Research on the application of phase-change heat storage in centralized solar hot water system

Solar hot water has attracted growing level of attention due to rapid growth of energy consumption in buildings. Centralized solar hot water system has been widely used in residential buildings in China because it is suitable for multi-story and high-rise buildings. The application of phase change thermal storage in distributed solar hot water system has been widely studied. By contrast, few researches have focused on centralized solar hot water system. This paper aims to investigate the centralized solar hot water system with phase change material (PCM) as thermal energy storage. Through experiments, stainless steel ball was selected as the unit to package paraffin wax, and expanded graphite was added to enhance the heat transfer performance. According to the working temperature of solar collector, two materials with melting temperature of 55 degrees C and 60 degrees C were selected to test the properties of heat storage, heat releasing and thermal insulation of the water tank with PCM at different levels of solar irradiation. In addition, solar fraction of the system was calculated based on the maximum daily demand of users. The results showed that the PCM can complete the heat storage within 2-4.5 h at 10.8 MJ/m(2), 15.1 MJ/m2 and 19.4 MJ/m2 solar irradiation. The average heat releasing rate was 78% for the 55 degrees C PCM and 83% for the 60 degrees C PCM. The average solar fraction of the system with 55 degrees C PCM was 75% and 80% for the system with 60 degrees C PCM. Similarly, control mode 1 is more appropriate to the buildings with low occupancy rate or low hot water demand. By contrast, control mode 2 is more appropriate to the buildings with high occupancy rate or large fluctuation of hot water demand. Finally, the results showed that the centralized solar hot water system with PCM performed better than that without PCM. The solar fraction was improved significantly. Meanwhile, it is more effective to use 60 degrees C PCM in centralized solar hot water system than 55 degrees C PCM. (C) 2018 Elsevier Ltd. All rights reserved.