Fog Harvesting

Source: www.climatetechwiki.org

Fog can be utilized as an alternative source of fresh water in dry regions and can be harvested through the use of simple and low-cost collection systems. This innovative technology is based on the fact that water can be collected from fog under favorable climatic conditions. Fog is defined as a mass of water vapors condensed into small water droplets at, or just above, the Earth's surface at low temperature. Fog has the potential to provide an alternative source of freshwater in this otherwise dry region if harvested through the use of simple and low-cost collection systems known as fog collectors. Captured water can then be used for agricultural irrigation and domestic use.

Description

Fog, a cloud that touches the ground, is made of tiny droplets of water, each cubic meter of fog contains 0.05 to 0.5 grams (half the weight of a paper clip) of water. Fog harvesting technology consists of a single or double layer mesh net supported by two posts rising from the ground. Mesh panels can vary in size. The material used for the mesh is usually nylon, polyethylene or polypropylene netting (also known as ‘shade cloth’) which can be produced to various densities capable of capturing different quantities of water from the fog that passes through it. When the fog rolls in, the tiny droplets of water cling to the mesh and as more and more cluster gets together, they drip into a gutter below that channels to a water tank. Fog collectors, which can also harvest rain and drizzle, are best suited to high elevation arid and rural areas, they would not work in cities because of the space constraints and water needs of an urban environment.

Fog collection projects have used from 2 to 100 fog collectors, and depending on the location, each panel can produce 150 to 750 liters of fresh water per day during the foggy season.

The collectors are positioned on ridgelines perpendicular to prevailing wind and capture and collect water when fog sweeps through. The number and size of meshes chosen will depend on the local topography, demand for water, and availability of financial resources and materials. Optimal allocation is single mesh units with spacing between them of at least 5m with additional fog collectors placed upstream at a distance of at least ten times higher than the other fog collector.

The collector and conveyance system functions due to gravity. Water droplets that collect on the mesh run downwards and drip into a gutter at the bottom of the net from where they are channeled via pipes to a storage tank or cistern. Typical water production rates from a fog collector range from 200 to 1,000 litres per day, with variability occurring on a daily and seasonal basis. Efficiency of collection improves with larger fog droplets, higher wind speeds, and narrower collection fibers/mesh width. In addition, the mesh should have good drainage characteristics.

The dimensions of the conveyance system and storage device will depend on the scale of the scheme. Storage facilities should be provided for at least 50 per cent of the expected maximum daily volume of water consumed. For agricultural purposes, water is collected in a regulating tank, transferred to a reservoir and then finally into an irrigation system that farmers can use to water their crops.

Operation and Maintenance

Operation and maintenance are relatively simple processes once the system has been properly installed. Nevertheless, an important factor in the sustainability of this technology is the establishment of a routine quality control programme which should include the following tasks:

  • Inspection of mesh nets and cable tensions to prevent loss in water harvesting efficiency and avoid structural damage.
  • Maintenance of nets, storage tanks, drains and pipelines to include removal of dust, debris, fungi and algae.
  • Where spare parts are not available locally, it is recommended that a stock of mesh and other components be kept in reserve as local supply might be restricted, especially in remote mountainous regions.

Drought caused by climate change is leading to reductions in the availability of fresh water supplies in some regions. This is having an impact on agricultural production by limiting opportunities for planting and irrigation. Fog harvesting provides a way of capturing vital water supplies to support farming in these areas. Furthermore, when used for irrigation to increase forested areas or vegetation coverage, water supplies from fog harvesting can help to counteract the desertification process. If the higher hills in the area are planted with trees, they will collect fog water and contribute to the aquifers.

Advantages of the Technology

Atmospheric water is generally clean, does not contain harmful micro-organisms and is immediately suitable for irrigation purposes. In a number of cases, water collected with fog harvesting technology has been shown to meet World Health Organization (WHO) standards. The environmental impact of installing and maintaining the technology is cheap and minimal. The construction process is not labour intensive, only basic skills are required and, once installed, the system does not require any energy for operation. Given that fog harvesting is particularly suitable for mountainous areas where communities often live in remote conditions, capital investment and other costs are generally found to be low in comparison with conventional sources of water supply.

Disadvantages of the Technology

Fog harvesting technologies depend on a water source that is not always reliable, because the occurrence of fogs is uncertain. Further, calculation of even an approximate quantity of water that can be obtained at a particular location is difficult. This technology might represent an investment risk unless a pilot project is first carried out to quantify the potential water rate yield that can be anticipated in the area under consideration.

Financial Requirements and Costs

The costs vary depending on the size of the fog catchers, quality of & access to the materials, labour, and location of the site. Small fog collectors cost between Rs.8,000 and Rs.21,000 each to build. Large 40-m² fog collectors cost between Rs.105, 000 and Rs. 157,500 and can last for up to ten years. A village project producing about 2,000 litres of water per day will cost about Rs. 1,575,000. Multiple-unit systems have the advantage of a lower cost per unit of water produced, and the number of panels in use can be changed as climatic conditions and demand for water vary. Community participation will help to reduce the labour cost of building the fog harvesting system.

Potential Areas for Fog Harvesting

Research suggests that fog collectors work best in locations with frequent fog periods, such as coastal areas where water can be harvested as fog moves inland driven by the wind. However, the technology could also potentially supply water in mountainous areas if the water is present in stratocumulus clouds, at altitudes of approximately 400 m to 1,200 m. In Pakistan fog harvesting can be done in the plain areas of South Punjab such as Lahore, Sargodha, Sahiwal, Multan etc. According to the International Development Research Centre, in addition to Chile, Peru, and Ecuador, the areas with the most potential to benefit include the Atlantic coast of southern Africa (Angola, Namibia), South Africa, Cape Verde, China, Eastern Yemen, Oman, Mexico, Kenya, and Sri Lanka. Right now, fog collection is still useful in a limited range of places ones with large amounts of fog and few other sources of water but that may not always be the case. Fog collectors could make a significant difference to the water supply of many arid regions.
A United Nations report notes that "fog collection technology appears to be an extremely promising and low-cost water harvesting system for drinking water, crop irrigation, livestock beverage, and forest restoration in dry land mountains.

Institutional and Organizational Requirements

Community participation helps to remove labour costs and also helps to ensure a sense of ownership by the community and a commitment to maintenance. A community management committee could be set up and consist of trained individuals responsible for repair and maintenance tasks, helping to ensure the long-term sustainability of the technology. In the initial stages, government subsidies may be required to buy raw materials and fund technical expertise.
A range of meteorological and geographic information is required for choosing a site to implement fog harvesting technology, including predominant wind direction and the potential for extracting water from fogs (such as frequency of fog occurrence and fog water content). A feasibility study and pilot-scale assessment should also be carried out to assess the magnitude and reliability of the fog water source. Some of this information can usually be gathered from government meteorological agencies but may require local meteorological stations and the use of a neblinometer (a device to measure the liquid water content) for collection of localized data.

Key Information Requirements for Assessing Fog Harvesting Suitability

Global wind patterns:

Persistent winds from one direction are ideal for fog collection. The high-pressure area in the eastern part of the South Pacific Ocean produces onshore, south-west winds in northern Chile for most of the year and southerly winds along the coast of Peru.

Topography:

It is necessary to have sufficient topographic relief to intercept the fogs/clouds. Examples on a continental scale, include the coastal mountains of Chile, Peru, and Ecuador, and, on a local scale, include isolated hills or coastal dunes.

Relief in the surrounding areas:

It is important that there are no major obstacles to the wind within a few kilometers upwind of the site. In arid coastal regions, the presence of an inland depression or basin that heats up during the day can be advantageous, as the localized low pressure area thus created can enhance the sea breeze and increase the wind speed at which marine cloud decks flow over the collection devices.

Altitude

The thickness of the clouds and the height of their bases will vary with location. A desirable working altitude is at two-thirds of the cloud thickness above the base. This portion of the cloud will normally have the highest liquid water content. In Chile and Peru, the working altitudes range from 400 m to 1,000 m above sea level.

Orientation of the topographic features:

It is important that the longitudinal axis of the mountain range, hills or dune system be approximately perpendicular to the direction of the wind bringing the clouds from the ocean. The clouds will flow over the ridge lines and through passes, with the fog often dissipating on the downwind side.

Distance from the coastline:

There are many high-elevation continental locations with frequent fog cover resulting from either the transport of upwind clouds or the formation of orographic clouds. In these cases, the distance to the coastline is irrelevant. However, areas of high relief near the coastline are generally preferred sites for fog harvesting.

Space for collectors:

Ridge lines and the upwind edges of flat-topped mountains are good fog harvesting sites. When long fog water collectors are used, they should be placed at intervals of about 4.0 m to allow the wind to blow around the collectors.

Crestline and upwind locations:

Slightly lower-altitude upwind locations are acceptable, as are constant-altitude locations on a flat terrain. But locations behind a ridge or hill, especially where the wind is blowing down slope, should be avoided.

Several challenges and issues have emerged from fog harvesting projects implemented to date:

  • Where fog is a seasonal source, water has to be stored in large quantities for dry season use
  • If not properly maintained, water quality becomes an issue during low-flow periods
  • Fog water collection requires specific environmental and topographical conditions, limiting its application to specific regions
  • Procurement and transportation of materials is hindered by remote locations and steep terrain
  • Strong winds and snow fall can result in structural failure during the winter season
  • Water yield is difficult to predict, requiring feasibility studies prior to large scale implementation
  • For harvesting to be effective, frequent fogs are needed and sufficient water collected for the investment to be cost-effective. This limits the technologies to areas with specific conditions.
  • There are few commercial producers of mesh currently in operation, with main suppliers located in the Chile. Therefore implementation and maintenance can be costly [due to import or transportation].

Opportunities for implementation:

Fog water collection has emerged as an innovative technology for mountainous communities without access to traditional sources of water. Still largely in a state of development, there is opportunity for research and development into fog harvesting technology and its potential to support agricultural production. Given the lack of mesh suppliers, using locally available materials for component parts presents an opportunity for local business development. This technology also provides an opportunity to restore natural vegetation and support agricultural practices through the sourcing of clear water for crops and livestock

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