Planar Dew Condensers

Abstract

In a context of increasing difficulty of access to fresh water, harvesting by passive means the atmospheric water vapor is a welcome supplement. A condensing plane with a tilt angle θ with horizontal is the simplest configuration for passive condensation and water collection. The present study is concerned with the main parameters that control dew condensation on a plane, that is radiative cooling and convective heat exchange with air.  Since cooling power is seen to decrease significantly when θ exceeds 20 -30° while it is the opposite for collection efficiency, the optimum θ-value is  around 30°. The effects of wind speed and wind orientation are numerically and experimentally investigated on a 30° tilted, 1 m2 planar condenser in a site (Ajaccio airport, France) where the wind orientation is well characterized, SW during the day and NE during the night. Surface temperatures are mapped by an IR camera and thermocouples. Dew volume is measured by a pluviometer and by wiping the condensing surface; it is compared with the value obtained on a horizontal disk protected by fairings used as a reference condenser. Plane/air heat transfer coefficients and dew volumes are very dependent on the condenser-wind orientation, a key factor. In particular, the condensed dew mass is larger for frontward wind than for backward wind, remaining however less than given by the reference condenser, by 30% and 50%, respectively. This study has thus consequences on the use of planar condensers for measuring dew and on the design of future dew condensers.

Keywords: Dew water harvesting, Dew measurement, Air flow numerical simulation, Heat exchange, Mass exchange