Dew Harvesting

What is it?

A passive method of extracting water from the humidity of the air, purely by condensation — with no energy input. It is a way of obtaining water (not of purifying existing, contaminated water, but of "precipitating" new water out of the air).

What is it good for?

It is useful in regions with little rainfall but significant nighttime humidity swings and clear skies (desert and semi-desert climates). Because it requires zero energy, it keeps working through any infrastructure failure — it needs neither electricity nor fuel.

The physics behind it

At night, every surface with an unobstructed view of a clear sky sheds heat toward space as long-wave infrared radiation (the effective radiative temperature of the clear night sky is around −270 °C — "colder" than anything on Earth) — this is radiative cooling. As a result, the surface can cool several degrees below the surrounding air. When the surface temperature drops below the air's current dew point, the water vapor in the air condenses onto it — this is dew.

Efficiency is governed by two material properties: emissivity within the atmospheric window (8–13 micron wavelengths) — the better a material radiates in this band, the more effectively it cools — and the wettability of the surface (on a hydrophilic surface the condensate spreads out in a thinner, more even film; on a hydrophobic surface it collects in beads).

History

An ancient, widespread practice — the Amish communities, the Mediterranean "dew ponds," and many desert cultures have used it for centuries. Research into modern, purpose-designed dew-collecting foils began in the 1990s (Daniel Beysens and the OPUR research group), aiming to find the optimal material composition for maximum nighttime cooling.

Simple version

A plain sheet of foil or mesh set out at night on a slope (so the condensed water runs off), with a collection gutter along its lower edge feeding into a storage container. No moving parts, no energy input — just the right material and an open view of the sky.

Advanced version

Purpose-designed foils optimized for radiative cooling (special polyethylene formulations) that maximize infrared emission in the atmospheric window while minimizing solar absorption during the day. Larger-area condensing systems built from multiple panels.

Industrial version

Large-scale dew-collection installations (e.g. experimental projects in Croatia and Israel), often combined with fog-net systems in coastal desert regions.

Building your own

A sloped panel (foil or mesh), a collection gutter along the lower edge, a storage container. Placement matters: an unobstructed view of the sky (shading by trees or buildings degrades radiative cooling), preferably in a spot sheltered from the wind. Test at night, under a clear sky.

Common mistakes

• An obstructed view of the sky (trees and buildings reduce radiative cooling)
• Poor material choice — some plastics have weak emissivity in the 8–13 micron atmospheric window
• Insufficient slope angle, so the condensate does not run off and evaporates before it can be collected
• Poorly sealed collection gutter, losing water to leaks

How to measure

A hygrometer (relative humidity, dew-point estimation), a surface thermometer (how far the panel cools below ambient temperature), and yield measurement (ml per m² of surface per night) — this is what makes different materials and designs comparable.

Videos

(TODO)

Downloadable PDF

(TODO)