Abstract: A numerical study on a packed bed thermal storage unit with phase change material（PCM)having different melting points is presented.Paraffin is used as PCM and injected into polycarbonate spheres as phase change capsules.Hot water is used as heat transfer fluid（HTF)flowing from the top down to melt PCM and store latent heat.The capsules are placed in series in the tank based on melting points.PCM having higher melting temperature is placed closer with hot water inlet.One-dimensional Schumann’s model with steady-state flow field assumption is adopted to calculate HTF temperature, while the phase change of PCM is simulated using apparent heat capacity method.The energy and exergy performance in charging process of two arrangement modes containing two, three and four kinds different PCMs are investigated and compared with those of packed bed using only one kind of PCM.The energy comparison results indicate that the thermal storage unit with multiple PCMs has much higher charge rate and higher thermal efficiency than single PCM storage bed.In the exergy comparison, the storage bed with higher average melting point can store more exergy than that has lower average melting point.The utilization of multiple PCMs can dramatically reduce charge time, improving the storage performance.

Key words: packed bed, phase change capsule, different melting points, energy, exergy

摘要： 对采用多种不同熔点相变材料（PCM）构成的堆积蓄热床进行了数值分析。热水作为换热流体（HTF）自上而下流经蓄热床，熔化相变材料、蓄积相变潜热。石蜡作为相变材料被注射入聚碳酸酯球壳内形成相变胶囊，根据熔点高低依次排放在蓄热床的不同位置，熔点越高距离热水进口越近。假定流场稳定，采用一维Schumann模型计算HTF温度，相变模拟采用显热容法。分别对两种排列方式下采用2种、3种以及4种相变材料的蓄热床的蓄热过程进行了基于热力学第一及第二定律的性能分析，并将结果与单相变材料蓄热床进行比较。基于热力学第一定律分析结果表明，采用多种相变材料构成的蓄热堆积床蓄热速度更快，能量效率更高。基于热力学第二定律分析表明，平均熔点更高的蓄热床能够储存更高的火用。结果表明采用多熔点相变材料构成的堆积蓄热床能够显著地缩短蓄热时间，改进蓄热性能。

关键词: 堆积蓄热床, 相变胶囊, 多熔点, 能量, 火用