Integration And Methods For CO₂ Power Cycle Driven By Concentrated Solar Thermal Energy

Concentrated solar power is one of the most important solar power technologies in China,however,the steam Rankine cycle is utilized for concentrated solar power generation,causing the low operation temperature,large solar collecting area and low solar-to-electrical efficiency.In order to realize the efficient conversion of solar thermal energy,improving the performance of power cycle driven by solar energy and exploring flexible and stable solar power generation system are urgent requirements in the field of concentrated solar power generation.Supercritical CO₂ power cycle,with lower consumption work in compression and higher cycle efficiency,driven by solar thermal energy has the potential to further improve the solar-to-electrical efficiency.Supported by the National Key R&D Program of China?No.2018YBT151005?,key program of the Chinese Academy of Sciences?No.KFZD-SW-418?,based on the efficient conversion of solar thermal energy,the dissertation carried out the researches,mainly including integration mechanisms and system integration of solar energy powered supercritical CO₂ Brayton cycle and CO₂-H₂ O cycle,and the experimental research of the key processes of turbine operation.Based on the thermodynamic characteristics of energy conversion process,the thermodynamic model of solar energy driven supercritical CO₂ power cycle was established,the relationship between the system performance and operation parameters was investigated to determined the system integration principles,and the experimental test for key process in supercritical CO₂ cycle was investigated.This research analyzed characteristics of CO₂ as a working fluid of power cycle,based on the thermophysical property,explores energy conversion rules and performance improvement mechanisms of CO₂ cycles,and investigated the relationship between the performance and operation parameters of solar powered CO₂ system to provide the theoretical basis for selection of cycle parameters.Moreover,the experimental verification of key process of turbine operation in supercritical CO₂ cycle was conducted to investigate the stability and generation performance of the turbine at high speed condition.On the issues of unstability of heat transfer fluid at high temperature and solar intermittence,utilizing supercritical CO₂ as heat transfer fluid improved the temperature of solar collection process,and the biomass energy was applied to hybrid with solar thermal energy to extend the operation time of the system.The cascadedsupercritical CO₂ cycle driven by solar energy and biomass was proposed,and the overall performance of the system was investigated by conducting thermodynamic analyses.The solar-to-electrical efficiency of the system at nominal condition is27.85%,with the advantage of efficient conversion from thermal energy to electricity.In this system,the cascaded supercritical CO₂ cycle was utilized to integrate complementary application of solar thermal energy and biomass to avoid the narrow temperature window of heat transfer process in supercritical CO₂ cycle and to take full use of biomass energy.The thermodynamic analyses of the system were conducted to analyze the reason of energy loss by accounting the energy and exergy balances and EUD analysis,and to investigate overall thermodynamic performance by carrying out the analyses of the system in representative days and whole year.The power cycle with CO₂-H₂O as a working fluid was proposed to improve the expansion ratio and specific work compared with supercritical CO₂ cycle.The expansion ration of the CO₂-H₂ O power cycle was increased by reducing pressure of condensing process,and the lower consumption work and higher stability in compression process were realized by compress liquid working fluid.The thermodynamic analyses were investigated,and the specific work and cycle efficiency were higher than supercritical CO₂ cycle.Compared with supercritical CO₂ cycle,the specific work of CO₂-H₂ O cycle at the design condition was increased by 39%,and the thermal efficiency was increased by 13%.Then,the solar thermal energy driven CO₂-H₂ O power cycle was established to investigate system performances at the nominal condition,in typical days and in whole year.In order to reduce the irreversible losses of recuperation in CO₂-H₂ O cycle,the two stage heating system,which integrates parabolic trough collector and tower collector,was proposed,applying solar thermal energy collected by parabolic trough collector to make up the lack of recuperation.Compared the two stage heating system with CO₂-H₂ O recuperation system,the solar-to-electrical efficiency of the two stage heating system was higher by optimizing the solar collector and reducing the heat loss in solar collection.