Fog collection is the harvesting of water from fog using large pieces of vertical mesh netting to induce the fog-droplets to flow down towards a trough below. The setup is known as a fog fence, fog collector or fog net. Through condensation, atmospheric water vapour from the air condenses on cold surfaces into droplets of liquid water known as dew. The phenomenon is most observable on thin, flat, exposed objects including plant leaves and blades of grass. As the exposed surface cools by radiating its heat to the sky, atmospheric moisture condenses at a rate greater than that of which it can evaporate, resulting in the formation of water droplets.
Water condenses onto the array of parallel wires and collects at the bottom of the net. This requires no external energy and is facilitated naturally through temperature fluctuation, making it attractive for deployment in less developed areas. The term 'fog fence' comes from its long rectangular shape resembling a fence, but fog collectors are not confined just to this structural style. The efficiency of the fog collector is based on the net material, the size of the holes and filament, and chemical coating. Fog collectors can harvest from 2% up to 10% of the moisture in the air, depending on their efficiency. An ideal location is a high altitude arid area near cold offshore currents, where fog is common, and therefore, the fog collector can produce the highest yield.
Historical origin
The organized collection of dew or condensation through natural or assisted processes is an ancient practice, from the small-scale drinking of pools of condensation collected in plant stems (still practiced today by survivalists), to large-scale natural irrigation without rain falling, such as in the Atacama and Namib deserts. The first man-made fog collectors stretch back as far as the Inca Empire, where buckets were placed under trees to take advantage of condensation.
Several man-made devices such as antique stone piles in Ukraine, medieval dew ponds in southern England and volcanic stone covers on the fields of Lanzarote have all been thought to be possible dew-catching devices.
One of the first recorded projects of fog collection was in 1969 in South Africa as a water source for an air force base. The structure consisted of two fences each 100m2. Between the two, 11 litres of water was produced on average per day over the 14 month study, which is 0.05 litres of water for every square meter. The next large study was performed by the National Catholic University of Chile and the International Development Research Centre in Canada in 1987. One hundred 48m2 fog fences were assembled in northern Italy. The project was able to yield on average 0.5 litre of water for every square meter, or 33L for each of the 300 villagers, each day.
In nature
Fog collectors were first seen in nature as a technique for collecting water by some insects and foliage. Namib Desert beetles live off water that condenses on their wings due to a pattern of alternating hydrophilic (water attracting) and hydrophobic (water repelling) regions. Redwood forests are able to survive on limited rainfall due to the addition of condensation on needles which drip into the trees' root systems.
Parts of a fog collector
The fog collector is made up of three major parts: the frame, the mesh netting, and the trough or basin.
The frame supports the mesh netting and can be made from a wide array of materials from stainless steel poles to bamboo. The frame can vary in shape. Proposed geometries include linear, similar to a fence and cylindrical. Linear frames are rectangles with the vertical endpoints embedded into the ground. They have rope supports connected at the top and staked into the ground to provide stability.
The mesh netting is where the condensation of water droplets appear. It consists of filaments knitted together with small openings, coated with a chemical to increase condensation. Shade Cloth is used for mesh structure because it can be locally sourced in underdeveloped countries. The filaments are coated to be hydrophilic and hydrophobic, which attracts and repels water to increase the condensation. This can retrieve 2% of moisture in the air. Efficiency increases as the size of the filaments and the holes decrease. The most optimal mesh netting is made from stainless steel filaments the size of three to four human hairs and with holes that are twice as big as the filament. The netting is coated in a chemical that decreases water droplet's contact angle hysteresis, which allows for more small droplets to form. This type of netting can capture 10% of the moisture in the air.