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is to produce fresh water for communities for their continuous health, development, and growth at an acceptable cost. To meet the challenge, large desalination systems, including dual-purpose power and desalination plants, have been built to reduce the cost of production of electricity and water. Thermal energy extracted or exhausted from power plants is used effectively in the desalination process. It is estimated that there are over 25,000 megawatts (MW) of power combined with desalination plants used in the cogeneration concept. However, not all water demands are coupled with the need for additional electric power.

A worldwide inventory shows that by the end of 1995 there were over 11,000 desalting units with total capacity of 20 million cubic meters per day. Desalination is already used in 120 countries around the world. The exponential growth of desalination can be illustrated by the fact that in 1971 total worldwide capacity was only 1.5 million cubic meters per day. In 1976 the total was 4 million cubic meters per day and in 1995 it was 20 million cubic meters per day. In the last 10 years, worldwide capacity grew from 12 to about 22 million cubic meters per day.

The Middle Eastern countries are the biggest users of desalination technology, with about 50 percent of the world’s capacity installed in the area. The dominant plant type is Multi Stage Flash (MSF), which accounts for 86.7 percent of the desalting capacity, while the Reverse Osmosis type accounts for only 10.7 percent. In the state of Hormozgan, in the southern part of Iran, about 45 percent of the fresh water produced from the sea for the cities and islands is through MSF, 20 percent through Multi Effect Distillation (MED), 31 percent through Vapor Compression (VC), and 4 percent through Reverse Osmosis (RO). Worldwide, 48.1 percent of the total installed or contracted capacity is based on the MSF principle, reflecting a continuing decline from the proportion reached in 1991 (51.5 percent). In comparison, the Reverse Osmosis process increased its share from 32.7 percent to 35.9 percent in the same period.

LARGE-SCALE DESALINATION PROCESSES

Desalination can be classified into phase-change and single-phase processes. The most commonly used phase-change processes are Multi Stage Flash (MSF), Multi Effect Distillation (MED), Vapor Compression (VC), and Solar Distillation. Highly developed single-phase processes are Reverse Osmosis (RO) and Electrodialysis (ED), which use membranes to separate impurities from water (Assimacopoulos, 2001).

Solar energy may be employed to produce fresh water from the sea. This may be accomplished in a large system or in a simple basin type desalination unit. For a large quantity of fresh water production, a unit was constructed in the city of Abu Dhabi on the Persian Gulf coast using solar energy (El-Nashar, 2001). The plant consists of three subsystems: the solar collector field, the heat accumulator, and the sea water evaporator. It is designed for an expected yearly



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