Phase change materials can improve the efficiency of energy systems by time shifting or reducing peak thermal loads. The value of a phase change material is defined by its energy and power density—the total available storage capacity and the speed at which it can be accessed. These are influenced by material properties but cannot be defined with these properties alone. Here we show the close link between energy and power density by developing thermal rate capability and Ragone plots, a framework widely used to describe the trade-off between energy and power in electrochemical storage systems (that is, batteries). Our results elucidate how material properties, geometry and operating conditions influence the performance of phase change thermal storage. This research sets a clear framework for comparing thermal storage materials and devices and can be used by researchers and designers to increase clean energy use with storage.

相变材料可以通过时移或降低峰值热负荷来提高能源系统的效率。相变材料的价值取决于其能量和功率密度——可用的总存储容量和访问速度。这些受材料属性的影响，但不能单独用这些属性来定义。在这里，我们通过开发热速率能力和 Ragone 图来展示能量和功率密度之间的密切联系，这是一个广泛用于描述电化学存储系统（即电池）中能量和功率之间权衡的框架。我们的结果阐明了材料特性、几何形状和操作条件如何影响相变蓄热的性能。