Building-integrated fog collectors

Building-integrated fog collectors (BIFCs) are façade, roof or shading elements that harvest atmospheric moisture by intercepting wind-borne fog directly on the surfaces of buildings.^[1][2] By embedding mesh or patterned condenser surfaces into the building envelope, BIFCs combine passive water production with shading and aesthetic functions, offering a compact alternative to ground-mounted fog nets in dense urban areas.^[3]

Concept and terminology

The expression “building-integrated fog collector” (BIFC) was coined by Caldas et al. (2018) when translating rural fog nets into ventilated double-skin façades.^[1] They framed a BIFC as “any cladding element that simultaneously fulfils a building-physics role (e.g. shading, weather protection) and passively intercepts atmospheric droplets.” Later reviews compare BIFCs with BIPV, arguing that water-harvesting façades can “stack” functions—solar control, water supply and architectural articulation—within the same envelope depth.^[3]

Because the collector is integral to the façade, wind-load design, fire safety and maintenance access become part of the BIFC definition. Recent literature therefore classifies BIFCs first by integration zone (façade, roof, sun-breaker) and only second by mesh type, giving rise to textile curtain walls, rotating mesh louvres and roof-top radiative fins.^[4]

Operating principle

Building-integrated fog collectors follow the classic capture → coalescence → collection sequence familiar from free-standing fog nets, but the near-wall airflow modifies each phase. Three-dimensional CFD modelling shows that when a porous mesh is mounted a few centimetres in front of a solid façade the streamlines accelerate to about 1.3 × the free-stream velocity; that boost in turn raises aerodynamic interception efficiency by roughly 15 %.^[5]

After interception, surface chemistry governs how quickly water can drain. Laboratory tests with Janus meshes—fibres that alternate super-hydrophilic and super-hydrophobic stripes—report drainage times 40–60 % shorter than on uniform nets and far less re-entrainment of run-off.^[6] The collected film is then channelled through purpose-built façade gutters; a recent European patent (EP 4170112 B1) hides those gutters behind bristle seals so that condensate is protected from wind and direct sun, limiting secondary evaporation losses.^[7]

Geometry has a comparable influence. CFD parametrics indicate that keeping the mesh 50–150 mm clear of the wall minimises wake recirculation and yields an additional 10–20 % theoretical gain.^[5] Orientation matters too: field monitoring of CloudFisher® panels shows a ≈ 25 % drop in daily output when the collector deviates more than 30° from the prevailing fog direction.^[8] Finally, several groups are experimenting with radiative-cooling back-plates and ETFE cushions that condense dew at night and extend harvesting into calm, humid hours; early prototypes confirm extra yield, but systematic performance data are still being gathered.^[4]

Historical development

Systematic work on building-integrated fog collectors began in the early 2000s, when design studios at the University of California, Berkeley and the Politecnico di Milano stretched Raschel-mesh sun-screens across test façades and discovered that the same textile could intercept wind-borne droplets, effectively coining the BIFC concept.^[1] The idea moved from sketches to instrumented trials between 2011 and 2014: at Berkeley’s coastal field site, 0.3 × 0.5 m resin-framed panels on a timber hut yielded ≈ 0.8 L m⁻² day⁻¹, while 1 m² reference nets collected 2.3–3.9 L m⁻² day⁻¹, establishing the first empirical baseline for façade integration.

Research from 2015 to 2020 focused on aerodynamic optimisation. In a controlled wind-fog tunnel (5–6 m s⁻¹, LWC ≈ 0.30 g m⁻³) Caldas and colleagues tested 1 m² double-skin modules and repeatedly logged 2–4 L m⁻² day⁻¹; three-dimensional CFD by Carvajal et al. reproduced efficiencies in the same range, confirming the design rules derived from the tunnel work.^[4][5] A major leap followed in 2021, when Li et al. unveiled a kirigami-shaped three-dimensional mesh that generated counter-rotating vortices; a 1 m² outdoor cassette harvested ≈ 14 L m⁻² day⁻¹ at only 2.5 m s⁻¹, seven times the yield of a flat panel under identical conditions.^[9] Full-scale validation arrived in 2023, as Politecnico di Milano installed the 3 × 5 m double-layer textile façade “Nieblagua,” which survived 12 m s⁻¹ gusts while maintaining continuous drainage—marking the first engineering proof of a curtain-wall BIFC.^[2]

Typologies and design strategies

Building-integrated fog collectors are grouped by where the mesh sits on the envelope and how it interacts with airflow. The most common configuration is a fixed mesh-screen façade: porous Raschel or monofilament textiles are tensioned across wind-ward elevations, providing both shading and water capture; trials on Peruvian school buildings delivered 2–3 L m⁻² day⁻¹ during the winter *Garúa* while adding under 12 kg m⁻² structural load.^[1][5] When façade maintenance and airtightness are critical, designers convert the mesh into the outer leaf of a ventilated double-skin. Politecnico di Milano’s 3 × 5 m “Nieblagua” mock-up hangs a hydrophilic basalt textile 120 mm in front of the waterproofed wall; six-month monitoring confirmed stable drainage and 2.8 L m⁻² day⁻¹ average yield without staining the cladding.^[2] The most dynamic option is the adaptive modular panel: a kirigami-shaped, 1 m² louvre mounted on a servo arm captured ≈ 14 L m⁻² day⁻¹ at only 2.5 m s⁻¹, nearly seven times the yield of a fixed mesh under identical conditions.^[9]

Together these fixed screens, ventilated cavities, radiative roof fins and kinetic panels illustrate that BIFCs are not a single product but a flexible family of envelope strategies whose geometry, materials and control logic can be tuned to climate, structural limits and architectural intent.

Key performance factors

Water yield depends first on the macro-scale conditions that feed a façade. Field measurements on the “Nieblagua” mock-up show a near-linear relationship between liquid-water content, on-coming wind speed and output, but also reveal that building wakes can sap 30–40 % of the incoming flux when the collector sits too close to a recirculation zone.^[2] Orientation adds another geometric penalty: computational-fluid-dynamics studies presented at FOGDEW 2023 predict a 15–20 % drop in daily yield once the panel normal departs more than 30° from the dominant fog direction, whereas rotating or multi-aspect modules recover most of that loss.^[10]

Micro-scale surface engineering can offset some of these aerodynamic constraints. Laboratory trials with Janus meshes—alternating super-hydrophilic grooves and slippery, fluorinated stripes—accelerate droplet coalescence and cut drainage times, raising net water collection by roughly 40 % compared with uniform meshes under identical fog-wind conditions.^[4]

Applications

Across climates, building-integrated fog collectors have moved from prototype to service water infrastructure. Pilot façades on elementary schools and municipal offices along Peru’s northern coast, for example, contribute 3–8 % of the buildings’ annual non-potable demand—mainly for toilets, cleaning and landscaping—and yield about 2.5 L m⁻² each winter day during the coastal *Garúa* season.^[1] Beyond water provision, BIFCs can deliver thermal and emergency benefits. A double-skin façade test cell in Milan circulated its harvested water over interior aluminium fins, dropping inner-surface temperatures by 3–5 °C and trimming peak HVAC energy by about 12 % while still collecting 2 L m⁻² day⁻¹.^[2]

Advantages

Building-integrated fog collectors bring several envelope functions onto the same surface: a porous mesh both shades glazing and harvests wind-borne droplets, so no extra plot area is required.^[1] Hydraulic integration can also support cooling: a Milan double-skin prototype routed condensate over interior aluminium fins, lowering the inner-surface temperature by 3–5 °C and trimming peak HVAC demand by about 12 % while still yielding 2 L m⁻² day⁻¹.^[2] Because Raschel HDPE mesh and tension-cable framing add less than 12 kg m⁻², most existing curtain walls can accept a retrofit without structural reinforcement.^[5]

Challenges

Field data underscore that performance is climate-limited: long-term monitoring on Peru’s north coast records seasonal swings of ± 60 % in daily yield, so retrofit programmes must pair BIFCs with auxiliary supplies during dry, fog-free months.^[1] Hardware durability imposes parallel limits: salt-spray ageing tests simulate five years of coastal exposure and show up to a 30 % loss in HDPE tensile strength, urging careful resin choice and UV stabilisation, while dust-fog cycling cuts drainage efficiency by more than 40 % on uncoated meshes; Janus-patterned surfaces retain roughly 60 % higher throughput under the same fouling load.^[5][6]

See also

• Fog collection
• Atmospheric water generator
• Building-integrated photovoltaics
• Green wall