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. (C) 2020 Published by Elsevier Ltd.