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In order to establish data to be used for comparison with future evolution related to climate change, physical, chemical and biological characteristics of dew and rainwater as collected during the 2005 dry season (plus a few data during the 2004 dry season) are reported. They have been collected in two characteristic tropical islands of French Polynesia, Tikehau (TKH), a low-lying coral atoll in the Tuamotu Archipelago, and the mountainous Tahiti island at the University of French Polynesia (TAH). Trade winds dominate the trajectory of air masses, ensuring constant temperature and humidity to the lower layers of the atmosphere where dew forms. In addition to the comparison of dew yields with a physical model using simple meteorological data (air and dew point temperatures, windspeed, cloud cover), the following parameters were studied: pH, electrical conductivity (EC), total dissolved solids (TDS), total hardness, suspended matter, ion concentration with major cations (Ca2+, K+, Na+, Mg2+, NH4+) and major anions (Cl−, SO42−, NO3−), reviviscible aerobe microorganisms and compared to two Polynesian spring waters (“Eau Royale” and “Vaimato”). Dew, with a chemical composition mainly consisting of Na+, Ca2+, Mg2+, Cl− and SO42−, exhibits much higher ion concentration than rain and compares well with the composition of local spring waters. The values of pH, EC and TDS are larger in dew than in rainwater. At TAH, the volume weighted mean (VWM) pH values of dew were 6.05 in 2004 and 5.23 in 2005, larger than the pH of rain (4.69). The VWM dew EC (at TAH, 203 μS·cm−1 in 2004 and 237 μS·cm−1 in 2005; 321 μS·cm−1 at TKH in 2005) and dew TDS (152 mg·L−1 at TAH and 225 mg·L−1 at TKH) were higher than the corresponding quantities in rain. Mean total hardness (TH) values of dew water are much higher than in rainwater (2.8 for dew versus 0.5 for rain at TAH and 5.7 for dew versus 0.5 for rain at TKH). Ions Na+, Mg2+ and Cl− are clearly of sea origin while the presence of Ca2+ is due to coral particles. From their chemical characteristics, dew and rainwater could be used as an alternative source of water in dry season but, due to the presence of reviviscible aerobe microorganisms at 22 °C and 36 °C (>300 CFU·mL−1), water must be disinfected to be potable.

The coastal region of south Morocco presents a chronically shortage of drinkable and fresh water. In 2007, only 49 mm of rain was recorded. However, measurements in the same year showed that the dew yield was on order of 40% of rain fall. In order to recover dew water in addition to rain water, a small village (Idouasskssou), 8 km from Mirleft and the Atlantic Ocean, was equipped with three pilot condensers of 136 m(2) total surface area. A local organization (IMIRJANE) collaborated to ensure a good integration of the project by the village inhabitants. All materials were from local shops. Only the special radiative and hydrophilic coating was coming from non local resources (www.opur.fr). Dew water production during six months, from 15 December. 2008 to 31 July. 2009 (137 dew events, 47% of days) was more than 3,800 L (28 mm, 0.2 mm/dew day). The devices not only condense dew water, they also harvest rain and fog, thus providing to the population a valuable water resource (during fall 2009, the collectors were the only source of water of the village).

The world's largest dew and rain collecting system, comprised of ridge-and-trough modules, was constructed in March 2006 at Panandhro in the semi-arid area of Kutch (NW India). The main goals were (i) to collect dew on a scale that could be beneficial to the local population (ii) to determine the efficiency of this new module shape, (iii) to determine whether results obtained from small measurement condensers can be projected to large condensers, (iv) to apply a computational fluid dynamic simulation to improve the condenser set-up. Preliminary studies performed with four standard plane condensers of 1 m(2) surface area, inclined 30 from horizontal, identified Panandhro as a promising site. The cumulated dew water during 192 days was 12.6 mm with a maximum of 0.556 mm/night. A large dew condenser (850 m(2) net total surface) was designed with 10 ridge-and-trough modules. The ridges are trapezoidal, 33 m long, 0.5 m wide at the top, 2.2 m wide at the base and sloping 30 from horizontal. The depth of the troughs between the ridges is 0.5 m. A 2.5 cm thick polystyrene foam rests on the surface as insulation with a radiative foil on top (similar to that developed by OPUR, see www.opur.fr).Numerical simulations using the computational fluid dynamic software PHOENICS were performed. The most profitable orientation was with the condenser oriented back to the wind direction, a configuration that lowers the wind velocity near the foil due to the combination of free convection and wind recirculation flows.A comparison of water yields over one year of measurements between four 1 m(2) plane condensers and a 850 m(2) ridge condenser showed a 42% lower yield on the large condenser. The difference is attributed mainly to folds in the plastic foil allowing water to fill the central ridge, thus decreasing radiative cooling. The output for 2007 was 6545 L, corresponding to 7.7 mm/day on average. The largest event was 251.4 L/night (0.3 mm). Such a condenser can also collect rain (and, to a lesser extent, fog). Chemical and biological analyses showed that dew water, once filtered and bottled, could be used for drinking after a light treatment to increase the pH. The price of this water could be lowered to reach 30% (dew only) or even 3% (dew plus rain) of the market prize. (C) 2011 Elsevier B.V. All rights reserved.

Radiation-cooled dew water condensers can serve as a complementary potable water source. In order to enhance passive dew collection water yield, a Computational Fluid Dynamics (CFD) software, PHOENICS, was used to simulate several innovative condenser structures. The sky radiation is calculated for each of the geometries. Several types of condensers under typical meteorological conditions were investigated using their average radiating surface temperature. The simulations were compared with dew yield measurements from a 1m2 30°-inclined planar condenser used as a reference. A robust correlation between the condenser cooling ability and the corresponding dew yield was found. The following four shapes were studied: (1) a 7.3m2 funnel shape, whose best performance is for a cone half-angle of 60°. Compared to the reference condenser, the cooling efficiency improved by 40%, (2) 0.16m2 flat planar condenser (another dew standard), giving a 35% lower efficiency than the 30° 1m2 inclined reference condenser, (3) a 30m2 30°-inclined planar condenser (representing one side of a dew condensing roof), whose yield is the same as the reference collector, and (4) a 255m2 multi-ridge condenser at the ground surface provided results similar to the reference collector at wind speeds below 1.5ms−1 but about 40% higher yields at wind speeds above 1.5ms−1.

An assessment of the potential for dew water to serve as a potable water source during a rainless season in a humid tropical climate was carried out in the Pacific islands of French Potynesia. The climate of these islands, in terms of diurnal and seasonal variations, wind and energy balance, is representative of the climate of the tropical Atlantic and Pacific oceans. Measurements were obtained at two characteristic sites of this region; a mountainous island (Punaauia, Tahiti Island) and an atoll (Tikehau, Tuamotu Archipelago). Dew was measured daily on a 30 degrees tilted, 1 m(2) plane collector equipped with a thermally insulated radiative foil. In addition, an electronic balance placed at 1 m above the ground with a horizontal 0.16 m(2) condensing plate made of PotyTetraFluoroEthylene (Teflon) was used in Tahiti. Dew volume data, taken during the dry season from 16/5/2005 to 14/10/2005, were correlated with air temperature and relative humidity, wind speed, cloud cover and visible plus infrared radiometer measurements. The data were also fitted to a model Dew formation in such a tropical climate is characterized by high absolute humidity, weak nocturnal temperature drop and strong Trade winds. These winds prevent dew from forming unless protected e. g. by natural vegetal windbreaks. In protected areas, dew can then form with winds as large as 7 m/s. Such strong winds also hamper at night the formation near the ground of a calm and cold air layer with high relative humidity. As the cooling power is tower than in the Mediterranean islands because of the high absolute humidity of the atmosphere, both effects combine to generate modest dewyields. However, dew events are frequent and provide accumulated amounts of water attractive for dew water harvesting. Slight modifications of existing rain collection devices on roofs can enhance dew formation and collection. Dew harvesting thus appears as an attractive possibility to provide the local population with a complementary - but on occasion, essential - water resource. (C) 2008 Elsevier B.V. All rights reserved.

Harvesting condensed atmospheric vapour as dew water can be an alternative or complementary potable water resource in specific arid or insular areas. Such radiation-cooled condensing devices use already existing flat surfaces (roofs) or innovative structures with more complex shapes to enhance the dew yield. The Computational Fluid Dynamic - CFD - software PHOENICS has been programmed and applied to such radiation cooled condensers. For this purpose, the sky radiation is previously integrated and averaged for each structure. The radiative balance is then included in the CFD simulation tool to compare the efficiency of the different structures under various meteorological parameters, for complex or simple shapes and at various scales. It has been used to precise different structures before construction. (1) a 7.32 m^2 funnel shape was studied; a 30 degree tilted angle (60 degree cone half-angle) was computed to be the best compromise for funnel cooling. Compared to a 1 m^2 flat condenser, the cooling efficiency was expected to be improved by 40%. Seventeen months measurements in outdoor tests presented a 138 % increased dew yield as compared to the 1 m^2 flat condenser. (2) The simulation results for 5 various condenser shapes were also compared with experimental measurement on corresponding pilots systems: 0.16 m^2 flat planar condenser, 1 m^2 and 30 degree tilted planar condenser, 30 m^2 and 30 degree tilted planar condenser, 255 m^2 multi ridges, a preliminary construction of a large scale dew plant being implemented in the Kutch area (Gujarat, India).

The present paper discusses the fog and dew water collection in Croatia. Zavizan, the highest meteorological station in Croatia(1594m) is chosen for collecting of fog water with a standard fog collector (SFC). The highest daily collection rate was 27.8 L / m2. The highest daily collection rate in days without rain was 19.1 l/m2. Dew is also a noticeable source of water, especially during the drier summer season. Dew condensers in Croatia have been installed on the Adriatic coast (Zadar) and islands Vis and Bisevo. We report and discuss the data collected since 2003. In the small Bisevo island, a special roof has been designed to improve the formation and collection of dew on a house. Data from April 2005 will be presented and discussed.

To determine to what extent dew water is potable without further treatment, a thorough set of chemical and biological analyses were performed on 10 samples of dew water collected on a large scale radiative collector (29.83 m(2)) in Ajaccio (Corsica Island, France), between 21 May 2002 and 5 Mar. 2003. Samples were collected following four protocols according to the dew volume amount and 48 parameters (ions, minerals, and bacteria) were analyzed and compared to French and European Union legislation and also World Health Organization (WHO) recommendations. Aluminum and Fe were the main pollutants whose concentrations were significantly larger than recommended. Their presence is due to local deposition of aerosols coming from the Sahara (a characteristic of the Mediterranean basin). A large number of biologically cultivable microorganisms were found, together with bacteria typical of fecal contamination. For dew water to be potable with respect to present legislation at the Ajaccio site, it should be disinfected and treated for turbidity.

In many regions and geographical settings, dew water collection can serve as a water source, supplementing rain and fog water collection. This is particularly useful when precipitation is low or lacking, especially in remote areas and islands in the dry season. A project called Dew Equipment for Water (DEW) was initiated for a 15.1m2 roof in the island of Biševo (Croatia), equipped with commercial plastic cover selected for its superior dew collection properties. Measurements of both rain and dew water will be performed over several years and data will be correlated with meteorological data collected in situ. Preliminary measurements during the period 21 April–21 October 2005 showed that dew water contributed significantly, 26% of the total collected water.

We report on a 1-year investigation (15 January 2002–14 January 2003) in Bordeaux, France, comparing the quality of dew water with respect to rain water. The following physico-chemical and bacteriological properties of dew and rain water were measured: pH, electric conductivity, cations (Na+, K+, Ca2+, Mg++, Zn++, Cu++), anions (Cl−, SO42−, NO3−, NO2−), hardness (TH, calcical, magnesial, permanent), complete alkalimetric title, dry residue and number of colony-forming unities (CFU) at 22 and 36°C. The CO32−, HCO3−, HPO42− concentrations were found negligible. The ionic concentrations are in general lower in dew than in rain with NO2− as a noticeable exception. The mean rain pH (5.4) is lower than dew pH (6.3). The major ions are from the nearby Atlantic Ocean (within 50km). Average ion concentrations are found below the World Health Organization (WHO) limit requirements for potable water; dew composition is close to low mineralized commercial spring waters for the analyzed ions. The biological analyses are concerned with CFU at 22 and 36°C. Dew is seen to exceed on various occasions the WHO limits.

Atmospheric vapour condensation and harvesting is proved to be an alternative or complementary potable water resource in several specific arid or insular geographic areas. Last tests were carried out in Vignola (Corsica Island, France) on small (1 m2) and large scale (30 m2) planar condensers. Nowadays, priority is given to real structures implementation as roofs and very large condensers, or innovative structures with enhanced dew yields like funnel shapes*. The computational fluid dynamic - CFD - software PHOENICS 3.5 has been programmed and applied to the radiative cooled condensers. The sky radiation is integrated and averaged for each structure. The radiative balance is then included in the simulation tool that permits to compare different structures efficiencies: for various meteorological parameters, for complex or simple shapes and for various scales. The program has been validated with experimental data of various pilot systems tested in outdoor. Then, it has been used to precise a 7.32 m2 funnel shape before realization. A 30deg tilted angle was computed to be the best compromise for funnel cooling. Eight months measurements in outdoor tests presented 137.04 % increased dew rate and total collected amount. Max dew amount was 3080 ml in one night, correspondent to a 0.472 mm dewfall. At least, the program will soon be used to expect dew yields of a several thousands square meter condenser implemented in the Kutch (Gujarat, India).