Dynamic Performances Of Photovoltaic-thermal Systems With Microencapsulated Phase Change Fluids

With the ever-increasing consumption of fossil energy,the development and utilization of renewable clean energy are developing and have broad application prospects.The research on the utilization efficiency and energy storage of solar thermal energy is a subject that adapts to the development of recent energy status.Its combination with the phase change energy storage can greatly increase the energy conversion efficiency of solar energy.In this paper,a composite energy storage system for photovoltaic/thermal(PV/T)collector and microcapsulated phase change fluid is designed.In this article,the first chapter briefly describes the current status and development trend of energy,and introduces the application of microencapsulated phase change materials and their suspensions,and introduces their potential for solar collectors in solar photovoltaic solar thermal panels;In chapter two,a microencapsulated phase change material(MPCM)was prepared and characterized,and some existing microencapsulation methods were introduced.In the third chapter,the discretization equations of MPCM were evaluated using numerical methods to evaluate the flow and heat transfer characteristics of suspension of MPCM(MPCS),and the application of numerical calculation software in this work is introduced.In the forth chapter,MPCS is introduced into the PV/T collector and two PV/T/MPCS collectors are introduced.The equilibrium solution of the collector is obtained by establishing a physical model and solved by software.In the fifth chapter,by changing the design parameters and the MPCM thermophysical parameters,a design scheme for optimizing the energy output is proposed,and its dynamic performance in the actual environment is given.The analysis proved the superiority of this module compared to traditional water cooling.Finally,some suggestions were made for the next research.The main conclusions of each chapter are as follows.1.Preparation and thermal-physical of microcapsulated phase change materialA shape-stabilized microcapsule phase change material was prepared by a sol-gel method.Octadecane is used as a phase change material for thermal energy storage.Silica is used as a shell material to prevent octadecan leakage and improve the thermal stability of the microcapsules.Experimentally,microcapsules having a particle diameter of 500 nm to 2 ?m were obtained.The crystal structure is stable.The MPCM sample melts at 28.32?.The latent heat is 227.66 kJ/kg.It solidifies at 26.22 ?and the latent heat is 226.26 kJ/kg.TGA analysis shows that the sample has good thermal stability and can be used as a shape-stabilized MPCM for thermal energy storage.2.Flow and heat transfer characteristics of microencapsulated phase change material fluidUsing MPCS may result in better heat transfer performance by controlling the temperature of the working fluid.The phase change process strongly influences thermal convection heat transfer.The most important parameters are mass concentration and Stefan number.The 20 wt%suspension with Stefan number of 1.5 can reduce the wall temperature increasing rate by about 50%over the wall temperature increasing rate of water.The maximum local thermal convection enhancement ratio reached 190%.Thinner wall thick MPCM,larger particle size,and smaller phase transition temperature range have the positive effect.Longer and wider rectangular tubes are more conducive to releasing the latent heat and heat transfer of the MPCM.For the same heat load,the pumping power consumption of MPCS can be greatly reduced compared to water,but the effect of excessively increasing the concentration of MPCM is not significant.3.Dynamic performances of PV/T/MPCS collectorTwo PV/T collectors were designed and built,and the micro-models of the photovoltaic panel module were studied using simplified 2D and 3D models,respectively.The electrical and thermal efficiency of the collector was numerically simulated using CFD FLUENT.Based on the results obtained,it can be concluded that the two collectors and their corresponding 2D/3D models can reliably predict and analyze electrical and thermal performance.Based on this simulation method,the design parameters can be optimized.The presence of MPCM helps to reduce the temperature of the coolant and thus increase the heat transfer rate.This also lowers the temperature of the PV cell.In the simulation,the temperature change of the fluid can be reduced by about 20%,and the PV temperature can be reduced by about 21%.Both thermal and electrical properties can be improved.4.Energy output optimization of PV/T/MPCSCooling fluid mass flow is the most important design parameter to optimize the performance of CPC PV/T collectors.For the plate PV/T/MPCS collector used,the PV/T collector configuration achieves the highest net efficiency at a MPCS concentration of 10 wt%with the latent heat of 175 kJ/s at the flow rate of 0.02 kg/s.At the height of 10 mm,the overall net efficiency of the PV/T collector of the MPCS reached 80.57%at 13:00,about 1.8%higher than that of the conventional water model.The thermal efficiency at noon can reach 80%.The available energy is highest in the morning,at 11.4%.For the CPC PV/T/MPCS collector used,mass flow of I g/s per channel was applied.When the flow is relatively low,the use of fins in the fluid passage is more helpful.Solar radiation was the lowest at 18:00,with the highest thermal efficiency,78.5%for MPCS,71.9%for water,and the highest electrical efficiency,11.8%for MPCS,consistent with water.When solar radiation was highest at noon,the thermal efficiency of MPCS and water dropped to 58.6%and 55.3%,respectively,and the electrical efficiency of MPCS and water dropped to 11.4%and 11.2%,respectively.