Theoretical And Experimental Study On The Construction Of Novel Thermodynamic Cycle Based On Hydrate Separation

Organic Rankine cycle is the main method of energy recovery in the field of medium and low temperature thermal energy,but the thermodynamic perfection of laboratory-level organic Rankine cycle is generally less than 50%,which greatly limits the engineering application of organic Rankine cycle.How to improve the thermal efficiency and the utilization rate of medium and low temperature thermal energy has become one of the research priorities in the energy field in recent years.Working fluid,thermodynamic process and cycle structure are coupled to each other,which jointly affects the performance of thermodynamic cycles.Traditional research has typically focused only on the choice of working fluids,ignoring the matching relationship between the working fluids,thermodynamic processes and cycle structures.The organic Rankine cycle with variable composition arises as the research on the influence of the physical properties of working medium on different thermal processes becomes more and more thorough.In the composition adjustable organic Rankine cycle,there are several working fluids available.By appropriately changing the composition of working fluids,the physical properties of working fluids could be optimized and better matched with thermodynamic processes,and the thermal efficiency and thermodynamic perfection are higher than that of traditional organic Rankine cycle.The key to realize the composition adjustable organic Rankine cycle is how to realize the composition regulation of working fluids economically and efficiently.Limited by the shortcomings of the existing composition regulator such as high investment cost,low efficiency,difficult regulation and vague separation mechanism,this study innovatively proposed a working fluid composition adjustment method based on hydrate and constructed the corresponding thermodynamic cycle.The hydrate separation is applied to the composition regulation process of working fluids,and the hydrate formation/dissociation process is used to replace the traditional composition regulation process.In this way,the hydrate formation/dissociation heat could be coupled with organic Rankine cycle,and the separation work of composition adjustment process could be used in the form of heat.At the same time,the optimization and matching of working fluid and thermodynamic process are realized.Compared with the traditional composition regulators,hydrate separation has the advantages of energy saving and environmental protection,wide adjustment threshold,etc.,opening up a new way to accurately adjust the composition of mixed working fluids.These are the core academic ideas of this study.The main research contents and innovative results are as follows:（1）In order to incorporate hydrates into the framework of thermodynamic cycle analysis,the thermodynamic description of entropy changes in gas hydrate formation/dissociation process was carried out,and the entropy change calculation process suitable for different gas hydrates was determined and modified.Limited by the current lack of working fluid hydrate studies,the T-?s plot of CO₂ and N₂/CO₂mixtures in the formation/dissociation process was determined and applied to the thermodynamic cycle construction.The results show that the dissociation entropy change of gas hydrate fluctuates significantly with the dissociation temperature and pressure.Under 1296 operating conditions,the CO₂ separation and purification system was analyzed for CO₂ recovery rate,efficiency,power consumption and low-grade cold energy consumption.（2）For the first time,a working fluid composition regulation method based on gas hydrate was proposed,and its feasibility was verified at the experimental level.The mechanism of working fluid composition regulation based on hydrate is clarified,that is,the difference in the hydrate formation kinetics of different working fluids.Based on this,considerable composition adjustment was achieved for the mixed working fluids of different initial components（30 mol%to 70 mol%R125）with an adjustment range of 10 to 19 mol%.The effects of stirred and unstirred,constant temperature and cooling modes on the separation effect were studied.It is found that in the constant temperature mode,stirring could accelerate the heat transfer and mass transfer in the hydrate generation kettle,forming a morphological characteristic and spatial distribution that is obviously different from that of unstirred cases,increasing the initial formation rate of hydrate by about 200%,and doubling the gas consumption in the same time.Compared with the constant temperature mode,the cooling mode slows down the formation dynamic of the working fluid hydrate,but improves the separation performance,reflects the significant influence of temperature on the hydrate separation effect,and provides guidance for the next step of optimizing the working conditions of the working fluid component adjustment of the hydrate method.（3）On the basis of the feasibility of working fluid composition regulation by hydrate method being verified,a new type of combined cooling and power system based on hydrate is proposed,which introduces the regulation ability of gas hydrates to the working fluid mixtures and the formation/dissociation heat of the reaction into the thermodynamic cycle at the same time,which can realize the conversion between various energy forms.A three-dimensional T-s-θdiagram is proposed,since there is no reversible process in the actual situation,by selecting the shortest path in the working fluid composition adjustment process to achieve the initial optimization of the cycle structure,the irreversible loss of the actual cycle can be greatly reduced,in addition,the three-dimensional diagram method can also reduce the optimization workload.The second law efficiency of the traditional organic Rankine cycle using pure R22 or pure R32 is 0.48,while the second law efficiency of the system could be increased by up to about 20%when using R22/R32 hydrate as the heat exchanging medium.（4）In the framework of the hydrate-based thermodynamic model of the combined cooling and power system,the working fluid pairs suitable for the new combined cold-power cycle is optimized.Based on the existing study on the physical properties of working fluid and working fluid hydrate,there are currently 20 types of mixed working fluids,and the first three mixed working fluids in order of environmental protection are R1234yf-R152a>R1234yf-R32>R32-R152a,of which the mixed working fluids with higher second law efficiency of the corresponding system are R1234yf-R152a and R1234yf-R32.When the hydrate exhibited natural hydration separation for the mixed working fluids,the second law of the system corresponding to R32-R22 and R22-R152a was more efficient,between 0.3862～0.5345 and 0.5411～0.5428,respectively,and the change trend was stable;when the hydrate showed reverse hydration separation of the mixed working fluids,the selections of the superior working fluid pairs were greatly increased,including R1234yf-R152a,R32-R143a,R32-R125,R32-R22,R143a-R22,R22-R152a,and R134a-R152a,corresponding to the highest efficiency of the second law between 0.6098 and 0.7654.