六水硝酸镁相变储热复合材料改性制备及储/放热性能研究

水合盐相变材料因具有较高的相变焓和较低的成本在中低温储热领域有着广泛的应用前景，但其在储放热过程中通常存在过冷度大和热循环稳定性差的问题。以六水硝酸镁为主要研究对象开展相变储热复合材料的改性制备及相变储热装置的研制，采用熔融共混法制备了以二水硫酸钙为成核剂的六水硝酸镁相变储热复合材料，利用差示扫描量热仪及步冷曲线法测试了相变储热复合材料的热物性和循环热稳定性。在此基础上设计并构建了储热量为152 kWh的相变储热装置和相变储热系统，并对其储/放热性能进行了测试。结果表明：添加了2%（质量）二水硫酸钙的相变储热复合材料具有较好的循环热稳定性，且在经过50次熔化-凝固循环后其过冷度一直保持在0.5℃内，相变温度保持在87℃左右，相变焓保持在150 kJ/kg以上；相变储热装置可实现高达27 kW的平均储热功率，在保证放热过程中出水温度不低于56℃的情况下，可实现8 kW的平均放热功率和92.3%的储-放热效率，可满足建筑采暖及日常生活热水需求。

Preparation and investigation of the thermal charging and discharging of modified magnesium nitrate hexahydrate composite phase change material

The hydrated salt phase change material has a wide range of application prospects in the field of medium and low temperature heat storage due to its high phase change enthalpy and low cost, but it usually has the problems of large subcooling and poor thermal cycle stability in the process of heat storage and release. Here, a modified magnesium nitrate hexahydrate is prepared as composite phase change material (PCM) for thermal energy storage, and the high-performance heat storage device using the modified PCM is designed and fabricated. The magnesium nitrate hexahydrate is modified by using calcium sulfate dihydrate as nucleating agent to synthesize composite PCM. The thermal properties and cycle thermal stability of the composite PCM are tested by differential scanning calorimeter (DSC) and thermal analysis method. Moreover, the composite PCM-based heat storage device and heating system with storage capacity of 152 kWh are designed and constructed, and their thermal performances are tested at different working conditions. The results show that the composite PCM with 2%(mass). Calcium sulfate dihydrate has good cycle thermal stability, and its supercooling degree can be kept within 0.5℃ after 50 melting-solidification cycles. The phase-change temperature is about 87℃ and the phase-change enthalpy is maintained above 150 kJ/kg. The PCM-based heat storage device achieves an average charging power up to 27 kW and discharging power of 8 kW. The thermal efficiency of heat storage-release is as high as 92.3%, and the heat storage device can ensure the outlet water temperature is higher than 56℃ during the heat release process, and thus can meet building heating and hot water.