| The winter climate in northern China is cold,and the production of overwintering crops in the north relies entirely on greenhouses.The maintenance of indoor temperature of solar greenhouse at night mainly relies on the heat released from the soil and the internal surface of the wall,and the wall with good heat storage and release performance can make fuller use of solar energy to accumulate heat for night use during the day.To this end,this paper researches and develops new thermal storage materials for greenhouse walls with the aim of improving the thermal storage capacity of greenhouse walls,and the main research contents and conclusions are as follows:（1）Preparation of paraffin/expanded graphite-acid modified diatomite composite phase change materials by atmospheric pressure impregnation method.Diatomite was used to shape encapsulate the paraffin wax to solve the problem of paraffin leakage and acid modification was selected for the treatment of diatomite.Scanning electron microscopy analysis was performed to analyze the reasons for the enhanced adsorption performance from a microscopic perspective.Expanded graphite was introduced as the thermal conductivity enhancement material,and a series of paraffin/expanded graphite-acid modified diatomite composite phase change material samples were prepared by varying the content of expanded graphite.The results showed that the paraffin loading in the composites increased with the addition of expanded graphite,with the highest loading of 62.7%for paraffin/10%expanded graphite-acid modified diatomite（10%EG-a DIAP）,which was 16.5%higher than paraffin/diatomite.（2）The structural property relationships of the homemade individual paraffin/expanded graphite-acid modified diatomite composite phase change material samples were investigated.The structural compatibility,phase change properties,thermal stability,thermal conductivity,thermal storage and discharge properties,and cyclic stability of the composite phase change materials were analyzed,and the best performance of the 10%EG-a DIAP was found among the produced samples,with good chemical stability,stable crystal structure,and destabilization temperature much higher than the application scenario temperature.The melting temperature of the produced 10%EG-a DIAP was 20.03℃,the latent heat of melting was 94.37 J/g,the solidification temperature was 18.62℃,the latent heat of solidification was 92.46 J/g,and the thermal conductivity was 0.593 W·m-1K-1,which was 114%higher than that of the sample without the addition of expanded graphite,and the thermal storage efficiency was 45.6%higher and the exothermic efficiency was 43.1%higher than that.After 200 cycles,the latent heat of melting of 10%EG-a DIAP was reduced from94.37 J/g to 91.04 J/g,and the latent heat of solidification was reduced from 92.46 J/g to 90.27 J/g,and the paraffin loss ratio was 0.41%,which was in a reasonable range.（3）To investigate the role of the prepared 10%EG-a DIAP composite phase change material in regulating the thermal environment of greenhouse.The composite phase change mortar mix was made of 10%EG-a DIAP and cement as the main raw materials and plastered evenly on the greenhouse walls.The temperature changes of each temperature measurement point under typical weather conditions were monitored and the data obtained were compared and analyzed.The average daily temperature was increased in all four locations of the test greenhouse:indoor,ground,at 10 cm from the ground and at 20 cm from the ground,but the 10%EG-a DIAP composite phase change material was more effective in regulating the indoor thermal environment during sunny days.In sunny weather,the average daily temperature of the test was about 1.7℃higher than that of the comparison greenhouse;in cloudy weather,it could be about 0.5℃higher.The temperature fluctuation in the test greenhouse was smaller than that in the comparison greenhouse.Under sunny weather,the maximum temperature in the test greenhouse decreased by 1.7℃～2.5℃and the minimum temperature increased by 1.2℃～1.8℃;under cloudy weather,the maximum temperature in the test greenhouse decreased by 1.2℃～2.6℃and the minimum temperature increased by 0.1℃～0.3℃.The composite phase change materials developed in this paper are used in greenhouse walls to make full use of solar energy,increase the temperature inside the greenhouse and improve the thermal environment inside the greenhouse,which can achieve the effect of"cutting the peaks and filling the valleys"and provide scientific design solutions for greenhouse wall optimization. |
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