Salt water is heated up by solar energy in a basin or at an wetted absorber surface; the water evaporates and condensates on a transparent colder material that should be opaque to long-wave radiation (Figs. 15.1-15.4). The condensate runs down on the sloped surface into a gutter. The aim is to use as much solar energy as possible, and to design a simple and cost-effective construction with low maintenance and running costs. Only commercially available construction components, if possible greenhouse components, should be used for the design. A completely sealed system and good insulation to the soil are prerequisites for high efficiency. Losses are due
C. von Zabeltitz, Integrated Greenhouse Systems for Mild Climates,
DOI 10.1007/978-3-642-14582-7_15, © Springer-Verlag Berlin Heidelberg 2011
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Fig. 15.1 The experimental type 1 consists of a plastic-film structure with a No – Drop film at the south side and a white PE film at the north side. The saltwater basin is covered with a black film and insulated from the soil at the bottom |
to reflection at the covering material and water surface, as well as due to leakage of the structure.
The productivity of desalinated water [l/(m2 day)] of the four systems depending on solar radiation is shown in Fig. 15.5. The active system type 3 had the highest productivity. The additional absorber inside the water basin of type 4 did not improve productivity. It is more suitable if the solar radiation gets through the whole water layer and is absorbed on the black film at the bottom. A part of the solar radiation will be absorbed directly by the water already.
The passive system 2 had a relatively good productivity. Type 1 with the plastic – film cladding had the worst productivity, and is not suitable for desalination systems, because the plastic film flutters through wind influence, and the condensed drops fall back into the saltwater basin.
Desalination systems do not work only during daytime, but a significant amount of water will be evaporated and condensed at night, because the water warms up during the day, evaporates at night and condenses on the colder cladding material. Figure 15.6 shows the condensed water production for day and night over a period of 48 days from end of May to beginning of July in Germany. The night-time production is higher than in daytime in the active systems.
Figure 15.7 shows the course of the temperatures for a 1-day period. The maximum water temperature of type 3 gets up to 50°C, while the temperature of
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Fig. 15.2 The experimental type 2 consists of a vertical back wall covered by an aluminium film for reflection of sun radiation, and an inclined glass roof. Black absorbent cloths were positioned vertically and parallel at 10 cm distance in the basin for absorption of solar radiation. One part of the black cloths is positioned above the water surface. Water can rise up by capillarity and keep the cloths permanently wet. In thatway, it is possible to significantly enlarge the evaporation surface. Type 1 and type 2 are passive systems without any water pump |
type 4 is lower than 45°C. The water temperatures correspond to the total productivity of the systems.
In addition, another passive system was designed and investigated, consisting of a water basin and two glass panes, 1.74 m long, on the south and north roof (Figs. 15.8 and 15.9). The productivity was 3 l/m2 day for a global radiation of
5.9 kWh/m2 day. Thus, the productivity was higher than in the other passive systems.
The following requirements should be fulfilled for simple solar desalination systems:
• The air volume in the system should be as small as possible.
• The construction components should be outside and not inside the system.
• The system must be absolutely sealed.
• The salt water should reach high temperatures.
• An air circulation inside can improve the productivity.
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Fig. 15.3 Experimental type 3. The salt water is pumped and distributed over a black irrigation cloth at the back wall of the active system |
