Analysis And Performance Optimization Of Supercritical CO₂ Brayton Cycle Coupled Organic Rankine Cycle Based On Solar Tower

The supercritical carbon dioxide（SCO₂）Brayton cycle was first proposed in the 1950s,and it was first studied in the United States,and it was proposed in the 1990s that it can replace the steam Rankine cycle for power generation.This paper takes the integrated system of solar tower and SCO₂ Brayton cycle coupled organic Rankine cycle（ORC）as the research object,establishes the dynamic model of the combined cycle,and puts forward a new combined cycle mode,studies the influence of the main key parameters on the system performance,and explores the cycle optimization direction.Firstly,four kinds of SCO₂ Brayton cycle coupled ORC system models based on solar tower are established by using Matlab software.They are:SCO₂ Brayton cycle coupled with ORC（SCBC/ORC）,SCO₂ recompression Brayton cycle coupled with ORC（SCRBC/ORC）,new SCO₂ Brayton cycle coupled with ORC,new SCO₂ recompression Brayton cycle coupled with ORC.Secondly,the mathematical models of the main heat source components such as the heliostat field and the receiver of the molten salt solar tower are established,the corresponding analysis and calculation formulas are given,and the ORC working medium is selected and compared.Then,taking SCRBC/ORC as an example,the effects of split ratio,turbine inlet pressure on system performance are studied,and then the effects of compressor inlet pressure,turbine inlet temperature on system performance are studied by taking the new SCBC/ORC as an example.Thus the optimization direction of the system is obtained.Finally,in order to obtain the balance of thermal performance and economy,the multi-objective optimization genetic algorithm is respectively used to optimize the efficiency and unit investment cost of the SCRBC/ORC system and the new SCBC/ORC system.The results show that,through the verification of the simulation model of the system established by Matlab,the circulation system established in this paper meets the simulation requirements,and the research in this paper is reliable.Compared with the single cycle,the thermal efficiency of SCRBC/ORC combined cycle can be increased by more than 2 percentage points,and the highest thermal efficiency can be reached by 11 percentage points;There is an optimal split ratio for SCRBC/ORC,and the position of the optimal split ratio for different turbine inlet pressure is different.When the optimal split ratio is taken,the thermal efficiency of 23.28MPa combined cycle is the highest;The change of inlet temperature of top cycle turbine will not affect the thermal efficiency of ORC,and the change of mass flow ratio will not affect the thermal efficiency of top cycle;Compared with the single cycle,the thermal efficiency of the new SCBC/ORC combined cycle system can be increased by more than 4percentage points,and the thermal efficiency can be increased by more than1.32 percentage points;In the process of changing cycle parameters,the higher thermal performance of the system is accompanied by higher unit investment cost,and the fixed entropy efficiency of compressor and turbine have great influence on the thermal efficiency of the system;The multi-objective optimization results show that there is no point to maximize the efficiency and minimize the unit investment cost at the same time.Through the multi-objective optimization method,several groups of pareto solutions can be found,which can provide some reference for engineering design.