| The solar greenhouse is widely used in northern China because of its excellent thermal insulation and energy saving characteristics.As the most important enclosure in the solar greenhouse,the wall has the functions of bearing,heat preservation and heat storage,which plays a vital role in maintaining the stability of the greenhouse temperature.Wall,as the most important enclosure structure in solar greenhouse,has the functions of bearing,heat preservation and heat storage,and plays a vital role in maintaining the stability of greenhouse temperature.On the one hand,the heat storage capacity of the wall is related to the heat storage characteristics of the wall materials,on the other hand,it depends on the thickness of the heat storage layer.Therefore,determining the thickness of solar greenhouse heat storage layer is of great significance to promote the improvement of solar greenhouse walls and the development of facility agriculture.Therefore,from the perspective of exploring the thickness of the regenerative layer of greenhouse wall,greenhouses were tested in this paper,model calculation and virtual simulation to control the solar radiation illuminance and the internal temperature of the wall.factors were studied.Firstly,on the basis of summarizing the previous models of solar radiation,a relatively perfect solar radiation model of solar greenhouse was established in this paper.Meteorological data,the laws of the earth and the movement of the sun,and the relationship between the sun’s rays and the angle of the front roof were all considered in this solar radiation model.Internal radiation change of the greenhouse was analyzed based on the model.Secondly,parametric model according to the actual size and correlation of the test greenhouse was created based on the solar radiation and air temperature.Thickness of the wall thermal storage layer in different months were simulated in this study.The solar radiation on the greenhouse floor and wall surface were used as the input condition,and the outdoor air temperature was the boundary conditions.Finally,the internal temperature changes of the solar greenhouse walls with different wall types were tested,and the thickness of the heat storage layer of the walls were calculated through the attenuation changes of the internal temperature of the wall.The main conclusions are as follows:(1)On the basis of summarizing the previous models of solar radiation,a relatively perfect solar radiation model of solar greenhouse was established in this paper.Meteorological data,the laws of the earth and the movement of the sun,and the relationship between the sun’s rays and the angle of the front roof were all considered in this solar radiation model.Internal radiation change of the greenhouse was analyzed based on the model.The model was verified in typical sunny days.The results showed that there were some differences between the calculated and measured solar radiation values,the MBE(mean bias error)was 63.46 W/m~2,the MAE(mean absolute error)was 63.48 W/m~2,the RMSE(root mean square error)was 79.18 W/m~2 and the coefficient of determination R~2was high,ranged from 0.95 to 0.99.The model can be used to calculate the solar radiation intensity of the wall and the ground of solar greenhouse in any area anytime.Highly accurate analysis of the radiation change and the variation of light transmittance in the greenhouse in different seasons were achieved in this paper.(2)Through the analysis of solar radiation model,it is found that the incident angle and light transmittance of the front roof of the greenhouse do not change obviously with time within 4 h around noon.At the same time,the incident angle at each position on the roof changed obviously,but the light transmittance did not change obviously.At other times in the morning and evening,the incident angle and light transmittance of each point on the front roof of the greenhouse change obviously with time.At the same time,the incident angle at each position on the roof changed little,but the light transmittance changed obviously.Compared with the roof angle,the light transmittance is more affected by the the sun orientation and the sun height angle.(3)Through the analysis of solar radiation model,it is found that the solar radiation on the surface of greenhouse wall and the ground changes with seasons.The spring and autumn equinox is the solar term with the longest solar radiation time between the wall and the ground in a year.At this time,the solar radiation illuminance on the wall surface is approximately equal to that on the ground.During the spring equinox to autumn equinox,the ground radiation illuminance is higher than the wall surface radiation illuminance.From the autumnal equinox to the vernal equinox,the solar irradiance on the wall surface is greater than that on the ground.Solar radiation in greenhouses in different regions is mainly affected by latitude.In low latitude areas and high latitude areas,the amount of solar radiation is large in winter and small in summer.(4)The azimuth change of the greenhouse is within 20°,which has little effect on the ground illumination,and mainly affects the size of the wall receiving the illuminance of the solar radiation.South-West can delay the heat storage time of the greenhouse wall,keeping the night temperature at a high level.The change in the surface inclination of the wall affects the solar irradiance of the wall surface and ultimately affects the heat storage capacity of the wall.When the height of the wall is constant,the surface of the wall is inclined by 10°,which can increase the surface area of the wall by 1.54%.When the surface of the wall is inclined to 20°,the surface area of the wall can be increased by 6.62%.Compared with the non-inclined wall,the solar irradiance of 10°and 20°per square metre can be increased by 0.40~1.32MJ/m~2 and 0.47~2.47MJ/m~2,respectively,which is3.91%~32.18%and 4.66%.~60.35%.(5)In this paper,brick wall greenhouse was taken as the research object,and CFD software wass used to simulate and verify the dynamic changes of the temperature field of greenhouse wall.It is found that taking the greenhouse wall,ground solar radiation intensity and outdoor air temperature as the input conditions,the internal heat conduction of greenhouse wall,convection heat exchange and radiation heat exchange with indoor and outdoor air were comprehensively considered,and CFD software was used to simulate the changes of the internal temperature field of greenhouse wall.The results were in good agreement,which showed that CFD was feasible and reliable to simulate the changes of the temperature field of greenhouse wall.(6)Three types of greenhouses,namely concrete back wall,brick wall and modular wall,which were currently used for external insulation,were selected.According to the test results from January to April,the internal temperature of the greenhouses increases as the temperature rised,with little difference in temperature fluctuation at different depths of different walls.By analyzing the temperature attenuation inside the wall and Logistic regression analysis of the attenuation factor,it is defined that the thickness of the heat storage layer of the greenhouse is when the temperature attenuation factor changes less than 0.01 at the adjacent 1 cm of the internal thickness of the greenhouse wall.It can be considered that the thickness of thermal storage layer of ceramsite concrete greenhouse wall is 16 cm,the thickness of brick wall greenhouse is over 32cm,and the thickness of modular greenhouse wall is over 25 cm.When the material with low thermal conductivity was used as the wall heat storage layer,the thickness of the heat storage layer was also small.(7)The wall can be divided into"thermal insulation layer,stabilizing layer and thermal storage layer".The thickness of each layer is related to the thermal properties of the thermal storage material and thermal insulation material of the wall.Judging from the test results in this paper,under the condition that the thermal properties of wall materials are unchanged,the thickness of each layer will not change much under the comprehensive influence of the external environment.To sum up,this study summarizes the method of using solar radiation illuminance as the boundary condition and initial condition for CFD simulation of greenhouse to simulate and calculate the temperature field change of greenhouse wall,and then determining the thickness of heat storage layer of greenhouse wall through further analysis of greenhouse data.The method can simulate the temperature field of the greenhouse wall and determine the thickness of the heat storage layer by CFD technology according to the climate environment in different regions,different materials and greenhouse structure types,which is of great significance for enriching the determination method and theory of the thickness of the heat storage layer of the solar greenhouse wall.The research results can provide theoretical reference and relevant data for the research of wall heat storage,roof optimization,crop planting,energy balance of enclosure structure in solar greenhouse.It can also provide basis and reference for optimizing greenhouse wall structure in other areas and promoting the improvement of solar greenhouse wall. |
PDF Full Text Request