Heat storage and release test of external hanging phase change energy storage device in greenhouses

Abstract: The purpose of the test was to verify and evaluate the long-period heat storage and release performance of phase change material (PCM) that covered on the solar greenhouse in severe cold regions. Firstly, the CaCl2·6H2O-based PCM was modified by the orthogonal tests. SrCl2·6H2O, CMC and H2O were added to alleviate the supercooling, phase separation, and high phase transition temperatures. The composite PCM of 3% CMC + 3% SrCl2·6H2O + 35.96% CaCl2 + 58.04% H2O was determined as the energy storage medium. Secondly, a traditional greenhouse was taken as a contrast in the Fuxin City, Liaoning Province of China. Four independent parts were divided to transform from the middle two greenhouse. The composite PCM was packaged into the PVC-U pipes, which covered on the inner surface of the north wall of the east independent greenhouse. A 51-day comparative test was then carried out (February 09th, 2020-March 31st, 2020). The heat storage and release performance of PCM greenhouse was evaluated from the aspects of the indoor air temperature, the heat storage and release performance of the PCM device, supercooling unsuitable growth rate, effective accumulated temperature, the comparative performance of PCMs before and after field test, as well as the cherry tomato growth data. The following results were obtained: 1) The T-history and DSC tests show that the latent heat of phase change was reduced by 11.5 % and 13.2 %, respectively, in the composite PCM during heat storage and release processes, compared with that before the field test. The maximum difference of phase transition temperature was 0.37 ℃, indicating the stable phase transition temperature of the PCM before and after the field comparative test. There were no obvious supercooling and phase separation. 2) The best heat storage and release performance of PCM was found in the typical sunny, followed by the cloudy, and the worst in the snowy days. The average temperature in the PCM solar greenhouse at night increased by 3.1 ℃, 1.9 ℃, and 0.9 ℃, respectively. 3) Some recommendations were given to fully utilize the heat storage and release performance of the PCM. The manual operation procedures should be considered, particularly on the environmental factors inside and outside the greenhouse and the thermophysical properties of the PCM. 4) The supercooling unsuitable growth degrees of the PCM solar greenhouse and the contrasted greenhouse were 1 244.7 h·℃ and 2 073.4 h·℃, respectively, during the field comparative test period. There was the 40 % reduction rate of the supercooling unsuitable growth in the PCM solar greenhouse. A quantitative evaluation was realized for the heat storage and release performance of PCM solar greenhouse. 5) The effective accumulated temperatures were 174.5, and 110.1 ℃·d, respectively, in the phase change and contrasted greenhouse. Anyway, the effective accumulated temperature of the phase change greenhouse increased by 58.4%, compared with the contrasted greenhouse. In brief, the PVC-U pipes packaged with the PCM as the energy storage devices covered on the solar greenhouse can be expected to improve the heat storage and release performance of solar greenhouse in severe cold regions. The finding can provide the data and theoretical support for the practical popularization, application, and long-period theoretical analysis for the heat storage and release performance of the PCM solar greenhouse.