| The No.1 central document for 2023 clearly points out the development of modern facility agriculture and the implementation of modernization and enhancement actions of facility agriculture.As a unique energy-saving agricultural facility building in China,the accelerated promotion of heliostats will help improve agricultural production efficiency.However,at present,most of the heliostats in Northwest China are built in the traditional way with a single earth or brick wall,which has limited heat storage capacity and poor regulation of indoor thermal environment by passive heat release,making the greenhouse crops prone to cold and frost damage and low yield.Therefore,in this paper,we design a solar greenhouse active-passive sensible heat storage and heating system,and study the heat storage and exothermic performance of the system by numerical simulation,and the main research contents and results are as follows:Firstly,by summarizing the research of domestic and foreign research scholars in heliostat,most of the heliostat research is based on passive heat storage technology,but it is difficult to achieve a significant increase of heat storage and release due to the natural accumulation/release method of this wall and limited by its own thermophysical properties,thus limiting the effect of temperature increase.The use of both phase change materials and aqueous media for active heat storage in heliostats can effectively improve the thermal environment and increase the energy use efficiency of greenhouses.However,in greenhouse design,in order to meet the needs of agricultural development,we need to consider the direction of economic energy saving.Therefore,for different heat storage materials,we need to consider their performance,cost and application effect in order to achieve a balance of economy and practicality.In this regard,sensible heat storage technology has better application prospects.Water is a substance with a large specific heat capacity in nature,which can absorb a large amount of heat,and the way to obtain it is relatively easy price is low compared with other heat storage materials initial investment.Therefore,we should give priority to the application of sensible heat storage technology in greenhouse design in order to achieve better economic and energy-saving effects and provide more reliable energy security for agricultural production.Secondly,this paper proposed a new active-passive sensible heat storage solar greenhouse heating system,in which water was used as the heat storage material and a coil system was used for heat energy storage and transmission.During sunny daytime,solar radiation will shine through the front slope roof to the heat storage tank walls,and these walls store the heat energy by heat conduction.When it is necessary to maintain a suitable temperature inside the greenhouse,the active heating system is activated,using solar air collectors outside to heat and transport the air to the thermal storage coils,releasing the thermal energy stored in the thermal storage tanks.At night,if the passive heat release method of the walls cannot ensure the normal growth of crops in the greenhouse,the active heating and passive heat release methods will be needed to work together to maintain or increase the air temperature in the greenhouse.Thirdly,a 1:1 scale 3D model was created using Space Claim according to the CAD dimensions,and the Poly-Hexcore bulk mesh was used to mesh the model,and the mesh quality was also evaluated by following the Skewness criteria and orthogonal quality control,and the final mesh quantity was selected to be 1698145 through mesh independence verification.The simulation method was also validated,and through error analysis of the simulated B box phase change material temperature,wall internal surface temperature and indoor air temperature,it was found that the numerical simulation values agreed well with the experimental data,and the maximum relative errors were analysed to be 13.43%,12.67% and12.53% respectively,with average relative errors of 5.09%,7.36% and 6.73% respectively,and the maximum absolute errors were 2.38°C,2.43°C and 1.36°C respectively,which verified the correctness of the simulation method and analyzed to determine the feasibility of the aqueous media chamber greenhouse.Fourthly,in order to select the best heat storage wind speed,numerical simulation was used to finally establish 8 m/s as the daytime heat release wind speed and 4 m/s as the nighttime heat release wind speed for the new solar greenhouse.With the determined wind speed,the indoor air temperature of the active-passive thermal storage greenhouse increased by 2.1℃ compared to that of the ordinary greenhouse,the effective daily temperature increased by 53.6%,and the suitable growing time of the crop increased by 6h.The active-passive greenhouse is more conducive to the increase of the indoor air temperature to reach above zero degree of crop biology,which is conducive to the healthy growth of the crop.The surface temperature of the wall is 2.37°C higher than that of ordinary greenhouse,the average temperature of the greenhouse at 60 mm from the surface of the wall is 1.58°C higher,the average temperature of the greenhouse at 195 mm from the surface of the wall is 2.6°C higher and the average temperature of the main passive greenhouse at 390 mm from the surface of the wall is 3.4°C higher than that of ordinary greenhouse.Fifthly,the overall heat storage of this active-passive sensible thermal wall and the ordinary greenhouse wall were 51.4MJ and 36.72 MJ respectively,with a difference of14.68 MJ,an increase of 40%.The heat release capacity of the active-passive thermal storage greenhouse wall was 35.56 MJ and that of the ordinary greenhouse wall was 15.43 MJ,with a difference of 20.13 MJ,an increase of 130%.It can be found that the heat storage and release capacity of the active-passive thermal storage greenhouse wall is significantly improved,which not only strengthened the heat storage capacity of the wall but also strengthened the heat release capacity at night.Sixth,through the study of different operating conditions and comprehensive analysis of wall surface temperature,indoor air temperature and heat release,the active air supply speed is recommended to be 3-4-2m/s,at which time the total heat release from the wall is 34.89 MJ,which is 125.6% higher than that of the common greenhouse,and the heat release efficiency is 67.8%,and the indoor air temperature is maintained above 9℃ and the average temperature is 10.4℃.Reduce the heat loss from the wall.And according to the comprehensive analysis of variable duration and start/stop,it is recommended to turn on the active heat release system at 0:00 and stop the active heat release system at 8:00 as the best time to reduce the heat loss of the wall and also ensure the normal growth of crops. |
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