_a is shown to be limiting, such that values over η_a > 50% are difficult to attain. By contrast the impaction efficiency, η_imp, of fog water droplets onto multiple n_R parallel screens of filaments may reach theoretical values of η_imp > 80%. Inclination of the impacting screens over the vertical may slightly reduce η_imp, but this may be compensated by a reduction in flow resistance, i.e. increased aerodynamic efficiency.