Loop heat pipe-based solar thermal facade water heating system: A review of performance evaluation and enhancement

To meet the demand for a high-efficiency, anti-freezing, and building-integrated solar water heating system, a versatile loop heat pipe-based solar thermal facade water heating system has been developed in recent years. To date, a comprehensive summary of evaluation indexes has not yet been reported for assessing the transient and steady-state performance of the loop heat pipe, which deserves to be determined because the overall thermal efficiency of the system is highly sensitive to the operational performance of the loop heat pipe. In addition, the heat transfer process within the system is analyzed along the three stages of the heat transfer path, namely, solar absorption, outdoor to indoor heat transportation, and water heating, to reveal the working mechanism of the system and clarify the factors influencing its behavior. In line with these three stages and practical requirements, the refined improvement strategies for system performance are elaborately summarized from three corresponding aspects, including the solar absorber structure, heat transfer fluid, and energy storage, to provide a reference for achieving a superior heat transfer effect. The particularity of the system lies in the involved heat and mass transfer phenomena and the latent heat thermal energy storage technology, which are the core issues worthy of in-depth exploration. Therefore, the system's potential for further development is outlined to promote market penetration. Through the current comprehensive analysis, the evaluation criteria of the operational characteristics of loop heat pipe and loop heat pipe-based solar thermal facade water heating systems as well as performance enhancement techniques are methodically expounded. This review is expected to lay a solid foundation for improving and extending the loop heat pipe-based solar thermal facade water heating system, along with achieving the targets for significant energy-saving and environmental benefits.