Concept
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 sustainable and renewable energy resources. All air well designs incorporate a substrate with a temperature sufficiently low so that dew forms. Dew is a form of precipitation that occurs naturally when atmospheric water vapour condenses onto a substrate. It is distinct from fog, in that fog is made of droplets of water that condense around particles in the air. Condensation releases latent heat which must be dissipated in order for water collection to continue. An air well requires moisture from the air. Everywhere on Earth, even in deserts, the surrounding atmosphere contains at least some water. According to Beysens and Milimouk: "The atmosphere contains of fresh water, composed of 98 percent water vapour and 2 percent condensed water (clouds): a figure comparable to the renewable liquid water resources of inhabited lands (12,500 km3)." The quantity of water vapour contained within the air is commonly reported as a relative humidity, and this depends on temperature—warmer air can contain more water vapour than cooler air. When air is cooled to the dew point, it becomes saturated, and moisture will condense on a suitable surface.