Optimization Studies for Integrated Solar Combined Cycle Systems

Abstract The integrated solar plant concept was initially proposed by Luz Solar International [1] as a means of integrating a parabolic trough solar plant with modern combined cycle power plants. An integrated plant consists of a conventional combined cycle plant, a solar collector field, and a solar steam generator. During sunny periods, feedwater is withdrawn from the combined cycle plant heat recovery steam generator, and converted to saturated steam in the solar steam generator. The saturated steam is returned to the heat recovery steam generator, and the combined fossil and solar steam flows are superheated in the heat recovery steam generator. The increased steam flow rate provides an increase in the output of the Rankine cycle. During cloudy periods and at night, the integrated plant operates as a conventional combined cycle facility. Two studies on integrated plant designs using a General Electric Frame 7(FA) gas turbine and a three pressure heat recovery steam generator are currently being conducted by the authors. Preliminary results include the following items: 1) the most efficient use of solar thermal energy is the production of high pressure saturated steam for addition to the heat recovery steam generator; 2) the quantity of high pressure steam generation duty which can be transferred from the heat recovery steam generator to the solar steam generator is limited; thus, the maximum practical solar contribution is also reasonably well defined; 3) small annual solar thermal contributions to an integrated plant can be converted to electric energy at a higher efficiency than a solar-only parabolic trough plant, and can also raise the overall thermal-to-electric conversion efficiency in the Rankine cycle; and 4) annual solar contributions up to 12 percent in an integrated plant should offer economic advantages over a conventional solar-only parabolic trough power plant.