Method for measuring thermodynamic disfigurement area in Chinese solar greenhouse by utilizing thermal infrared image

Abstract: The Chinese solar greenhouse is a typical greenhouse without heating system, in which the heat insulating layer and the defect of air tightness of the envelope structure are the major factors affecting insulation and heat storage performance. Compared with temperature measurement method of single point, the thermal infrared imaging technology can be used to measure the temperature of the whole area, which can be seen in the form of a color picture. Therefore, the thermal infrared imaging technology can be used as an effective method to detect temperature difference area. The purpose of this study is to develop an efficient method to detect and quantitatively analyze the area of thermodynamic disfigurement in solar greenhouse. The tested solar greenhouse, with length of 52 m and width of 8 m, locate in Yangling, Shanxi province (108°4′E, 34°16′N). The north wall of solar green house, equipped with 16 ventilation vents, was structured by clay-brickwith width of 1 m and polystyrene board with width of 10 cm. E4 thermal mapper, which was perpendicular to the north wall with a distance of 1.5 m, was used to obtain infrared images of the north wall in solar greenhouse. The data that were collected in a typical sunny day (from Dec. 15, 2015, 15:30 to 16:30, the heat insulation sheet was rolled down at 16:30) were used to analyze the accuracy of different area measurement methods. FLIR Tools software was used to extract temperature data in thermal infrared image of the north wall. LabVIEW 13.0 software was used to read and select the interested infrared image area, and the corresponding temperature data were imported. Root mean square (RMS) and histogram were used to set different temperature thresholds. The measured data and calculated data from different area measurement methods were compared. The results showed that the temperature of different regions of the north wall in solar greenhouse can be displayed in the thermal infrared images, the surface temperature of the ventilation vent was lower than the inner surface temperature of northern wall, and the maximum temperature difference was up to 8.4 ℃. Locations of thermodynamic disfigurement could be quickly detected and positioned. The histogram of thermal infrared image showed that the surface temperature of thermal region and the proportion of the temperature interval had a gradual increase and then a decrease after reaching the peak point, and finally a sudden increase. In these three types of the area measurement methods, histogram method showed best results with the minimum average relative error (ARE) of 5.4%, followed by manual extraction method with ARE of 6.0% and the RSM method with ARE of 11.8%. Based on these results, an efficient method for measuring the area of the thermal region in solar greenhouse was developed to quantitatively analyze the entire thermodynamic disfigurement area and the value of the heat loss in solar greenhouse, which will help the further optimization of the solar green house and supply constructive recommendations for farmers and business leaders.