A comparative study of two large radiative dew water condensers
Résumé
In order to improve the yield of dew condensation from atmospheric vapor, two large (30 m2 in area) insulated plane radiative condensers, inclined at 30°, were installed in Ajaccio (Corsica island, France; latitude 41°55′N, longitude 8°48′E). Prototype P1 was elevated such that the underside was open and exposed. Prototype P2, however, was enclosed on all sides and closer to the ground. Both used a special radiative foil that enhances dew formation. The period of observation for P1 was July 22, 2000–November 11, 2001, and for P2 was December 10, 2001–December 10, 2003. All data were compared with respect to the same horizontal calibration plate of polymethylmethacrylate (Plexiglas) placed at 1 m above the ground on a sensitive recording balance. Water yield of both prototypes were compared and correlated against meteorological data (cloud cover, relative humidity, wind speed, condenser temperature and air temperature). Both prototypes exhibit improved performances when compared with the calibration plate: more dew days (+16% and +15% for P1 and P2, respectively); decrease of the humidity threshold (−3% and −4.4% for P1 and P2); increase of dew yields for wind speeds up to 3 m s−1. A model of the mass and thermal exchanges with the ambient air was used. Two adjustable parameters (heat and mass transfer coefficients) are used in the model. The values of these parameters were found larger than the values obtained in continental sites where dew forms with weak wind, thus emphasizing the peculiarities of dew formation in windy islands. When data are reduced with the calibration PMMA data, prototype P1 provided average water yields slightly larger than the enclosed prototype P2, a result that can be attributed to the influence of surface thermal radiation.