| Photo-thermal conversion phase change material,as an energy-saving novel material,can convert solar energy into heat and store it as latent heat in the phase change material to release on demand,which has far-reaching implications in dealing with global warming and energy crises.Plastic greenhouses,which are a widespread agricultural production facility in southern China,have a simple agricultural film support structure and strong light transmission to provide natural photo-thermal conversion conditions.If photothermal conversion phase change material can be employed in plastic greenhouses,the problem of overcooling in the greenhouses at night will presumably be solved,and energy efficiency will improve.Therefore,in this thesis,woody carbon-based composite phase change material with photo-thermal conversion performance were prepared,and the prepared woody carbon-based composite phase change material were used in small plastic greenhouses as the test object to investigate the effect of their thermal storage and temperature control performance on plastic greenhouses.First,the composite phase change material with good photo-thermal conversion performance was prepared.The carbonized wood substrate was made by delignification pretreatment using Blasa wood as the carbon source,and it was found that the carbonized wood under different carbonization holding time retained the natural pore structure of wood,and the porosity of carbonized wood could be as high as 92.03%.The photo-thermal conversion composite phase change material was prepared by adsorption of paraffin wax by vacuum impregnation.The effective impregnation rate of paraffin wax was 85.25 wt.%,the enthalpy was 206.3 J/g,and the photo-thermal conversion efficiency could reach 90%.Further,deeply carbonized wood at different carbonization temperatures was used as the support material and thermoplastic elastomer was used to pretreat the paraffin wax by melt blending method to achieve the purpose of improving the thermal conductivity and deformation effect of the composite phase change material.The shape stability and thermal reliability of the composite phase change material were good after 200 accumulation and exothermic cycles.The thermal conductivity of the samples was jointly influenced by the effective adsorption rate of carbonized wood matrix and paraffin wax.CW300/OP44E/SEBS thermal conductivity was improved by 27.8% compared to CW-1/OP44 E,and CW500/OP44E/SEBS thermal conductivity was improved by 73.9% compared to CW300/OP44E/SEBS,with a maximum photo-thermal conversion efficiency of 91%.Finally,the experimental scheme of photo-thermal conversion composite phase change material for heat storage and temperature control in plastic greenhouses is designed in Chengdu area as an example.The experiments were conducted using a combination of phase change material and insulation cotton.The temperature of phase change material,air temperature,and soil temperature in the plastic greenhouses were monitored hourly for 24 hours in sunny,cloudy,and overcast conditions.The following conclusions were obtained:(1)The main heat storage phase of the day for phase change material was 11:00–15:00,which had a significant effect on mitigating temperature fluctuations in the arches.(2)In cloudy and overcast conditions,the photo-thermal conversion phase change material was more advantageous in temperature regulation than ordinary phase change material.(3)Due to the shading effect of the photo-thermal phase change material arrangement on the greenhouse space,the soil overheating was better than that of the control greenhouse,which was more favorable to the crop root growth.(4)For sunny conditions,increasing the amount of phase change material can extend the phase change the exothermic length to 7.4 hours in the insulation phase.The greenhouse with phase change material can raise the temperature by 5.2 ℃in the insulation phase.(5)When combined with daytime ventilation,it can reduce the accumulation of daytime heat in the greenhouse.The greenhouse with phase change material can shorten the daytime overheating time by 46.2% and increase the nighttime temperature of the greenhouse by 39.3% compared with the control one.The effect of "peak-shifting and valley-filling" is obvious,which confirms the heat storage and temperature control performance of photo-thermal conversion phase change material in plastic greenhouses. |
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