Study On Optimization Design And Performance Analysis Of Supercritical CO₂ Brayton Cycle Power Generation System

With the development of society and the increasing demand for electricity,improving the efficiency of power generation units has important practical significance.The supercritical CO₂（S-CO₂）Brayton cycle has been paid much attention in recent years due to its simple system and high power generation efficiency.This thesis takes the S-CO₂ Brayton cycle power generation system as the research object.The optimization design and performance analysis for coal-fired,solar-coal supplementary and solar power generation system are studied.And the design method of power generation system from traditional fossil energy drived to solar energy as the main energy is built.The mathematical models of the S-CO₂ Brayton cycle power generation system are built,which mainly includes the S-CO₂ Brayton cycle,S-CO₂ coal-fired boiler subsystem,solar tower subsystem and thermal energy storage subsystem.The performance evaluation model of the S-CO₂ Brayton cycle power generation system is established from the thermal,exergy,environmental and economical performances.Firstly,a novel S-CO₂ coal-fired boiler heating surface arrangement is proposed.The furnace adopts the single furnace double-tangential circle arrangement,and increases the proportion of reheat radiant heat exchange surfaces.Both the working medium pressure drop and reheated temperature maintenance of the novel boiler are obviously better than those of the conventional boiler.The effects of coal type and excess air coefficient on the novel boiler performances are analyzed.The results show that the lower the moisture content and the higher the lower heating value of coal,the higher the boiler efficiency.When the excess air coefficient is 1.2～1.4,the best efficiency zone of the boiler is reached.Based on the novel boiler,a novel S-CO₂ coal-fired power system is proposed.The novel system extends the air preheater surface area,and both the evaporator and precooler are arranged before the cooler.Compared with that of the reference system,the boiler efficiency of the novel system increases by 1.1%,and the power generation efficiency increases by 2.6%.The effects of the economizer split ratio,the main compressor split ratio and the operation load on the novel system performances are revealed.Based on the S-CO₂ coal-fired power plant,three novel solar tower aided S-CO₂ coalfired power generation systems with different integration schemes are proposed.The three schemes are analyzed and compared.Finally,using solar energy to heat the reheated working medium is selected.Integrating solar energy on the pure coal-fired power generation system can reduce the standard coal consumption rate and CO₂ emission rate.Compared with the pure S-CO₂ coal-fired power system,the standard coal consumption rates of novel system is reduced by 10.54 g/kWh.In addition,influences of both the solar energy input and operation load on the performances of novel system are studied.The results show that the boiler efficiency and the solar-to-electricity efficiency increase with an increasing solar energy input,the power generation efficiency,CO₂ emission rate and standard coal consumption rate decrease with an increasing solar energy input,the system performance declines with a decreasing operation load.To increase the proportion of solar energy input,a novel S-CO₂ high ratio solar-coal supplementary power generation system is proposed,which takes solar energy as the main heat source.The daily and annual performances of S-CO₂ high ratio solar-coal supplementary power generation system are studied.The results show that on the summer and winter solstices,the proportions of solar generating power are 100%and 57.15%,respectively.The system performances with the change of the load rate are revealed.The exergy efficiency for the novel system increases with an increasing operation load rate.The changes in energy,exergy,environmental and economic performances of the S-CO₂ high ratio solar-coal supplementary power generation system with solar multiple and thermal energy storage capacity are analyzed.The results show that with the increase of the solar multiple,the energy and exergy performances for the novel system decrease,the environmental performance increases,and the economic performance decreases after an increasing.As the thermal energy storage capacity increases,the energy,exergy and environmental performances increase,the economic performance decreases.To improve the solar generating power and solar energy utilization rate,a novel SCO₂ solar tower power generation system integrated with steam Rankine cycle is proposed.The daily and monthly performances of the novel system are compared with those of the conventional system.Compared with the conventional system,both the power output and solar-to-electricity efficiency of the novel system are improved.The influences of the steam Rankine cycle power output on the performances of the combined cycle,thermal energy storage subsystem,solar tower subsystem and the novel power generation system are studied.The results show that,for low-direct normal irradiance（DNI）,the steam Rankine cycle power output corresponding to the optimal thermal performance is 15 MW;for medium-DNI and high-DNI,those are 20 MW.For low-DNI,the steam Rankine cycle power output corresponding to the optimal economic performance is 5 MW;for mediumDNI and high-DNI,those are 15 MW.In addition,compared with low-DNI,medium-DNI and high-DNI are more suitable for novel system.