Condensate Harvesting is the harvest of water vapor in the air that freely condenses on surfaces, which are colder than the air temperature at your site.
When water vapor in the air (humidity) comes into contact with a colder surface, the water changes from a gas to a liquid and collects on that colder surface. This water vapor in the air that becomes liquid is referred to as condensate. Common examples are the drops of condensate that form on the outside of a cold glass of iced tea on a hot, humid day; condensate collecting on the cold coil of an air conditioner or refrigerator unit; or the condensate that forms on a metal roof cooled by the night air in early humid morning hours.
Passive condensation does not require the consumption of energy generated at a power plant to create the cool surface on which the moisture condenses. Examples include condensate forming on a night-cooled metal roof in the early hours of a humid morning, or the ancient practice of creating a small pile of cool rocks (with ample air space between the rocks – so they can cool off at night) on which condensate will form in the early humid morning hours to passively irrigate an adjoining plant. An air well is a condensate-harvesting structure that works similar to the pile of rocks.
Active condensationdoes require the consumption of energy generated at a power plant to create the cool surface on which the moisture condenses. Examples include moisture condensing on outside of a glass of an iced drink (energy had to be generated and consumed to make the ice), and condensation on an air conditioning unit (lots of energy has to be generated and consumed to run the air conditioner).
Ample condensate typically collects on the cold coil of air conditioner and refrigerator units, which is often wastefully discarded via a drain or pipe (see photo below). The hotter and more humid the climate, and/or the more moisture (such as from respiring and perspiring people) in an air-conditioned building; the more condensate air conditioners, refrigerator units, ice machines, and freezers will discharge.
How much condensate does an air conditioner discharge:
In a dry climate or season:
a home air conditioner can generate 0.25 gallons (1 liter) of condensate/day
a large commercial air conditioner can generate 500 gallons (1,900 liters)/day
In a humid climate or season:
a home air conditioner can generate 18 gallons (68 liters) of condensate/day
a large commercial air conditioner can generate 1,000 gallons (3,750 liters) or more/day
Quality of condensate
This condensate is basically distilled water, containing low amounts of minerals such as salt, but may contain bacteria. Condensate is great for watering plants, but don’t drink it unless you adequately treat the water. Comparatively, municipal or well water is often relatively high in salt — which can be toxic to plants and soil life — thus condensate can help dilute those salts, making for a healthier growing environment (though salt-free rainwater does an even better job of this).
How to harvest and use condensate
To irrigate with condensate simply direct the condensate (ideally with free gravity—no pumps) to vegetation. It typically should not puddle on the surface, but rather quickly infiltrate into the root zone of the plants to reduce the chance of mosquitoes, bacteria growth in the water, or water loss to evaporation. More organic matter, diverse soil life, and vegetation in the soil increase the rate of infiltration into the soil.
I like to plant a relative oasis of plants (local natives are the easiest to succeed with) at the “spring” where condensate discharge pipes outlet into the landscape. And I like to first route the pipes and outlet where such an oasis can provide the maximum benefits (such as passive cooling, a verdant entryway, etc.).
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See the book Rainwater Harvesting for Drylands and Beyond, Volume 1 3rd Edition to estimate how much condensate your air conditioner may discharge, the water needs (expenses) of potential plants for your condensate “spring” to grow a sustainable oasis in balance with your free on-site waters’ budget (income), and how to situate vegetation for free summer cooling and winter heating of associated buildings and outdoor spaces.
Other free on-site waters can include rainwater, stormwater, snow, fog, greywater, and dark greywater. The new, full-color editions of Rainwater Harvesting for Drylands and Beyond Volumes 1 and 2 show you how to harvest all of these to great effect, along with case studies of success.