## 1 Introduction  
According to [1], it is predicted that by 2100, solar technologies will supply approximately 70% of the world's energy consumption. Conventional low- and medium-temperature collectors are used to collect heat, for space heating or hot water supply [2, 3]. The absorber of the solar collector has a solid surface of thermally stable polymers, aluminum, steel or copper, to which a matte black or wavelength selective coating is applied. This solid surface is supported by a fluid pipe coil, placed in an insulated casing with transparent glazing. The working fluid circulates through the tube and absorbs heat indirectly from the absorber, through conduction and convection. Therefore, a large amount of heat is lost to the environment by radiation and convection and other components by conduction, as stated by [4]. Due to the existing thermal resistance, to convert the incoming radiation into the internal energy of the transport medium, an alternative design idea is proposed, in which the working fluid is directly exposed to the incident radiation and the heat is absorbed volumetrically within the transport medium, rather than within a thin surface layer. With this, the thermal resistance, when converting solar energy into thermal energy of the working fluid, is significantly reduced in a direct absorption system. The direct absorption solar collector concept was first proposed by [PERSON] and [PERSON] in 1975 [5]. It consists of a container with a transparent cover, a channel where the thermal liquid circulates, and insulation to reduce heat loss; it is easy to build and has low cost [6]. Unlike other collectors, the working fluid directly absorbs radiation instead of an absorber plate or tube, so it must have the ability to absorb broadly in the solar spectrum (ultraviolet, visible and infrared) [6; 7]. Various works have been developed to improve the performance of direct absorption solar collectors, as can be seen in [8; 9; 10]. [PERSON] et al. [11] developed a volumetric solar collector, in which three types of semi-transparent liquid suspensions of microparticles were used: graphite (black), carborundum (grey), and silicon dioxide (white), in diethyl phthalate (colorless liquid). Measurements of the absorption coefficient of each suspension demonstrated the effectiveness of the use of fine particle additives. Further investigations on a high flux direct absorption collector, using salt, demonstrated the need to add particles to salt, which is a relatively weak absorber in the visible and near-infrared regions of the solar spectrum, to increase its absorption capacity [12]. Meanwhile, it was shown that the addition of organic and inorganic micron-sized chromophores, such as black dyes or carbon black in low-flow collectors, to salt eutectics in high-flow systems, caused thermal and photochemical degradation [13], which can lead to several practical drawbacks, such as reduced absorption, fouling and clogging of surfaces, erosion and abrasion of pipes and pumps over time [14]. The stability of nanofluids is a fundamental criterion for choosing a thermal fluid, because if the nanofluid is unstable, the nanoparticles will agglomerate, so that they will have a greater tendency to settle, which will produce a reduction in thermal and radiative properties [15; 16]. There are various factors that affect the stability of a nanofluid; for example, the concentration, characteristics of the nanoparticle (such as shape and size), the base fluid used, pH, among others. At a higher concentration of nanoparticles, the distance between them is smaller, so it is more likely that they will collide and, therefore, agglomerate. Forms of nanoparticles that have a larger surface area tend to agglomerate more easily, due to their greater possibility for collisions. Base fluids, which are polar, contain ions that allow the formation of electrical layers in each nanoparticle, which allows repulsion between two neighboring nanoparticles.