Heat pipe based solar collectors (HP–SCs) have emerged as a promising way to improve the performance of solar thermal collectors (STCs) and PV/T collectors, a technology that leads to a proliferation of scientific studies and literature. To date, several review papers have been published summarizing studies relevant to either HP-STCs or HP-PV/T. As the update development of HP-STCs, HP-PV/T technology shows common features from structur. Heat pipe based solar collectors (HP–SCs) have emerged as a promising way to improve the performance of solar thermal collectors (STCs) and PV/T collectors, a technology that leads to a proliferation of scientific studies and literature. To date, several review papers have been published summarizing studies relevant to either HP-STCs or HP-PV/T. As the update development of HP-STCs, HP-PV/T technology shows common features from structural design to operation principles. There exist some degrees of inevitable relationships between such two kinds of HP-SCs. Therefore, it is meaningful to conduct a comprehensive overview of studies about these two kinds of HP-SCs. This paper aims to summarize the classification, performance evaluation and optimization, and effective improvements related to both HP-STCs and HP-PV/T collectors. Diverse performance evaluation and optimization methods, and effective improvement measurements for various HP-SCs are presented and discussed. This is followed by conclusive remarks of their application situations, performance analyses and comparison, and effective improvements. Finally, new research directions and potentials in this field are identified and recommended.••••A background of HP-SCs including HP-STCs and HP-PV/T, their features and obstacles.••The classification, performance evaluation and optimization, improvement methods.••Conclusive remarks of the review works.••New research directions and potentials for future studies are recommended.Heat pipeSolar collectorSolar thermalSolar photovoltaicThe rapidly rising energy consumption in building sector in pursuit of better indoor environment has heightened the need for renewable energy. Of particular interest is the solar energy application. Solar energy technologies have been well developed for many years and are taken as the most feasible renewable solutions for the building application. Solar energy is chiefly utilized in two proven technologies, i.e. solar thermal technology and solar photovoltaic (PV) technology.Solar thermal collectors (STCs) play crucial role in determining the operation performance of solar thermal technology. There are two kinds of commonly used STCs: Flat Plate STCs (FP-STCs) and Evacuated Tube STCs (ET-STCs). The FP-STCs have crucial advantages like simple structure, high operation stability, easy integration with buildings, low maintenance cost and so on. However, their broader application is limited by several significant challenges including the large thermal loss, potential freeze issues, and the relative low efficiency. To a certain degree, ET-STCs could overcome the limitations faced by FP-STCs since this kind of collectors can achieve a much higher efficiency even in cold climates and the other unfavorable weather conditions. Despite above proven advantages, ET-STCs suffer from some restrains, e.g. the overheating problem and its corresponding impacts, the frangibility of glass tu. The structures and operation principles of heat pipes would play significant impacts on performances and applications of HP-SCs [,, ]. Considering different structures, heat pipes can be mainly categorized into integral HP (IHP), loop HP (LHP), and pulsating HP (PHP). IHP is basically a sealed tube with a specific amount of working fluid that transfers heat through continuous evaporation-condensation cycle. Deviating from IHP, LHP has a separate evaporator and condenser, thus eliminating an entrainment effect occurring in between. LHP allows a high thermal flux to transport over a distance of up to several tens of meters in a horizontal or vertical position owing to its capillary or gravitational structure [39,40]. Typically, a PHP comprises a serpentine channel of capillary dimension, which has been evacuated, and partially filled with the working fluid. The variety of heat pipes, in terms of their structural and technical characteristics, is briefly summarized in Table 1.Table 1. Summary of technical characteristics of heat pipes. Types Schematic diagram Description SourcesIHP High effective thermal conductivity. Low thermal and hydraulic resistances. .
