## 3 Results and Discussion  
The hourly recorded water temperatures at the collector inlet and outlet, as illustrated in Figure 6, provide insight into the system's thermal behavior. Starting from 26.4 \({}^{\circ}\)C at the inlet and 28.1 \({}^{\circ}\)C at the outlet, the temperatures steadily rose, reaching peak values of 61.1 \({}^{\circ}\)C and 65 \({}^{\circ}\)C, respectively, at 1:00 p.m. Subsequently, both temperatures remained relatively constant at around 65 \({}^{\circ}\)C for the duration of the test, indicating a thermal equilibrium where heat gain and loss were balanced. The narrowing temperature difference between the inlet and outlet temperatures is a result of the closed-loop water circulation nature of the system, demonstrating its ability to maintain stability in operation. Figure 7 depicts the mass flow rate of water at the thermal collector inlet throughout the test period. Ranging from 3.18 g\(\cdot\)s\({}^{-1}\) to 3.45 g\(\cdot\)s\({}^{-1}\), the flow rate is inversely related to water temperature. As temperatures increased, the mass flow rate decreased due to water expansion within the system. This phenomenon reduces both the amount and rate of heat absorption, contributing to a drop in efficiency at higher water circulation temperatures. Careful adjustment of the flow rate can lead to higher photothermal conversion efficiency. The control strategy is not developed for this experiment. Figure 6: Hourly recorded inlet and outlet water temperature. The photothermal conversion efficiency, shown in Figure 8, was calculated based on a closed-loop water circulation system, allowing for gradual heating of the water over the course of the day. Efficiency peaked at 24% in the first hour, gradually declining as the day progressed. This decline can be attributed to the narrowing temperature difference between the inlet and outlet temperatures over time, coupled with increasing solar irradiance towards midday. After 1:00 p.m., the efficiency gradually declined and nearly reached zero by 2:00 p.m. This decrease was largely due to minimal temperature differences between the inlet and outlet, as shown in Figure 6. When the temperature difference approaches zero, the energy absorbed by the collector (\(Qa\)) also approaches zero, leading to an efficiency of zero according to Equation (2). Although using metal tube may reach a similar maximum temperature more quickly due to its absorptivity and thermal conductivity, achieving the same efficiency with a polymer tube is feasible with appropriate mass flow rate adjustments. Increasing the residence time of water in the collector tube could lead to a higher collector outlet temperature. Since the experiment was performed in a closed-loop water circulation, the maximum temperature was reached at a faster rate as compared to an opened-loop water flow with a fixed inlet temperature. Additionally, the absence of a transparent cover, called \"glazing\", limits the system's performance. With glazing, more heat could be retained in the collector, enhancing heat transfer from the enclosed air to the collector tubes. The collector efficiency \(\eta_{th}\) plotted against (\(T_{i}-T_{a}\))/\(l_{s}\) as shown in Figure 9. The slope of this curve, \(-F_{R}U_{L}\), explained in Equation (2), represents the rate of heat loss from  
Figure 8: Hourly photothermal conversion efficiency with respect to solar irradiance. Figure 7: Water mass flow rate through thermal collector tubes. the collector. Since this collector is unglazed, steeper slopes can be seen especially at the small temperature difference between inlet water and ambient temperature. The maximum collector efficiency, \(F_{R}\tau\alpha\), significantly depends on the optical properties of the collector. The moderately selective black paint applied on the thermal absorbers improved the overall thermal efficiency due to increased radiation absorption. Overall, this polymer tube solar water heater has demonstrated comparable performance to devices using metal tubes.