###### Abstract  
The present review paper aims to document the latest developments on the applications of nanofluids as working fluid in parabolic trough collectors (PTCs). The influence of many factors such as nanoparticles and base fluid type as well as volume fraction and size of nanoparticles on the performance of PTCs has been investigated. The reviewed studies were mainly categorized into three different types of experimental, modeling (semi-analytical), and computational fluid dynamics (CFD). The main focus was to evaluate the effect of nanofluids on thermal efficiency, entropy generation, heat transfer coefficient enhancement, as well as pressure drop in PTCs. It was revealed that nanofluids not only enhance (in most of the cases) the thermal efficiency, convection heat transfer coefficient, and exergy efficiency of the system but also can decrease the entropy generation of the system. The only drawback in application of nanofluids in PTCs was found to be pressure drop increase that can be controlled by optimization in nanoparticles volume fraction and mass flow rate. parabolic trough solar collectors; nanofluid; thermodynamic performance; entropy generation; exergy analysis +
Footnote †: journal: Applied Soft  
Solar energy has many advantages over other types of renewable energy. Most importantly, solar energy, compared to other types of renewable energy resources, is considered the greatest resource, as the energy provided to Earth by the Sun in an hour can be more than the whole amount of the energy consumed by human beings in a year [4]. However, solar energy is currently providing rather a small portion of the total energy supply; this energy can be the most useful energy in the future. Solar energy can be separated to two major types, solar photovoltaic, which directly convert the Sun's light to electricity, and solar thermal collectors which concentrates the Sun's light to generate heat and use the heat to run heat engines [5]. The conversion efficiency of Photovoltaic (PV)cells are typically in the range of 10-20%, as much as the radiation heat up the cells, the efficiency goes down [5], which by considering the price is not affordable. However, besides its cost, one cannot deny that solar energy in some cases is the only option for remote areas and this advantage makes it an asset. Another way to harness solar energy is the use of solar thermal collectors. The direct absorption solar collector (DASC) was the pioneer in this type of solar collectors introduced in the 1970s, instead of simple flat plate type collectors [6]. Solar thermal collectors work based on concentrating and absorbing the sunlight, converting to heat, and transferring this heat to a working fluid which can be liquid or gas. Working fluids can be water, oil, molten salt, air nitrogen, or helium flowing through the collectors [7]. Enhancing heat transfer and thermal conductivity through these working fluids can cause increasing efficiency. In recent years, research along this issue got popularized among scientists. It was suggested to add solid nanoparticles to common working fluid to enhance the effective thermal conduction. Using nanoparticles instead of particles with sizes in order of micrometer diminishes the issues associated with erosion and corrosion of components as well as the sedimentation and clogging of particles [8]. This mixture considerably increased the heat transfer rate and the suspension called nanofluid [9; 10; 11]. Nanofluid, which consists of particles with a dimension less than 100 nm dispersed in a base fluid, can be defined as suspensions of NPs. This nanofluid is able to enhance conductivity and heat transfer coefficient of working fluid. [PERSON] et al. [9], by introducing and discussing a new idea on using nanoparticles, became pioneers in efficiency enhancement of working fluid in thermal systems. Their results revealed the benefits of using copper-based nanofluid.