Study On The Coupling Effect Of Ground Heat Storage And Indoor Thermal Environment In Plastic Greenhouses During Winter In Hot Summer And Cold Winter Regions

Along with the fast advancement of science&technology and the the economic level,the demand of citizens for"clothing,food,housing and transportation"is getting higher and higher.Meanwhile,in recent years,the country has gradually increased the importance of promoting rural revitalization,developing facility agriculture,as well as energy conservation and emission reduction,and promoting green development.In Chinese hot-summer and cold-winter regions,the commonly used plastic greenhouses,due to their ultra-thin lightweight envelope characteristics,and almost no heat storage and insulation capacity,so the indoor thermal environment can’t meet the normal growth of crops,may lead to low-temperature frost damage.In this background,it is very significant for agricultural engineering practice,to provide an energy-efficient heating method for the plastic greenhouses in the region,which can alleviate the problems of traditional heating of plastic greenhouses with high energy consumption and environmental pollution,and at the same time,improve the solar energy utilization.In this paper,a plastic greenhouse in Chengdu,a representative city in the hot-summer and cold-winter regions,was selected for the study.Firstly,we conducted research and on-site measurements on the plastic greenhouse,to measure the thermal environment parameters and outdoor meteorological parameters of the greenhouse.The measured data were used to investigate the indoor thermal environment of the plastic greenhouse,to explore the indoor ground heat storage and release,and to provide practical data support for the energy consumption simulation stage later.Based on the measured results,the physical model of heat transfer in the plastic greenhouse was established,and the mechanism of indoor thermal environment formation in the plastic greenhouse was analyzed.The heat balance analysis was carried out by combining the structural characteristics of the plastic greenhouse itself.Then,EnergyPlus,a building energy simulation software,was used to build a numerical model of the experimental greenhouse,which was used to numerically study the dynamic thermal environment of the indoor plastic greenhouse and the ground heat storage and release.Finally,based on the above analysis,we proposed to optimize the thermal storage performance of indoor ground by using PCMs.The optimization index of the ground was established,the impact of different thermal parameters of PCMs on the indoor thermal environment was simulated and analyzed by EnergyPlus,and the optimization strategy was proposed.The main conclusions of the study were as follows.(1)the same vertical surface of the plastic greenhouse,in the daytime,the air temperature increased with the increase in height,and the temperature change meet upper>middle>lower.At night,on the contrary,the air temperature decreased with the increase in height,and the temperature change meets lower>middle>upper.At the same time,the changing trend of surface temperature and the changing trend of indoor and outdoor air temperature was basically the same throughout the day,while the temperature at 10 cm underground was in a stable state throughout the day,with very little fluctuation,ranging from 15.1℃to 15.8℃,with a temperature difference of less than 1℃.(2)The heat transfer process of the ground in the greenhouse was mainly the absorption of solar radiation;convection heat exchange with indoor air,radiation heat exchange with the inner wall of the mulch layer,and heat transfer between the shallow soil and the adjacent deep soil.(3)In the sunny weather condition,during sunrise,under the main effect of solar radiation,the heat absorbed by the ground of the plastic greenhouse was greater than the heat that it dissipated to the surrounding environment,and the ground carried out heat storage.At the same time,the air temperature was higher than the ground temperature,and the maximum values were 18.35°C and 16.14°C,respectively.After sunset,there was no other source of heat on the indoor ground,and the ground was exothermic.The average heat flux per unit time was 45W/m~2～50 W/m~2,real-time heat release was about-1118.86k J,and the average heat storage conversion rate of the ground was only 35.6%.At night,the indoor air temperature was continuously lower than the ground temperature,and the indoor air temperature varied from2.52°C to 4.62°C,the indoor ground temperature varied from 7.87°C to 9.67°C,and the maximum temperature difference at night was 5.54°C.(4)The thermal storage properties of the ground were optimized by using phase change materials,and the values of the ground optimization evaluation indexes were 0.067,0.007,0.042,0.022,and 0.647 regarding the five thermal property parameters of thermal conductivity,specific heat capacity,density,latent heat value,and phase change temperature,respectively.The optimization effect of each thermal property parameter on the ground is from strong to weak in the order of phase change temperature>thermal conductivity>density>latent heat value>specific heat capacity.The conclusion and optimization strategy of this paper can provide data support and theoretical reference for the study of the indoor thermal environment of plastic greenhouses in hot summer and cold winter regions.