Two important issues for commercializing dish

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Two important issues for commercializing Dish-Stirling systems are cost and reliability. Presently installation costs of such systems are very high, at around $ 10 000 per kW. This is because very few systems have been built. However, it is estimated that the cost of a system could reduce to $ 2500 per kW if there is a production of 500 units per year. Central receiver system In a central receiver system, solar radiation reflected from an array of large mirrors is concentrated on a receiver situated at the top of a supporting tower. The mirrors are called heliostats and they are placed on the ground around the tower. Their orientation is individually controlled so that throughout the day they reflect beam radiation on the receiver. A fluid flowing through the receiver absorbs the concentrated radiation and transports the heat to the ground level where it is used to operate a thermodynamic cycle like the Rankine or the Brayton cycle. Molten salts, water (converted to steam) and air have been used as the heat transfer fluids. Because of the use of a receiver placed at the top of a tower, a central receiver system is also referred to as a power tower. A schematic diagram of a typical central receiver system using a molten salt as the heat transfer fluid is shown in Fig. 3.22. The molten salt used frequently is a mixture of 60 per cent sodium nitrate and 40 per cent potassium nitrate. Cold salt at 290°C is pumped from a tank at ground level to the receiver at the top of a tower where it is heated by the concentrated radiation to a temperature of 565°C. The salt flows back to another tank at ground level. In order to generate electricity, hot salt is pumped from the hot tank through a steam generator where superheated steam is produced. The superheated steam then goes through a Rankine cycle to produce mechanical work and then electricity. The heliostat array can be sized to collect more power than is required by the electricity generation system. In that case, the excess thermal energy in the form of excess salt at 565°C accumulates in the hot tank and serves as a thermal storage.
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32 3.13.5 Distillation In many small communities, the natural supply of fresh water is inadequate in comparison to the availability of brackish or saline water. Solar distillation can prove to be an effective way of supplying drinking water to such communities. The principle of solar distillation is simple and can be explained with reference to Fig. 3.23, in which a conventional basin-type solar still is shown. The still consists of a shallow air-tight basin lined with a black, impervious material which contains the saline water. A sloping transparent cover is provided at the top. Solar radiation is transmitted through the cover and is absorbed in the black lining. It thus heats up the water by about 10° to 20°C and causes it to evaporate. The resulting vapor rises, condenses as pure water on the underside of the cover and flows into condensate collection channels on the sides. An output of about 3 litres/m 2 with an associated
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