During this period it operated fully automatically

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early 1989 for a period of 32 months, the plant was run regularly on a daily basis. During this period, it operated fully automatically for an average of 8.9 hours every day and generated 44 MWh annually. The results obtained showed that the plant as a whole was capable of reliable operation over extended periods of time
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30 Medium Temperature Systems Among solar thermal-electric power plants, those operating on medium temperature cycles and using the line-focusing parabolic collector technology at a temperature close to 400°C have proved to be the most cost effective and successful so far. A schematic diagram of a typical plant is shown in Fig. 3.21. The first commercial plant of this type (Solar Electric Generating System-SEGS I) having a capacity of 14 MW was set up in 1984. Since then six plants of 30 M W capacity each (SEGS II to VII), followed by two plants of 80 MW each (SEGS VIII and IX), were installed and commissioned. All these plants were set up by LUZ International in California, which has a total installed capacity of 354 MW. The collector array for SEGS IX has an area of 483 960 m 2 . The cylindrical parabolic collectors used have their axes oriented north-south. The absorber tube used is made of stainless steel and lias a specially developed selective coating. It is surrounded by a glass cover with a vacuum. The collectors heat a synthetic oil (Dowtherm-A) to a temperature of 390°C with a collector efficiency of about 0.7 for beam radiation. The synthetic oil is used for generating superheated high pressure steam which executes a Rankine cycle with an efficiency of 38 per cent. The plant generally produces electricity for about eight hours a day and is coupled with natural gas for continuous operation. The installed cost of this type of plant has reduced over the years because of the increasing installed capacity. However it is very high. SEGS VIII which commenced operation in 1990 is reported to have cost $ 4000 per kW. Because of the high capital cost, no new large-size plants have been erected after 1991. However SEGS I - IX have continued to operate and valuable operating experience extending to more than 15 years has been obtained. High Temperature Systems Two concepts have been experimented with in the case of high temperature systems. These are the paraboloid dish concept and the central receiver concept. Paraboloid dish System In the paraboloid dish concept, the concentrator tracks the sun by rotating about two axes and the sun's rays are brought to a point focus. A fluid flowing through a receiver at the focus is heated and this heat is used to drive a prime mover. Typically, Stirling engines have been favoured as the prime movers. For this reason, such systems are referred to as Dish-Stirling Systems. Because
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31 of the limitations on the size of the concentrator, paraboloid dish systems can be expected to generate power in kilowatts rather than megawatts. Thus they can be expected to meet the local power needs of communities, particularly in rural areas.
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