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Roofs as dew collectors: III. Special Polyethylene foil on a school in Sayara (NW India)
Abstract
The arid, coastal region of northwest India is chronically short of drinking water. At the same time, measurements have shown the presence of significant dew resources. To access dew for human, the use of large condensers will be required. Mounting large condensers on frames is expensive in windy areas. One way to reduce the costs is to mount the condensers on roofs of large buildings-Condenser-on-Roof or (CoR). Many schools and warehouses provide such an opportunity. A project was accordingly carried out to demonstrate this. A 360 m 2 roof of a school was equipped with a special condensing foil similar to Nilsson' and OPUR' in village Sayara near the coast. Condensation does not occur on bare surfaces of reinforced concrete of which the roof was made. It was therefore, thermally isolated. The dew yield was measured during the dry season-October 2005 to May 2006. The objective was to determine the quantity of useful water that can be harvested and to compare it with yield from another large roof nearby in village Suthari, made of (un-insulated) galvanized iron sheets. The metal sheet itself was used as condenser without the foil. During the period, the cumulative dew yield from the school roof was 3107 L (8.6 mm). This value is nearly two times of that obtained on galvanized iron roof (4.7 mm). In addition, the use of thermal insulation led to reduced heat gain by the class rooms during the day, making these more comfortable. The yield from iron roof was less but so was the cost of installation which only required gutters. Both the examples indicate the potential of using existing roofs to make cost-effective dew harvesters.
... Size is constrained by the roof area, pitch is often less steep than required (30°) for dew collection, elevation is higher than optimal and orientation unchangeable. The largest working COR type system built in the area at the time had an area of 360 m 2 (Sharan et al., 2007c). The dew yield was nearly 50% of the yield from the 1 m 2 measuring units mounted on frames. ...
... As mentioned earlier, large working installations tend to yield less dew per unit area than small units. COR at the small nearby village of Sayara (Sharan et al., 2007c) yielded nearly half that of the smaller units placed nearby, the same ratio as in this study. The COG Panandhro yield was 41.8% less than the small units on the site. ...
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 m2 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 m2 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).
... High yields have been obtained by Beysens et al. (2013) with periodical structures of origami shapes. Roofs condensing dew water have been built and operated successfully by Beysens et al. (2007) and Sharan et al. (2007a; 2007b). Large dew condensers have been developed and field-tried in India in the recent years by Sharan (2011) and Sharan et al. (2011). ...
This study presents experimental and theoretical results of winter dew harvest in Mexico City in terms of condensation rate. A simplified theoretical model based on a steady-state energy balance on a radiator-condenser was fitted, as a function of the ambient temperature, the relative humidity and the wind velocity. A glass sheet and aluminum sheet white-painted were used as samples over the outdoor experiments. A good correlation was obtained between the theoretical and experimental data. The experimental results show that there was condensation in 68% of the winter nights on both condensers. The total winter condensed mass was 2977 g/m(2) and 2888 g/m(2) on the glass sheet and aluminum sheet white-painted, respectively. Thus, the condensed mass on the glass was only 3% higher than that on the painted surface. The maximum nightly dew harvests occurred during December, which linearly reduced from 50 g/m(2) night to 22 g/m(2) night as the winter months went by. The condensation occurred from 1:00 a.m. to 9:00 a.m., with maximum condensation rates between 6:00 a.m. and 7:00 a.m. The dew harvest can provide a partial alternative to the winter water shortage in certain locations with similar climates to the winter in Mexico City, as long as pollution is not significant.
This paper describes the development of technology to harvest (condense and collect) dew water for use of people living in coastal area of Gujarat province, north-west India. Potable water is severely scarce in the area, such that extra supply of even tens of liters a day could add to water security of the households. A R&D program was initiated in the year 2001-02, after it was noticed that dew occurred more frequently and in larger quantity here than inlands. Dew water is extracted from air using condensers made of special plastic film which cool rapidly at night by radiating heat to atmosphere. Dew condensing efficiency of the plastic film appears to be comparable to blades of grass, natural vegetation. A four year R&D program led to design of large working systems after testing of prototypes and pilot units. Large dew harvest systems have been erected on roofs (condenser-on-roof), open ground (condenser-on-ground). Technology is simple, uses material available in the area and is amenable to do-it-yourself procedure.
This paper describes the development of dew harvesting systems for use in the semi-arid coastal region of northwest India. These systems were developed to ameliorate the drinking water problem in the region, especially for the people living near the coast where groundwater is of poor quality and surface sources are scarce. Although the amount of dew is much less (20-30 mm) than the rains (200-300 mm), it is a more consistent water source. Dew nights number approximately 100 while rainy days number 15-20. Dew occurs over a seven month period (October to April), while rain occurs over a four month period (June -September). Although engineered specifically to harvest dew, the designed systems also harvest rain, providing varying amounts of potable water throughout the year. A four-year R&D program led to development of three types of systems -condenser-on-roof (CoR), condenser-on-ground (CoG) and Roof-as-Condenser (RaC). The CoR and CoGs systems employ condensers made of plastic film insulated on the underside. CoRs are constructed over the roof of buildings while CoGs are constructed on open ground. The RaCs use the metal roof of buildings as the condenser itself. The CoR and CoGs gave higher output but required higher investment. The RaCs gave lower output but required only a small investment in collection and storage. Examples of working installations are presented. The benefits to the region, the learning accrued, and the partnerships created in the course of work are briefly discussed.
Kothara, the site of our greenhouse installation is 85 km west of Bhuj (23° 15'N, 69° 48'E). Dew condensation on greenhouse cladding and assorted other surfaces at the site was frequently noticed. Accordingly, arrangement was made to measure the quantity of condensation. Greenhouse is a saw-tooth structure of 6 m span and 20 m length. It is east-west oriented. Area of the slightly curved roof surface is 124 m2• Condensate flows down the roof and north face into a rectangular channel set at the base along the length. A pit was dug into the ground at each end of the channel. A bucket was placed in each pit to collect the condensate. Water collected in the buckets was measured each morning for 365 days continuously. There was only one day when it had rained. Data of that day was excluded.
Atmospheric humidity can be condensed as dew and used for example in small-scale irrigation. In arid locations, the most favourable conditions for dew collection persist in the late night and around sunrise. We study the possibility to use a dew collector for condensing atmospheric water vapour by exploiting the effect of radiative cooling. In particular, we study pigmented polymer foils with high solar reflectance and high thermal emittance. Suitable pigments are a mixture of TiO2 and BaSO4 particles or a novel SiO2/TiO2 composite. We calculate the condensation rate under different climatic conditions and report on initial field tests.
The recovery of clean water from dew has remained a longstanding challenge in many places all around the world. It is currently believed that the ancient Greeks succeeded in recovering atmospheric water vapour on a scale large enough to supply water to the city of Theodosia (presently Feodosia, Crimea, Ukraine). Several attempts were made in the early 20th century to build artificial dew-catching constructions which were subsequently abandoned because of their low yield. The idea of dew collection is revised in the fight of recent investigations of the basic physical phenomena involved in the formation of dew. A model for calculating condensation rates on real dew condensers is proposed. Some suggestions for the ‘ideal’ condenser are formulated.
Moisture in the air can be condensed as dew and used for drinking and irrigation. The radiative cooling properties of polymer foils can enhance the performance of dew collecting surfaces. The main restrictions in condensing water in warm and arid locations are climatic factors, the dew collector design, and the optically selective and adhesive properties of the condensing surface itself. This paper concerns observations of dew formation on radiatively cooled pigmented polyethylene foils. The experiments were carried out in Sweden and in arid Dodoma, Tanzania. The results are in agreement with thermodynamical calculations, though the variation is large in the daily measured dew water volumes. This variation is caused by the hourly and daily changes in wind-speed, cloud cover, dry bulb temperature, and dew point temperature. The results are compared with earlier outdoor observations in Tanzania.