## 4 Conclusions  
The results obtained in this study emphasize the need for a detailed analysis of the performance characteristics of solar collection systems, such as the flat plate collector (FPC), the heat pipe evacuated tube collector (HPETC), and the heat pipe flat plate collector (HPFPC) under various operating conditions. The most important conclusions include the following.  
The heat transfer coefficient in the HPFPC decreases by 28% and 41% when 30% and 50% antifreeze concentrations are used. Overall, the integration of heat pipes in flat plate solar collectors can enhance efficiency by as much as 6.5% compared to traditional FPCs, highlighting the importance of fluid composition and operating conditions in optimizing thermal performance.  
The results show that the use of antifreeze fluids in flat plate or evacuated tube collectors results in a minimal efficiency change of approximately 0.5%. By contrast, utilizing antifreeze in the HPFPC leads to more significant reductions in efficiency, specifically by 10% and 32% for 30% and 50% ethylene glycol mixtures, respectively. However, at a fluid temperature of 30 \({}^{\circ}\)C with antifreeze, the efficiency is diminished by 10% and 15% for 30% and 50% antifreeze concentrations, respectively. Notably, at a temperature of 50 \({}^{\circ}\)C, the efficiency of both systems converges. At higher fluid temperatures of 80 \({}^{\circ}\)C, efficiency enhancements of 13% and 8% are recorded for antifreeze concentrations of 30% and 50%, respectively. When water is used as the working fluid in the HPFPC, efficiency gains of up to 13% and 21% are observed at fluid temperatures of 30 \({}^{\circ}\)C and 80 \({}^{\circ}\)C, respectively.  
The influence of fluid properties on system performance is highlighted, as well as the need for careful consideration of working fluid composition and operating conditions in improving thermal efficiency across different solar collector systems. Future work should be oriented on the study of several nano-fluids, long-term performance, and the economic viability of these systems.  
Conceptualization, J.M.C., G.G.-U. and A.Z.; methodology, J.M.C., G.G.-U., J.E.D.L.-R. and A.Z.; software, J.M.C., J.B., R.B., R.M.-M. and A.Z.; validation, J.E.D.L.-R., J.B., G.R. and A.Z.; formal analysis, J.M.C., J.E.D.L.-R., G.G.-U. and A.Z.; investigation, J.M.C., G.R. and A.Z.; resources, J.B., G.R., S.E.C. and A.Z.; data curation, J.M.C., J.E.D.L.-R., G.G.-U., G.R. and A.Z.; writing--original draft preparation, J.M.C. and A.Z.; writing--review and editing, J.M.C., G.G.-U., O.G. and A.Z.; visualization, G.R., O.G. and S.E.C.; supervision, A.Z.; project administration, A.Z.; funding acquisition, G.R., S.E.C. and A.Z. All authors have read and agreed to the published version of the manuscript.  
The financial support of this study by the science research grant SIP20240693, and by the Instituto Politecnico Nacional, IPN, is greatly appreciated.  
Not applicable.  
The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding authors.  
The financial support of this study by the science research grant SIP20250693, by the Instituto Politecnico Nacional, IPN, is greatly appreciate. The Authors also acknowledge the financial support given for the postgraduate students by the National Board of Science and Technology, CONACYT, of Mexico.  
The authors declare no conflicts of interest.