## 1 Introduction
[10] reported that a heat storage system with a modular configuration meets the required storage capacity for the evaporation section and is thus a promising option for application in direct steam generation plants. [PERSON] and [PERSON] [11] compared the characteristics of a standard glass plate collector and vacuum tube collector by evaluating the efficiencies based on corrected and quasi-static conditions. [PERSON] and [PERSON] [12] demonstrated that the use of a thermal energy storage system is essential in solar energy systems due to variable solar inputs. [PERSON] et al. [13] studied the effect of nanofluids on the performance of solar collectors and water heaters and found a dependency on the nanofluid volume fraction and particle size. [PERSON] et al. [14; 15] reported the use of alumina (Al\({}_{2}\)O\({}_{3}\)) nanofluid as the working fluid for flat-plate solar collectors and demonstrated its higher efficiency than water. [PERSON] et al. [16] investigated nanotechnology in general and reported that nanotechnology is more innovative, precise, and future-oriented than other technologies. [PERSON] [17] reviewed the thermal properties of graphene, carbon nanotubes, and nanostructured carbon materials with different irregularities and assessed their potential applications in thermal management. [PERSON] et al. [18] studied the dispersibility and thermal conductivity of single-walled carbon nanotubes (SWCNTs) and reported that nanofluid dispersions of shorter SWCNTs have a higher thermal conductivity than resin-pristine SWCNT composites. [PERSON] et al. [19] confirmed that thermal conductivity increased with nanofluid concentration and showed that the efficiency of the nanofluids improved in the order of multi-walled carbon nanotubes (MWCNTs), CuO, Al\({}_{2}\)O\({}_{3}\), TiO\({}_{2}\), and SiO\({}_{2}\) based on their thermal performance in a U-type solar collector. [PERSON] and [PERSON] [20] experimentally examined pool boiling using \(\gamma\)-alumina nanofluids and confirmed that the heat transfer performance increased with \(\gamma\)-alumina particle concentration. [PERSON] et al. [21] observed the changes in pressure and continuous transfer performance of alumina and MWCNT nanofluids in chevron plate heat exchangers and reported that the nanofluid viscosity increased with nanoparticle concentration, leading to a pressure drop significantly higher than that of water. [PERSON] et al. [22] measured the thermal conductivity of alumina nanofluids depending on the diameter of the alumina particles and temperature and reported a higher thermal conductivity with smaller particles and higher temperatures. [PERSON] et al. [23] studied the thermal properties of alumina nanofluids for use as a new phase-change material in cooling systems and concluded that the addition of alumina nanoparticles alone reduced the solidification time of the fluid and thus improved the performance of the refrigeration system. [PERSON] et al. [24] examined the performance of a U-type solar collector according to the concentration and particle size of alumina nanofluids and reported that the nanofluid thermal conductivity increased with increasing nanofluid concentration and decreasing alumina nanoparticle size. [PERSON] et al. [25] analyzed the thermal performance of enclosed-type evacuated U-tube solar collectors using a MWCNT nanofluid as the working fluid and demonstrated that the heat transfer coefficient between the tube and working fluid is higher than that of water. Huminic and Huminic [26] investigated the heat transfer properties of two-phase closed thermosyphons using SiC nanofluids and confirmed that increasing the SiC nanofluid concentration increases the thermal conductivity and heat transfer performance compared with water. Contrary to previous studies in which the nanofluid was found to change the efficiency of the thermal solar energy system, in this study, alumina nanofluids were applied with different concentrations as the working fluid in a thermal solar energy collection system.