Preparation Of Composite Phase Change Material For Solar Greenhouse And Optimization Of Wall Structure Composition

The solar greenhouse occupies an important position in the facility agriculture.The heat storage wall of the solar greenhouse maintains a stable indoor temperature through heat storage and release to ensure crop growth.However,the north wall of the traditional solar greenhouse mostly uses clay and brick walls as heat storage materials,which has problems such as environmental damage,waste of resources,and high construction costs.It can no longer meet the development needs of modern solar greenhouses.It is urgent to develop new,low-cost,energy-saving and environmentally friendly Thermal storage wall material.Phase change material（PCM）is to store and release heat by absorbing and releasing heat through the physical phase change process,which can maintain the temperature in the greenhouse to be stable near its phase change point,so as to achieve the purpose of regulating room temperature and promoting crop growth,becoming a new type of heat storage Research hotspots of wall materials.In this study,the binary system of phase change materials Na₂SO₄·10H₂O and Na₂HPO₄·12H₂O was used as the heat storage substrate,and the influence of the two-phase composition ratio on the phase change latent heat,melting point temperature,crystallization temperature and subcooling degree of the binary system was studied;KCl was used as Phase change temperature regulators and super absorbent resins are used as carriers to modify the binary phase change system;use physical methods to prepare binary system composite shape-setting phase change materials,and use 3 factors and 5 levels orthogonal test to determine the binary system composite The optimal ratio of various parameters of the shaped phase change material.The selection of materials for each layer of the phase change greenhouse and the design of the greenhouse structure were studied in order to obtain a light-weight assembled solar greenhouse structure for the phase change greenhouse.Use comsol to simulate the greenhouse to obtain the optimal thickness of each layer,and finally perform a proportional reduction greenhouse model for verification.The main conclusions are as follows:（1）Modification test of Na₂SO₄·10H₂O-Na₂HPO₄·12H₂O system.With the increase of Na₂HPO₄·12H₂O,the crystallization point,subcooling degree and latent heat value of phase change of Na₂SO₄·10H₂O fluctuate,and the melting point temperature is close to the theoretical value.With the increase of KCl,the crystallization point,melting point and latent heat value of phase change of the binary system gradually decrease.The degree of subcooling fluctuates;when the superabsorbent resin increases,the degree of subcooling of the binary system first decreases and then increases.The crystallization temperature and phase change latent heat value first increase and then decrease.The phase change material has no phase stratification and no liquid.（2）Compound shaping process parameter optimization and performance test.The optimal combination of optimal parameters is KCl content 6.11 wt%,resin content 6.99wt%,Na₂SO₄·10H₂O content 18 wt%;the verified results show that the latent heat value of phase change is 145.18 J/g,melting point is 21.45℃,crystallization point is 12.91℃,and degree of subcooling.At 1.55℃,the test result is basically consistent with the optimized result.The final prepared composite shape-stable phase change material has no liquid phase leakage after macroscopic heating.After 100 cycles,the latent heat of phase change is 127.8 J/g,which is attenuated by 11.2%compared with one cycle.It has good thermophysical properties and cycle stability.Observed by the field emission scanning electron microscope（SEM）microscopic morphology,most of the composite inorganic phase change salt is wrapped in the three-dimensional network structure of the resin.（3）Based on comsol greenhouse heat transfer simulation and greenhouse model test research.The material of the insulation layer is EPS foam polystyrene board;the insulation layer is cork;the wall body suitable for phase change heat storage materials is designed;comsol simulation is carried out,the thickness of EPS foam polystyrene board is 60 mm,and the thickness of cork block is 200 mm.The variable material thickness is 20 mm;the experiment is conducted through a scaled-down greenhouse model,and the difference between the lowest indoor temperature and the lowest outdoor temperature in a day of sunny,cloudy,rainy,extreme sunny,and extreme cloudy phase change greenhouses is at least higher than that of the other two greenhouses 4.402℃,1.08℃,0.35℃,2.606℃and2.106℃,the difference between the average indoor temperature at night and the average outdoor temperature at night in the phase change greenhouse is at least 5.242℃,1.348℃,0.052℃,4.119℃and 1.544 compared to the other two greenhouses℃.Therefore,the phase change material can increase the indoor temperature of the greenhouse under various weather conditions.Among them,the improvement effect is the best on sunny days,followed by cloudy days,and the worst on rainy days;under extreme weather,sunny days are the best and cloudy days are the worst.