Passive-dew-condenser is a device designed to extract moisture from humid air without the use of external energy. Under a clear sky at night , the condenser surface gets cooled simply by radiative exchange with sky, becoming colder than the dew-point of the surrounding humid air. When that happens some of the vapour contained in the air condenses on its surface. The condenser surface is set at an angle from the horizontal to permit draining by gravity. When conditions are favorable condensation could occur over good part of the night yielding significant amount of water which has been tested and found potable. Water only needs to be filtered and sterilized by UV light or boiling. Samples of dew water from a part of Kutch are shown in Figure 1.

Figure 1: Dew water from Kothara - Kutch obtained using passive dew

In the interior regions of India, dew occurs only (or mostly) in winter months. In coastal arid regions such as Kutch in Gujarat, dew occurs over a period of 8 months from October to May. In fact, measurements have shown that dew occurs more frequently and in larger quantity in summer months compared to others. It also happens that good quality water is scarce in these regions. Plastics has been successfully used to develop dew harvest systems for people living in coastal villages of Kutch to enable them to harvest dew and use it as a supplementary source of water. Suitably designed large cost-effective dew condensers can enable humans too to extract moisture from air. Plastics appear to be the most promising material to achieve this. Besides the plastics, metal sheets used for roofing can also be used as condensers.

Development of condensers
The development work was spread over a three year period and has passed through three stages. In the first stage systematic measurement of daily dew fall was made at Kothara, Kutch over a one year period. In the second stage test condensers were made and tried for one year. In the third stage pilot scale condensers were made and tried.

Test
Test condensers were made using several materials including polyethylene (PE) film, polyethylene mixed with titanium-oxide and barium-sulfate (PETB) film, fiber reinforced plastic (FRP) sheet and poly-carbonate sheet. The PETB film was first developed and tried for dew condensation in France by Nilsson, Beysens and others. The film was made to specifications. Galvanized iron and aluminum sheets were also included in the trials.

Condensers made of plastic

A condenser has three components: a panel, mounting frame and collection accessories. The panel is made of two sheets bonded together with adhesive. The sheet on top is made of material being tested for its suitability to construct condensers, indicated above. This sheet is insulated with styrene foam sheet of 25mm thickness on the underside. Panels are squares of 1x1 m. Panel is mounted on metal frame made of welded angles. The collection accessories (channel and tube) are also supported on the frame. The panel is mounted with a 30 angle with the horizontal. Flow is channeled via a flexible rubber tube into a plastic bottle securely placed on the ground.

All the test condensers were installed in a coastal village - Kothara, Kutch - and daily dew condensation was measured for one whole year. Figure 3 shows the test site at Kothara.

Measurements showed that PETB film gave the highest yield {Table 1(a)} :
Monthly dew yield from north oriented Test condensers (Kothara-04-05)

Table 1(b) : Dew nights from north oriented condensers (Kothara-04-05)

Trials in Kutch confirm the findings of Nilsson and Beysens from France. Metal sheets give lower yields specially the new commercial aluminium sheets. FRP and polycarbonate also give good yields but being expensive these are not suited for suited for large practical harvest systems.
Pilot
After analysis of the year-long measurements, PETB was selected to make larger pilot condensers for further trial. Figure 4 shows the pilot PETB condenser. It consists of two basic modules, each of 9m2 surface area or 3x3 m in size. The condenser surface is made of PETB sheet of 400 micron thickness. Condenser is insulated in the same manner as were the smaller ones. The condenser modules are placed on a sand bed that has a slope of 30o from the horizontal.

Commercial
Two types of models are ready for commercialization. The smaller model is for individual families living in coastal arid areas. It is called Dew Family-20. The larger model is for communities and for commercial bottling plants. It is called Dew Community-X.
Dew Family - 20 is sized for an average output of about 20 liters per day during the season. It will be built using 300 m2 plastic (PETB) film. Total collection of dew water over the season of 8 months would be approximately 5000 liters. The system will also harvest rain. The systems could be installed at or near the user's home (Figure 5).
Dew-Community systems will be larger and will serve a community by bottling dew and selling at affordable price locally. These will be built on 'waste-land' and other locations along the coast.
Dew harvesting be an attractive option in areas where (a) dewfall occurs frequently, in appreciable quantity over a long season and (b) where there is shortage of potable water.

Figure 5(a) Roof over roof dew harvest system On a tiled roof house - Front view



Figure 5 (b) Roof over roof dew harvest system On a tiled roof house - side view