SPT-sCO₂ Cycle Power Generation System Thermal-economy-environment Analysis And Optimization

At present,solar power tower（SPT）technology uses the steam Rankine cycle for power geneartion,and its overall power generation efficiency is relatively low.In contrast,supercritical carbon dioxide（sCO₂）Brayton cycle is considered as an effective alternative due to its high cycle thermal efficiency,small equipment size,compact system,and low water consumption.It can achieve higher power generation efficiency under the same turbine inlet temperature and lower heating pressure.Therefore,the application of sCO₂ Brayton cycle can potentially improve the efficiency of SPT.In addition,the evaluation of the SPT-sCO₂ Brayton cycle integrated system should not only be based on a single indicator of thermal efficiency,but also should be evaluated by multiple criteria in terms of its economics and environmental impact.This thesis proposes a simultaneous optimization model based on the mathematical programming method.The model considersCO₂ property equation of state,thermodynamic processes of unit operations,thermoeconomics and life cycle emissions of the SPT-sCO₂Brayton cycle integrated system.Economic and environment multi-objective analysis and optimization are performed to capture the tradeoffs of different system design.It provides a theoretical basis for the conceptual design and engineering application of SPT-sCO₂ Brayton cycle integrated system.The thermodynamic model of the SPT-sCO₂ Brayton cycle integrated system is established and its accuracy is verified.The model is applied to the optimization design of power generation efficiency of two systems based on Simple Regenerative Brayton Cycle（SRBC）and Recompressed Brayton Cycle（RBC）to prove the effect of the optimized design.Moreover,the influence of turbine inlet temperature and compression ratio on the thermal efficiency of the integrated system is analyzed.The results show that the thermal efficiency of SPT-RBC integrated system is 4.51%higher than that of SPT-SRBC;due to the heat loss of the receiver,the thermal efficiency of the two integrated systems reaches the maximum when the turbine inlet temperature reaches 823 K and 901 K respectively;and the tradeoff between turbine output work and compressor energy consumption also suggests that the optimal compressor inlet pressure is notably higher than the CO₂’s critical point（7.4 MPa）.The life cycle emission inventory of SPT-sCO₂ system is established by using the life cycle assessment method,and the environmental emission of SPT-sCO₂ system in the whole life cycle is obtained.The results show that the GWP,AP and EP of the SPT-sCO₂ system are far lower than that of the coal-fired system,and the environmental impact potential of each is2.42%,29.99%and 0.01%of coal-fired system,respectively,which has good environmental performance.A multi-objective optimization model is formulated with the levelized cost of energy（LCOE）and total environmental impact potential（TEIP）of the integrated system as the objective functions.Simultaneous optimization of system operating parameters obtained the optimally designed SPT-sCO₂ system Pareto solution,and analyzed the trade-off relationship between TEIP and LCOE Pareto curve shows that the inevitable cost of reducing LCOE is to increase TEIP,by increasing the total environmental impact potential,the LCOE of the integrated system can be reduced by 4.12%;the Pareto equilibrium solution is obtained by analyzing the Pareto curve,and the LCOE and TEIP are 0.707 yuan/k Wh and 307.25×103m PE90,respectively,which achieves a relative balance between TEIP and LCOE.The thermodynamic model of solar power driven sCO₂ Brayton cycle integrated with ORC was established,and the sensitivity analysis and optimization of key parameters were carried out.The feasibility and advantages of intercooling Brayton cycle（IBC）and combined cycle design are verified by comparing independent cycle system and recompression system.The results show that thermal efficiency of SPT-IBC integrated system is 0.57%higher than that of SPT-RBC,and the thermal efficiency of combined cycle is 0.81%and 0.39%higher than that of single cycle respectively;among the ORC work fluids studied,isobutane has the largest increase in combined cycle,followed by R134a and R152a,and R245fa is the smallest.