Heat conduction law of hydroponic nutrient solution as heat storage medium

In order to grasp the characteristics of heat transfer of hydroponic nutrient solution as heat storage medium and the related heat storage and preservation performance, nutrient solutions of leaf vegetable production systems were used for experimentation. The experiment was conducted at the solar greenhouse of Agricultural Engineering Institute of Shanxi Agricultural University. In this study, SH-16 road temperature inspection was used in leaf vegetable production systems and sensor elements placed at several different depths of different positions to monitor solution temperature. The regulation of temperature change of nutrient solutions in the system under porous planting plates were discussed. The experimental results showed that temperature change in nutrient solution at different depths were significantly different. The largest amplitude of temperature variation of nutrition solutions was at surface layer. The deeper the nutrient solution, the smaller was the variation. The highest temperature of nutrient solution in autumn decreased rapidly with increasing nutrient solution depth, and happened at 14:00, 16:00, 17:40 and 20:00 for the solution depths of 0 cm, 5 cm, 10 cm and 15 cm respectively. Based on the daily range of temperature at different depths, nutrient solutions were divided into three temperature layers-heat exchange layer (daily range of temperature > 3℃), heat buffer layer (daily range of temperature of 1-3℃) and heat stability layer (daily range of temperature at 0-1℃), which were located in the solution layers of 0-5 cm, 5-10 cm and 10-15 cm, respectively. When the nutrient solution depth of leaf vegetable production system was 21.5 cm, the relationship between daily temperature difference and solution depth (0 cm, 5 cm, 10 cm, 15 cm were expressed as 1, 2, 3 and 4 in the function) was described with the logarithmic function y=-2.619lnx + 4.215 2. That indicated that daily temperature difference at 20 cm below solution surface was 0℃. The above results indicated that energy was conducted on a layer-by-layer basis in the nutrient solution of hydroponic system.