Property and heat storage performances of Glauber's salt-based phase change materials for solar greenhouse in Qinghai-Tibet plateau

Abstract: Thermal energy storage (TES) is considered as one of the most important energy storage methods, and it can reduce the imbalance between solar heat supply and consumption and help to save costs. The phase change materials (PCMs) are the materials storing and releasing latent heat during the phase change. Low temperature PCMs are widely used in greenhouse study. Glauber salt (NaSO4•10H2O) based composite PCMs affect the greenhouse temperature change directly and their phase change temperature is suitable for greenhouse application, but the problems of supercooling phase separation and low thermal conductivity exist in their application. Related studies have showed that borax is the best substance which can decrease the supercooling problem, and CMC (RnOCH2COONa) is effective to reduce their phase separation. In order to enhance its thermal conductivity, we chose graphitic material as the heat-removing agent. In this paper, we studied the properties of Glauber salt based phase change energy storage material and it’s performance of heat storage in winter in the Qinghai-Tibet Plateau region. Firstly, we prepared the Glauber salt based composite PCMs in laboratory, and then their thermal properties were measured by the differential scanning calorimetry (DSC). Samples were put in an alumina pan and heated from -10 to 40 ℃ at a rate of 1.0 ℃/min in purified nitrogen atmosphere (50 mL/min). The thermal cycling tests were carried out in an oven (Themo SCIENTIFIC HAAKE A 40) by heating and cooling the samples from 10 to 40 ℃ repeatedly. Thermal conductivities were determined by the transient plane source (TPS) method (Hot Disk, TPS 2200). The tested simple solar greenhouse was located in Xining City, Qinghai Province (101°44' E，36°43' N). It was 3.5 m long and 2.7 m wide. The wall included 5 cm flexible foam (polyurethane) and 20 cm gypsum brick, as well as 0.1 cm iron sheet. The test period was from December 20th, 2015 to February 4th, 2016. The heat insulation sheet of the simple solar greenhouse was rolled up and down at 9:00 am and 5:00 pm on every sunny day, respectively. The temperatures of outdoor, and indoor air and gypsum wall, and solar irradiation were measured continuously at a time interval of 10 min. The data collected on a typical sunny day were used to study the heat-retaining properties in greenhouse with PCMs. When the temperature was higher than PCMs freezing and melting point, the PCMs released heat and froze. This paper explored the property about its undercooling, phase separation, thermal conductivity and phase change cycle. Two simple greenhouses were built, and the change of temperature was analyzed. Results showed that when adding the 4% borax (mass fraction), the supercooling of PCMs almost disappeared; adding 1.5% CMC, the phase separation did not exist; adding 1% graphite powder, the thermal conductivity significantly improved. Latent heat of PCMs measured by DSC was 127 kJ/kg. After the phase changing cycle of 300 times, PCMs still could keep good performance, which was suitable for greenhouse to keep warm in cold climate. The test results of greenhouse temperature showed that when the mass of phase change materials was 25, 35 and 45 kg, the lowest greenhouse temperature increased by 1, 3.6 and 4.4 ℃, respectively before sunrise in the morning. The test results also showed that when the mass of phase change materials was 25, 35 and 45 kg, the highest temperature of gypsum back wall in noon decreased by 1.1, 2.2 and 3.0 ℃ and the lowest temperature before sunrise in the morning increased by 0.4, 2.4 and 4.0 ℃ respectively compared with the contrast greenhouse. We can conclude that the adding of the PCMs can obviously increase the lowest temperature inside the greenhouse and the back wall, and it is suitable for solar greenhouse heating application in extremely cold climate environment.