Optimization Design Method Of A Cascaded Phase Change Storage Based On Entransy Analysis

The energy storage technology can effectively improve the stability and flexibility of the energy system,which plays a role of peak shifting.So,it makes a difference in the optimization of China's energy structure.Phase change energy storage technology utilizes a large amount of latent heat absorbed/released by phase change materials in the phase change process to store energy.It has attracted many attention due to its high energy storage density and stable energy storage temperature.However,phase change energy storage technology also has some problems such as low thermal conductivity of phase change material and stubborn structure of phase change energy storage.Therefore,heat transfer enhancement of phase change energy storage process is a hot topic in the current research.In order to optimize the structure of phase change energy storage,the preparation method of composite phase change materials and the mechanism of flow and heat transfer in the phase change energy storage process is studied in this paper,which aims to provide a reference for the promotion and application of phase change energy storage technology.The specific research contents are as follows:(1)According to the theory of phase equilibrium and the second law of thermodynamics,the prediction model of the phase change temperature and phase change enthalpy of composite phase change materials are established.Then,the model is verified through the thermal physical property tests.The results show that the predicted values and measured values coincide well.The deviation between measured phase change temperature and predicted values is just 1 ?.Meanwhile,the deviation of phase change enthalpy is within 4%.On this basis,the flow and heat transfer model of phase change energy storage process is established.The influence of natural convection on phase change process is investigated in this model.It was found that compared with melting process,natural convection has less influence on heat transfer during solidification process.Finally,a comprehensive evaluation system of phase change energy storage process is established based on the entransy theory,which lays a foundation for the optimization of phase change energy storage.(2)In order to improve the performance of the phase change cold storage packed bed used in air conditioning,the flow and heat transfer model was established and the experimental verified.Then,the construction method of cascade packed bed is optimized and evaluated by entransy theory.The results showed that the RMSE of the heat transfer temperature difference in the whole process was only 0.3?,which was18.1% lower than that of the single-stage packed bed.For the cascade construction method,the layers of 3-6,the temperature difference of 2.5? and the dimensionless diameter of 0.1 are optimal values.With those parameters,the energy storage rate of the cascade cold storage packed bed is 13.5% higher than that of the single-stage cold storage packed bed.(3)After the construction of cascaded phase change packing bed,the construction method of shell and tube phase change energy storage is also studied.By comparing characteristics of flow and heat transfer,it is found that the phase change time of cascaded inner flow storage is 11.4% less than that of single stage storage;as for outer flow storage,the time reduction is 4.9%.When the volume fraction of PCM is 0.3,the phase change time for the inner flow structure is only 33.9% of that of the outer flow structure.Moreover,the stage of 3,the temperature difference of 2.5?and the length setting of 7:8:9 are better choices for the inner flow structure.The energy storage rate is increased by 14.1% compared with the single-stage energy storage.(4)Based on the cascaded phase change packing bed,the packing mode of each stage was further studied.Firstly,four kinds of ordered packing schemes are summarized based on spatial lattice.And then,the flow and heat transfer characteristics of the pack bed under different packing schemes are researched by numerical simulation.The results show that the face-centered cubic packing has the best temperature-velocity synergy while the simple cubic stacking has the best pressure-velocity synergy.Therefore,the face-centered cubic packing has the best heat transfer performance and the simple cubic stacking has the lowest pressure drop.In general,face-centered cubic is an optimal choice.Not only its storage capacity is53.1% higher than that of simple cubic,but also its energy storage rate is also 6.1%higher than that of simple cubic.(5)Based on the cascade shell and tube storage,the configuration of fins is optimized by topology optimization.The optimization results show that the configuration of fins in shell and tube phase change storage be designed as a dendritic bifurcated configuration with some fractal characteristics.Compared with the straight fins,the phase change completion of the topology structure is improved by 20%.If the topological optimization is carried out based on 2-tube or 4-tube storage,the energy storage rate can be increased by 10.4% and 23.9% again.Finally,the two-dimensional structure is extended to the three-dimensional space.The results show that the three-dimensional topology optimization configuration is better than the two-dimensional stretching configutation.