Among all the available solutions to the current high energy demand and consequent economic and environmental problems, solar energy, without any doubt, is one of the most promising and widespread solutions. However, conventional solar systems face some intractable challenges affecting their technical performance and economic feasibility. To overcome these challenges, increasing attention has been drawn towards the utilization of heat pipes, as an efficient heat transfer technology, in conventional solar systems. To the authors' knowledge, despite many valuable studies on heat pipe solar collectors (mainly during the last decade), a comprehensive review which surveys and summarizes those studies and identifies the research gaps in this field has not been published to date. This review paper provides an overview of the recent studies on heat pipe solar collectors (HPSCs), their utilization in different domestic, industrial, and innovative applications, challenges, and future research potentials. The concept and principles of HPSCs are first introduced and a review of the previous studies to improve both energy efficiency and cost effectiveness of these collectors is presented. Moreover, a concise section is dedicated to mathematical modeling to demonstrate suitable methods for simulating the performance of HPSCs. Also, the latest applications of HPSCs in water heating, desalination, space heating, and electricity generation systems are reviewed, and finally, some recommendations for future research directions, regarding both development and new applications, are made.

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Invention of evacuated tube heat pipe solar collectors (HPSCs) was a huge step forward towards resolving the challenges of conventional solar systems due to their unique features and advantages. This has led to their utilization in a wide range of solar applications surpassing other conventional collectors. However, relatively low thermal efficiency of heat pipe solar (HPS) systems is still the major challenge of solar industry evidenced by numerous studies conducted mainly during the last decade to improve their efficiency. To date, several review papers have been published summarizing studies relevant to utilization of HPSCs in various thermal applications. However, to the authors' knowledge, a comprehensive review which surveys and provides an overview of the studies undertaken to improve the thermal performance of HPS systems (mainly during the last decade) by implementing different strategies has not been published to date. This review paper summarizes all the proposed strategies to improve the thermal efficiency of different industrial, domestic, and innovative HPS systems. First, the concept, structure, and operational principles of HPSCs are introduced concisely. Then, novel structures and designs of HPSCs aiming to increase the thermal efficiency of the collector as the most important component of the solar system is reviewed. This is followed by a comprehensive review of various methods to store solar energy more efficiently, increase solar system's operation time, increase overall efficiency by turning the solar system into a multipurpose system, enhance heat transmission in the solar system, and implement new solar loop and heat pipe working fluids with better heat transfer characteristics. Finally, research gaps in this field are identified and some future research trends and directions are recommended.

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