A review on loop heat pipe for use in solar water heating

• Introduce the concept, working principle, and classification of loop heat pipe.
• Analyze the heat transfer limit and thermo-fluid process of the loop heat pipe.
• Present the application of the loop heat pipe in solar water heating system.
• Review and analyze significant research works regarding loop heat pipe technology.
• Propose further works to be done in promoting the solar water heating system employing loop heat pipe.

In this paper, a comprehensive review on the loop heat pipe (LHP) for use in solar water heating (SWH) will be carried out. LHP is an efficient heat transfer device that could transport thermal energy over long distances, up to several tens of meters. It has some unique features over conventional heat pipe, e.g., gravity-unaffected and flexibility in its design and installation, which makes LHP particularly suitable for applications in SWHs. A significant literature review of the LHP technology will be illustrated, and the review results will be analyzed in terms of the performance characteristics and research methodology. For the performance characteristics, the steady-state and transient behavior of the LHP device will be examined. The research methods, i.e., theoretical analyses and computer simulations, and experimental investigations, will be analyzed. Although substantial work has been done in LHP studies, the opportunities for further work will be identified. This review will help recognize the problems remaining in the existing SWHs, remove the barriers to solar applications, identify new research subjects to improve the performance of the SWHs, expand solar thermal market potential throughout the world and broaden the application of LHP in solar water heating.