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[[Image:Dew on a flower.jpg|thumb|left|Fig 2: Dew on a flower]] | [[Image:Dew on a flower.jpg|thumb|left|Fig 2: Dew on a flower]] | ||
Interestingly, many desert and costal plants and animals depend on this source [4]. Naturally, the phenomenon of dew formation takes place due to night sky radiation cooling. On a clear night, heat energy is radiated away from the earth’s surface into the sky. As the surface temperature drops below the dew point, water vapor will condense into droplets. Dew is formed both from water vapor in the atmosphere in the form of dewfall, as well as from water in the soil in the form of dewrise. | Interestingly, many desert and costal plants and animals depend on this source [4]. Naturally, the phenomenon of dew formation takes place due to night sky radiation cooling. On a clear night, heat energy is radiated away from the earth’s surface into the sky. As the surface temperature drops below the dew point, water vapor will condense into droplets. Dew is formed both from water vapor in the atmosphere in the form of dewfall, as well as from water in the soil in the form of dewrise. | ||
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The amount of dewfall can be approximated from a complex turbulent fluid analysis; however it is far simpler to consider it as an energy balance problem [5]. The process of condensation releases heat, and thus raises the surface temperature. | The amount of dewfall can be approximated from a complex turbulent fluid analysis; however it is far simpler to consider it as an energy balance problem [5]. The process of condensation releases heat, and thus raises the surface temperature. | ||
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Furthermore, it follows that the maximum possible condensation rate for combined dewfall and dew-rise during nighttime is [10, 11, 7, 8]: | Furthermore, it follows that the maximum possible condensation rate for combined dewfall and dew-rise during nighttime is [10, 11, 7, 8]: | ||
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<math> D_f + D_r = \frac{s}{s+\gamma}\frac{Q^*}{\lambda}</math> | <center><math> D_f + D_r = \frac{s}{s+\gamma}\frac{Q^*}{\lambda}</math></center> | ||
Where | Where | ||
:Df = rate of dewfall (kg | :Df = rate of dewfall (<math>\frac{kg}{m^*s}</math>) | ||
:Dr = rate of dew rise (kg m−2 s−1) | :Dr = rate of dew rise (kg m−2 s−1) | ||
:s = slope saturation curve (Pa K−1) | :s = slope saturation curve (Pa K−1) | ||
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== Types of AWVPs == | == Types of AWVPs == | ||
Currently, there are three main methods of taking moisture out of the | |||
[[File:AWVP Flow chart.jpg]] | [[File:AWVP Flow chart.jpg]] | ||
Revision as of 19:36, 15 April 2010
Introduction
As the world’s population increases, fresh water supplies are being tapped out. Desalinization has become a necessary means of acquiring water; however current methods are usually quite costly and use fossil fuels. This is inappropriate for the developing world, where there is a serious need for increased fresh water availability. Atmospheric or atmospheric water vapor processing (AWVP) is an emerging technology in which the atmospheric water vapor is condensed and collected [1, 2, 9]. AWVP is a relatively new technology.
Advantages
- It is at an early stage in development, but has the potential to provide environmentally acceptable alternatives [12, 13] to standard water supplies.
- Many AWVP designs favor decentralization of water distribution and avoidance of huge capital costs for infrastructure [13].
- AWVP can be made appropriate, community-managed and community-maintained for developing countries [review].
- AWVP methods are competitive with desalination plants and simpler and less expensive to operate and maintain.
- The amount of water produced would vary according to installation size, and be suitable to provide potable water to individuals or even thousands of people.
- AWVP production can take place in a wide variety of locations. Thus, expensive water distribution infrastructure can be reduced or avoided.
Disadvantages
[NEEEDS TO BE DONE]
Background information
Dew Formation
Interestingly, many desert and costal plants and animals depend on this source [4]. Naturally, the phenomenon of dew formation takes place due to night sky radiation cooling. On a clear night, heat energy is radiated away from the earth’s surface into the sky. As the surface temperature drops below the dew point, water vapor will condense into droplets. Dew is formed both from water vapor in the atmosphere in the form of dewfall, as well as from water in the soil in the form of dewrise.
The amount of dewfall can be approximated from a complex turbulent fluid analysis; however it is far simpler to consider it as an energy balance problem [5]. The process of condensation releases heat, and thus raises the surface temperature.
Where
- R = net incoming radiation flux (a negative term)
- G = flux of total heat from soil to surface
- C = flux of sensible heat from air to surface
- F = flux of water vapor from air to surface
- λ = latent heat of vaporization
- Q = heat capacity of the air-grass layer per cm2
- T = mean temperature of layer
- M = heat released by vegetable metablism per cm2
Furthermore, it follows that the maximum possible condensation rate for combined dewfall and dew-rise during nighttime is [10, 11, 7, 8]:
Where
- Df = rate of dewfall ()
- Dr = rate of dew rise (kg m−2 s−1)
- s = slope saturation curve (Pa K−1)
- Q* = net radiation (W m−2)
- γ = psychrometric constant (66 Pa K−1)
- λ = latent energy for vaporization (J kg−1)
Studies have demonstrated that the above modle can accurately and reliably predict dew rates [11].
The maximum amount of dewfall possible on a given night is limited by the amount of radiative cooling possible, and is estimated to be approximately 1mm [6, 7]. In actuality, this number is usually much lower, ranging from 0.17 – 0.45 mm per night [8].
Types of AWVPs
Currently, there are three main methods of taking moisture out of the
