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Papa Earth and Rangi Sky created the trees and the shrubs, the plants and the flowers, the birds and the butterflies and all other animals in the sky, on the earth and in the sea. It was very crowded and a tight place to be, and Papa’s en Rangi’s children were desperate to get more breathing space. After many of them tried and failed, their child Tane, God of the forests, succeeded to part Heaven from Earth, and ever since Rangi and Papa grieve ceaselessly for each other. Papa’s sighs form the soft mists that rise from the earth, Rangi’s tears are the source of the morning dew.
– Maori Myth
inspiringfuture In arid areas, where rain is rare and every drop of water is precious, many plants and animals have become highly specialized in capturing moisture from the sky.
The Welwitschia can survive rainless years using its long leaves to gather dew and fog and channel it to its root system. The Thorny Devil, who lives in the central Australian desert and thanks his name to his impressive thorn-covered skin, also found a way to quench his thirst. Dew condenses on the thorns and is drawn by capillary action along the thorn’s grooves to eventually end in this prickly creature’s mouth. But there’s not only passive ways of collecting and distributing water. The Australian mouse covers a large area around its burrow with pebbles, apparently for the sole purpose to harvest dew as water supply.
Humans also have a history collecting dew. The alchemists for example attributed high importance to dew and used it in many experiments in search of the philosopher’s stone. An example of its use can be found in the alchemical work Mutus Liber, where one of the illustrations shows some dew collectors (cloths stretched among four stakes) and a couple wringing the dew water out of a cloth.
There can be several reasons to start dew harvesting. Maybe you just like the idea and love to experiment, or you would like to use rain for irrigation, showering and doing laundry, and think it’s a nice idea to get your drinking water from dew. Maybe you live in an area where water is scarce and you want to use everything you can get. Or you think bigger and you are looking for ways to bring water to villages that are threatened by drought. There could even be some among you that think money can be made out of everything, and selling dew water might be big business (crazy enough, you won’t be the first!)
Whatever reasons you may have, there are many ways to harvest dew. Some collecting methods have questionable results, but you might find ways to improve them.
You can choose to use only natural or recycled materials or to get some stuff from your local store. Of course you can also choose to buy a ready made device like an atmospheric water generator that can run directly from a solar panel or from your home electricity.
what will be covered in this article:
What is dew? Some facts to help you understand the principles behind dew harvesting.
Ways of harvesting and creating dew collectors, advantages and disadvantages of different systems. The focus will be on clean drinking water. Dew harvesting for agricultural purposes will be covered in a future article.
Dew water quality.
What is dew?
The air around us can contain between just above 0 to around 5% of water vapor. This water vapor will condensate on surfaces when the surface temperature reaches the dew point. With a relative humidity of 100% the dew point is the same as the air temperature, and the lower the relative humidity gets, the larger the difference between air temperature and dew point (the dew point getting lower). To give an example; when the air temperature is 20°C the dew point is around 6°C at a relative humidity of 40%, and around 16°C at a relative humidity of 80%.1)
But why and when do we get this difference in temperature, and why does this result in formation of dew drops?
All objects around us are continuously exchanging heat by radiation, in the eternal natural search for an equilibrium. The quantity of heat being radiated from an object is dependent on the nature of the object and it’s surface.
During the day the earth receives heat from the sun, while at the same time it also radiates heat into space. It receives much more though than it radiates. After sunset the balance shifts, and the earth radiates much more heat than it receives, with the result that the surface cools down.
Different objects with their different textures, like stones, metals and plants have different ‘radiation power’ and as a result they will show different temperatures.
Everything that interrupts radiation, be it a large tree or a cloud, will diminish radiation substantially. When there is a complete cloud cover almost all radiation from the earth will be radiated back, and differences in temperature between air and object will be negligible.
When a surface reaches the dew point, the air closely surrounding it also cools down, and as colder air can hold less water vapor, condensation will occur.
In natural circumstances dew can start forming at a minimum rel. humidity of around 75%
and at a maximum daily rate of around 0.8L/m2
When you collect dew the chances are very small that you will ever get more than 0.6L/m2
Fog occurs when water vapor condensates on particles in the air (cloud condensation nuclei). This will mostly happen when the dew point is very close to the air temperature, and thus when humidity is very close to 100%. Fog can be harvested using fog nets that catch the tiny water drops, and while the yields are likely to be much larger than the yields of dew harvesting, occurrence of fog is more location dependent than occurrence of dew.
The Air Wells – different ways of harvesting dew
A distinction can be made between High Mass Air Wells, Radiative (low mass) Air Wells and Active Air Wells.
1. High Mass Air Wells
High Mass Air Wells apparently thank their existence to a misconception. An engineering guy named Friedrich Zibold one day ran into thirteen large piles of stones near the city Feodosiya in the Ukraine. Each pile had a diameter of over 30 meters and was about 10 meters high.
He associated these piles with the remains of 75-millimeter diameter terracotta pipes that seemed to lead to wells and fountains in the city. Zibold concluded that the piles originally were used as condensers that took care of the water supply for the whole city.
To verify his hypothesis Zibold made a comparable construction. There are no public records of his results but the maximum daily production was later estimated to be 360 liters, in spite of the fact that it is generally believed now that the piles that inspired Zibold were actually ancient burial mounds.
Zibolds experiment became an inspiration for many. However, the best known high mass air wells (Chaptal’s collector, Klaphake’s collector, Knapen’s Arial Well, see picture) only produced up to a few liters a days.
So the construction of high mass air wells is apparently based on a kind of myth, and besides, it seems to be very hard to create something that functions well. Should those facts keep us from experimenting with such huge constructions? If you have lots of time, many stones available on your site, and your life doesn’t depend on the water you try to harvest, why not give it a go?
All high mass constructions were based on valid principles. The biggest problem you will face is the cooling of the mass that has to collect the dew.
Zibold and his successors made a serious mistake, as their collectors, which reached high temperatures, did not cool efficiently. Our theoretical studies, which take into account the different exchanges between the ground and the atmosphere, show that the yield decreases dramatically when the mass to surface ratio increases. This is what happened in the case of massive condensers such as Chaptal’s and Knapen’s.
Zibold’s condenser owes its relative success to two elements in its construction: the stack formation of the sea pebbles allowed radiative cooling of the outer layers and prevented anything but the slightest thermal contact between the pebbles. The condensation mass to surface ratio therefore proved to be relatively important.
– D. Beysens, I. Milimouk, “Pour les ressources alternatives en eau”
Sécheresse, Vol. 11, n° 4, December 2000.
Despite of the problems and inefficiencies you may have to face, it is possible to make a construction that delivers enough water for your drinking needs. So for the ones who prefer a big and solid creation made out of stone instead of the “light mass” dew collectors that have the name to be much more efficient, here a short description of Zibold’s and Knapen’s air wells:
“Zibold’s dew condenser. (a) is a truncated cone of beach pebbles 20m in diameter at the base and 8m in diameter at the top. (b) is a concrete bowl; a pipe (not shown) leads away from the base of the bowl to a collecting point. (c) is ground level and (d) is the natural limestone base.” – Gaius Cornelius
“Zibold’s condenser was surrounded by a wall 1 meters high, 20 meters wide, around a bowl-shaped collection area with drainage. He used sea stones 10–40 centimeters in diameter piled 6 meters high in a truncated cone that was 8 meters in diameter across the top. The shape of the stone pile allowed a good air flow with only minimal thermal contact between the stones” – Nelson, Robert A. (2003). “Air Wells, Fog Fences & Dew Ponds – Methods for Recovery of Atmospheric Humidity”. Rex Research
“Warm, moist outdoor air enters the central chamber, as the daytime temperature rises, through the upper ducts in the outer wall. It immediately strikes the chilled core, which is studded with rows of slates to increase the cooling surface. The air, chilled by the contact, gives up its moisture upon the slates. As it cools, it gets heavier and descends, finally leaving the chamber by way of the lower ducts. Meanwhile the moisture trickles from the slates and falls into a collecting basin at the bottom of the well.” – Popular Science, March 1933
Now you know about the principles and the weak points of these constructions you might have ideas to make it more efficient. If you decide to build a High Mass Air Well construction, please let us know about your plans, progress and results!
2. Radiative Air Wells
Most radiative collectors have a condensing surface, which is backed by a layer of insulating material (like Styrofoam) and fixed on a frame at an angle of 30° and around 2-3 meters above the ground. At the bottom you’ll find a collecting gutter.
The condensing surface should be made of a light material (so it can’t hold onto heat) that is hydrophobic (water repelling) so the condensed moisture drips down easily.
The OPUR (International Organization For Dew Utilization) is doing research and many projects with these kind of condensers and developed a special condensing foil.2)
The amount of dew that can be harvested with this type of collector is of course very dependent on local circumstances. The maximum measured yield is around 0.6 liter per square meter surface. A study in Morocco3) showed a yield of 18.85 mm in a year. During this year there were 178 “dew days” which means that the average yield on a dew day was around 0.1 liter / m2 / day, or the average yield on any day (including days without any dew collection) around 0.05 liter / m2 / day.
This may sound as mighty little, but if we compare the total of 18.85 mm to the rain that was collected over the same period (48.65 mm), the contribution of dew gave an increase in water yields of almost 40%. With 178 dew days compared to 31 rain days it even proves to be quite a reliable source.
To develop new materials that can (among other things) be used to effectively collect water, researchers have been inspired by nature, e.g. by the Lotus flower (with its hydrophobic leaves) and the Namib Desert Beetle.
“The Namibian Beetle (Stenocara gracilipes) lives in one of the driest deserts in the world, the Namib on the southwest coast of Africa, but obtains all of the water it needs from ocean fog due to the unique surface of its back. Microscopic bumps with hydrophilic tips and hydrophobic sides cover its hardened forewings, which it aims at oncoming fog each morning. Water droplets materialize out of thin air on its back, then slide down channels into its awaiting mouth.“ – asknature.org
The MIT (Massachusetts Institute of Technology) and NBD Nanotechnologies are developing coatings based on the Namib Desert Beetle. When asked about the current state of affairs Daniel Beysens, Physicist and expert on Dew Utilization who collaborates with MIT through MIT-France said:
“We tested some coatings, with mitigated success. The only problem is manufacturing large surfaces, at low cost and without aging effects. For the time being, only plastic foils (OPUR for testing, commercial for large surfaces) and painted metallic surfaces with OPUR additives are low cost and give long term efficiency.”
To make a radiative air well you are not restricted to using square planes and condensing foil:
Gabin Koto N’Gobi’s Dew Collector
The lack of water in the Northern region of his home country Benin motivated Gabin Koto N’Gobi to design a dew collector.
His prototype is made out of local materials which makes it sustainable and accessible. It harvests up to 4 liters of water per night.
Arturo Vittori’s “WarkaWater”
The WarkaWater is designed to collect dew, fog and rainwater. With it’s beautifully designed 9 meters tall framework made out of bamboo with a special water collecting fabric hanging inside, it is a great example of functional art.
The project was conceived in Ethiopia and inspired by the crude fact that women and children in the rural areas have to walk several hours to collect water of questionable quality.
These designs might make you wonder about the effect of different shapes on the efficiency of dew collecting. During summer and fall 2009 experiments have been done (Pessac, France) to get an answer to this question. The results were published in “New Architectural Forms to Enhance Dew Collection” (Daniel Beysens, Filippo Broggini, Iryna Milimouk-Melnytchouk, Jalil Ouazzani, Nicolas Tixier)
60° cone angle (30° from horizontal)
the 30° angle has been found to give the best cooling efficiency. This angle also allows water to easily flow by gravity as the gravity forces are only reduced by 50% with respect to vertical.
YIELDS: an average of 22% larger than the planar reference condenser (30% at wind speeds below 1.5 m/s to 0% above 3 m/s). The gains are larger for low dew yields.
b. inverted pyramid
Here the surface also has an angle of 30° from horizontal
YIELDS: an average of 20% larger than the planar reference condenser. The gains are larger for low dew yields, these increased gains are lower though than with the conical shape.
As these shapes are somewhat unpractical when making constructions to collect dew on large scale, tests were also made with hollow shapes that could be ’tiled’ on a bigger surface, like a roof. In this way the collecting surface can be increased without needing more horizontal space nor an unpractical increase of vertical space.
YIELDS: an average of 10% larger than the planar reference condenser.
Note that part of the formed dew can’t be collected because of the areas where the angle is too low for the drops to flow off (the flat tops of the egg bumps)
YIELDS: an average of 120% larger than the planar reference condenser, with much higher gains for low dew yields (up to 400%)
So we can see that the geometrical shape of the collectors is of great influence to the amount of dew that can be collected. The big advantage of hollow shapes lies in the fact that influences of the wind are decreased, and thus the cooling is increased.
3. Active Air Wells / Atmospheric Water Generator
While the working of a passive air well is based on naturally occurring temperature variations, the active air well requires an external energy source. In most cases they appear to be quite inefficient, requiring a considerable amount of energy to produce relatively little water, but an important advantage is that it is much less dependent on external factors like temperature variations and humidity. And when power is available in abundance (for example from solar energy) the only big disadvantage left is the price tag.
The working of a water generator can be based on different principles. One method to extract water from the air is that of cooling air to the dew point. This method will become very inefficient though when relative humidity drops under 30%, or the air temperature drops under circa 18°C.
A more efficient method under such circumstances involves the use of desiccants. A desiccant is a substance (like Silica gel) that absorbs water and is generally used to remove moisture. The generator then extracts the water from the desiccants and purifies the water.
A comparable method can also be found in nature:
The ability to absorb water vapor from the atmosphere enables ticks to survive without drinking water for many months. The tick rehydrates using a three-stage process. First, it uses its foremost pair of legs to detect microregions of high humidity, such as those surrounding water droplets. Once a suitable water source is detected, the tick secretes a hydrophilic solution from its mouth. Once it is saturated, the tick draws the now hydrated secretion back into its mouth. The secretion is a hygroscopic salt solution. Once ejected from the mouth, the solution dries at low ambient humidities, leaving a crystalline substance behind. When the humidity increases, the hydrophilic crystalline substance dissolves and is swallowed back into the body of the tick. The adaptation allows exophilic ticks to absorb water vapor from close to saturation down to 43% relative humidity. Mites and soil-dwellings use a similar mechanism to absorb water vapor.
The Atmospheric Water Generators that you can buy nowadays start at a price of around $1000 (in ideal circumstances producing up to 40 liters of water a day, for real life we might have to adjust the expectations to between 5 and 20 liters a day), but there are also systems available that can generate several thousands of liters a day. The small ones have a power rating of around 500W, but machines that can produce 5000 liters a day have a power rating more than 100 KW, they weigh over 4000kg and cost at least $60,000
As the main reason for harvesting dew is to create an alternative source for potable water, it is important to have a look at the quality of dew water.4)
Atmospheric Water Generators are supplied with filters, and one can generally assume that the quality of the produced water is good.
Using a radiative collector (in areas without industrial pollution) the dew water will most often comply to WHO (World Health Organization) recommendations. The probable presence though of small amounts of animal and/or vegetal bacteria (coming for example from the excretions of insects drinking the dew water) might ask for a light antibacterial treatment like boiling or microfiltration.
Because of the large differences in quality and composition of water used for drinking around the world (whether it is tap, table, rain or dew water) it is impossible to make good comparisons. Your collected dew could be of better quality and/or taste than certain tap or table waters, or dew collected on a different spot for that matter5), but worse as well.
If you want to learn more about the quality of your water you can always test it with a water quality test kit or send a sample to a laboratory.
A short word about dew compared to rain, collected on the same location
Dew water has a different quality than rain water, partly because the formation process and thus exposure time to the environment is much shorter for dew, and partly because of the differences in the atmospheric composition in aerosols (small particles) and gas at high elevation (cloud) and low elevation (dew)
Dew water has in general a slightly higher pH than rain water because less gases will be absorbed, and a higher mineralization due to salt from marine origin and / or the enhanced deposition of fine dust coming from the dry, arid soil.
to calculate the dew point:
Td = 243.12 * A / (17.62 – A)
A = Log(RH / 100) / Log(2.718282) + (17.62 * Ta / (243.12 + Ta))
the OPUR condensing foil is 0.39 mm thick and made of 5.0 vol %
of TiO2 microspheres of 0.19 μm diameter, and 2.0 vol. % of BaSO4 of 0.8 μm diameter embedded in a matrix of low-density polyethylene (LDPE). It also contains approximately 1 vol % of a surfactant additive non-soluble in water. This material improves the mid-infrared emitting properties to provide radiative cooling at room temperature and efficiently reflects the visible (sun) light
The quality of dew water (based on chemical, physical, and biological characteristics) is of course dependent on many factors; what kind of materials, constructions or devices were used to collect the water? Where was it collected? Was it close to a city, or a sea, in the desert or in the mountains?
Whatever these and other factors may be, in general we can assume that the pH of dew water is between 6 and 7.5 (common for drinking water is between 6 to 8.5) which not only depends on location, but also on seasonal variations. The mineralization (or total dissolved solids, TDS) is between 35 and 560mg/L (WHO recommendation is below 1000mg/L)
5) ”The quality of dew water was the object of a few studies. Chemical analyzes were carried out in Chile, in the USA, in Japan, in Jordan, in India, in France, the characteristics of which can
be quite different. Dew water was very corrosive in Chile with a high ionic concentration, very acidic in Japan with a high concentration of sulphates and nitrates, and only slightly alkaline and slightly mineral-bearing in Jordan and France.” – Dew and Rain Water Collection in South Croatia – Daniel Beysens, Imad Lekouch, Marina Mileta, Iryna Milimouk and Marc Muselli
The dew-drop and the mist, or, an account of the nature, properties, dangers, and uses of dew and mist– C. Tomlinson
Essay on Dew and several appearances connected with it – W.C. Wells
On the History of the Scientific Exploration of Fog, Dew, Rain and Other Atmospheric Water – Detlev Möller
New Architectural Forms to Enhance Dew Collection – Daniel Beysens, Filippo Broggini, Iryna Milimouk-Melnytchouk, Jalil Ouazzani, Nicolas Tixier
Pour les ressources alternatives en eau – D. Beysens, I. Milimouk, Sécheresse, Vol. 11, n° 4, December 2000.
Dew and Rain Water Collection in South Croatia – Daniel Beysens, Imad Lekouch, Marina Mileta, Iryna Milimouk and Marc Muselli
Dew, fog, and rain as supplementary sources of water in south-western Morocco – I. Lekoucha, M. Muselli, B. Kabbachia, J. Ouazzanib, I. Melnytchouk-Milimouk, D. Beysens
Influence of temporal variations and climatic conditions on the physical and chemical characteristics of dew and rain in South-West Morocco – I. Lekouch, B. Kabbachi, I. Milimouk-Melnytchouk, M. Muselli, D. Beysens
Leveraged Innovation Management: Key Themes from the Journey of Dewrain Harvest Systems -Mukund Dixit , Girja Sharan