## 3 Results and Discussion
### Influence of Temperature and Working Fluid on Performance  
The influence of the fluid temperature on the useful heat and efficiency of the collector, with respect to the mass flow rate at three different temperatures, is presented in Figure 6. In the range of mass flow evaluated, as shown in Figure 6a,b, the almost constant efficiency and useful heat can be seen. However, if these performance parameters are compared at different fluid temperatures, it is observed that the efficiency decreases from 0.70 to 0.58 and 0.48 with the flat plate collector for temperatures of 30 \({}^{\circ}\)C, 40 \({}^{\circ}\)C, and 50 \({}^{\circ}\)C, respectively. While for the evacuated tube collector, the efficiency decreases from 0.83 to 0.80 and 0.75 for the same temperatures of 30 \({}^{\circ}\)C, 40 \({}^{\circ}\)C, and 50 \({}^{\circ}\)C.  
When evaluating the performance of the HPFFC, as shown in Figure 6c, a clear trend emerges indicating that both efficiency and useful heat increase with an increase in the mass flow rate. For instance, at a fluid temperature of 40 \({}^{\circ}\)C, the efficiency rises from 0.38 to 0.55, while the useful heat increases from 300 W to 400 W, respectively. However, the figure also illustrates that both efficiency and useful heat decrease with rising temperature. Specifically, at a mass flow rate of 0.045 kg/s, the efficiency declines from 0.73 to 0.50 and further to 0.23 for fluid temperatures of 30 \({}^{\circ}\)C, 40 \({}^{\circ}\)C, and 50 \({}^{\circ}\)C, respectively. This information underscores the significant impact of temperature and mass flow on the performance of solar collectors.  
Figure 7 shows the efficiency, with respect to the mass flow rate of the five working fluids evaluated for the three types of low temperature solar collectors. Based on the observations presented in Figure 7a, the efficiency of the FPC operating at a mass flow rate of 0.045 kg/s demonstrates only a marginal decrease of 0.2% when utilizing a mixture of  
water and 30% ethylene glycol, and a slightly greater reduction of 0.4% when the mixture is composed of 50% ethylene glycol. Notably, the reduction in efficiency is even less pronounced when propylene glycol is employed. Similarly, as illustrated in Figure 7b, the efficiency of the HPETC exhibits decreases of 0.1% and 0.2% for the same concentrations of ethylene glycol solutions. From the analysis of these two figures, the variations in efficiency are negligible when utilizing either pure water or mixtures containing less than 50% ethylene glycol or propylene glycol in both flat plate collectors and evacuated tube collectors.  
However, as illustrated in Figure 7c, the efficiency of the HPFPC at a mass flow rate of 0.045 kg/s experiences a significant decline, dropping from 72% to 48% when a mixture containing 30% ethylene glycol is utilized. If the concentration of ethylene glycol is increased to 50%, efficiency further decreases to 36%. By contrast, the reduction in efficiency when using propylene glycol mixtures is relatively minor. This observed decrease in performance for the flat plate solar collector incorporating heat pipes is attributable to the integration of two distinct technologies: the flat plate collector, which inherently operates at a lower efficiency, and the heat pipes, which provide enhanced efficiency. Consequently, the combination of these two technologies results in a combined efficiency. To improve the efficiency of heat pipe flat panel solar collectors (HPFPCs) using mixtures of water with  
Figure 6: Results of efficiency and useful heat with respect to the mass flow rate of the fluid at different temperatures for the three types of solar collectors: (**a**) flat plate, (**b**) evacuated tubes, (**c**) flat plate with heat pipes.  
ethylene glycol and propylene glycol, one could insert improved surfaces into the heat pipe or place heat sinks on the condenser, or even experiment with nanofluids.