The efficient exploitation of solar irradiation is one of the most encouraging ways of handling numerous environmental concerns. Solar collectors are suitable devices that capture solar irradiation and convert it into thermal energy and electricity. In the last years, the nanofluids used in solar thermal systems have been studied as a useful technique for enhancing the solar collectors’ performance and establishing them as viable and highly efficient systems. The present review paper aims to summarize and discuss the most important numerical and experimental studies in nanofluid-based solar systems for application at low and medium temperature levels, while the emphasis on the fundamental physical phenomena that occur. In the first part, numerous numerical models and the principal physical phenomena affecting the heat transfer rate in the nanofluid have been analyzed. More specifically, the importance of different forces in nanofluid flows that exist in particulate flows such as drag, lift (Magnus and Saffman), Brownian, thermophoretic, Van der Waals, electrostatic double-layer forces are considered. Moreover, an overview of the thermophysical properties, physical models, heat transfer models, and evaluation criteria of nanofluids are included in this work. In the second part, which is the main part of this work, a comprehensive review is performed to gather and discuss the new advantages in the nanofluid-based solar collectors that operate at low and medium temperatures. More specifically, the examined solar systems are the flat plate collectors, the evacuated tube collectors, the direct absorption collectors, and the thermal photovoltaic systems, while the investigated applications are space-heating, space-cooling, household hot water production, desalination, industrial activities, and power generation. The aforementioned collectors and applications are the most usual in the real systems, indicating the importance of the present work. Moreover, the emphasis is given in the thermal, exergy, economic, and environmental evaluation of the studied systems, as well as in the discussion of the possible limitations of the use of nanofluids like the lack of long-term stability, the agglomeration of nanoparticles, and the increased pumping work due to the increased pressure drop. Finally, it is found that the nanofluid utilization usually enhances the collector efficiency up to 5%, while higher enhancements can be found in thermal photovoltaics. Moreover, it is concluded that there is a need to emphasize issues such as stability and the use of eco-friendly solar systems. Lastly, the field's future trends are highlighted, and a clear image of the present situation and the next steps in the field are given.

有效利用太阳辐射是处理众多环境问题的最令人鼓舞的方法之一。太阳能收集器是捕获太阳辐射并将其转换成热能和电能的合适设备。在过去的几年中，已经研究了用于太阳能热系统的纳米流体，作为增强太阳能收集器性能并将其建立为可行且高效的系统的有用技术。本综述旨在总结和讨论基于纳米流体的太阳能系统在中低温度下的应用中最重要的数值和实验研究，同时重点介绍发生的基本物理现象。在第一部分，分析了许多数值模型和影响纳米流体传热速率的主要物理现象。更具体地，考虑了存在于颗粒流中的纳米流体流中不同力的重要性，所述颗粒流例如为阻力，升力（Magnus和Saffman），布朗力，热泳，范德华力，静电双层力。此外，这项工作还包括纳米流体的热物理性质，物理模型，传热模型和评估标准的概述。在第二部分，即这项工作的主要部分，进行了全面的综述，以收集和讨论在低温和中温下运行的基于纳米流体的太阳能收集器的新优势。更具体地说，所检查的太阳能系统是平板集热器，真空管集热器，直接吸收式集热器和热光伏系统，而研究的应用领域包括空间加热，空间冷却，家用热水生产，淡化，工业活动和发电。前面提到的收集器和应用程序是实际系统中最常用的，表明了当前工作的重要性。此外，重点在于对所研究系统的热，火用，经济和环境评估，以及在讨论使用纳米流体的可能局限性（例如缺乏长期稳定性，纳米颗粒的团聚）方面的讨论。 ，以及由于压降增加而增加的泵送功。最后，发现纳米流体的利用通常可将集电极效率提高到5％，而在热光伏技术中可以找到更高的增强。此外，得出的结论是，需要强调诸如稳定性和使用生态友好型太阳能系统之类的问题。最后，突出显示了该领域的未来趋势，并给出了该领域的现状和下一步的清晰图像。