Abstract
Atmospheric water harvesting is being studied across the globe as a reliable source of freshwater. This easily modifiable, simple design can be used in both humid and arid environments and can be implemented without additional dependence on alternative energy sources. These studies have identified the contact angle of the condensation collected by the net of the fog collector as being one of the most influential factors for increasing interception efficiency. The purpose of this study was to further investigate the degree to which this contact angle affects interception efficiency and the resulting water yield. It was assumed that a contact angle in the range of 45-60 degrees would provide the highest yield by creating enough adhesion force to allow for the collection of larger fog droplets while still maintaining minimal surface tension to prevent clogging.
This hypothesis was tested on a small-scale vertical mesh net/frame made from scrap wood, threaded bolts, and flexible stainless-steel wire. The net was placed inside a vented environmental chamber which maintained constant internal conditions of 100% relative humidity and 24.5 C (+- 0.1) throughout each test prior to the introduction of a simulated fog stream directed at the net for 30 minutes. A dowel-based rotation system allowed for the frame to be tested at multiple angles while ensuring its structural integrity. As a result of user error and frequent design alterations, the data obtained from these tests was clearly skewed. The only true consistency in the data showed the 90-degree contact angle having the best interception efficiency which did not match the initial hypothesis proposing 60 degrees would produce the highest yield. Overall it could also be assumed that fog collection devices are most effective when implemented on a large scale for the purpose of serving an entire community’s non-potable water needs, especially in rural areas. However, future research on this topic may include testing larger contact angles, alternative materials, and the impact of including multiple mesh layers and/or shapes.
This hypothesis was tested on a small-scale vertical mesh net/frame made from scrap wood, threaded bolts, and flexible stainless-steel wire. The net was placed inside a vented environmental chamber which maintained constant internal conditions of 100% relative humidity and 24.5 C (+- 0.1) throughout each test prior to the introduction of a simulated fog stream directed at the net for 30 minutes. A dowel-based rotation system allowed for the frame to be tested at multiple angles while ensuring its structural integrity. As a result of user error and frequent design alterations, the data obtained from these tests was clearly skewed. The only true consistency in the data showed the 90-degree contact angle having the best interception efficiency which did not match the initial hypothesis proposing 60 degrees would produce the highest yield. Overall it could also be assumed that fog collection devices are most effective when implemented on a large scale for the purpose of serving an entire community’s non-potable water needs, especially in rural areas. However, future research on this topic may include testing larger contact angles, alternative materials, and the impact of including multiple mesh layers and/or shapes.
Background
Access to clean water is a basic human right, yet despite this approximately 4.5 billion people around the globe struggle to acquire reliable sources on a daily basis. The consequences of inadequate access and sanitation of available water sources are especially harmful to children as global estimates put the annual deaths of children under five years old at approximately 340,000 (Jarimi, Powell, & Riffat, 2020). Current estimates predict that despite asian countries occupying 60% of the global population they only possess 36% of available water (Ismail & Go, 2021). Southeast Asia is especially vulnerable to water scarcity associated with rising global temperatures and changing rainfall patterns. Solutions for addressing these growing demands for freshwater that have limited environmental impact will have on the climate will only become increasingly necessary.
The most pressing challenges to achieving widespread access to clean water in Southeast Asia include both variability in climates and size of local river basins. Although all regions in Southeast Asia maintain warm temperatures year round (25 C), the amount of rainfall they experience is directly related to the presence of monsoons throughout the region giving some areas excess rainfall while others experience a dry, temperate climate. These climate factors combined with many urban cities in these regions being dependent on proportionally small river basins often lead to excess pollution, flooding, and often result in the over extraction of groundwater (Yuen & Kong, 2009). Global studies have shown that atmospheric humidity can be captured through passive collection techniques to provide a source of freshwater that is both reliable and sustainable. Additionally, atmospheric water harvesting has been proven successful in both humid and arid regions making it applicable for the varied demands of different climates.
Although Southeast Asia receives adequate rainfall, impacts of climate change including rising temperatures, thermal alterations, pollution, decrease in biodiversity and changes to rainfall patterns have prevented access to water resources. This is not only an issue for the human populations, but for animal and plant species as well. Because of this, passive and noninvasive methods of water harvesting are becoming increasingly attractive solutions for a growing water demand.
The most pressing challenges to achieving widespread access to clean water in Southeast Asia include both variability in climates and size of local river basins. Although all regions in Southeast Asia maintain warm temperatures year round (25 C), the amount of rainfall they experience is directly related to the presence of monsoons throughout the region giving some areas excess rainfall while others experience a dry, temperate climate. These climate factors combined with many urban cities in these regions being dependent on proportionally small river basins often lead to excess pollution, flooding, and often result in the over extraction of groundwater (Yuen & Kong, 2009). Global studies have shown that atmospheric humidity can be captured through passive collection techniques to provide a source of freshwater that is both reliable and sustainable. Additionally, atmospheric water harvesting has been proven successful in both humid and arid regions making it applicable for the varied demands of different climates.
Although Southeast Asia receives adequate rainfall, impacts of climate change including rising temperatures, thermal alterations, pollution, decrease in biodiversity and changes to rainfall patterns have prevented access to water resources. This is not only an issue for the human populations, but for animal and plant species as well. Because of this, passive and noninvasive methods of water harvesting are becoming increasingly attractive solutions for a growing water demand.
Climate risk index in Southeast Asia: Ranking for 1999–2018 (Ismail & Go, 2021).
Fog collectors are one of the most simple methods for atmospheric water harvesting. This passive system can collect water found in the low-lying clouds and is particularly effective in mountainous areas near the coast (a geographic staple of many Southeast Asian countries). The most simple fog collectors consist of a section of mesh suspended between two vertical posts positioned at a right angle to the fog-bearing wind. As the fog comes in contact with the mesh, condensation will continue to collect until the resulting water droplets become heavy enough to be collected in a drain or tank system beneath the mesh. The most popular type of mesh for fog harvesting is Raschel mesh which consists of woven high-density polyethylene fibers and is commercially available. Fog collectors have also been used in conjunction with agricultural practices as the use of a higher density mesh can provide adequate shade to crops while also providing additional water. Despite their success, water collected from fog often has higher concentrations of contaminants as its purity is directly related to levels of air pollution, however the World Health Organization has determined that fog water samples can still meet standards for drinking water as well as domestic and agricultural use (Ismail & Go, 2021). Despite this observation, it has been noted that fog water may be more suited for domestic, agricultural, and reforestation purposes as a way to reduce the strain on traditional freshwater sources.
Fog collectors are one of the most simple methods for atmospheric water harvesting. This passive system can collect water found in the low-lying clouds and is particularly effective in mountainous areas near the coast (a geographic staple of many Southeast Asian countries). The most simple fog collectors consist of a section of mesh suspended between two vertical posts positioned at a right angle to the fog-bearing wind. As the fog comes in contact with the mesh, condensation will continue to collect until the resulting water droplets become heavy enough to be collected in a drain or tank system beneath the mesh. The most popular type of mesh for fog harvesting is Raschel mesh which consists of woven high-density polyethylene fibers and is commercially available. Fog collectors have also been used in conjunction with agricultural practices as the use of a higher density mesh can provide adequate shade to crops while also providing additional water. Despite their success, water collected from fog often has higher concentrations of contaminants as its purity is directly related to levels of air pollution, however the World Health Organization has determined that fog water samples can still meet standards for drinking water as well as domestic and agricultural use (Ismail & Go, 2021). Despite this observation, it has been noted that fog water may be more suited for domestic, agricultural, and reforestation purposes as a way to reduce the strain on traditional freshwater sources.
The basic concept of fog collector (Park et al., 2013)
Recent studies on using fog collectors for atmospheric water harvesting focus on increasing the amount of liquid water captured by the device. Improvements to the design that prevent losses resulting from fog passing around the water collector, through the openings of the mesh, and through the droplets bouncing back into the air are the targets of modern studies hoping to increase the fog interception efficiency. These efforts to improve collection efficiency have been most heavily associated with characteristics of the mesh such as the shade coefficient as well as influencing the shape of the net to help improve the aerodynamics of the mesh (de Dios Rivera, 2011). Other studies have identified the contact angle of the liquid on the surface where the condensation meets the surface of the mesh. These studies identified that the best fog harvesting performance was observed on oil-infused surfaces with a large receding contact angle at a high supersaturation.
Popular methods for testing improvements to fog collector design involve a simulated fog stream intercepted by small-scale sections of mesh or mesh material (Azeem et al., 2022). Some studies conduct their experiments within an environmental chamber to control for temperature and humidity that would otherwise influence data by means of evaporation and condensation (Park et al., 2013). However, due to the simplicity of their design several studies have conducted fog collection experiments in the field at much larger scales without the use of an environmental chamber (Jarimi, Powell, & Riffat, 2020). Overall these studies identified that the best fog harvesting performance was observed on oil-infused surfaces with a large receding contact angle at a high supersaturation.
Recent studies on using fog collectors for atmospheric water harvesting focus on increasing the amount of liquid water captured by the device. Improvements to the design that prevent losses resulting from fog passing around the water collector, through the openings of the mesh, and through the droplets bouncing back into the air are the targets of modern studies hoping to increase the fog interception efficiency. These efforts to improve collection efficiency have been most heavily associated with characteristics of the mesh such as the shade coefficient as well as influencing the shape of the net to help improve the aerodynamics of the mesh (de Dios Rivera, 2011). Other studies have identified the contact angle of the liquid on the surface where the condensation meets the surface of the mesh. These studies identified that the best fog harvesting performance was observed on oil-infused surfaces with a large receding contact angle at a high supersaturation.
Popular methods for testing improvements to fog collector design involve a simulated fog stream intercepted by small-scale sections of mesh or mesh material (Azeem et al., 2022). Some studies conduct their experiments within an environmental chamber to control for temperature and humidity that would otherwise influence data by means of evaporation and condensation (Park et al., 2013). However, due to the simplicity of their design several studies have conducted fog collection experiments in the field at much larger scales without the use of an environmental chamber (Jarimi, Powell, & Riffat, 2020). Overall these studies identified that the best fog harvesting performance was observed on oil-infused surfaces with a large receding contact angle at a high supersaturation.
Other studies have attempted to increase interception efficiency through changes to the mesh net itself. Instead of the common Raschel mesh which is prone to clogging on the horizontal crisscrossing thread patterns, fog collectors utilizing a strictly vertical mesh are observed as exerting less frictional force on the developing water droplets (Azeem et al., 2022). These surface forces are key players in predicting the degree to which fog collectors can supplement freshwater sources. The ideal mesh material is one that has enough adhesion force to encourage water droplets to collect enough mass to be pulled into the collection basin through forces of gravity while simultaneously avoiding as much surface tension as possible to avoid the droplets remaining stagnant on the net. This can sometimes be achieved through the use of coated mesh materials, however even these studies are abandoning the traditional Raschel mesh for the more effective vertical alternative (Park et al., 2013) (Shi et al., 2018). These vertical arrays are often referred to as "fog harps".
These forms of vertical mesh systems are based on biomimicry. Designs have been largely inspired by the textural and chemical adaptations used by various plants and animals to increase water intake. There are many species throughout nature that harvest fog water, most of which share a specific physical attributes that make their techniques so effective. By observing these creatures researchers have identified surface wettability and geometric structures as being the essential components of passive fog collection (Zhong et al., 2021).
Plants are especially efficient at pulling water out of thin air. The plants that benefit the most from fog collection often share specific physical characteristics of long narrow leaves which grow in circular formations. These plants are so effective in their atmospheric water harvesting due to the combination of these factors which allow for maximum interception of wind and the condensation it carries. Not only does the wind/fog stream catch on the leaves, their narrow shapes encourage collection and provide a simple path for gravitational forces to pull the collected water towards the plant's roots. Additionally, because the leaves grow in circular clusters, the plant is able to intercept wind through dual layers of narrow leaves regardless of the direction it blows (Richter, 2020)
Solutions such as these seem to be the simplest to replicate in developing nations since they do not require chemical absorption processes or the construction of elaborate geometric designs.
Plants are especially efficient at pulling water out of thin air. The plants that benefit the most from fog collection often share specific physical characteristics of long narrow leaves which grow in circular formations. These plants are so effective in their atmospheric water harvesting due to the combination of these factors which allow for maximum interception of wind and the condensation it carries. Not only does the wind/fog stream catch on the leaves, their narrow shapes encourage collection and provide a simple path for gravitational forces to pull the collected water towards the plant's roots. Additionally, because the leaves grow in circular clusters, the plant is able to intercept wind through dual layers of narrow leaves regardless of the direction it blows (Richter, 2020)
Solutions such as these seem to be the simplest to replicate in developing nations since they do not require chemical absorption processes or the construction of elaborate geometric designs.
Sources
Azeem, M., Noman, M. T., Petru, M., Shahid, M., Khan, M. Q., & Wiener, J. (2022). Surface wettability of vertical harps for fog collection. Surfaces and Interfaces, 30, 101842.
de Dios Rivera, J. (2011). Aerodynamic collection efficiency of fog water collectors. Atmospheric Research, 102(3), 335-342.
Ismail, Z., & Go, Y. I. (2021). Fog‐to‐Water for Water Scarcity in Climate‐Change Hazards Hotspots: Pilot Study in Southeast Asia. Global Challenges, 5(5), 2000036.
Jarimi, H., Powell, R., & Riffat, S. (2020). Review of sustainable methods for atmospheric water harvesting. International Journal of Low-Carbon Technologies, 15(2), 253-276.
Park, K. C., Chhatre, S. S., Srinivasan, S., Cohen, R. E., & McKinley, G. H. (2013). Optimal design of permeable fiber network structures for fog harvesting. Langmuir, 29(43), 13269-13277.
Richter, A. (2020). Fog collection in buildings as a method to meet the future water needs.
Shi, W., Anderson, M. J., Tulkoff, J. B., Kennedy, B. S., & Boreyko, J. B. (2018). Fog harvesting with harps. ACS applied materials & interfaces, 10(14), 11979-11986.
Yuen, B., & Kong, L. (2009). Climate change and urban planning in Southeast Asia. SAPI EN. S. Surveys and Perspectives Integrating Environment and Society, (2.3).
Zhong, L., Zhu, L., Li, J., Pei, W., Chen, H., Wang, S., ... & Zheng, Y. (2021). Recent advances in biomimetic fog harvesting: focusing on higher efficiency and large-scale fabrication. Molecular Systems Design & Engineering, 6(12), 986-996.
de Dios Rivera, J. (2011). Aerodynamic collection efficiency of fog water collectors. Atmospheric Research, 102(3), 335-342.
Ismail, Z., & Go, Y. I. (2021). Fog‐to‐Water for Water Scarcity in Climate‐Change Hazards Hotspots: Pilot Study in Southeast Asia. Global Challenges, 5(5), 2000036.
Jarimi, H., Powell, R., & Riffat, S. (2020). Review of sustainable methods for atmospheric water harvesting. International Journal of Low-Carbon Technologies, 15(2), 253-276.
Park, K. C., Chhatre, S. S., Srinivasan, S., Cohen, R. E., & McKinley, G. H. (2013). Optimal design of permeable fiber network structures for fog harvesting. Langmuir, 29(43), 13269-13277.
Richter, A. (2020). Fog collection in buildings as a method to meet the future water needs.
Shi, W., Anderson, M. J., Tulkoff, J. B., Kennedy, B. S., & Boreyko, J. B. (2018). Fog harvesting with harps. ACS applied materials & interfaces, 10(14), 11979-11986.
Yuen, B., & Kong, L. (2009). Climate change and urban planning in Southeast Asia. SAPI EN. S. Surveys and Perspectives Integrating Environment and Society, (2.3).
Zhong, L., Zhu, L., Li, J., Pei, W., Chen, H., Wang, S., ... & Zheng, Y. (2021). Recent advances in biomimetic fog harvesting: focusing on higher efficiency and large-scale fabrication. Molecular Systems Design & Engineering, 6(12), 986-996.