This paper presents a novel composite phase change material (PCM) with two phase transition temperatures (PTTs), which can be used in solar water heating system (SWHS) to accomplish the demand for seasonal thermal energy storage. A three-dimensional (3D) numerical model of evacuated tube collector (ETC) is established. An outdoor test rig has been fabricated to validate the numerical analysis. Differential scanning calorimetry (DSC) is employed to measure the thermal properties of PCMs. The numerical analysis shows that the natural convection effect cannot be ignored. The temperature gradients calculated in the axial direction during the solid sensible heating and phase change periods are respectively 0.02% and 1.7% of that in the radial direction in summer, 0.1% and 1% in winter, which indicates that the heat transfer is essentially in a two-dimensional (2D) way in these two periods. The DSC test illustrates that the enthalpy and PTT of the composite PCM are mainly affected by the component of single PCM with high PTT. Both the test and simulation show that CA/62, the combination of capric acid (CA) and #62 paraffin, can reach two quite different PTTs, to make it a suitable one in terms of seasonal thermal energy storage in SWHS.

中文翻译：

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一种用于太阳能热水系统季节性热能存储的新型复合相变材料

本文提出了一种具有两个相变温度 (PTT) 的新型复合相变材料 (PCM)，可用于太阳能热水系统 (SWHS)，以满足季节性热能存储的需求。建立了真空管集热器（ETC）的三维（3D）数值模型。已经制造了一个室外试验台来验证数值分析。差示扫描量热法 (DSC) 用于测量 PCM 的热性能。数值分析表明，自然对流效应不容忽视。计算得到的固体显热和相变期间轴向温度梯度夏季分别为径向温度的0.02%和1.7%，冬季分别为0.1%和1%，这表明这两个时期的传热基本上是二维（2D）方式。DSC 测试表明，复合 PCM 的焓和 PTT 主要受高 PTT 的单一 PCM 成分的影响。测试和模拟均表明，癸酸 (CA) 和 #62 石蜡的组合 CA/62 可以达到两种截然不同的 PTT，使其成为适合 SWHS 季节性热能储存的一种。