Harps under Heavy Fog Conditions: Superior to Meshes but Prone to Tangling

Abstract

In arid yet foggy regions, fog harvesting is emerging as a promising approach to combat water scarcity. The mesh netting used by current fog harvesters suffers from inefficient drainage, which severely constrains the water collection efficiency. Recently, it was demonstrated that fog harps can significantly enhance water harvesting as the vertical wire array does not obstruct the drainage pathway. However, fabrication limitations resulted in a very low shade coefficient of 18% for the initial fog harp prototype and the field testing was geographically confined to light fog conditions. Here, we use wire-electrical discharge machining (wire-EDM) to machine ultrafine comb arrays; winding the harp wire along a comb-embedded reinforced frame enabled a shade coefficient of 50%. To field test under heavy fog conditions, we placed the harvesters on a closed-circuit test road and inundated them with fog produced by an array of overlying fog towers. On average, the fog harps collected about three times more water than the mesh netting. During fog harvesting, the harp wires were observed to tangle together due to the surface tension of water. We developed a rational model to predict the extent of the tangling problem for any given fog harp design. By designing next-generation fog harps to be anti-tangling, we expect that even larger performance multipliers will be possible compared to the current mesh harvesters.

Optimizing Fog Harps

“…40,41 This enhancement was attributed to an increased droplet sliding efficiency (i.e., reduced onset time) under light fog conditions 40,42 and an increased aerodynamic efficiency (i.e., anticlogging) under heavy fog. 41 The concept of using an electric field to enhance fog harvesting 43 has also been recently extended to a fog harp. 44 However, previous fog harp studies have been limited in their conclusions, as they only optimized the wire diameter 39 and did not vary the wire material, shade coefficient (SC), or the wire length.…”

“…Physical observations (Movies S5 and S6 in Supporting Information) agreed excellently with the model, with the only exception being the fourwire tangling case, while the model predicted five. While the wire tension was not varied, a previous work validated the model's prediction of a longer tangling length for higher-tension wires 41. The regime map does not consider values below a pitch of P = 0.254 mm, as having a pitch smaller than the wire diameter is not possible.…”

Highly Efficient Multiscale Fog Collector Inspired by Sarracenia Trichome Hierarchical Structure

“…BJMM-V and BJMM-H adopted a widely used fog collection system, which combined BST with JM-fabricated fog harps. [27,28] For this type of fog collection system, the wind quickly flowed through the intervals between neighboring harps, in which the wind only blew through a single BST. For BJMP, the wind was twisted inside the jungle structure of the BST to contact with several BSTs, and the fog harvesting was enormously enhanced.…”

“…[18][19][20][21][22] Conical structures are usually combined with fog harps to construct fog collectors for highly efficient fog harvesting. [23][24][25][26][27][28] However, the velocity of directional water transport on these conical structures remains of ≈0.5 mm s −1 , which limits further enhancement of fog harvesting by fog collectors.Fortunately, a more efficient fog harvesting and transport mode was discovered on Sarracenia trichomes that has a unique hierarchical microchannel structure around the needle-shaped trichomes (Figure 1a). [29] A thin water film is automatically formed on the hierarchical microchannel structure to generate superslippery capillaries, which remarkably enhances the water transport capability and further reinforces the fog harvesting efficiency of trichomes.…”

“…[18][19][20][21][22] Conical structures are usually combined with fog harps to construct fog collectors for highly efficient fog harvesting. [23][24][25][26][27][28] However, the velocity of directional water transport on these conical structures remains of ≈0.5 mm s −1 , which limits further enhancement of fog harvesting by fog collectors.…”