page_title: Air well (condenser)
section_title: Radiative
hierarchical_section_title: Air well (condenser) / Types / Radiative
caption_attribution_description: English: Properties that have metal roofing as this one , can be used to harvest dew water simply by adding gutters and for increased output a layer of insulation underside. Without the insulation the output is nearly half of that from plastc condensers.
context_page_description: An air well or aerial well is a structure or device that collects water by promoting the condensation of moisture from air. Designs for air wells are many and varied, but the simplest designs are completely passive, require no external energy source and have few, if any, moving parts.
  Three principal designs are used for air wells, designated as high mass, radiative, and active:
  High-mass air wells: used in the early 20th century, but the approach failed.
  Low-mass, radiative collectors: Developed in the late 20th century onwards, proved to be much more successful.
  Active collectors: these collect water in the same way as a dehumidifier; although the designs work well, they require an energy source, making them uneconomical except in special circumstances. New, innovative designs seek to minimise the energy requirements of active condensers or make use of renewable energy resources.
context_section_description: A radiative air well is designed to cool a substrate by radiating heat to the night sky. The substrate has a low mass so that it cannot hold onto heat, and it is thermally isolated from any mass, including the ground. A typical radiative collector presents a condensing surface at an angle of 30° from the horizontal. The condensing surface is backed by a thick layer of insulating material such as polystyrene foam and supported 2–3 metres (7–10 ft) above ground level. Such condensers may be conveniently installed on the ridge roofs of low buildings or supported by a simple frame. Although other heights do not typically work quite so well, it may be less expensive or more convenient to mount a collector near to ground level or on a two-story building.
  A 550 square metres (5,900 sq ft) radiative condenser illustrated to the left is built near the ground. In the area of northwest India where it is installed dew occurs for 8 months a year, and the installation collects about 15 millimetres (0.59 in) of dew water over the season with nearly 100 dew-nights. In a year it provides a total of about 9,000 litres (2,000 imp gal; 2,400 US gal) of potable water for the school which owns and operates the site.
  Although flat designs have the benefit of simplicity, other designs such as inverted pyramids and cones can be significantly more effective. This is probably because the designs shield the condensing surfaces from unwanted heat radiated by the lower atmosphere, and, being symmetrical, they are not sensitive to wind direction.
  New materials may make even better collectors. One such material is inspired by the Namib Desert beetle, which survives only on the moisture it extracts from the atmosphere. It has been found that its back is coated with microscopic projections: the peaks are hydrophilic and the troughs are hydrophobic. Researchers at the Massachusetts Institute of Technology have emulated this capability by creating a textured surface that combines alternating hydrophobic and hydrophilic materials.
caption: Structures with metal roofing, such as this one, can be used to harvest dew water simply by adding gutters and, for increased output, a layer of insulation underside. Without the insulation the output is nearly half of that from plastic condensers.
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