## 1 Introduction
The use of surfactants that induce changes in the surface also allows one to obtain stable nanofluids. In the case of nanofluids synthesized by the two-step method, which are usually more unstable than those of the one-step method [17], dispersion using mechanical agitation and ultrasonication is recommended, the latter being more effective in achieving that nanofluids remain stable for a longer period of time. Various authors, such as [18; 19; 20], have shown that the use of water-based nanofluids improves the efficiency of solar thermal collectors. In the case of the direct absorption solar collectors, the improvement in thermal efficiency using nanofluids has also been demonstrated [21; 22]. The advantage of heating within the volume of the liquid is mentioned in [23], where the authors studied the efficiency of the direct absorption solar collector using nanofluids. The configuration had dimensions of 3 \(\times\) 5 cm, with a channel depth of 150 \(\upmu\)m. Three different types of nanoparticles were used for the study: graphite (30 nm in diameter),carbon nanotube (6-20 nm in diameter and 1000-5000 in length), and silver (20 and 40 nm in diameter). Measurements were taken by varying particle size or volume fraction. They found that variability in size, shape and volume fraction can lead to maximum spectral absorption. Efficiency improvement of up to 5% was seen using the nanofluid. The experimental and numerical results showed a rapid initial increase in efficiency with an increase in volume fraction; however, a slight reduction in efficiency was observed for a further increase in volume fraction. Using this method, the need for a hot surface that only transfers heat to a small area of the liquid is eliminated. In [24], the authors observed that the presence of nanoparticles increases the absorption of incident radiation by more than 9 times, with respect to pure water. In [25], the authors noted that, depending on the particle material, size, shape, and volume fraction, a fluid can change from being nearly transparent to sunlight (in the case of water, alcohols, oils, and glycols), to being a very efficient volumetric absorber of sunlight. They present some benefits, including the improvement of thermophysical properties of conventional working fluids. The system considered can absorb more than 90% of the incident radiant energy, in addition to improving the performance of the collector by up to 10%. [PERSON] et al. [26] experimentally investigated the efficiency of a low-temperature solar collector under outdoor conditions, using various concentrations of water-based Al\({}_{2}\)O\({}_{3}\) nanoparticle suspensions (0.001% to 0.05% vol.) as working fluid. The researchers reported an improvement in solar collector efficiency compared to using water. Instantaneous efficiency versus reduced temperature plots showed that increasing the concentration of nanoparticles beyond 0.005% vol. had an inverse effect on efficiency. A maximum improvement in efficacy of 39.6% was observed using 0.005% vol. of nanofluid. [PERSON] et al. [27] experimentally studied the efficiency of a low-temperature solar collector with applicability for domestic hot water systems, using CuO nanoparticles in water as the base fluid, under real outdoor conditions. The effect of the emissivity of the inner surface of the bottom wall, the inlet temperature of the nanofluid, the flow rate and the concentration, on the efficiency of the collector was evaluated. The use of a black background inner surface was shown to increase efficiency up to approximately 11.4% more than the reflective inner surface. Collector efficiency was reported to have a direct relationship with nanofluid concentration and flow rate. Nanofluids with volumetric fractions of 25,100 ppm could improve collector efficiency by 91.7% compared to base fluid. Water-based carbon nanoparticles improve the efficiency of solar collectors, as shown in [28; 29]. On the other hand, as seen in [30], carbon nanomaterial can be obtained at low cost.