Affordable, effective and sustainable "fog nets" to capture droplets and combat water scarcity in deserts. An article by Denis Terwagne, Professor and Chairman of the Centre de Recherche en Physique, Faculty of Science, in The Conversation.
In 2022, 2.2 billion people still have no access to drinking water services. With global warming, the scarcity of drinking water has become a major challenge for our future, and this problem is of course intensifying first and foremost in arid and overpopulated regions. A recent United Nations report highlights the crucial importance of exploring "unconventional" water sources such as atmospheric water, as current resources are overexploited and insufficient.
When it comes to finding original solutions, nature can be an incredible source of inspiration. Indeed, over the course of evolution, certain species of trees and insects have developed ingenious ways of capturing water in the atmosphere in the form of vapour or mist.
California’s giant redwoods, for example, capture more than a third of the water they consume through the fog intercepted by their needles and drained at their base. The Socotra dragon tree, for its part, captures and ingests fog water directly through its rosette-shaped leaves. Some desert cacti, mosses and beetles use their structure, geometry and surface affinity with water to help collect fog.
Nets to capture the fog
Faced with these fascinating solutions, mankind is not to be outdone: fog nets have been in use since the early 90s, notably by the NGO Fog Quest. We’re looking to develop frugal ways of making nets that are more efficient, more economical and easier to manufacture than those currently available.
In some desert areas, such as Chile and Morocco, people use "fog nets" to capture the water droplets in the "advection fogs" that form and move over certain mountainous deserts close to ocean coasts. Driven by prevailing winds, warm, moist ocean air condenses into tiny droplets of water as it cools on contact with the cooler atmosphere of these mountainous regions.
For several decades, scientific and technical advances in the design of large, affordable nets have led many communities in need to deploy nets to capture wind-borne fog. These "cloud" or "fog" nets resemble fishing nets strung in the air. The best nets can collect up to 15% of the water contained in the fog. Depending on where the nets are placed, they can collect between 10 and 100 liters of water per square meter per day.
In water-stressed regions prone to advection fog, the real challenge is to develop affordable, effective and durable fog nets. Inexpensive nets, such as the raschel mesh used for food packaging, are accessible, but their effectiveness is limited and they cannot withstand strong winds. Stronger, high-performance nets, such as the Aqualonis FogCollector commercial net, are harder to produce and much more expensive.
Faced with these scientific and technical challenges, researchers are mobilizing to explore these passive solutions for improving nets by working on their efficiency, aerodynamics or even the nature of the material making up the net and its affinity with water. The "passive" aspect means that no external energy input (electrical or fossil fuel) is required, unlike other methods of harvesting fresh water, which are often highly energy-intensive, such as certain seawater desalination methods.
Improve cloud nets by adapting fiber geometry
Our recent research has resulted in kirigami mist nets, designed by simply cutting and folding plastic sheets. This type of net is inexpensive, has a simple geometry and outperforms most existing nets.
The origin of this efficiency lies in the geometry of the fiber used. While most fog nets use cylindrical fibers, we use flat fibers.
A drop placed on a surface will take on a shape that balances the internal and external pressures of the drop, as well as the excess (Laplace) pressure coming from the curvature of the drop’s liquid interface.
As the cylindrical fiber has a curvature, a drop will not be able to spread out, as to compensate for the curvature of the fiber, it will have to keep the shape of a pearl. Thanks to the flat geometry of the fibers used in our nets, mist drops spread out completely, rapidly forming a fine, highly stable liquid film over the entire surface of the net, which favors water collection.
Our tests showed that, under equivalent experimental conditions, our Kirigami fog net collected eight liters per square meter in one hour, while the commonly used fiber harp and rashel mesh nets collected only three and two liters per square meter respectively.
Compared with one of the best fog nets on the market (the Aqualonis FogCollector 3D-2013 ), our Kirigami net has a comparable stationary efficiency, but performs much better dynamically, i.e. when fog is of short duration or low intensity.
An approach designed to be easy to scale under frugal conditions
The simplicity and scalability of kirigami make it a promising candidate for low-cost field applications (after all, it’s just a sheet of plastic with cut-outs). We are currently testing large-scale prototypes in collaboration with the Moroccan NGO Dar Si Hmad, which owns the world’s largest field of fog nets, and initial results are very encouraging.
In addition to this new manufacturing method, we have also developed a precise, well-controlled laboratory test method for quantifying the effectiveness of our Kirigami fog net and comparing it with other nets under the same conditions.
Measuring the effectiveness of a fog net is often subjective and not yet standardized.
Our test rig consists of a wind tunnel producing a laminar air flow into which a mist is generated and sent using piezoelectric transducers. Fillet samples, placed in the flow on ultra-precise scales, enable us to measure collection efficiency in real time. This enables us to distinguish dynamically between the different contributions to efficiency.
Creatively constrained research to meet societal challenges
To guarantee the accessibility and scalability of the solutions we developed, we worked under constraints: in the context of "frugal science" (science that aims to design scalable and creative solutions while taking into account the cost/performance ratio) within a "fab lab" environment (a digital manufacturing laboratory).
By fostering interdisciplinary collaboration and prototyping, fab labs play a key role in creating innovations to meet global challenges, in line with the United Nations’ Sustainable Development Goals.
Denis Terwagne , Physicist and interdisciplinary experimenter, Professor, Université Libre de Bruxelles (ULB ) This article is republished from The Conversation under a Creative Commons License.
