Daily change and math-expression method of outside temperature in winter for greenhouse environmental analysis

Abstract: Hourly, outdoor meteorological data are essential for the accurate dynamic simulation and analysis of the greenhouse thermal environment. Most meteorological stations have only published the four times (six/eight times) observation results, but hourly data have always been considered as classified files and not published. Also not published is the continuous observation temperature data in research or production, which should be included in the change principle and in greenhouse environment analysis. In this paper, we examined the daily change rule of outside air temperature in January for greenhouse environment analysis. According to observation data from 2009 to 2011, measured and recorded by the thermal recorder RS-12, we acquired the temperature data of each 10 min interval from 14 stations in North China. For each station, we adjusted the data to reflect true solar time, and using the hourly temperature at the true solar hourly time, we calculated the hourly temperature changing coefficient β. We then analyzed the hourly change of outside temperature in five provinces of North China for the month of January, and summarized a mathematical method for hourly air temperature by using β when the daily maximum and minimum temperature were known.The results showed: In the same province, although the measured temperature is quite different in various years, β is always on the similar change trend and value at the same hour, so the outside temperature daily change can be described by the same coefficient in each province. By comparing β of the five provinces in North China, we found that β are similar, and R2 reached 0.9704, so we can also show the hourly change of outside temperature by the same β. Based on the results above, we summed the hourly changing coefficient (1-24h) of outside temperature in January in North China. When we contrasted β with heating ventilation and air conditioning (HVAC), the two curves were different in minimum value time and rise/fall speed, which is related to seasons. The values of the outside air temperature hourly were changing coefficient β from 1 to 24h is -0.35, -0.37, -0.40, -0.44, -0.46, -0.48, -0.50, -0.39, -0.09, 0.12, 0.28, 0.39, 0.46, 0.50, 0.49, 0.41, 0.20, 0.02, -0.08, -0.15, -0.19, -0.24, -0.27, -0.31, respectively. The winter coefficient was examined by Xinkaikou Town in Tianjin in January of 2011, and Tongzhou District in Beijing in January of 2012. The simulation results of hourly air temperature outside by this method were similar to the actual measurements, the R2 all reached 0.98, and average error were all 0.3℃. On this basis, using the Fourier series expansion, we stated a mathematical expression for calculating outside temperature at any time. According to the daily maximum and minimum temperature outside, we could determine the outside temperature at any time and provide full outdoor temperature data for greenhouse environment simulations accurately.