| As a unique agricultural facility in China,solar greenhouse has solved the problem of vegetables supply in winter in northern China,and made a great contribution to agricultural efficiency and farmers’income and rural economic development.At present,the walls of solar greenhouse in China are mostly made of ordinary building materials with constant physical properties and passive heat release through sensible heat.However,due to the limitation of the heat transfer capacity of wall materials,there still exist the problem of thick walls and poor heat storage and release performance,which leads to less heat release at night and too low air temperature in the greenhouse.In this paper,based on the previous research of our research group,the flat plate micro heat pipe array was made into blocks and applied to the phase change heat storage wall of solar greenhouse,and a new flat plate micro heat pipe array phase change heat storage greenhouse wall was proposed.Through the method of combining experiment and numerical simulation,the heat transfer characteristics of micro heat pipe array suitable for solar greenhouse heat storage wall,the influence of heat conduction fin on the heat transfer performance of the wall,the influence of the thickness and position of the phase change plate,the time of insulation closing and the thickness of the wall bearing layer on the heat transfer performance of greenhouse were studied.Firstly,an experiment bench was set up to test the thermal performance of the micro heat pipe array.The effects of bending radius,bending angle,evaporation length and heat pipe length on the heat transfer characteristics of the micro heat pipe array were analyzed.The results show that the average equivalent thermal conductivity of heat pipes increases with the increase of the bending radius of the micro heat pipe array,and the average temperature difference between the evaporation section and the condensation section of the heat pipe with the bending radius of 40mm is the largest.The temperature difference of 500mm heat pipe is1.58℃and 1.47℃when the heating power is 10W and 20W,and the temperature difference of 750mm heat pipe is 0.26℃and 0.32℃when the heating power is 10W and 20W,respectively.The maximum temperature difference on the surface of 500mm heat pipe is within 2℃,while the maximum temperature difference on the surface of 750mm heat pipe is within 1℃.By comparing the heat pipes of two lengths,it can be found that under the same heating power,the larger the heat pipe length is,the better the temperature uniformity is.When the bending Angle of heat pipe is 90°,its maximum average equivalent thermal conductivity is 293320 W/(m·K),and its working range also increases.Taking micro heat pipe array as the application background in solar greenhouse,the heat pipe was bent twice.The length of heat pipe was 500mm,the evaporation section was 208mm,and the condensation section was 96mm.The equivalent thermal conductivity of heat pipe was71708W/(m·K).The heat pipe with the length of 750mm,the length of evaporation section350mm,and the length of condensation section 150mm has an equivalent thermal conductivity of 34050W/(m·K).Secondly,the 3D micro heat pipe array phase change heat storage wall was established and compared with the experimental values.The results show that:At 14:00,17:00,4:00 and9:00 of the next day,the overall temperature of the wall of the experimental value is 23.4,24.9,18.0,16.0℃respectively,while the overall temperature of the wall of the simulated value is 23.6,25.8,18.5,16.1℃,and the relative errors are 0.8%,3.6%,2.8%,0.6%respectively,and the relative errors are all less than 5%.By comparing the different height,spacing and thickness of fins,it is concluded that the combination form of fins with height of25mm,spacing of 12.5mm and thickness of 1mm is the best,and its heat storage is19.7MJ/m~3 and heat release is 19.45MJ/m~3,compared with the wall without heat conduction fin,the wall increases by 4.76 MJ/m~3 and 4.93 MJ/m~3 respectively.According to fin temperature cloud can be seen in the block,with the increase of fin height,at 14:00,40℃temperature is greater than the area of the increase in the thickness direction,and the next day at 4:00,less than 20℃temperature increase in the thickness direction of the area,so the fin height mainly affecting wall and block temperature distribution along the thickness direction.At 14:00,the area with temperature higher than 34℃in the height direction increases with the increase of spacing;at 4:00 the next day,the area with temperature lower than 20.8℃increases with the increase of spacing in the height direction,indicating that the fin spacing affects the temperature distribution in the height direction.Fin thickness has little influence on temperature distribution,which mainly affects the heat input and output of the wall,thus affecting the overall heat storage and release,and the fin combination form can be applied to the research of wall optimization.Thirdly,a 3D model of the solar greenhouse with phase change heat storage wall with micro heat pipe array is established,and the experimental data and outdoor meteorological parameters are used as boundary conditions for simulation calculation.The results show that the experimental results have the same trend with the simulation results,and the experimental results are consistent with the simulation results.The maximum errors of the air temperature in the greenhouse and the inner surface temperature of the wall are 3.3℃and 6.95℃respectively.During the whole heat storage and release period,the average relative errors between the air temperature in the greenhouse and the inner surface temperature of the wall are 2.5%and 3.7%respectively.It can be used for subsequent optimization calculation.Fourthly,in order to study the influence of the thickness of PCM on the heat storage and release of the greenhouse wall and the air in the greenhouse,a PCM wall solar greenhouse with microheat pipe array with different thickness of PCM was established.With the increase of the thickness of PCM,the heat storage and release of the wall and the heat storage and release rate of the wall increased.However,when the thickness of PCM plate is larger than40mm,the heat storage and release of the wall and the heat storage and release rate of the wall are basically unchanged.The heat storage and release of the wall with the thickness 40mm of PCM plate is 6.59 and 3.44MJ/m~3 respectively,while that of the wall without PCM plate is4.72 and 2.37 MJ/m~3respectively,which are 39.8%and 45.5%higher than that of the wall without PCM plate.When the insulation was closed,the temperature in the greenhouse with the thickness of phase change plate 20,40 and 60mm was 9.95,10.18 and 10.14℃,respectively.Compared with the air temperature in the greenhouse without phase change plate9.58℃,the increase was 0.37,0.6 and 0.56℃respectively,and the increase gradually decreased to basically unchanged.Through the wall temperature variation analysis,different time at temperature for 6～7℃parts during the whole heat storage and release stage,the temperature is always constant,part of the temperature of 7～8℃temperature is gradually rising,although its heat storage in the ascent,but the thickness of the wall makes the temperature wave quickly exhausted and cannot be transmitted to the air in the greenhouse,so the contribution rate to the increase of the temperature in the greenhouse is small.The results show that the average temperature in the greenhouse at night is 10.44℃and 10.4℃,when the insulation is closed at 16:30 and 17:00,Compared with the average air temperature of10.18℃in the greenhouse at night when the insulation was closed at 16:00,the average air temperature increased by 0.26℃and 0.22℃respectively.Therefore,the optimal thermal insulation period suitable for Gansu is 16:30～17:00Fifth,in order to study the influence of the position of PCM on the heat storage and release of the greenhouse wall and the air in the greenhouse,two greenhouses with different positions of PCM were established based on the thickness of PCM 40mm.The results show that:Phase change plate under greenhouse interior wall surface through direct absorption into the solar radiation in greenhouse can absorb more heat,so as to better play the role of"peak",results in lower in the daytime temperature in greenhouse and in the heat due to phase transition plate storage quantity of heat more can be used at night to emit more heat to improve the air temperature in greenhouse,Thus better play the role of"shifting".The heat storage capacity was 6.59MJ/m~3,and the heat release capacity was 3.44 MJ/m~3,which increased by 15.1 MJ/m~3.The average air temperature in the greenhouse was 10.13℃,which increased by 0.27℃.Therefore,it is better to place the phase change plate on the inner wall surface of greenhouse.Sixth,in order to study the influence of wall thickness on heat storage and release of greenhouse wall and air in the greenhouse,six three-dimensional greenhouse models with different wall thickness were established and simulated.The results show that:with the decrease of wall thickness,the heat storage and release increases first and then decreases,reaching the maximum value when the total thickness is 590mm,and the heat storage and release are 8.91 and 4.81 MJ/m~3,respectively.When the total thickness is reduced to 540mm,the heat storage and heat release are 5.82 and 3.48 MJ/m~3,respectively,with a relative decrease of 34.7%and 37.7%.During the daytime heat storage stage,the average air temperature in the greenhouse with the total wall thickness of 590mm～810mm is 18.28℃,and the average air temperature in the greenhouse with the thickness of 540mm is 17.36℃,which is 0.92℃lower,and the average air temperature in the greenhouse at night heat release stage is 10.33,9.85℃,respectively,relative decrease of 0.48℃.Therefore,the total thickness of the wall is 590mm,the thickness of insulation board,brick layer and phase change board is70mm,480mm and 40mm respectively.Seventh,in order to study the application effect of the greenhouse structure proposed,ordinary brick wall greenhouse and MHPA wall greenhouse model were established,and compared with the obtained wall structure.The results show that MHPA and phase change board have positive effects on the greenhouse wall heat storage and release,heat storage and release rate and air temperature in the greenhouse.The wall heat storage of MHPA greenhouse and MHPA phase change greenhouse is 7.51 and 8.91MJ/m3 respectively,which is 1.32 and 2.72MJ/m~3 higher than that of ordinary greenhouse,while the heat release is 3.86and 4.8 MJ/m~3 respectively,which is 0.57 and 1.51 MJ/m~3 higher than that of ordinary greenhouse.The average air temperatures in the greenhouse at night were 9.73℃and 10.11℃respectively,which were 0.27℃and 0.65℃higher than the ordinary greenhouse 9.46℃In this paper,the structure of micro heat pipe array phase change heat storage wall solar greenhouse is optimized,and the results can provide reference for the wall construction of solar greenhouse in Gansu area. |
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