Tikalon Blog by Dev Gualtieri

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Fog Water Harvesting

December 2, 2010

Last week, I winterized my lawn mower and commissioned my snow blower for a new season. As I did these tasks, I was reminded of how much water falls on the Northeastern region of the US where I live. The summer rains make our lawns grow so rapidly that we need to cut them once a week. Winter snowfalls help me get some winter exercise through snow removal. Sure, we complain, but we're actually blessed by having enough water for drinking, cleaning and recreation. Some regions of the world aren't as lucky. Global warming is making things worse for some regions; and, as I wrote in a previous article (The Water Equivalent of Energy, June 1, 2010), industrial pollution has destroyed a lot of the world's potable water.

Water doesn't come to us by rainfall and snowfall alone. In the Northeast it's very common to awaken to find a heavy morning dew , so much so that we need to squeegee our automobile windows before we can drive. From my casual observations during these squeegee operations, there must be at least a liter of water covering the exterior surface of my mid-size American automobile (my wife drives a smaller Japanese car ). After poking around online for some data, I did a spherical cow calculation of the amount of dew water collected by my car. Sure, a "cubic car" calculation is more in line with spherical cows, but in this case, I approximated the geometry of the car as two rectangular parallelepipeds , one on the other. Ignoring the underside of the chassis, there's about 220 square feet exposed to the air, or about 20 square meters. If we claim a liter of water on the surface, then the collection efficiency is about 50 milliliters per square meter. Of course, such a heavy dew occurs only occasionally.

Minimal water demand for humans is about forty liters per day for a non-industrialized society, and many times this for technological societies. It makes sense to examine fog harvesting as one method to meet these requirements. Trees are natural water harvesters. Charles Darwin planted trees on Ascension Island , a dry volcanic island, and after two decades their leaves were harvesting enough water to grow crops that fed hundreds of British troops. Trees with needle-like leaves are the best harvesters, especially the ones that have leaves oriented vertically, rather than horizontally. California Redwoods (Sequoia sempervirens) are so adept at fog harvesting that they create wet microclimates at their base.[1]

Dew on a nasturtium leaf

A drop of dew cupped in the leaf of a nasturtium
(Tropaeolum majus).
(Photo by "Elucidate")

A recent article in Science[1] describes successful fog water harvesting in Lima, Peru .[1] Lima is the world's second largest desert city (the first is Cairo, Egypt ). The Lima fog harvesting system is built from 8 meter wide x 4 meter high plastic nets tied to poles at each end. In the dry season (January - April) not much water is collected, but May-November each net can catch hundreds of liters of water (the record is 590 liters in a single day). A modified double net that has two parallel net planes connected by sections of vertical netting shows a yield of 300 liters per day when averaged through all seasons, with a top rate of 2650 liters in a single day.[1]

A similar system is installed at Copiapó , a city in the Atacama Region of Peru.[2] As an indication of the dryness of this place, the Atacama Cosmology Telescope is located there, since water vapor in the atmosphere would interfere with its millimeter wave measurements. The billion dollar Atacama Large Millimeter Array is being constructed there, also. However, the Peruvian coast in that region does have fog, and the nets collect about a liter and a half of water per square meter per day to irrigate aloe vera plants. The net fog harvesting system was reportedly invented by Robert Schemenauer, a Canadian, in the 1990s.[2] Schemenauer is Executive Director of FogQuest , a non-profit Canadian charity that implements water projects, including fog harvesters, for rural communities in developing countries.

What properties are important to fog collection systems? Netting is preferred to solid sheets for the obvious reason that a net allows air to flow through it. Materials with a low heat capacity are preferred, since they will lose heat rapidly as the air temperature decreases. The closer a material is to a black body the better. Materials with an emissivity close to one radiate most effectively. Of course, black bodies are better at absorbing heat, also, so the fog collectors should be isolated or insulated from sources of heat, such as the ground. Hydrophilicity is good, since the first step is to nucleate a water layer on the surface. Since air moves faster at higher elevations , maximal water collection is achieved at a height.

For these reasons, the type of material used in dew collector construction can have big effect on efficiency. In one experiment[3], the monthly dew yield was measured under identical conditions using solid sheets of galvanized iron , aluminum and titanium-oxide -barium-sulfate -filled polyethylene . The filled polyethylene harvested about 16.0 liters, galvanized iron harvested about 13.4 liters, and aluminum harvested only 7.8 liters. Biomimetic materials can be useful in the future. The Namib Desert Beetle collects dew water on its back, which has alternating hydrophobic, wax-coated regions and hydrophilic, non-waxy regions.[4] Spider webs are also good dew collectors, and they've inspired at least one dew collector design. [5]

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