Thermaldynamic Analysis Of Supercritical Brayton Cycle Using CO₂-Based Binary Mxitures For Solar Power Tower System Application

Solar energy,which is abundant and convenient to develop,will become the main energy source in the future.In the field of solar thermal utilization,the mode of solar power tower（SPT）integrated with the supercritical CO₂（S-CO₂）Brayton cycle can achieve outstanding thermodynamic performance.To maintain the efficiency advantages of the S-CO₂ Brayton cycle at high ambient temperature,using CO₂-based binary mixtures to increase the condensation temperature and retain the advantage of the CO₂ at the critical point is one of the viable options for the S-CO₂ Brayton cycle in SPT plant application.Therefore,this paper has carried out a systematic study on the thermodynamic performance and operation behavior of the S-CO₂ Brayton cycle in the SPT system enabled by CO₂-based binary mixtures under design and off-design conditions.Firstly,propane,butane,and hydrogen sulfide are selected as the second additives of CO₂.Based on the thermodynamic equilibrium equation,a recompression Brayton cycle model for CO₂-based binary mixtures is established,and the accuracy of the model is verified.A comprehensive thermodynamic analysis under the typical operation conditions,such as turbine inlet temperature,turbine inlet pressure,and main compressor inlet temperature,is conducted based on the optimal split ratio.The results show that the thermal efficiency and exergy efficiency of the cycle after adding the second additives are better than the original at high condensing temperature,and the performance of CO₂-propane cycle remains the most stable,which is the best choice in the SPT plants.Secondly,the targeted structural optimization for the original cycle is adopted by adding a second high temperature regenerator（HTR）to reduce the exergy loss and using the organic Rankine cycle（ORC）for waste heat recovery.Meanwhile,the improved power cycle subsystem is integrated with the heliostat field and the direct-heated receiver.The results show that the thermal efficiency is improved by increasing the bottom cycle,but the kind of organic working fluid has little effect on the overall cycle performance of the SPT plant.Adding a second HTR and setting a bypass flow device can reduce the exergy loss,thus improving the overall cycle efficiency.Thirdly,an optimization model aiming at the thermal efficiency of the cycle is established based on a genetic algorithm.The optimal operating conditions of CO₂ and CO₂-propane are obtained respectively,and a systematic evaluation based on exergy analysis is carried out.The results show that the SPT plant overall thermal efficiency with double HTR（34.95%）is higher than the ordinary recompression Brayton cycle（34.21%）under the same working conditions.The cycle performance of CO₂-propane becomes more significant when the cooling temperature increases,the SPT thermal efficiency using CO₂-propane is 2.08%higher than CO₂ at 50°C.Lastly,a two-tank thermal energy storage system and an auxiliary fossil fuel back-up system are connected in parallel in the direct-heated SPT plant for solving the intermittence and instability of solar energy.In the same time,the corresponding operation strategy is proposed by flexibly scheduling solar,molten salt,and fossil fuel energy supply,and the off-design performance and operation behavior of the SPT plant before and after adding propane is compared.The results show that the operation strategy can realize all-weather energy supply for the direct-heated SPT plant,and adding propane to CO₂ can prolong the time for the system to effectively use solar energy,which is 0.3 hours more than that of the CO₂ system at the summer solstice and 0.2 hours more than that of the CO₂ system at the winter solstice.