الفهرس 
CHAPTER (1) INTRODUCTIONOnly 14 pages are availabe for public view
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Abstract
Seawater desalination is an important technology adopted to overcome the freshwater shortage problem. Since desalination requires a considerable amount of energy, new systems are being proposed and constructed to use low temperature energy sources such as renewable energies. In this investigation, a novel system operating under natural vacuum has been designed, fabricated and assembled. The system utilizes natural forces of gravity and atmospheric pressure to create a vacuum under which water can be evaporated at lower temperatures than that by simple conventional techniques. This would allow the use of low temperature energy sources, such as solar energy. The uniqueness of the system is in the way natural forces are used to create vacuum conditions. The proposed desalination system consists of a solar heated evaporator and, a condenser. The evaporator and the condenser are kept at 10 m above ground level and are connected by pipes to a saline water supply tank and a freshwater tank, respectively. An experimental test rig has been designed and fabricated. Detailed experimental investigation has been carried out to examine the system feasibility under different cooling techniques. The cooling methods include air cooling (both natural and forced), external water cooling of condenser surface, and internal water cooling. The system design has been modified and an upgraded set-up has been developed. The system performance has been assessed all around the year under actual weather conditions. The hourly productivity as well as the cumulative daily productivity has been measured.
It has been found that, the average pressure for the three cases; natural convection, forced convection and external cooling of condenser surface; are 0.123, 0.107 and 0.092 bar (abs) respectively. It has been also found that the cumulative productivity for the three cooling techniques are 2.7, 5.2 and 6.6 liter/m2.day. Highest cumulative productivity is obtained under external cooling as the condenser exhibits lowest pressure. The best cooling mechanism is the external cooling as it achieves the highest performance ratio with an average value of 74 %. This performance ratio is more than double of that of the conventional solar still.
In the initial design of the test-rig, the fresh water contained considerably high salt concentration. It was attributed to the saline water bubbles carried away to the condenser during boiling. The problem has been solved by adding a liquid separator between the exit of the evaporator and the inlet to the condenser. It is found also that the condenser cooling process is most efficient at a flow rate of 2 liter/min. Moreover, the average pressure for the cooling techniques (cooling water and free convection) are 0.0725 and 0.115 bar (abs) respectively. The fill ratio in evaporator affects the system performance and the best performance is attained in case of ¼ filling ratio at the cooling water flow rate of 2 liter/min. The system performance is more feasible when forced water cooling is adopted. At 2 liter/min cooling flow rate, a performance ratio as high as 0.88 has been achieved and a cumulative productivity of about 10 liter/m2.day has been obtained. The system productivity is 300% higher than the productivity of solar still. The proposed system is competitive with conventional solar desalination systems of similar collection areas.