Thermodynamic Analysis And Optimization Of Supercritical CO₂ Brayton Cycles Integrated With Solar Power Tower

In order to pursue higher economic benefits,the parameters of electric power units have been trending toward higher temperature and higher pressure.However,when the inlet temperature of the turbine exceeds 650°C,the efficiency of the ultra-supercritical steam Rankine cycle is difficult to further increase.Therefore,exploring new forms of thermodynamic cycles is of great significance for further improving cycle efficiency and electricity supply system stability.Non-toxic,odorless,non-self-igniting,widely used and low-cost CO2 has received extensive attention as a new thermal cycle medium.Among them,the Brayton cycle with supercritical carbon dioxide as the working medium has received special attention.According to the working characteristics of solar power tower and supercritical Brayton cycle,the system is divided into four parts: heat collecting system,turbine,heat exchanger and compressor through system modeling and theoretical analysis.The working model of supercritical CO2 Brayton cycles integrated with solar power tower is built to calculate the thermodynamic cycle efficiency under different heliostat field layout and different parameter configurations,and integrates the working process of the two subsystems to optimize the overall coupling system.The different cycle structure forms are compared and analyzed,and efficiency differences between all the cycle forms are obtained.Based on the changing trend of efficiency,the calculation and optimization of the solar power tower driven Brayton cycle in off-design operating conditions for recompression cycle are carried out,and the sliding pressure optimization principle and the optimal backpressure principle are introduced to guide the operation process.For the solar power tower driven Brayton cycle system,the overall system efficiency is decided by the heat collecting efficiency and Brayton cycle efficiency together,as the turbine inlet temperature increases,the operating temperature of the solar power receiver increases,and the efficiency of the thermodynamic cycle increases,but due to the increased loss of the receiver,the heat collecting efficiency will decrease.Therefore,with the increase of the turbine inlet temperature,the system efficiency will firstly rise and then fall.Considering that the turbine exhaust pressure,turbine inlet pressure and precooler outlet temperature have slight effect on the heat collecting efficieny,the overall system efficiency is decided by the cycle efficiency.The cycle efficiency increases first and then decreases with the turbine exhaust pressure increases.Peak appears slightly above the critical point of carbon dioxide,indicating that for the supercritical carbon dioxide Brayton cycle,although the operating pressure of the system is above the supercritical pressure,yet the backpressure should be as close as possible to the critical point pressure,so that the cycle efficiency reaches the maximum;as the temperature of the precooler decreases,the decrease of the cold fluid inlet temperature of the regenerative heater increases the heat exchange amount of the regenerative heater,which in turn causes the temperature of the fluid at the inlet of the precooler to decrease,and the release heat of the precooler is reduced and the cycle efficiency of the entire system is increased.Under partial load conditions,the lower the turbine inlet pressure,the higher the system efficiency until the pressure reaches the lowest value.At this time,the corresponding control valve of the turbine is fully open,indicating that the Brayton cycle is consistent with the Rankine cycle,and the turine control valve opening should be kept as far as possible,and operates under fully sliding pressure;under partial load conditions,the precooler temperature or system load rate is higher,the best back pressure is higher,and the optimal back pressure is above the critical pressure(7.38 MPa).