A Performance Study On CO₂-based Mixture Working Fluids Used For The Dry-cooling Supercritical Brayton Cycle

Supercritical carbon dioxide Brayton cycle is known for its high thermal efficiency and compact structure,and has huge application potential in the field of solar thermal power generation.In arid and water-scarce areas,dry cooling is the most practical choice for CSP systems,but its power generation efficiency is significantly affected by ambient temperature,and there is a negative correlation between the two.In response to this problem,the use of high critical temperature CO2-based mixture working fluid is expected to become one of the feasible solutions for the CSP system based on the supercritical carbon dioxide Brayton cycle.To this end,this paper takes dry cooling supercritical recompression Brayton cycle with a rated output power of 50MW as the object to study the effect of mixture working fluid on the thermodynamic characteristics of the cycle under full condition and the heat transfer characteristics of the recuperator.Firstly,Matlab software was used to establish a CO2-based mixture working fluid supercritical Brayton cycle model under design conditions and mixture working fluid thermal properties calculation model.To raise the critical temperature of CO2 with considering the thermal stability,corrosiveness and environmental safety factors,five gaseous additives suitable for CO2,such as cyclohexane,butane,isobutane,propane and hydrogen sulfide,were screened out.The optimal operating parameters of the basic mixture working fluid include main compressor inlet pressure,the split coefficient and the thermal conductivity distribution ratio of the high and low temperature recuperators.On this basis,the sensitivity analysis of parameters such as main compressor inlet temperature,turbine inlet temperature,main compressor outlet pressure and the total thermal conductivity of the high and low temperature recuperators was further carried out.Studies have shown that the five CO2-based mixture working fluids can significantly improve the cycle thermal efficiency,and CO2-H2S has the best performance.Increasing the inlet temperature of turbine and the outlet pressure of main compressor can improve the cycle thermal efficiency.The former is more effective,while the latter has a negligible effect on the cycle thermal efficiency when it reaches more than 30MPa.Secondly,based on the design conditions,the model of CO2-H2S mixture supercritical Brayton cycle under off-design conditions was established.The cycle performance was studied with the split coefficient,spindle speed,main compressor outlet pressure,main compressor inlet temperature and turbine inlet temperature.The results show that when main compressor inlet temperature deviates from the design conditions,a lower inlet temperature will result in higher cycle thermal efficiency.Increasing inlet temperature reduces the net output work,and its decrease is greater than that of lower inlet temperature.Under the rated load,main compressor inlet temperature rises,the cycle thermal efficiency shows a downward trend,and decreases greatly at fractional load;the turbine inlet temperature increases,the cycle thermal efficiency increases accordingly,and fractional load also leads to significantly reduce.Finally,Fluent 16.0 software was used to establish a two-channel three-dimensional model of a printed circuit board heat exchanger(PCHE),and the flow and heat transfer characteristics of the CO2-H2S mixture working fluid in the channel were numerically simulated under high and low temperature conditions.The results show that,with the higher thermal conductivity,the convection heat transfer coefficient of cold flow is much higher than that of heat flow under low temperature conditions.With the increase of mass flux,the heat transfer capacity of the fluid is greatly improved,but it will also cause more pressure drop,especially the pressure drop will have a greater increase for higher mass flux.Based on the numerical simulation results,the origin software was used to fit the correlation of the convection heat transfer and the resistance of the straight channel.The deviation between the prediction results of the two correlations and the numerical simulation results was within 3.5%and 3%,respectively.