Study On Calculation Model Of Charge/discharge In The Phase Change Process Of Shaped Hydrated Salt

The problems of energy consumption and environmental pollution promote the utilization and development of renewable energy.Solar energy is rich in resources and is one of the most potential renewable energy sources,but it has the characteristics of randomness,instability and intermittence.Therefore,heat storage technology has become one of the key technologies for the utilization of solar energy resources.Phase change heat storage technology has developed rapidly in recent years,among which inorganic hydrated salt phase change material(PCM)has attracted much attention because of its large volume heat storage density and low cost.Supercooling and phase separation are the main obstacles to the application of hydrated salt in short-term heat storage system.The composite setting of hydrated salt by adding porous matrix and other materials can significantly reduce the supercooling degree and phase separation,and may also improve the thermal conductivity.Therefore,the hydrated salt composite phase change material shaped by expanded graphite(EG)is studied in this paper.Numerical simulation is an important means to carry out the research on the heat storage performance and application of PCM.However,due to the complex crystallization process of hydrated salt and the phase change hysteresis,the materials will experience a variety of different working modes in the process of heat storage and release,and will show different states even at the same temperature,which increases the difficulty of simulation.The existing enthalpy method and equivalent heat capacity method models for simulating the conventional solid-liquid phase change process,solidification and melting model or multiphase flow model in CFD software can not solve the simulation problems of supercooling and crystallization process.At present,the only one-dimensional crystal growth calculation model of monomer hydrated salt phase transition is only for the cooling process,without considering the phase change hysteresis;Moreover,the supercooling degree of monomer materials is significantly greater than that of shaped composites,so it is doubtful whether the calculation method of crystal growth is suitable for shaped composites;In addition,in the actual phase change heat exchanger,only considering one-dimensional process can not meet the requirements.Therefore,this paper systematically studies the calculation method of the phase change process of shaped hydrated salt materials.Taking sodium acetate trihydrate(SAT)/ disodium hydrogen phosphate dodecahydrate(DHPD)-EG composite PCM as the research object,the material was prepared and its basic data were tested.The crystallization growth rate test experiment and casing heat PCM heat exchanger test-bed were designed and built.According to the charge/discharge characteristics found in the experiment and the calculation formula of crystallization growth rate obtained,a complete calculation model considerting crystallization growth,phase change hysteresis and partial phase change under dynamic conditions is established under a cylindrical coordinate system.Then,the accuracy of the model is verified by experimental data;On this basis,several calculation models for simplifying the crystallization growth process were proposed,and their calculation deviations are evaluated;Finally,using the calculation model established in this paper,a solar short-term heat storage system is simulated,and the performance of the system is evaluated.The specific work contents are as follows:(1)SAT/DHPD-EG composite PCM with different ratios(Hereinafter refereed to as shaped SAT)were prepared.The supercooling degree and phase change temperature of each sample were measured by cooling curve method.The leakage of materials with different ratios under the molten state was tested,and the appropriate samples were selected for charge/discharge experiment.The experimental system of heat storage and release of casing phase change heat exchanger is established,and the changes of temperature field under different working conditions are obtained,which provides experimental verification data for the numerical model of shaped hydrated salt established in this subject.In view of the fact that the existing test methods of crystal growth rate are only applicable to transparent monomer hydrated salt,an experimental method for testing the crystal growth rate and solidification time of opaque composites is proposed.Based on the experiment,the fitting calculation formulas of the two parameters of crystal growth rate and solidification time of shaped SAT are obtained.(2)Considering the phase change hysteresis and crystallization process,a two-dimensional phase change numerical model of shaped SAT heat exchanger in cylindrical coordinate system is established.The simulation results of the numerical model are compared with the experimental data to find out the shortcomings of the model.The simulation strategy of crystallization process,the control strategy of crystallization growth direction and the modeling and solution process of the numerical model are further improved.Based on the experimental data,the improved numerical model is verified and the accuracy is improved;The comparison of calculation time shows that the improved model increases the calculation speed by about 1500 times.(3)Aiming at the established numerical calculation model of shaped hydrated salt phase change process,three simplified models such as single-growth model,no-growth model and no-supercooling model are proposed.The accuracy of the simplified models are evaluated by comparing with the experimental data.The accuracy from high to low is complete model,single-growth model,no-growth model and no-supercooling model.Under 99 sets of boundary conditions,396 numerical simulations are carried out.Through the comparative analysis of the simulation results,the maximum relative deviations of the cumulative output heat of the three simplified models are 3.0%,9.0%and 32.7% respectively.(4)Under the actual application conditions,the PCM heat exchanger will switch between heating and cooling conditions.If the PCM is in the process of phase change when the condition changes,partial phase change may occur.In view of this situation,based on the numerical calculation model,the curve scale method is used to model partial phase change conditions,and the conversion and connection between various conditions are realized,so that the model can update the current conditions of each grid and solve them according to the current state and the boundary conditions under the dynamic HTF(heat transfer fluid)inlet boundary conditions.(5)A solar heating system with PCM heat exchanger and water tank in series is proposed,the numerical simulation model of the system is built,and the phase change numerical model established earlier is included.The influence of PCM physical parameters on the operation effect of the system is simulated and studied,and the matching optimization between system components is carried out.