Warming effect comparison between substrate warming system and air warming system by active heat storage-release in Chinese solar greenhouse

Abstract: In order to improve the utilization efficiency of the heat from active heat storage-release system in Chinese solar greenhouses, the solar energy can be utilized to improve the temperature of the substrate. Based on the active heat storage-release system, we designed the active heat storage-release substrate warming system (AHSSWS) in order to provide theoretical basis for multiple applications of the active heat storage-release system. The AHSSWS was composed of active heat storage-release plates, water tank, heating pipes under substrate bags and submersible pump. In the daytime, when the water circuited in the system, the solar radiation and thermal energy in the air were absorbed. Then the heat would be released to the substrate in the soil through the heating pipes embedded. The experiment was conducted from December 16, 2016 to February 16, 2017. Tomato was used as the model plant in the greenhouse. The AHSSWS was compared with the active heat storage-release air warming system (AHSAWS) in order to find out which was better to lower the heat consumption for unit production. Effects of 2 heating approaches on the substrate temperature, air temperature indoor, plant height and stem diameter of tomato on different dates and tomato yield were contradistinguished. Moreover, collected and released heat, COP (coefficient of performance), power consumption and facility investment of 2 systems under diverse weather conditions were analyzed and compared. The results showed that compared with AHSAWS, AHSSWS designed could increase the substrate temperature by 2.5-5.3 ℃, but there was not obvious difference on air warming. Compared with air heating, substrate heating treatment could significantly increase the height, fresh weight and dry weight of tomato plant. In the case of continuous sunny days, the COP of AHSSWS was 1.5-1.9 and that of AHSAWS was 3.0-4.0. In the case of continuous cloudy days, the COP of AHSSWS was 0.5-0.9 and that of AHSAWS was 1.0-2.2. The difference between water temperature and substrate temperature in the AHSSWS was smaller than the AHSAWS, and the heat transfer mode was mainly thermal conduction when the heat was released to the substrate, while the heat transfer mode was mainly thermal convection when the AHSAWS was heating air. So compared with the AHSAWS, the AHSSWS had slightly lower heat storage efficiency, heat saving rate and average COP. The average daily electricity consumption of the AHSSWS was 0.031 kW·h/(m2) during both sunny days and cloudy days while the average daily electricity consumption of the AHSAWS was 0.026 kW·h/(m2). Tomatoes were picked about 5 times and the yield under the treatment of AHSSWS was 6.73 kg/m2. At the same time the yield under the treatment of AHSAWS was 3.83 kg/m2. Last but not least, the heat consumption for unit production under the AHSSWS was 0.7 kJ/kg, which was 1.0 kJ/kg lower than that of the treatment with AHSAWS. If tomatoes could be sold at the price of 2.7 yuan/kg, there would be favorable for productors using the AHSSWS compared with the AHSAWS. The results of this study provide the basis for the efficient application of active heat from active heat storage-release system and the application of the AHSSWS in tomato cultivation in Chinese solar greenhouses in winter and spring.