Steps to Harvesting Water from the Clouds
One of the more innovative methods of gathering freshwater is harvesting it from clouds, but hasn’t been unproven. When the base of the cloud is in contact with the ground, it is called fog. Fog collectors work best in coastal areas where the water can be harvested as the fog moves inland driven by the wind.
What is Fog?
Fog can form at lower humidity, and sometimes it does not form with the relative humidity at 100%. The air will be supersaturated if moisture is added beyond this point, which presents conditions favorable for condensation in the form of rain or fog.
Fog formation requires all of the elements that normal cloud formation requires, the most important being condensation nuclei, in the form of dust, aerosols, pollutants, etc., for the water to condense upon. When there are exceptional amounts of condensation nuclei present, especially hygroscopic (water seeking) particles such as salt, the water vapor may condense below 100% humidity.
Sea fog is a fog caused by the peculiar effect of salt. Sea fog is common along coastlines where airborne salt particles are generated from the salt spray produced by breaking waves. These salt particles act as the nuclei for water to condense. Unusual to sea fog is that, due to the hygroscopic nature of salt, condensation can occur with humidity as low as 70%.
Fog typically contains about 0.2 grams of water per cubic meter; the amount of water that can be harvested depends on surface area of the collector, efficiency of collection, and wind speed.
The keys to the feasibility of artificial fog catchers are meteorology and topography, as well as a community base to maintain the system. First, moisture-laden air must be driven by wind to a geographic barrier, such as a mountain range that intercepts the clouds. These conditions occur most commonly in a place where there is onshore winds air mass, which creates appropriate conditions. The same combination can occur with interior mountain chains. Secondly, modest in quantity, the more cost-effective fog-collecting site is best located near the point of use.
In order to implement a fog-harvesting program, the potential for extracting water from fog first must be investigated. The occurrence of fog can be assessed from reports compiled by government meteorological agencies. To be successful, this technology should be located in regions where favorable climatic conditions exist. Since fog and clouds are carried to the harvesting site by the wind, the interaction of the topography and the wind will be influential in determining the success of the site chosen. The following factors affect the volume of water that can be extracted from fog and the frequency with which the water can be harvest:
- Frequency of fog occurrence is a function of atmospheric pressure and circulation, oceanic water temperature, and the presence of thermal inversions.
- Fog water content is a function of altitude, seasons, and terrain features
- Design of fog water collection systems takes into account wind velocity and direction, topographic conditions, and the materials used in the construction of the fog collector.
Prior to implementing a fog water harvesting program, a pilot-scale assessment should be undertaken of the proposed collection system and the water content of the fog at the proposed harvesting site. The following factors should be considered in selecting an appropriate site for fog harvesting:
- Global wind patterns
- Relief in the surrounding areas
- Orientation of the topographic features
- Distance from the coastline
- Space for collectors
- Crestline and upwind locations
Technical Description of a Fog Collector
Fog collectors are a simple concept. The surface of fog collectors is usually made of fine-mesh nylon or polypropylene netting (e.g., shade cloth). It should be made of a relatively fine material that loses heat rapidly, with a black UV-stabilized fabric normally preferred. The fabric should shed its moisture load as rapidly as possible. To more effectively shed the water, vertical fibers are most important than horizontal. The efficiency of various mesh densities was tested. The fog collectors equipped with Raschel netting, with 35-45 percent coverage, mounted in double layer extracts, provided about 30-40 percent of the water from the fog passing through the collection netting. Other materials used with good results are air conditioning filter material and aluminum shade mesh.
Wind and Site
The role of the wind in fog water collection is important and can be leveraged to obtain optimal water harvesting.
Because of the variation in local fog occurrence, a test installation should be provided to confirm the viability of a fog collection system before construction is begun. The length of the test should run at least half a fog season. The results of this test should be compared to the minimum duration fog season that would make this technology cost-effective. In general, fog harvesting has been found to be cost-effective in arid regions when compared to alternative conventional options.
In a mountainous environment, the optimum elevation for the most productive collection of water is obtained from the wetter upper half of a stratoform cloud. For a coastal installation, care must be taken to assure that the water collected will not be contaminated with ocean salt. Collection from the upper elevations of the cloud, no less that 658 ft above the ground, will improve the quality of the collected water.
The storage cistern should be sized to provide at least 50% of the expected maximum daily volume of water consumed. But because the occurrence of fog is not perfectly predictable from day to day, it may be necessary to store additional water to meet demands on days when no fog water is collected.
Water quality can best be assured by maintaining the cleanliness of the system. Occasional purification of water in the storage tanks may be necessary if the water is used for drinking without first boiling. The normal contamination source could be expected to come from bird or rodent residue. Heavy metal contamination, commonly caused by proximity to industrial or agricultural sites, is best avoided by judicious selection of the fog-collecting site.
Advantages of a Fog Harvesting System
A fog collection system can be easily built or assembled on-site. Installation and connection of the collection panels is quick and simple. Assembly is not labor intensive and requires little skill. Some advantages are as follows:
- No energy is needed to operate the system or transport the water
- Maintenance and repair requirements are generally minimal
- Capital investment and other costs are low in comparison with those of conventional sources of potable water used, especially in mountainous regions
- It has the potential to create viable communities in inhospitable environments and to improve the quality of life for people in mountainous rural communities
- The water quality is better than from existing water sources used for agriculture and domestic purposes
Fog collection is an ancient and proven technology that is a viable source of clean water suitable for agriculture and for domestic use. Problems may occur in the maintenance and ongoing servicing of these systems to keep them operational. Providing sufficient storage volume, to bridge the occurrences when fog is not available, is also an important issue that determines its value as a reliable source of water.
Bob Boulware is President of Design-Aire Engineering, Inc. Bob is a past president of the American Rainwater Catchment Systems Association (ARCSA) and an Accredited Rainwater Systems Design Professional. He is a 30+-year member of ASHRAE and past president of the Central Indiana Chapter of American Society of Plumbing Engineers (ASPE) and serves on the ASPE National Standards Committee. Mr. Boulware is a member of the International Association of Plumbing and Mechanical Officials (IAPMO) Alternative Water Sources Committee, and helped to develop the Green Plumbing Supplement to the upcoming editions of the Uniform and the International Plumbing Codes. Mr. Boulware has taught Environmental Design for mechanical and electrical systems at Ball State University and plumbing design at IUPUI. Follow us @daengineering on Twitter & www.daengineering.com. Bob can be reached at firstname.lastname@example.org.
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