| Solar greenhouse is an original facilities agriculture construction with solar energy as the main energy in China.It provides a powerful guarantee for anti-season vegetables growing in the north of China,and it has also played a important part in the vegetables’ safety and stability supply for urban and rural residents.Northwest China is a suitable region to develop greenhouse industry,with the advantage of abundant solar energy resources.Although existing active heat storage walls all have good heat storage effect,and it already take a signifcant role in improving the night indoor thermal environment,there are still some problems,such as low heat storage and low heat storage efficiency.In addition,it is week in reasonable control method considering the actual situation to achieve the best heat stoage effect.Therefore,based on the demand of the indoor thermal environment of crops of the solar greenhouse in the northwest of China,this paper proposes an active-passive phase change thermal storage wall heating system in the greenhouse,through the experiment combined with numerical simulation method of the system under the different operation control strategy and parameters of performance and reliability of a series of research,the main research results as follows:1)This paper summarizes the current achievements of the domestic and foreign researchers in heat storage and release of the greenhouse at night,analyzes some problems still exist.The results show that most of the domestic and foreign studies on the solar greenhouse heat storage wall are only aimed at the theoretical and experimental research stage of heat storage,and cannot fundamentally solve the temperature problem of the thermal environment in the solar greenhouse.Although some studies have adopted active heat storage research,the best effect cannot be achieved under the actual operating conditions.Most of the literatures only focus on some parts such as wall heat storage,greenhouse ventilation or indoor temperature field,and there are few reports on how to improve the active heat storage and release by operating and controlling the irregularity of the overall solar radiation time distribution of the active heat storage and release system in solar greenhouse.In this paper,an active and passive phase change heat storage wall heating system for solar greenhouse is proposed.The performance and reliability of the system under different operation control strategies and parameters are studied by combining experiment and numerical simulation.2)The experimental platform of the solar greenhouse active and passive phase change heat storage wall heating system was built.The heat collection performance of the collector and the active heat storage and release performance of the wall under the control strategy of constant wind speed operation were studied,and the heat storage and release effect of the solar greenhouse heating system under the control strategy of constant wind speed operation was studied,and the following conclusions were drawn: When the wind speed at the outlet of the variable frequency fan is 8.23m/s,the heat collected by the collector and the active heat stored by the wall are 37.40 MJ and 20.20 MJ respectively,and the active heat storage efficiency of the wall is 54.03%.Later,with the increase of the wind speed at the outlet of the variable frequency fan,the increase of the heat collected by the collector and the active heat stored by the wall is small,and the active heat storage efficiency of the wall is always maintained at about 54%.By analyzing the performance of active heat release under the control strategy of constant wind speed,it is found that when the wind speed is 4m/s,3m/s and 2m/s,the active heat release from the wall is 4.40 MJ,3.29 MJ and 1.52 MJ in sunny night(22:00～ 10:00 next day),and 3.92 MJ,2.60 MJ and 1.44 MJ in cloudy night,respectively.However,under different active heat release wind speeds,the greater the wind speed,the greater the active heat release rate of the wall.At the same time,the indoor wall surface temperature,air temperature and soil temperature under different wind speeds in sunny days and cloudy days were compared with those in common greenhouse.3)According to a series of data of active and passive phase change heat storage wall heating system in solar greenhouse studied in this paper under the control strategy of constant wind speed operation,the correlation formula between active heat storage QAS and wind speed v at the outlet of the converter and solar radiation intensity I was obtained.4)A 1:1 3D model was created according to the actual size and correlation of the experimental greenhouse,and the grid quality was controlled according to the standard of Skewness(grid distortion degree).In this study,the grid Skewness was below 0.75,indicating good grid quality.In order to balance calculation time and simulation accuracy,grid independence was verified.Finally,the grid number of 2493991 was selected for simulation calculation.The maximum relative error and average relative error were 7.15% and 1.89%respectively.At the same time,the error analysis of simulated B box PCM,wall inner surface temperature and indoor air temperature showed that the numerical simulation values were in good agreement with the experimental data.The maximum relative errors were 10.56%,11.67%,10.61%,the average relative errors were 5.09%,7.36%,6.73%,and the maximum absolute errors were 1.68℃,1.73℃,1.48℃,respectively.5)Under the control strategy of constant wind speed,when the active heat release wind speed is 4m/s,3m/s and 2m/s,the total heat release of the wall is 40.58 MJ,38.86 MJ and36.84 MJ,which are 29.44%,23.96% and 17.51% higher than that of the passive heat release,and 164.88%,153.66% and 140.47% higher than that of the ordinary wall.Under the control strategy of time-divided variable wind speed,combined with the above analysis and the effect of wall heat release and improvement on indoor air temperature,it is recommended that the active wind speed is 3-2-4m/s.At this time,the total heat release of the wall is 37.33 MJ,which is 19.07% higher than that of passive heat release and 143.67% higher than that of ordinary wall,and the indoor air temperature is always maintained above 10℃.Under the constant wind speed variable length operation control strategy,combined with the above analysis and the wall heat quantity and effect on the improvement of the indoor air temperature,recommend active heating time is 8 h,in exothermic wind speed of 3 m/s,2 m/s,wall total heat quantity were 35.9 MJ,34.6 MJ,the passive heat increased by 14.51% and10.37% respectively,increased by 134.43% and 125.85% respectively than ordinary wall,indoor air temperature could remain above 9.4 ℃,under the different operation control strategy of indoor air temperature is relatively passive heating and common greenhouse were increased.6)Through the study of different operation control strategies,it is found that under the time-divided variable wind speed operation control strategy,the active heat release wind speed of 3-2-4m/s can ensure that the indoor greenhouse at night is above 10℃,which reaches the expected control goal. |
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