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European Journal of Scientific Research ISSN 1450-216X Vol.20 No.4 (2008), pp.882-890 © EuroJournals Publishing, Inc. 2008 http://www.eurojournals.com/ejsr.htm Modeling and Temperature Controller Design for Yazd Solar Power Plant Aref Shahmansoorian Islamic Azad University of IEslamshahr E-mail: a_shahmansoorian@iiau.ac.ir Abdolvahed Saidi National Iranian Oil Company-OGPC Department E-mail: saidi@nioc.org Abstract The Yazd Integrated Solar Combined Cycle (ISCC) Power Plant consists of two gas turbines that are generating power synchronous to Iranian national electrical network. One steam turbine will be supplied by gas units, while a parabolic through solar field is integrated with the system, as a combined cycle. In this paper an integrated model of the solar field with the purpose is presented, and a multivariable generalized predictive temperature controller is proposed for the system. As it is illustrated in the simulation results, such a control strategy can robustly regulate both the temperature of outlet oil, and the temperature of outlet steam water of the solar boiler, despite the variation of the inherent time delays of the system and external disturbances. Keywords: ISCC power plant, Solar Field, heat exchanger, modeling, multivariable GPC. I. Introduction The feasibility studies and technical specification of an integrated solar combined cycle power plant has been recently completed, and this type of power plant will be constructed in the city of Yazd in near future [15]. This power plant is designed with two steam turbines, two gas turbines and a solar field which supplies excess steam for steam turbines. The solar field consist of parabolic through collectors, solar boiler, pumps, control valves and an expansion vessel. The solar field itself possess 84 loops with eight collectors in each loop[16]. The collector has parabolic mirrors which focus the sun beams directly on the oil pipe, and the solar collectors are equipped with a sun tracker system. The oil enters the solar field with the temperature of 230ºC and depart from it with the temperature of 391ºC. The outlet oil from solar field enters a special heat exchanger named solar boiler, in which the accumulated heat in the solar field exchanges the supplied water into superheated steam[17]. The solar boiler consist of three exchangers that preheating, evaporating and superheating the water in three stage. The oil enters the superheater stage at 391ºC/15bar and depart from preheater stage at 293ºC/11bar. The supplied water enters the preheater with 235ºC/105bar and depart the superheater with 380ºC/102 bar. The inverse direction heat exchanger is chosen in order to increase the rate of heat transfer. The necessary pressure for oil is provided with 3 pumps, which one of them is standby. The type of tank in the solar field is an expansion vessel that the inlet pressure is fixed with
Modeling and Temperature Controller Design for Yazd Solar Power Plant 883 nitrogen. The control valves are regulating the oil flow for temperature control. Figure (1) illustrates the schematics of an ISCC power plant [12]. Figure 1: Integrated Solar Combined Cycle Power Plant II. Solar Field Modeling In order to have a tractable and complete model of the solar field it is necessary to model the heat exchanger and the solar collectors in detail [1]. In this section the proposed models for these components are studied. A. Solar Collector Model The designed solar collector consists of double steel/glass pipe with vacuum insulation. The collector mirrors absorb a fraction of solar beam energy, and focuses the rest on the aforementioned oil pipes at the center of parabolic through. The heat energy is transferred to the oil through convection, conduction and radiation [14]. Figure 2: circuit model of heat transfer Since the radiation heat transfer relates the rate of heat transfer to the fourth power of the temperature, in order to compensate for the temperature losses, a global coefficient of thermal losses is defined as below: Qloss Hl = (1) ( Ts −Tamb) In which T s and T are the oil pipe surface and the ambient temperatures, respectively. The amb Heat loss factor, H can be obtained from calibration experiments, and can be assumed constant with a l sufficient degree of accuracy. Considering the heat loss in the collectors as elaborated, the dynamic equation of solar field heat transfer is given as following [4]: ∂Ts ρ η IG H G T T ) LH ( T T ) (2) Cs As = opt − l ( s − amb − t s − f ∂t s This equation contains the absorbed energy of the oil pipe, the transferred energy to the oil and the heat loss to ambient. The left hand side of the equation determines the temperature variation of the metal pipe surface, in the condition while the thermal equilibrium has not been reached. The first term
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