Page - 80 - in Clean Water Using Solar and Wind - Outside the Power Grid

80 Clean Water Using Solar and Wind: Outside the Power Grid low investment costs, low maintenance and low energy requirements. It is certainly environmentally friendly. However, an SSD system has low productivity; but for small-scale water demands, those of a small village or a household, SSD is a viable option due to the high cost of water transportation. It also offers a complement to electric power- driven water supply so that the available electric power can be used for other purposes. The key function of an SSD system can be illustrated by a typical greenhouse. A constant volume of brackish or saline water is enclosed in a basin with a dark bottom. The basin can be constructed of concrete or some fibre-reinforced plastic. The roof is a transparent material like glass or plastic. The brackish or saline water is fed to the basin. The distillate is collected at the lower end of the roof. As the sunrays pass through the glass roof they are absorbed by the blackened bottom and will heat the water. The vapour pressure will increase and the water vapour is condensed on the underside of the roof. It will run down into troughs that conduct the distilled water to a storage basin. The process is a direct application of the greenhouse effect. Furthermore, the roof encloses all the vapour, prevents its loss and keeps the wind from reaching and cooling the contaminated water. As the saline water is distilled there will be increasingly higher salt concentrations in the basin. Therefore, the systems need flushing, which preferably should be done during the night. There are several designs of SSD systems and some of the more popular designs are described and reviewed by Kalogirou (2005) and Al-Karaghouli and Kazmerski (2011). A typical still production is about 3–4 l/m2 per day (Daniels, 1977), but 8–10 l/m2 per day has been reported in simple and practical designs. The real advantage of the SSD system is its simplicity and low cost. Since it is less efficient it needs much more space than the desalination methods described in 5.3 to produce a given volume of water. 6.2 SOLAR WATER HEATING Solar thermal collectors or solar water heating (SWH) have been used for a long time as heating sources for water. The technology is simple and extensively proven. For cooking this may remain the backbone of Downloaded from https://iwaponline.com/ebooks/book-pdf/520710/wio9781780409443.pdf by IWA Publishing user