## 1 Introduction
[9] began a research in 2008 to use nanofluids for industrial cooling, which might save a significant amount of energy and reduce emissions. [PERSON] et al. [10] demonstrated that a specific class of nanofluids can be employed as a smart material that acts as a heat valve to control the flow of heat. The nanofluid can easily be constructed in a low state, in which it conducts heat poorly, or in a high condition, in which heat dissipation is more efficient. The potential efficiency advantages that come up with using a nanofluid as a working fluid could increase the performance of solar collectors. [PERSON] [11] investigated the flow and thermal behavior of nano-encapsulated phase change materials (NEPCMs) dispersed in a liquid over a vertical flat plate. The finite difference method with a colocation technique as the grid adaptation and automatic error control was utilized to integrate the governing equations. Free convective flow and heat transfer of a suspension of Nano Encapsulated Phase Change Materials (NEPCMs) in an enclosure was studied by [PERSON] [12]. The governing equations are transformed into non-dimensional form and solved by the finite element method. [PERSON] et al. [13] examined the thermal energy storage of the so called solar salt (60% NaNO\({}_{3}\) and 40% KNO\({}_{3}\)) was improved by adding a phase change material composed of Al-Cu alloy nano-encapsulated with an aluminium oxide layer naturally formed when exposed to oxygen. Several experimental and theoretical studies conducted on the thermal conductivity of nanofluids are represented and investigated by [PERSON] et al. [14]. Based on the reviewed studies, various factors affect thermal conductivity of nanofluids, such as temperature, the shape of nanoparticles, concentration, etc., furthermore, it is concluded that there are some novel approaches in order to obtain nanofluids with more appropriate thermal properties including using binary fluids as the base fluid or utilizing hybrid nanofluids. To explore convective fluxes of nanofluids, several authors have developed a variety of models and group theory methodologies. In [15], the authors demonstrate an insight of the many flat-plate solar-energy air heating collector designs and performance evaluation approaches for low-temperature solar-energy crop drying applications, as well as the suitability of each design and materials requirements. [PERSON] et al. [16] constructed a unique solar air collector with a pin-fin integrated absorber to improve thermal efficiency. Their design offered many benefits, including high thermal efficiency, a wide range of flexibility, extended durability, and reasonable pricing. [PERSON] et al. [17], [PERSON] et al. [18], [PERSON] et al. [19], and [PERSON] et al. [20] explored nanofluids, which are nanoparticle suspensions in fluids, that demonstrated considerable increase in their properties at low nanoparticle concentrations. Solar energy is currently one of the most important sources of clean renewable energy and it is being studied in a variety of sectors. Many authors have recently proposed a non-homogeneous equilibrium model, whereas [PERSON] et al. [21], [PERSON] et al. [22], [PERSON] [23], and [PERSON] [24] have published several numerical studies on homogeneous modeling of natural convection heat transfer in nanofluids. The dispersed and continuous phases are combined and modeled as a new continuous phase in which the velocity is constant. The homogeneous flow model is the most straightforward method for evaluating multiphase flows. [PERSON] [25] used nanofluids on parabolic trough collector (PTC) systems. The efficiency of the PTC system employing Al\({}_{2}\)O\({}_{3}\)/synthetic oil nanofluid as heat transfer fluid (HTF) with non-uniform heat flux distributions is investigated using a multi-field coupling simulation based on the finite element method (FEM). Ref. [26] described the most recent advancements in the use of nanofluids as a working fluid in parabolic trough collectors (PTCs).