Investigation Of The Effects Of Compressor Inlet Temperature On The Cycle Efficiency In S-CO₂ And CO₂-Based Binary Mixtures Brayton Cycle

Supercritical carbon dioxide（S-CO₂）is an environment friendly gas medium with low viscosity,high density,high diffusion coefficient and moderate critical parameters.It has good properties such as chemical stability,nuclear physical properties,safety,non-toxicity and low cost.It can be used as working substance in thermodynamic cycle systems.S-CO₂ Brayton cycle is smaller in size and simpler than steam Rankine cycle.Besides,cycle efficiency of S-CO₂ Brayton cycle is higher.And the S-CO₂ Brayton cycle has become one of the most promising thermodynamic cycles for various energy conversion systems.If the compressor inlet temperature is close to pseudo-critical region,S-CO₂ recompression Brayton cycle will reach its highest efficiency.However,when operation environment of naval vessels with S-CO₂ recompression Brayton cycle changes（in different ocean space）,the compressor inlet temperature will change,and cycle efficiency will change greatly.In order to make Brayton cycle adapt different ambient,it’s necessary to study the effect of compressor inlet temperature on thermal efficiency and exergy efficiency,and proposing new cycle layouts and control methods to solve the problem.This paper established thermodynamic analysis model of S-CO₂ recompression Brayton cycle.The model was verified by the results of Massachusetts Institute of Technology.By analyzing the parameter analysis results of the model,it’s found that increasing the compressor inlet pressure and decreasing the split flow ratio（the mass flow rate flows through the recompressing compressor divides by total mass flow rate）can decrease the rangeability of efficiency.Combined with physical property changes of CO₂in critical region,this paper proposed modified recompression Brayton cycle.Based on recompression Brayton cycle,this cycle added a recuperator that connected with main compressor.Without excessive increase of system complexity,this cycle can decrease the inlet temperature of recompressing compressor,and keep the recompressing compressor work close to the critical temperature of CO₂.The compressor work and loss of compressor can be decreased.Modified recompression Brayton cycle can reach similar efficiency as recompression Brayton cycle,and can decrease the variation amplitude of cycle efficiency when compressor inlet temperature increases.This cycle increases adaptability to different temperature environments.The critical temperature of CO₂ is 31°C.To reach high efficiency,the compressor inlet temperature in S-CO₂ recompression Brayton cycle should be close to critical temperature.However,this cycle cannot adapt ambient temperature with large changes.Changing the critical parameter of working substance by adding other gases can help cycle to adapt different temperature environments.This paper studied combination scheme of multiple mixtures,and found using CO₂-Kr mixtures as working fluid can achieve higher efficiency and decrease the variation amplitude of cycle efficiency.This paper performed molecular dynamics analysis about CO₂-Kr mixtures to study the material properties and its effect on cycle.The simulation results are verified by Radial distribution function（RDF）and density.The results indicate CO₂-Kr mixtures will not change the microstructure and stability of CO₂ molecule.However,CO₂-Kr mixtures have lower aggregation degree at high density than that of pure CO₂.The temperature of maximum isothermal compressibility will change for CO₂-Kr binary mixtures.For Brayton cycle with CO₂-Kr binary mixtures,to take advantage of the compressibility of fluid,the compressor inlet temperature of Brayton cycle should be set equal to pseudocritical temperature.This can help to increasing cycle efficiency and reducing temperature sensitivity.The critical temperature of working substance can be changed by adding other gases.However,other properties will also change.Critical temperature is not the only properties that can affect cycle efficiency.This paper selected several typical additive gases（CO,O₂,CH₄,Kr and SO₂）,and discussed the feasibility of using mixtures physical properties as criterion to judge if CO₂-based binary mixtures can improve thermal efficiency combined with physical properties and recompression cycle process.Results show that within the temperature range of compressor,the specific heat capacity should be higher than S–CO₂.In this condition,compressor works in the high-density region of mixtures.The compression work can decrease and thermal efficiency will increase.At the same temperature ranges,CO₂-based binary mixtures that have larger entropy difference can achieve higher net work per unit mass than CO₂,and can achieve higher thermal efficiency than CO₂.When the maximum temperature of cycle is fixed,lower maximum enthalpy of CO₂-based binary mixtures can reach higher cycle efficiency.The criterion proposed in this paper can help to select proper addictive gases for recompression Brayton cycle rapidly,and provide reference for evaluating the adaptability of recompression cycle to different temperature environment.