Integration Mechanism And All Working Conditions Characteristics Of Integrated Solar Combined Cycle System

The Integrated Solar Combined Cycle(ISCC)system,as a new type of solar energy utilization method,has the advantages of low investment and high efficiency,and is receiving more and more attention.In this paper,the off-design operating characteristics of the integrated solar combined cycle system are studied,and the ways to improve the offdesign performance from the perspectives of performance evaluation index and system cycle optimization are explored,the specific research content is as follows:In this paper,a mathematical model of the ISCC system is simulated,mainly including trough and tower solar subsystems,gas turbine combined cycle and supercritical CO2 Brayton cycle.The performance evaluation model of the ISCC system is established in terms of both energy and exergy performance.In this paper,an in-depth study of the operational characteristics of the ISCC system is first carried out.Based on the meteorological data of four typical days,the thermal performance characteristics of the ISCC system under different loads on different typical days are analyzed.The results show that the thermal performance of the ISCC system is negatively related to the ambient temperature for any of the top cycle conditions and positively related to the solar irradiation intensity for any of the bottom cycle conditions.For the whole ISCC system,the overall system operating efficiency is optimal on the summer solstice,while the overall power generation is the highest on the winter solstice.In order to solve the comprehensive evaluation problem of different ISCC systems with different capacities,different integration methods and different solar operating temperatures,this paper proposes a new evaluation index based on the comprehensive consideration of fuel saving factor and efficiency enhancement factor.The evaluation index is also uscd to analyze the performance of different types of practical cases.The results show that this new evaluation index is related to two factors(ratio of the energy grade of working medium in the solar thermal collector and solar thermal exergy proportion in the total input heat exergy of ISCC system),and that for a certain energy grade ratio of the working medium at the cold and hot end in any solar energy collector,it is always possible to find the solar thermal exergy proportion corresponding to the best solar integration effect in the ISCC system.The new evaluation index provides valuable guidance for ISCC systems with different capacities,different integration methods,different operating temperatures of the solar collector system,etc.In order to find the optimal solution among many integration schemes,this paper proposes a general optimization method for solar integration schemes based on thermodynamic principles and the ideas of temperature matching and cascade utilization.The method takes photoelectric efficiency and system efficiency as the objective functions and uses genetic algorithms to optimize the performance of complementary systems with different DNI irradiances and different gas turbine loads,and derives the distribution ratios of solar thermal energy in complementary systems under the integration scheme with the optimal objective function,providing guidance for the selection of ISCC system integration schemes.The results show that as the gas turbine load decreases,the solar energy integration position will gradually change from the top cycle to the bottom cycle;when the gas turbine load changes from 100%to 75%,the optimal model of photoelectric efficiency prefers two-stage integration,which can integrate solar energy up to 141.3MW,which is greater than the maximum value of 127.1 MW of the optimal model of system efficiency.In the selection of the solar integration position in the bottom cycle,the photoelectric efficiency optimal model prefers the high pressure boiler and the system efficiency optimal model prefers the high pressure superheater.In order to improve solar power generation and utilization,a new ISCC system is proposed by replacing the steam Rankine bottom cycle in the conventional system with a supercritical CO2 Brayton cycle.Then the thermal performance analysis,exergy analysis and sensitivity analysis are carried out for the new ISCC system integrated with two different solar collectors:trough type and tower type.The results show that the ISCC system with parabolic trough solar collectors has higher optical,photothermal and photoelectric efficiencies than the ISCC system with solar tower collectors due to the different solar collector methods and the lower collector temperatures at the integration positions.The solar collector is the least exergy efficient component of the ISCC system.Sensitivity analyses have resulted in the optimum intercooling pressure for the bottom cycle of both new ISCC systems reaching maximum power output and system efficiency at 11.3MPa.