Climate classification of solar greenhouse based on the calculation method for indoor and outdoor temperature difference at night

Abstract: Various types of solar greenhouses with complex configurations have been widely used in China in recent years. Evaluating the thermal insulation performance of a solar greenhouse can greatly contribute to accurately arranging the cultivation stubble of horticultural crops in winter. The risk of low temperature can also be reduced, according to the optimal classification of solar greenhouses. In this study, a comprehensive model was proposed to clarify the relationship between vegetable planting and the climate types of the solar greenhouse using an indoor-outdoor temperature difference at night. An experiment was also conducted in the winter periods of 2011-2014 (from November to March of the following year) in the facility agricultural cluster in the northern Tianjin, located at North China Plain with an annual average temperature of 12.3 ℃, the average sunshine hours of 2 527 h, and the average precipitation of 564 mm. Three typical solar greenhouses were selected to observe the indoor and outdoor microclimate. A calculation model was also established using the indoor-outdoor temperature difference, according to the different structure configurations, building materials, and heat conduction in the solar greenhouse. The thermal insulation performance of solar greenhouses was evaluated to determine the climate classification. The results showed that: 1) The thermal insulation constant (γc) was an excellent indicator to evaluate the thermal insulation performance of the solar greenhouses. The specific values were 20.34, 15.84, and 13.21 for the three typical solar greenhouses, respectively. 2) The calculation model well simulated the temperature changes in the solar greenhouses with the coefficient of determination (R2) higher than 0.70, and the root mean square error (RMSE) 1.968-3.857 ℃, indicating an outstanding performance of the model. 3) The extremely minimum temperature was obtained ranging from 3 ℃ to 14.4 ℃ for the different types of solar greenhouses using the minimum temperature, humidity, and wind speed, according to the climate observation from 1960 to 2020 with the climate guarantee rate of 80%. According to the growth and development indicators of fruits and vegetables, a new classification standard was proposed for the solar greenhouses, including the cold-tolerant leafy vegetables, leafy vegetable suitability, fruit-leaf mixed type, fruit-vegetable suitable, and temperature-loving fruit-vegetable type. Moreover, this classification was verified in the actual application of greenhouse renovation and evaluation in the planting base of Hengfeng vegetable cooperative in Houjiaying, Jizhou City, Tianjin in the winter of 2016-2018, as well as in the two parks of Juhong and Qingtai manor, Wuqing City, Tianjin of China in the winter of 2019-2020. Hence, these findings and recommendations can provide strong support to guide the vegetable planting in the different types of solar greenhouses.