The collection efficiency of a massive fog collector

Abstract

Very large (48 m 2) fog-water collectors are being used on the coastal mountains in northern Chile to generate water. The microphysical characteristics of the high elevation fog (camanchaca) have been examined and the collection efficiency of the collectors measured. The camanchaca exhibits characteristics of clouds, reflecting its source as a marine stratocumulus deck. Droplet mean volume diameters (MVD) in ten cases ranged from 10.8 to 15.3 μm. Droplet concentrations were typically 400 cm −3 with fog liquid water contents ranging from 0.22 to 0.73 g m −3. The large fog-water collectors consist of a double layer of mesh made from a 1-mm wide flat polypropylene ribbon. The theoretical collection efficiencies of a 1-mm wide ribbon, for droplets with the observed MVD, at wind speeds from 2 to 8 m s −1, are 75 to 95%. The field measurements of the collection efficiency of the mesh at the centerline of a large collector gave values of ∼66% (3.5–6.5 m s −1; 11 μm MVD). This is in good agreement with the theoretical value for a single ribbon once the areal coverage of the mesh is taken into account. At lower windspeeds, the measured collection efficiencies dropped to ∼26% (1.9 m s −1; 15 μm MVD). A simple parameterization of the mesh collection efficiency allowed some properties of meshes to be examined, e.g. the mesh shows a marked decrease in droplet collection as the ribbon width increased while maintaining a constant percentage areal coverage. The measured water output from the large collector was 2.9 times lower than predicted using the measured amount of water removed at the centerline and the wind speed 6m upstream. This implies a large-collector efficiency of only ∼20%. This low value may result from a lowering of wind speed as the fog approaches the mesh, a reduced collection efficiency away from the centerline, and water losses in the system.