Study On Optimization Of Heat Transfer In Phase Change Composite Wall Of Solar Greenhouse

Solar greenhouse is an agricultural building facility for growing vegetables in winter,with solar energy as the main heat source,which is widely used in the cold areas in the north of China.Therefore,the emergence of solar greenhouse not only solves the problem of lack of fresh vegetables in winter in the north,but also increases farmers’income.Due to complex climatic factors,traditional solar greenhouses often suffer from problems such as cold damage and frost damage during actual production,thus reducing production efficiency and causing economic losses.Combining the concept of green environment protection and efficient use of solar energy,increasing the heat insulation,heat storage and heat release capacity of greenhouse walls themselves is an effective way to improve the thermal performance of heliostats.As a new type of energy storage material,phase change material absorbs heat from the environment during the process of material transformation and releases it when needed,thus achieving the purpose of controlling the environmental temperature.Therefore,combining with the working principle of heliostat production,phase change materials with suitable phase change range are selected and applied to heliostats,so that they can absorb more heat in the greenhouse during daytime and release it at night,thus significantly improving the heat storage and release capacity of heliostat enclosure structure and improving the temperature environment for crop growth.At present,the research on phase change materials for heliostat walls mainly focuses on the ratio,application form and practical application effect of phase change materials for heliostats,but there is a lack of theoretical research on the heat storage and heat transfer characteristics of phase change walls.Therefore,this project aims to regulate the thermal environment in the greenhouse by improving the heat storage capacity of the north wall of the heliostat,and uses numerical simulation and experimental verification to reveal the heat transport law in the heat storage and exothermic process of the phase change composite wall in the heliostat,so as to provide theoretical guidance for the optimal design of the phase change composite wall.Firstly,the optimization analysis of the phase change wall in heliostat is carried out,and the optimal location and thickness of the phase change material layer are investigated.The physical model was established according to the construction method of heliostat back wall,and the heat transfer model of phase change composite wall was established based on CFD,and the effect of four laying positions and seven different thicknesses of phase change composite wall on its heat transfer performance was simulated and studied.The results show that when the phase change material layer is laid on the interior side,the temperature change of the inner surface of the wall is the most obvious and the wall has the best thermal insulation and heat storage effect.The thicker the phase change material was laid on the north wall of the greenhouse,the higher the surface temperature inside the wall and the smaller the heat loss.From the perspective of economy,the thickness of 40mm of phase change material layer is selected as the optimal thickness of the wall.Secondly,the influence of the thermal property parameters of the phase change material on the heat transfer performance of the phase change composite wall was studied.The influence of density,specific heat capacity,thermal conductivity and latent heat of phase change on the heat storage and discharge performance of phase change composite wall was analyzed by numerical simulation using single-factor and composite-factor controlled variable method,and the accuracy of numerical simulation results under the same conditions was verified by experiments.The results show that the integrated error analysis（IA）value between the numerical simulation results and the experimental results under the same experimental conditions is 0.96,which indicates that the numerical simulation results of the phase change composite wall have high accuracy and reliability.The thermal property parameters of the phase change material have obvious influence on the thermal storage and exothermic performance of the composite wall,and the maximum temperature of the inner surface during thermal storage is inversely proportional to the variation law of the thermal property parameters,while the maximum temperature of the inner surface during exothermic storage is positively proportional to the variation law of the thermal property parameters.The increase of thermal conductivity can significantly increase the maximum temperature of the inner surface of the phase change wall in the exothermic phase.The influence of single thermal property on the change of heat storage and exothermic properties of phase change composite wall was ranked from strong to weak:change of density>change of thermal conductivity>change of latent heat of phase change>change of specific heat capacity.In the analysis of the influence of composite factors on the heat storage and heat release performance of phase change composite wall,it is found that n-Octadecane and Ca Cl₂-6H₂O phase change materials are more suitable for the phase change composite wall model constructed in this study.N-Octadecane can be stable at a lower temperature,while Ca Cl₂-6H₂O can be stable at a higher temperature.Finally,the dynamic changes of indoor temperature environment of phase change composite wall heliostat and common heliostat were compared and analyzed.The results show that the average temperature of phase change composite wall solar greenhouse was 1℃lower than that of ordinary solar greenhouse during daytime.After the insulation was put down,the temperature inside the composite wall solar greenhouse was always higher than that inside the normal solar greenhouse,and the average temperature at night was 1.5℃higher than that of the normal solar greenhouse.When comparing and analyzing the soil temperature in the phase change composite wall solar greenhouse and the normal solar greenhouse,it was found that the soil temperature in the phase change composite wall solar greenhouse changed relatively smoothly.After the insulation was opened,the soil temperature of the phase change composite wall solariums warmed up at a lower rate.Therefore,the phase change composite wall can effectively reduce the fluctuation of air temperature and ground temperature in the greenhouse,and finally realize the effect of"cutting the peak and filling the valley"on the temperature environment in the greenhouse.