Designing a Fog Collector Instead of Copying One: Droplet-Capture Physics, Shade-Coefficient Optimisation and an Honest Seasonal Yield for an Arid Coastal Climate

Fog collectors are usually copied, not designed: mesh from a catalogue, yield from hope. This paper designs a 40 m2 collector from droplet-capture physics. For a stated lognormal fog droplet spectrum over 5-40 um (number geometric mean 9 um, geometric standard deviation 1.65, computed volume median 18.29 um), Stokes numbers and single-fibre impaction efficiencies are computed for fibre diameters 0.3, 0.5 and 1.0 mm at wind speeds 2, 5 and 8 m/s: the volume-weighted impaction efficiency at 5 m/s is 0.8885 for the 0.3 mm fibre against 0.7308 for 1.0 mm. Total collection efficiency, computed as a pressure-drop-based aerodynamic efficiency times a single-layer filter-theory deposition efficiency times a drainage efficiency of 0.90 (class 0.85-0.95), peaks at a shade coefficient of 0.50 for every fibre, reaching 0.3213 for the chosen 0.3 mm fibre (0.3102 at 0.5 mm, 0.2873 at 1.0 mm). The design is a 40 m2 double-layer panel, each layer at shade 0.293 with pitch 1.886 mm. For a stated arid coastal fog climate class (200 fog days per year, 8 fog hours per day), yield is 4.627 L/m2 per fog day at liquid water content 0.1 g/m3, 13.88 L/m2 at 0.3 g/m3 and 23.134 L/m2 at 0.5 g/m3; the panel delivers 555.2 L per fog day and 111,041 L per season in the central class (37,014 L in the low class). A 40 m2 rain roof at the same site (150 mm/year rainfall, runoff coefficient 0.85) delivers 5,100 L per year, so the fog panel's central season is 21.77 times the roof's yield and its low-class season 7.26 times. Structure at a stated 35 m/s design gust: total wind force 30.01 kN on four guyed posts, guy tension 6.37 kN per cable, 389.6 kg of post steel and 2.3 m3 of footing and anchor concrete. All results are simulated, not measured; computations are reproducible from the deposited compute.py and results.json.