Study On Thermal Performance Of Thermal Storage Wall With Micro-heat Pipe Arrays In Solar Greenhouse

The solar greenhouse is a kind of protected horticulture building which relied on solar energy for the cultivation of off-season vegetables.Due to the instability of solar energy and the cold weather in the northwestern region and the large temperature difference between day and night,the passive heat storage method of the solar greenhouse wall cannot maintain the thermal environment required for crop growth,resulting in frequent occurrence of crop freezing damage.To excavate the heat storage and release capacity and improve the utilization of solar energy and land,this paper focuses on the construction method of the micro-heat pipe arrays storage wall.Thermal performance of micro-heat pipe arrays heat storage wall and array pattern of micro-heat pipe blocks on the north wall using the methods of experimental test and numerical simulation.Firstly,through the analysis of the existing greenhouse wall structure in China,it is found that the physical and the variable material walls can strengthen the heat storage and release of the wall to a certain extent,there is still a thick temperature stability zone in the wall,and the active heat storage method will increase the cost.In view of the above shortcomings,combined with the bionics"chord"function and the heat transfer technology of micro-heat pipe arrays,a construction method of heat storage wall with micro-heat pipe arrays in solar greenhouse is firstly proposed.The micro heat pipe blocks were constructed of a curved"B"shaped micro-heat pipe arrays and cement.Micro-heat pipe blocks are arranged inside the north wall,and the solar energy is transferred through the heat transfer capability of the micro-heat pipe arrays to the wall.This method directly deepens the heat transfer depth,increases the internal temperature of the wall,and enhances the overall heat storage and release of the wall.Secondly,based on the construction concept of micro-heat pipe arrays heat storage wall,the basic heat transfer performance is tested experimentally.The results show that at the heating temperature of 35°C,45°C,55°C,65°C,the response time of the"B"shaped micro-heat pipe arrays is 20s,50s,100s,150s later than that of the flat micro-heat pipe arrays.However,considering that the daytime solar irradiation time is usually 10:00 a.m.-17:30 p.m.,under the 7.5 hour heat storage time,the influence of bending on the response time lag caused by the micro-heat pipe arrays is small.The axial maximum temperature difference is only about 1℃lower than the flat one,indicating that the curved micro-heat pipe arrays has good heat transfer performance.Thirdly,the heat storage and release performance of the micro-heat pipe block and the ordinary block are experimentally studied.The results show that,under the stable condition of400W/m~2,the maximum effective heat storage capacity of the micro-heat pipe block is 7.76%higher than that of the ordinary block;under the changing condition of 400-800-400W/m~2,the maximum effective heat storage capacity of the micro-heat pipe block is 15.61%higher than that of the ordinary block.This indicates that the heat storage performance of the micro-heat pipe block is better than that of the ordinary block under the condition of stable radiation intensity or changing conditions.Fourthly,under the condition that the simulated values of the micro-heat pipe block and the ordinary block are in good agreement with the experimental values,a two-dimensional model of greenhouse with micro-heat pipe arrays heat storage wall is established and compared with the ordinary greenhouse.The simulation results show that the air temperature distribution in the greenhouse with micro-heat pipe arrays heat storage wall is more uniform under typical sunny days.The maximum surface temperature of the micro-heat pipe arrays heat storage wall is 22.09℃,which is 16.02%higher than that of the ordinary wall,and the minimum value is 11.93°C,which is 3.47%higher than that of the ordinary wall.The maximum heat storage and heat release capacity of the micro-heat pipe arrays heat storage wall is 48.91%and 21.63%higher than that of the ordinary wall respectively,the average heat storage rate and heat release rate of the micro-heat pipe arrays heat storage wall is 29.25%and10.74%higher than that of the ordinary wall.Fifthly,a 3D micro-heat pipe array regenerative wall model is established,the temperature distribution characteristics of the greenhouse wall of the uniform array and grouped array of micro-heat pipe blocks are discussed through simulation.The results show that the micro heat pipe blocks of the two arrays have good heat storage and release performance,and can increase the temperature of the brick wall part to a certain extent.In contrast,the micro heat pipe blocks of the grouped array make the heat too concentrated,so the heat transfer area in the longitudinal direction is small,and there is a lag in time.The micro-heat pipe arrays heat storage wall proposed in this paper can fully utilize solar energy,improve the heat storage and release capacity inside the wall,and improve the thermal environment in the solar greenhouse,which provides a reference for the modernization of the solar greenhouse in the northwest.