Optimization of active heat storage and release system in solar greenhouse

Active Heat Storage-release System (AHS) is a solar thermal utilization system, which collects and stores solar energy through the water circulation in the daytime, and the energy is released at night. To improve the stability and reliability of the operation of AHS, based on the sixth generation of AHS, the circulation pipeline, water supply mode, solar energy collection board were optimized and improved. The field test lasted 87 days was done to investigate the heating effect, heating performance, control strategy of circulation, and the distribution of indoor air temperature during the heating process of improved AHS under different weather conditions. The test period was from November 24th, 2019 to February 29th, 2020. The results showed that, during the experiment time, the improved AHS was in stable operation mode, the energy-saving, and high heating efficiency, the optimization was effective. In three different weather conditions of solar radiation intensity, the average temperature in the experimental area was 2.7, 2.2, and 1.9 ℃ higher than that in the control area, and the heat collection capacity was 4.6, 3.7, and 2.6 MJ/m2 respectively, heat release capacity was 4.1, 3.4, and 3.1 MJ/m2 respectively. The average heat collection power was 183.1, 146.5, and 105 W/m2 respectively, and average heat release power was 163.2, 134, and 121.1 W/m2 respectively. The average heat collection efficiency of improved AHS was 56.5%, 68.2%, and 73.8% respectively and the average coefficient of performance was 3.8, 3.1, and 2.8 respectively. Compared to electric heating, the energy conservation rate of improved AHS was 73.5%, 67.1%, and 63% respectively. Compared with the sixth generation AHS, when total heat energy collected was similar, the daily average power consumption was reduced by 20%, the energy-saving efficiency was increased by 6.7%, and the coefficient of performance was increased by 0.8. The water flow rate was reduced, so the average heat collection efficiency was reduced by 10.3%. If the structure of AHS is improved in the future, the diameter of the aluminum alloy finned pipe is advised to increase to improve the heat collection efficiency. During the heating period at night, the influence of improved AHS on temperature distribution in the experimental area was very obvious, because the board was located in the north wall, and the north side of the greenhouse was closed to the lower temperature roof, the temperature difference between the north and the south in the greenhouse was large. The closer to the north wall at the same height, the temperature is higher. Under different weather conditions, the maximum average temperature difference between north and south parts in the canopy was 2.8, 2.6, and 2.4 ℃ respectively. This temperature difference was getting smaller with height because of the influence of crops on heat shielding and greenhouse shape. Through the analysis of water temperature change during the daytime, it was suggested to adopt the joint control method of temperature and time for control strategy optimization. The application and promotion of AHS could be supported by the basis of theory and experimental results in this study.