1. Introduction
One of the important and technically viable applications of solar energy is the distillation of seawater coastal areas and small islands, and brackish wells water in remote towns and villages. Such utilization of solar energy offers an almost cost-free alternative for supplying potable water, particularly for small communities in arid regions [1], which are characterized by low demand of water and abundant solar radiation [2,3]. This advantage is generally offset by the low productivity of commonly used solar stills. An early large-scale solar still was built in 1872 to supply a mining community in Chile with drinking water [4]. Therefore, extensive efforts have been carried out through numerous investigations to improve the efficiency of solar distillation process. Various approaches have been suggested to achieve such goal, either through improving designs of conventional stills, or developing unique designs through adjustment of stills operating parameters [5]. The methods, of solar water desalination are classified according to the way in which solar energy is used, given in [6]; also, other approaches were adapted in which connection of stills with assisting external systems or elements showed different degrees of improvement in the stills yield [7,8].
2. Design of the Solar Still
A schematic diagram of the designed solar still is shown in Figure 1. The actually fabricated solar still is shown in Figure 2. The basin area of the still is 0.25 m2, fabricated using galvanized iron sheet of 18 gauge thickness. The bottom and sides of the basin are insulated by 5 cm thick fiber-glass surrounded by a wooden frame of 5 cm thickness. The surface of the basin is painted black to absorb maximum solar radiations. On the inside rim of the basin two troughs have been fixed with each one extends along two sides of the basin. The main function of these troughs is to collect the distillated water falling from the glass cover which is fixed above the tray. Each trough is titled at a small to permit the distillated water to leave the stiller through water to a graded flask.
The function of the glass cover is to permit solar radiation to pass to the stiller and elevate the temperature of the hot water in the tray. This enhances the evaporation process of water in the basin, which will condense on the inner surface of the glass covers whose temperature is definitely lower than that of the water. Four used glass covers are used on the four sides of the tray. Each side is equilateral triangle. The four sides when assembled and contained in frame pyramid shape of the stiller as shown
Figure 1. Schematic diagram of the designed single basin solar still (drawn not to scale).
Figure 2. The fabricated solar still being tested in outdoor conditions.
in Figure 2. To evaluate the performance of the water distillation process, the ambient temperature is measured along with the hourly solar radiation and the amount of distillate. All measured parameters are recorded every hour starting from 6 a.m. to 5 p.m. daily.
3. Measurements and Results
Investigation of the proposed stills performance was carried out through measurements of different parameters for three solar stills contain different amount of water in each one. For the first still the amount of water was 3 liter and this still was marked M1 as model number one. The second still tested for 6 liter and named model M2 while the third one tested for 9 liter with a model name M3. The solar stills were tested outdoors at Damascus University-in Syria. The measured parameters concerning the distillation process are the hourly distillated quantity of water for each model. Also the ambient conditions of solar insulation and ambient temperature were recorded every hour over the day-light during the period of performance tests. The wind speed has been determined from the recorded official weather data for Damascus city. All these measured parameters have been used to evaluate the performance of these stills.The obtained results are presented and discussed below. The ambient temperature, the value of distillated water of the three models of the stills, the solar radiation and wind speed on selected day in the month of November 2010 are as following. Table 1 lists the measured hourly values of ambient temperature, amount of distillated water produced in these the stills, and solar radiation and wind
Table 1. Ambient temperature, distillated water, solar radiation and wind speed as function of time (on November 14, 2010).
speed on November 14, 2010. The variation of these parameters along the day from (6 am) in the morning until the time of sunset is shown in Figures 3-7.
Figure 3 shows that ambient temperature increases with time until it reaches its maximum value around noon time then decrease gradually as shown in the figure.
Figure 4 shows that the solar radiation increases with time until reaching a maximum value around noon time.
After reaching its peak value, the solar radiation decreases with time until it diminishes near sunset as shown in the figure. The shape of solar insulation curve is very much similar to the expected theoretical curve by forecast for clear day except in afternoon around (3 p.m.) where some cloud might caused the depression at that time.
Figure 5 shows the variation of the wind speed during the day-light of November 14, 2010. It increases for (8 km/h) in the early morning up to (11 km/h) by the end of the day.