Transient Simulation And Control Strategies Of Supercritical Carbon Dioxide Brayton Cycle

Supercritical carbon dioxide(S-CO2)Brayton cycle has the characteristics of compact structures,simple layout of the system and high energy density.Meanwhile,for the heat source which can keep high temperature,its efficiency is higher than that of Rankine cycle.Therefore,nuclear energy,solar energy and other heat sources use it as the preferred energy conversion system in the future.At present,researches on SCO2 recompression Brayton cycle mostly focuses on steady-state simulation and optimization of system parameters under design conditions.However,researches on transient simulation are quite few,especially when covering control strategies and heat load change.In addition,there are few transient simulations about the cycle startstop process.Therefore,the transient simulation of the S-CO2 recompression Brayton cycle needs to be carried out urgently.In this thesis,Modelica language is used to develop a transient model of S-CO2 recompression Brayton cycle.The heat exchanger adopts the printed circuit heat exchanger(PCHE),and the turbomachinery adopts a transient model based on operating characteristics,which makes the model closer to the actual operating condition.According to the comparison with the results of existing steady-state model,the accuracy of the model developed in this paper is verified.Then,different control strategies are applied,such as main compressor throttling control,precooler bypass control,turbine bypass control and so on,to control the parameters of the cycle and analyze the impact on the parameters when adopting different control strategies.At the same time,for the operating conditions of load decrease from 100%thermal load to 80%and 50%respectively,different transient responses brought by different control strategies are discussed,and the use of combined control strategies can effectively balance the advantages and disadvantages of different control methods.In addition,the overshoot and response time of different adjustment methods are also compared.Through the comparison,it is found that the main compressor shaft speed control has advantages in the compared control methods.Turbine bypass and throttling control are both acceptable control methods.Finally,for the shutdown process of the cycle,a transient simulation is performed.The simulation results show that the shutdown scheme is reasonable.