## 4 Conclusions  
With the consideration of using renewable energy in the industry, direct absorption solar collectors have been reported for a variety of applications such as water heating. However, solar collectors have limited efficiency due to poor absorption characteristics of the working fluid. Therefore, in this study, the solar heat collection system was investigated by conducting quantitative performance experiments \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**September 24** & **DW** & **AI2O3 0.1 (wt%)** & **AI2O3 0.3 (wt%)** & **AI2O3 0.5 (wt%)** \\ \hline T\({}_{\text{in}}\) (\({}^{\circ}\)C) & 39.79 & 39.98 & 41.35 & 41.75 \\ T\({}_{\text{max}}\) (\({}^{\circ}\)C) & 21.66 & 21.78 & 21.76 & 21.52 \\ \(\Delta\)T (\({}^{\circ}\)C) & 18.13 & 18.21 & 19.59 & 20.23 \\ \(\Delta\)T Enhancement (\%) & & 0.4 & 8.1 & 11.6 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Changes in DW and alumina nanofluid temperatures on 24 September 2018. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**September 22** & **DW** & **AI2O3 0.1 (wt%)** & **AI2O3 0.3 (wt%)** & **AI2O3 0.5 (wt%)** \\ \hline T\({}_{\text{in}}\) (\({}^{\circ}\)C) & 22.59 & 22.64 & 22.66 & 22.55 \\ T\({}_{\text{max}}\) (\({}^{\circ}\)C) & 36.13 & 36.60 & 37.89 & 37.97 \\ \(\Delta\)T (\({}^{\circ}\)C) & 13.54 & 13.97 & 15.23 & 15.42 \\ \(\Delta\)T Enhancement (\%) & & 3.1 & 12.5 & 13.9 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Changes in DW and alumina nanofluid temperatures on 22 September 2018. Figure 6: Temperature and solar radiation according to accumulated time (24 September 2018). using DW as the base fluid and three alumina nanofluids at 0.1, 0.3, and 0.5 wt% and measuring the solar radiation in a real environment. The characteristics of the alumina nanofluid were experimentally studied by using an actual solar collector system. It shows that dispersibility is a very important factor for absorbing the solar heat source. As a result, the following conclusions were obtained. 1. The UV-vis absorbance of the nanofluids increased with alumina concentration and reached a maximum for the 0.5 wt% nanofluid, demonstrating its potential for solar thermal systems. 2. The electrical conductivity of the nanofluids increased with alumina concentration. The electrical conductivities of the 0.1, 0.3, and 0.5 wt% alumina nanofluids were 21.3%, 46.2%, and 73.2% higher, respectively, than that of DW. 3. The thermal conductivity of the nanofluids increased with alumina concentration and with temperature. The thermal conductivities of the 0.1, 0.3, and 0.5 wt% alumina nanofluids were 0.32%, 0.62%, and 0.95% higher, respectively, than that of DW. 4. In a real thermal solar collection system, the temperature increases for the 0.1, 0.3, and 0.5 wt% alumina nanofluids were 3.1%, 12.5%, and 13.9% higher, respectively, than that of DW. The optimum fluid condition was determined as 0.3 wt% alumina due to greatest thermal increase by concentration. In general, the fluid temperature increased with the concentration of alumina in the nanofluid, but significantly better results can be obtained when optimizing the fluid dispersibility and stability. This study shows that there are various factors that must be considered in future research on the utilization of thermal solar energy in the development of alternative energy sources. Aside from alumina nanofluids, using multiple types of nanoparticles and various dispersion methods could further improve the efficiency. We believe that this study will aid in finding solutions for the utilization of solar thermal energy, which has been developed at a slower pace than solar photovoltaic power due to efficiency issues, to ultimately develop an optimal solar system. [PERSON] and H.J. ; conceptualization, S.K. ; data curation, S.K. ; formal analysis, methodology, Y.L. and H.J. ; investigation, Y.L. ; validation, Y.L. ; writing--original draft, J.-T.P. ; resources A.D.; supervision, writing-review and editing, S.K. and A.D. All authors have read and agreed to the published version of the manuscript.