Classification of Solar Distillation Systems

On the basis of various modifications and modes of operation introduced in conventional solar stills, solar distillation systems are classified as passive or active. In active solar stills, an extra-thermal energy by external equipment is fed into the basin of a passive solar still for faster evaporation. The external equipment may be a collector-concentrator panel, waste thermal energy from any industrial plant, or a conventional boiler. If no such external equipment is

Basin-type design

Single-sloped cover design

Basin-type design

Single-sloped cover design

«IMiiilM;

Greenhouse-type design

Inflated plastic cover design

«IMiiilM;

Greenhouse-type design

Inflated plastic cover design

Inclined glass cover design

Inclined glass cover design

V-shape plastic cover design

FIGURE 8.2 Common designs of solar stills.

used, then that type of solar still is known as a passive solar still. Types of solar stills available in literature are conventional solar stills, a single-slope solar still with passive condenser, a double-condensing chamber solar still, a vertical solar still (Kiatsiriroat, 1989), a conical solar still (Tleimat and Howe, 1967), an inverted absorber solar still (Suneja and Tiwari, 1999), and a multiple-effect solar still (Adhikari et al., 1995; Tanaka et al., 2000a; 2000b).

Other researchers used different techniques to increase the production of stills. Rajvanshi (1981) used various dyes to enhance performance. These dyes darken the water and increase its solar radiation absorptivity. With the use of black napthalamine at a concentration of 172.5 ppm, the still output could be increased by as much as 29%. The use of these dyes is safe because evaporation in the still occurs at 60°C, whereas the boiling point of the dye is 180°C.

Akinsete and Duru (1979) increased the production of a still by lining its bed with charcoal. The presence of charcoal leads to a marked reduction in start-up time. Capillary action by the charcoal partially immersed in a liquid and its reasonably black color and surface roughness reduce the system thermal inertia.

Lobo and Araujo (1978) developed a two-basin solar still. This still provides a 40-55% increase in the freshwater produced as compared to a standard still, depending on the intensity of solar radiation. The idea is to use two stills, one on top of the other, the top one made completely from glass or plastic and separated into small partitions. Similar results were obtained by Al-Karaghouli and Alnaser (2004a; 2004b), who compared the performance of single- and double-basin solar stills.

Frick and Sommerfeld (1973), Sodha et al. (1981), and Tiwari (1984) developed a simple multiple-wick-type solar still, in which blackened wet jute cloth forms the liquid surface. Jute cloth pieces of increasing lengths were used, separated by thin black polyethylene sheets resting on foam insulation. Their upper edges were dipped in a saline water tank, where capillary suction provided a thin liquid sheet on the cloth, which was evaporated by solar energy. The results showed a 4% increase in still efficiency above conventional stills.

Seawater

Wick Distillation

Distilled water OUT Brine OUT

Ínsuiatlon

FIGURE 8.3 Schematic of a cascaded solar still.

Seawater

Ínsuiatlon

FIGURE 8.3 Schematic of a cascaded solar still.

Distilled water OUT Brine OUT

Evidently the distance of the gap between the evaporator tray and the condensing surface (glass cover) has a considerable influence on the performance of a solar still that increases with decreasing gap distance. This led to the development of a different category of solar stills, the cascaded-type solar still (Satcunanathan and Hanses, 1973). This consists mainly of shallow pools of water arranged in a cascade, as shown in Figure 8.3, covered by a slopping transparent enclosure. The evaporator tray is usually made of a piece of corrugated aluminum sheet (similar to the one used for roofing) painted flat black. Thermodynamic and economic analysis of solar stills is given by Goosen et al.

(2000). Boeher (1989) reported on a high-efficiency water distillation of humid air with heat recovery, with a capacity range of 2-20 m3/d. Solar still designs in which the evaporation and condensing zones are separated are described in Hussain and Rahim (2001) and El-Bahi and Inan (1999). In addition, a device that uses a "capillary film distiller" was implemented by Bouchekima et al.

(2001) and a solar still integrated in a greenhouse roof was reported by Chaibi (2000). Active solar stills in which the distillation temperature is increased by flat-plate collectors connected to the stills are described by Kumar and Tiwari (1998), Sodha and Adhikari (1990), and Voropoulos et al. (2001).

8.3.2 Performance of Solar Stills

Solar stills are the most widely analyzed desalination systems. The performance of a conventional solar distillation system can be predicted by various methods, such as computer simulation, periodic and transient analysis, iteration methods, and numerical methods. In most of these methods, the basic internal heat and mass transfer relations, given by Dunkle (1961), are used.

Dunkle's (1961) procedure is summarized by Tiwari et al. (2003). According to this procedure, the hourly evaporation per square meter from a solar still is given by

where

Pw = partial vapor pressure at water temperature (N/m2). Pg = partial vapor pressure at glass temperature (N/m2). hcw = convective heat transfer coefficient from water surface to glass (W/m2-°C)

The partial vapor pressures at the water and glass temperatures can be obtained from Eq. (5.21). The convective heat transfer coefficient can be obtained from h d

k where d = average spacing between water and glass surfaces (m). k = thermal conductivity of humid air (W/m-°C). C = constant. " = constant.

Gr = Grashof number (dimensionless). Pr = Prandl number (dimensionless).

The dimensionless quantities are given by

Solar Stirling Engine Basics Explained

Solar Stirling Engine Basics Explained

The solar Stirling engine is progressively becoming a viable alternative to solar panels for its higher efficiency. Stirling engines might be the best way to harvest the power provided by the sun. This is an easy-to-understand explanation of how Stirling engines work, the different types, and why they are more efficient than steam engines.

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