On the integration of phase change materials with evacuated tube solar thermal collectors

Evacuated tube solar collectors (ETSCs) have gained a significant share of the solar thermal collector (STC) market. Compared to other collector types, ETSCs cover a relatively wide range of operating temperatures, mostly offer higher thermal efficiency, and are available at reasonable prices. But similar to other solar energy technologies, ETSCs are suffering from two main drawbacks associated with intermittency of solar radiation. Phase change materials (PCMs) have been widely used to overcome this challenge. If properly designed and utilised, PCMs can reduce the energy fluctuations and store the solar thermal energy during the daytime and release it in the absence of sunlight. There have been many studies conducted on the integration of PCMs with ETSCs, but the lack of a comprehensive systematic review study focused on such integrated energy systems has remained to be a gap in the literature. This gap is addressed by the present review study. Based on both theoretical and experimental results reported in the literature, the present study focuses on PCM assisted ETSC systems from different perspectives such as integration types, design parameters, and performance. Four main types of integration between PCMs and ETSCs are identified and advantages and disadvantages of each type compared to the others are discussed. Furthermore, state of the art is also clarified, the knowledge gaps are identified, and a madmap for further studies on these energy systems is provided accordingly.