### Heat Pipe Flat Plate Solar Collectors  
Heat pipe flat plate solar collectors (HPFFPCs) incorporate multiple heat pipes as heat carriers, covered by a flat plate glass instead of the cylindrical glass tubes used in traditional evacuated tube collectors (HPETCs). Solar radiation penetrates the flat glass cover and heats the metallic absorber, which then transfers the heat to the fluid inside the heat pipes. The lower end of the tube is heated, causing the internal fluid to vaporize and the vapor to move to the upper end, where it condenses. The condensate returns to the hot end by gravity. In the condensation zone of the heat pipe, heat is released to the circulating heat transfer fluid inside an insulated manifold. This heat transfer process efficiently raises the temperature of the fluid in the manifold [28]. Figure 1c shows the concept of a flat collector incorporating heat pipes. There are several studies on this type of collector, as it is relatively new compared to commercially available collectors. An experimental study on HPFFPCs was reported by [29; 30], observing the influence of tube geometry and fill percentage on the collector's performance. They varied the inlet temperature and mass flow for the climate of Cairo. They indicated that the best performance was obtained with an elliptical shape and a specific fill ratio. [PERSON] et al. [31] reported theoretical and experimental results of an HPFFC with ethanol as a heat transfer fluid, finding an increase in the temperature of water in the tank as well as overall efficiency if the vacuum is maintained. These and other works are condensed in [28], in which the thermal performance of three-dimensional HPFFPCs is simulated through a parametric study. They indicated the values of several geometry parameters to improve useful heat and instantaneous efficiency. [PERSON] et al. [32] offered energy and exergy analysis of the HPFFC, illustrating the impact of diverse design and operating parameters on a whole system in Morocco. They have also shown results for the HPFFC working with several nanofluids. [PERSON] et al. [33; 34] modified the flow structure in a horizontally placed rectangular channel, traversed by an incompressible, turbulent flow of air in forced convection. For the study, the authors created singularities in the flow by inserting vertical baffles and rotating them around the vertical axis. This improved heat transfer in the rectangular duct and increased system efficiency. Regarding the variety of heat transfer fluids, in [35] the optimal selection of heat transfer fluids (HTFs) for solar thermal applications using multi-criteria decision-making  
Figure 1: (**a**) Flat plate solar collector. (**b**) Heat pipe evacuated tubes solar collector. (**c**) Heat pipe flat plate solar collector. (MCDM) methodologies is investigated. Sixteen HTF alternatives were analyzed based on their thermophysical properties as well as environmental, safety, and economic criteria. By employing approaches, such as the Technique for Order Preference by Similarity to Ideal Solution (TOPSIS) and grey relational analysis, this study aimed to establish an efficient method for identifying the most suitable HTF. The findings indicated that ethylene glycol and propylene glycol are among the five most suitable options after deionized water. Likewise, to mitigate the risk of freezing, the use of antifreeze solutions, such as ethylene glycol or propylene glycol, is essential [7,10]. It was mentioned in [10] that propylene glycol was used as a thermal fluid due to its properties for freeze protection and corrosion inhibition. The study also highlighted that propylene glycol has a low risk in terms of health, environment, fire, and corrosion, making it a suitable choice for solar thermal systems that may be exposed to extreme temperatures. According to the Ashrae Handbook [36], a minimum of 30% ethylene glycol or 35% propylene glycol is recommended to prevent system damage.