| Solar greenhouse is an agricultural facility building that can provide suitable growth environment for off-season crops in the cold season.However,the heat storage of traditional greenhouse walls affected by the material thermal properties and the heat storage mode is very low.In addition,the heat loss of the greenhouses is very high because of the cold environment during the nighttime.Under the combined effects of the above reasons,it's difficult to maintain the thermal environment required for the crop growth.In order to solve the above problems and improve the thermal environment of the greenhouse,this study combining experiments and numerical simulation analysis,focused on the construction method of the active-passive phase change thermal storage wall and its effect on improving the thermal environment of the greenhouse.A series of studies have been carried out.First,a series of improve methods of heat storage performance of traditional walls have been analyzed.The analysis found that placing phase change materials in the wall can maintain a stable effect on the thermal environment in the solar greenhouse,but it was still difficult to effectively develop the heat storage performance inside the wall by passive heat storage.Active heat storage can effectively increase the heat storage of the wall,thereby improving the thermal environment of the greenhouse.Combining the advantages of the above research,this paper proposed an active-passive phase change heat storage wall.In this wall,the phase change material was enclosed in the wall by 6 stainless steel boxes,and the concentrating solar air collectors and the boxes were connected by stainless steel pipes.During the daytime,the solar energy was collected by the CSACs and stored in the tanks,whereas during the nighttime,the stored energy was released into the indoor environment of the CSG through a passive heat mode of the north wall or an active heat mode of the system.Secondly,the effect of the active-passive phase change thermal storage wall of the thermal environment in the greenhouse was tested and compared with the ordinary greenhouse.The experimental data under the conditions of 3 typical sunny days were selected for analysis.The experimental results showed that the inner surface temperature of the active-passive wall increased by 2.96? at night,the soil surface temperature increases by 2.73? and the temperature inside the greenhouse at night increases by about 1? under passive heat release conditions and 1.34? under active heat release conditions.The daily effective accumulated temperature was increased by 71.06%-84.2%,and the daily uncomfortable time was effectively reduced by 3-4 hours.The energy consumption of the greenhouse system was also measured.The daily power consumption of the system was about 0.5 kWh-1 kWh,so the system is energy efficient.Thirdly,a numerical model with the same structure as the active-passive greenhouse was established and the actual material parameters and boundary conditions were taken into account for calculation.To ensure that the simulation results are accurate enough to analyze the thermal characteristics,the results were compared with the experimental measured data.The results showed that the maximum error,the average error and was the average relative error were 1.99?,1.04? and 6.09%,respectively.Therefore,this numerical model can be used to analyze the heat transfer characteristics of the active-passive phase change thermal storage greenhouses.Fourth,15 kind of phase change material with peak phase change temperatures of20.5?,25.5?,30.5?,35.5?,40.5? and heat of fusion of 217.2 J/g,162.9 J/g,108.6J/g were selected to simulation.The results showed that the heat storage efficiency of the wall increasing with the decreasing of the melting point and the increasing of the heat of fusion of phase change material.With the cumulative solar radiation of 19.80 MJ/m~2,the maximum heat storage capacity and heat release capacity were 28.70 MJ and 7.12 MJ.Fifth,three heat release conditions with air speed of 2 m/s,3 m/s,and 4 m/s and a passive heating case were chosen for numerical study.The results showed that the total heat radiation increased with the increasing of air speed.The heat release with air velocity of 2 m/s,3 m/s,and 4 m/s were 13.31 MJ,14.49 MJ,15.55 MJ,respectively,and the heat release was10.80 MJ under passive mode.Therefore,active heat release can effectively increase the air temperature in the greenhouse by 1.1?.The analysis found that the air wind speed of 3 m/s and 4 m/s had little difference on improving the thermal environment inside the greenhouse.Therefore,3 m/s was finally determined as heat release air velocity of the greenhouse at the nighttime.At last,a kind of phase change material with the heat of fusion of 217.2 J/g and the peak phase change temperature of 20.5? was selected as the heat storage element,and 3 m/s was selected as the heat release wind speed at nighttime for simulation.Comparing with the ordinary greenhouse,the results showed that the heat storage of wall was increased by 28.09MJ,the heat release was increased by 9.84 MJ,the heat efficiency was increased by 3.84%,the minimum indoor temperature at nighttime was increased by 3.59?,and the uniformity of the indoor temperature was increased by 26.6%,with the cumulative solar radiation of 19.80MJ/m~2 and environment temperature of 0.1-8.9?.Not only a new type of greenhouse wall has been proposed in this paper,but also a numerical model of the active greenhouse were established,which provided a reference for the modernization of greenhouse development in Northwest China and numerical simulation of the greenhouses. |
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