Low Temperature Forecast Model And Defense Technology In Solar Greenhouse In Tianjin

Solar greenhouse is the main greenhouse type in northern China. It can be utilized enough radiation thermal resources for producing vegetable in greenhouse to meet the demands of market in winter. Low temperature is the main agricultural meteorological disaster in the process of production in solar greenhouse in Tianjin. Vegetables are difficult to meet the demand of normal growth and development, vegetables yield and quality are affected under the stress conditions of greenhouse and even the physiological mutations can be caused when the environment is severe, it has an important sense to predict the inside low temperature and study the defensive measures.Based on the collected data of microclimate in the solar greenhouse and meteorological elements outside in Xinkou, Tianjin during2010-2012, the characters of the temperature inside greenhouse were analyzed organized, heat flow relationship and heat loss in different surfaces of solar greenhouse were studied, the modeling of greenhouse low temperature had been done based on energy balance and principal components regression, heating effect of intelligent electric heater was estimated in this paper. The research results can provide support to the greenhouse environment optimize regulatory and production management decision-making. The main conclusions are as follows:(1)The air temperature in solar greenhouse has obviously diurnal variation in sunny and cloudy days, the air temperature was low all day in sunless days. The minimum temperature was happened in the before or after sunrise, the maximum temperature was appeared at13:00, velocity of temperature falling increased after15:00, the temperature was declined mildly after17:00. Soil and backwall in solar greenhouse were main heat accumulators in different weather conditions, translucent membrane and backroof were main heat dissipation areas. The thermal energy loss rate showed the rule that soil<backwall<backroof<translucent membrane. The average heat loss rate of every surfaces in solar greenhouse showed the rule that February>January>December.(2)The main heat exchange way of air in solar greenhouse were heat convection with soil, backwall, backroof, translucent membrane and crop canopy, and heat exchange with outside air caused by ventilate. The evening temperature forecast model inside the solar greenhouse in winter has been set up based on principle of energy balance, which the correlation coefficient was0.647, RMSE was within2℃, the average relative errors was about20%. In addition, the correlation coefficient of low temperature measured value and predicted value was0.569, RMSE was1.8℃, the average relative error was16.5%, the average absolute error was1.5℃.(3) The errors of predicted cloud cover and wind velocity were reasonable by using cloud cover coefficient method and standard of wind velocity conversion, the determination coefficients of simulation value and measured value were0.969and0.799respectively. The minimum temperature forecast model inside the solar greenhouse in winter has been set up, which has high accuracy verified by historical data and application, and the correlation coefficient was0.857. The average absolute errors of the forecast minimum temperature inside the solar greenhouse in different weather conditions were less than1℃, the average relative errors were within10%, the RMSE was0.8℃during the whole winter by using the method of principal component regression. Compared with the energy balance model, the imitative effect of principal component regression model was better.(4) The heating effect was studied by using the electric warm-air machine to resist the low temperature disaster. The temperature in the greenhouse rises significantly and uniform by using electric heaters. The average temperate in heated greenhouse was4.23℃above contrast greenhouse during heating and the heating effect about average minimum temperature was particularly notable; The average temperate in heated greenhouse was4.24℃above contrast greenhouse in the condition of low temperatures and4.48℃in the condition of spare sunlight, the heating effect was better than normal weather conditions. The optimum condition of heating the greenhouse by warm-air machine is outside temperature range in-12℃～-4℃. The warm-air machine is an economic benefited heating measure for its little investment and low energy consumption.