The second generation of the solar desalination systems

In Solar Energy Laboratory, Mechanical Engineering Department, Faculty of Engineering & Technology— Sebha University, an investigation has been conducted of the productivity of solar desalination system working on the basis of evacuation. Solar energy intensity has been concentrated by means of concave mirror, which reflects the sunrays to the focus of the concave, where the still is located. The apparatus has been designed and fabricated in our laboratory, the experimental model consists of elliptical, metallic container (to resist the imposed outer pressure) located in the focus of concave reflector. The still works under vacuum of 562.5 torr (25 kPa absolute) to reduce the normal boiling point of the income water. A condenser has been used to condense the outlet vapor—and working as a water trap—before interring the vacuum pump. The experiment was carried out during the period from 15 April to 15 May 2003. The water productivity of the offered still was found about 20 l/d per unit area of the reflector. The experimental results showed a significant improvement of the productivity of desalinized water, about 303% compared with the other thermal solar stills. Moreover, the increase of the performance ratio is about 900% more than the roof-type desalination solar systems. These results encourage us to adopt the offered procedure to manufacture large-scale solar desalination plants to provide the rural regions with drinkable water.

Fig. 3. The productivity of the still as a function of daily solar radiation intensity. are: the performance ratio (PR), which defined as the ratio of the latent heat of evaporation of the product water to the energy required by the desalination system and the daily yield (Pd), which defined as the total volume of distillate, in liters, produced by the still in a day per unit heat collection area [4]. The obtained results demonstrate clearly the higher value of (PR) and (Pd) of the offered still compared with that of roof-type still. The improvement of (PR) and productivity are about 900% and 300%, respectively, which are due, mainly to the effect of vacuum applied. Accordingly, the boiling point will be reduced, e.g., the boiling point of water is 60°C at 180 torr, which We have successfully shown the effect of the vacuum on the performance of the roof-type solar still, and the main points of our conclusions are as followed: • The reduction in power required to product 1 kg of fresh-water will be 90.1%. • The improvement in the performance ratio PR will be 900%. • Under the same condition, the water production will be 20.11 kg/m2 d, compared with the most effective existing solar still productivity of 5 kg/m2 d. • Economically the excess in productivity will lead to reducing the unit cost of the water. • Furthermore, the vacuum pump in the still, may be operated by means of photovoltaic system to provide the rural villages with drinkable water. 5. Future investigations We have to continue the development of this system by concentrating on the following aspects: • Technical development with the aim of designing a high performance apparatus. • Modeling of the heat and mass transfer and hydrodynamics processes in the apparatus.