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Using solar energy to produce fresh water is one of the best solutions for the world’s problems: fresh water and energy. After the high development in the middle of the last century, solar water distillation systems are not widely used because of low productivity, about 2-5 liters/m2/day. At the beginning of research, a conventional solar still has been studied with the main purpose is to understand the operating principle and its disadvantages. An improved system using decompressed boiling is then investigated. This new system has a lot improvement compared to the conventional one. It works in vacuum condition in which pressure is less than that of atmosphere to take advantages of the un-concentrated solar energy and stimulate the rate of evaporation by boiling phenomenon. The system has two stages; one is on the top of the other separated by a copper plate. In the bottom stage, heat released from condensation on the copper plate is reutilized to evaporate water in the upper stage. At the top the upper stage, there is a cooler to absorb the energy from condensation. The productivity is expected to have more than 5 times compared to the conventional solar still. The main work in this thesis is to calculate the productivity and other parameters of the proposed system. The calculation is programmed using iteration method. There are three iteration loops using energy balance applied for three main components of the system where: energy input, heat transferred between two stages and energy output. The result has an acceptable agreement when compared to the experiment. At normal heater input temperature (40-50oC), the temperatures are quite the same with the experiment. When the heater input temperature is low (30oC), there is a gap in water temperatures in comparison, about 1.2oC. And productivity difference in the maximum case is less than 15%.