Research And Optimization Of High Temperature Phase Change Regenerator For Solar Thermal Utilization

As an important renewable energy source,solar energy is widely used in thermal power generation and other fields.However,the dispersion,discontinuity and instability of solar energy make it difficult to match the supply and demand,which reduces the efficiency of solar thermal power generation.Therefore,it is necessary to develop heat storage technology to store the temporarily redundant energy,so that it can operate continuously and be used efficiently.In this paper,the heat storage characteristics of the high temperature phase change heat.exchanger of the cuboid unit are studied deeply,which has strong theoretical value and application value.Firstly,a three-dimensional numerical model of the high temperature phase change heat exchanger was established,and the heat storage characteristics and heat transfer law of the traditional cylindrical unit heat storage model were studied by numerical simulation.Due to the disadvantage of the slow phase change speed of the cylindrical unit heat storage model in the later stage of heat storage and discharge,the heat storage model of cuboid unit was proposed,and the heat storage and discharge processes of two different models were compared and analyzed Then,based on the established simulation model of cuboid unit heat storage,the influence of dimensionless numbers such as Fo number,Ste number,Re number and different phase change materials on the heat storage characteristics was explored,and the universal heat exchange criterion of cuboid unit heat storage model was proposed.Finally,the cuboid unit heat storage unit was optimized from the aspects of the structure and cascade heat storage.The influence of different size ladder units on the heat transfer performance was compared.The cascaded heat storage process of different phase change materials was studied,and the mechanism of improving the heat transfer performance of the multi-stage phase change materials was revealed.Above study found that:(1)In the heat storage and discharge process of the cylindrical heat storage unit,there are shortcomings of slow rate and long time at the end of melting and solidification.The average liquid phase rate of PCM from 0.8 to complete melting accounts for 30.1%of the total melting time;the average liquid phase rate of PCM from 0.2 to complete solidification accounts for 32.9%of the total solidifying time.(2)The heat storage performance of the cuboid heat storage unit model is better than that of the cylindrical heat storage unit in the heat storage and discharge process.The overall trend of the heat storage and discharge process of the two is similar,but the time spent by the cuboid heat storage unit model in the heat storage and discharge process is 12.1%and 10.6%less than that of the cylindrical heat storage unit model,respectively,while the heat storage and discharge efficiency is increased by 33.2%and 10.1%.(3)The flow state of the cuboid heat storage unit model during heat storage and discharge is better than that of the cylindrical heat storage unit.The cylindrical unit model has a large velocity loss at the inlet,and the overall flow velocity in the flow direction is low,resulting in a very slow flow of the heat storage unit near the outlet,which causes a flow dead zone.The cuboid unit has a high flow velocity in the flow direction,and the outer surface of the PCM can guide the flow of HTF.This unit form can eliminate the flow dead zone between the cylinder units,and the overall flow resistance is small.(4)For the cuboid heat storage unit model,as the Fo number increases,the Nu number decreases exponentially,then tends to be flat,and finally slowly decreases;increasing the Ste number and Re number can increase Nu number and reduce the melting time;but increasing the Ste number will increase the heat storage efficiency,increasing the Re number will reduce the heat storage efficiency;although increasing the melting point of PCM can increase the heat storage,it will reduce the heat storage efficiency.In the selection of PCM,the phase change material with appropriate melting point should be selected on the basis of comprehensive consideration of the thermal properties of HTF and the working conditions of heat storage and discharge;the cuboid heat storage unit model The heat exchange criterion is:Nu=14.6382 Re0.2158Pr1,3.(5)Because the heat transfer and flow conditions of the HTF of the ladder-type heat storage model can better match the PCM volume.The complete melting time of the ladder-type model is 11.5%less than that of the cuboid heat storage model,and the average Nu number is 7.8%higher.Reducing the cross section area ratio of the near inlet and outlet of the ladder can improve the melting rate of PCM.At the same time,it can also enhance the heat storage efficiency and average Nu number.PCM cascade heat storage can enhance the consistency of PCM melting.Compared with the single heat storage model,when the complete melting is arrived,the Fo number of cascade heat storage is reduced by 5.2%.Heat storage increased by 10.2%.average Nu number increased by 8%,and heat storage efficiency increased by 12.8%.The total melting time of PCM is reduced by 20.8%compared with that of cuboid heat storage model,and the heat storage performance is greatly improved.