| 【Object】The high humidity prevailing in greenhouses at night provides favourable conditions for leaf condensation,and Leaf Wetness Duration(LWD)plays an important role in crop disease early warning systems as a quantitative parameter for its description.In this experiment,a 2D microclimate distribution model and LWD estimation model were constructed for the solar greenhouse at night,on this basis,the effect of natural ventilation on cucumber leaf condensation in the solar greenhouse was simulated in order to provide a reference for dehumidification and control.In addition,a PI controller based on indoor and outdoor environment and vent ratio data is constructed to automatically regulate the internal temperature of the solar greenhouse.【Experiments】(1)LWD estimation model in the solar greenhouseThe model is based the CFD(Computational Fluid Dynamics)method to account for the influence of greenhouse clusters,and two greenhouses in the front and back(3 m apart)and a part of the outdoor area were used as an indoor calculation domain to develop a 2D solar greenhouse microclimate distribution model at night.Simulation of the transient distribution of environmental factors such as air temperature,air relative humidity and wind speed is achieved by discretizing the mass,momentum,energy and turbulence complementary equations as well as the component transport equations.Using the simulated humidity data as input parameters,a specific threshold method is used to construct a model of cucumber leaf condensation in the solar greenhouse,and the simulated LWD is considered to be the duration of simulated condensation in each zone.(2)Automatic control of vents based on room temperatureThe indoor temperature was considered as the control object,and the ARX temperature model was used to estimate the indoor temperature under different vent angles,The PI controller was built in matlab and the Simulink simulation software was used to simulate and calculate the control effect.【Results】(1)LWD estimation model in the solar greenhouseThe experiments were conducted at the National Experiment Station for Precision Agriculture in Xiaotangshan,Beijing,China,where the temperature and humidity in the solar greenhouse were simulated at night from May 31 to June 1(cloudy)and from June 3 to June 4(clear)in 2021,respectively,and the environmental data were collected by sensor devices for the validation of the model results.Through verification and analysis,the average absolute errors of temperature and humidity at the canopy(0-1.5 m)in cloudy were 0.47 ℃ and 0.25% respectively;Mae were 1.06 ℃ and 6.69% respectively.The average absolute errors of temperature and humidity above the canopy(2.5 m)were 1.13 ℃ and 2.71% respectively;Mae were 2.23 ℃ and 6.69%,respectively.The average absolute error of temperature and humidity at the canopy(0-1.5 m)in clear were 0.97 ℃ and 3.32 % respectively;Mae were 1.93 ℃ and 7.91 % respectively.The average absolute errors of temperature and humidity above the canopy(2.5 m)were 1.15 ℃ and 5.2 %respectively;Mae were 3.06 ℃ and 17.88% respectively.The LWD estimation results show that the humidity threshold method for determining leaf condensation is 98.58 % accurate on clear and 81.79 %accurate on cloudy,and the average error of the LWD estimation model is 0.166 h on clear and 1.944 h on cloudy.The simulation results show that 2 h ventilation can effectively reduce leaf condensation to achieve the purpose of dehumidification,but will significantly reduce the indoor temperature(1.8 ℃).(2)Automatic control of vents based on room temperatureThe simulation results show that the PI control system based on the ARX temperature model can be applied to the automatic temperature control in the solar greenhouse,it requires a long regulation time(500s on average to reach steady state after control),but the weaker temperature fluctuation range(Less than 0.5 °C)after approaching the target temperature is still suitable for the growth of most crops. |
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