Water from the Air

Sustainable Water from the Air

An air well or aerial well is a structure or device that collects water by promoting the condensation of moisture from air.

High mass: Passive. High-mass air wells were used in the early 20th century, but are not very effective. They use a large mass which evens out the temperature between night and day, causing warm moist air to condense on the mass which is still cool from the night.
Radiative: From the late 20th century onwards, passive low-mass, radiative collectors proved to be much more successful. 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^
Active: Basically dehumidifiers; works well, but require energy, so not generally economical. New, innovative designs seek to minimise the energy requirements of active condensers or make use of renewable energy resources

The most basic and somewhat effective version is dew collection with mulch, cover crops (e.g. grass) or horizontal mesh. The key is thermal connection with the earth to provide a cool surface for the water to condense onto, combined with gravity to transport the water deeper, and then thermal insulation from the next days sun. Mulch collects more if it has a larger surface area (e.g. straw, fine particle mulches) but retains more if it has a lower surface area (e.g. bark, large chips).

Moisture Collection via Adsorption over Day/Night cycles

Google Photo's album and links for a very low cost system inspired by research from MIT which was able to produce more than 3 cups per square yard per day. Research is ongoing.

Dew Collection

Another system is fog or dew collection. Here, the moisture already coming out of suspension in the air is simply collected and encouraged to aggregate by structures or coatings which alternate hydrophilic^ and hydrophobic^ surfaces. The water collects on the surfaces which attract water, and when that space is filled, will "overflow" or run off that surface and not stick to the surfaces which repel water. Efficiency improvements of over 10 times are said to be possible over standard surfaces.

Since saturated air is hydrophobic, a mesh works as such a surface, but has the problem of the droplets passing through the opening of the mesh and not being captured. This can be improved by charging the air with one polarity and the mesh with the other.^ Like powder coating paint.

Another improvement is to use only the vertical wires in the mesh, which avoids the droplets becoming "stuck" on the horizontal wires.^

Air carries more water as temperature increases. For example, at 80'c 1kg of air will be saturated with about 0.26 Kg more water than it will at 30'c.
0.03 Kg/m³ of vapor at 30'C and 0.29 kg/m³at 80'C