To ensure proper functioning of the collectors during testing, several conditions must be met. First and foremost, it is crucial to check for any damage or leakage in the collectors. Secondly, the glass surface of the collectors must be cleaned thoroughly. Thirdly, any external factors that may affect the testing, such as heat sources or shadows, should be avoided. Fourthly, it is recommended to align the collector towards the south to maximize solar energy capture. Additionally, the water flow rate should be adjusted in accordance with the applicable standard, which is m ̇=0.02kg/m2 for the European standard EN 12975-2-2006. For our solar collector, the flow rate is m ̇=123 l/h. Finally, the collectors should be tested under natural conditions of clear sky and temperature within the range specified by the standard used to determine the collector's performance.
To assess the performance of our solar collectors, we compared our experimental test results with those of a previous study conducted by Brahimi and Koussa (2016) on flat fluid-circulation solar collectors using copper pipes manufactured by Thermo Cad. The solar collector tested by them had identical dimensions and was tested under similar conditions to our collectors.
3.1 Time constant calculation
To assess the thermal inertia of our solar collectors, we conducted a test to determine the time constant, which measures the system's responsiveness to changes in temperature. The test was performed during the heating phase of the collector in accordance with European standards. First, the collector was completely insulated until the solar radiation exceeded 750W/m2. The water was then circulated in the collector to ensure that the temperature of the water at the inlet was equal to the ambient temperature. Next, we removed the cover and measured the water temperatures at the inlet and outlet of the collector, the ambient temperature, and the global inclined solar radiation. We conducted the test on both collectors to ensure the accuracy of our results.
The tests were carried out under average irradiation of \({I}_{G}\) = 800 W/m2.
The time constant is defined as the time required for the collector to reach 63% of its maximum temperature rise. As shown in Fig. 4, the time constants for our collectors are as follows:
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The collector with insulant without starch: τ = 167 s
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The collector with insulant with starch: τ = 158 s
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The Thermo Cad collector: τ = 142 s
3.2 Time constant calculation
Solar irradiation is a critical factor as it drives the operation of the solar water heater by providing the primary energy for heating the water. Therefore, it is essential to assess the impact of this parameter on the performance of the solar water heaters being studied.
Figure 5 illustrates the relationship between efficiency and solar irradiation intensity for the two solar collectors under investigation, as well as for the Thermo Cad collector studied by Brahimi and Koussa (2016). The figure shows that an increase in solar irradiation leads to a decrease in efficiency for all collectors. As the solar irradiation intensity increases, the amount of heat collected by the water in the tubes of the collector exceeds the amount of heat dissipated through the collector's various walls. The figure indicates that the performance of the starch-free collector is comparable to that of the Thermo Cad collector, except for instances of low radiation where our collector outperforms the Thermo Cad collector.
The results presented in Fig. 6 are consistent with the basic principles of heat transfer. An increase in solar irradiation intensity leads to a higher rate of heat transfer from the solar collector to the water flowing through it, resulting in a higher temperature difference between the input and output of the collector. The lower thermal conductivity of starch-based insulation can explain why the collector with starch-based insulation has a lower temperature difference at all levels of solar irradiation intensity. However, this lower thermal conductivity may also result in a slower heat transfer rate, which can lead to better performance under low levels of solar irradiation, as observed in the figure.
The temperature of the collectors is influenced by solar irradiation intensity, as illustrated in Fig. 7. As the intensity of irradiation rises, the temperature of both collectors also increases, indicating greater absorption of solar energy by the collectors and transfer to the water. The temperature increase is evident in both collectors, indicating their efficiency in capturing and converting solar energy. These results emphasize the significance of solar irradiation as a critical factor in the performance of solar water heaters.
In Fig. 8, we can see how solar irradiation intensity affects the temperature of the glass in both solar collectors. As shown in the figure, an increase in irradiation intensity results in a rise in the glass temperature. This increase in temperature can cause heat losses through the glass, potentially decreasing the overall efficiency of the solar collectors. Thus, it is important to take into account the impact of glass temperature when designing and operating solar water heating systems.
Figure 9 depicts the impact of solar irradiation on the temperature of the insulation material. It is observed that with the rise in the intensity of the irradiation, the temperature of the insulation marginally increases for both collectors. This temperature increase can be attributed to the increase in the ambient temperature, which ranges from 21 to 22.5°C during the experiment. Although the rise in insulation temperature is small, it may still have a negative impact on the efficiency of the collector as it promotes heat losses through the insulation. Therefore, it is essential to ensure that the insulation material used in solar water heaters has a low thermal conductivity and a high resistance to heat transfer. This will minimize the heat losses and hence enhance the overall performance of the system.
3.3 Effect of inlet temperature
This section discusses the experiments conducted to study the impact of water inlet temperature on the performance of the two collectors under investigation. The tests were conducted under an irradiation level of 900 W/m2 to meet the European standard requirements. The input temperature of the water was varied between 25 and 65°C, and the performance of the collectors was recorded for each input temperature. The efficiency of the collectors was found to be dependent on the water inlet temperature, with higher temperatures resulting in higher efficiency. However, there is a limit to the temperature increase beyond which the efficiency starts to decrease. This is because the increase in temperature leads to an increase in the outlet temperature, which eventually approaches the maximum possible temperature. Moreover, the increase in temperature results in an increase in the heat loss from the collector, leading to a reduction in the efficiency. Overall, the experiments demonstrate the importance of optimizing the water inlet temperature for achieving maximum efficiency in solar water heating systems.
Figure 10 presents the results of evaluating the impact of water inlet temperature on the efficiency of the collectors. As shown in the figure, there is a decline in efficiency as the inlet temperature increases. This finding is consistent with the results reported by Hakem et al. (2008). The decrease in efficiency can be attributed to the fact that as the inlet temperature increases, less thermal energy is required to heat the water to the desired output temperature, leading to a reduction in the collector's efficiency. Therefore, careful selection of the inlet temperature is crucial for optimizing the performance of solar water heating systems.
Figure 11 shows the impact of inlet water temperature on the temperature difference between the input and output of the collectors. As the inlet water temperature increases, the temperature difference between the input and output of the collectors decreases. This finding is consistent with the previous observation that the efficiency of the collectors decreases as the inlet water temperature increases. This indicates that less energy is recovered from the water when the temperature of the water entering the collector is higher.
The impact of inlet water temperature on the temperature of different components of the collectors can be further understood by analyzing the temperature of the glass, insulation, absorber, and cover. These analyses can help explain the behavior of the two collectors.
Several studies have reported similar findings regarding the decrease in efficiency and temperature difference with increasing inlet water temperature. For instance, Hakem et al. (2008) found a decrease in efficiency as the inlet temperature increased in a solar water heater system.
Therefore, it can be concluded that the inlet water temperature is a critical parameter in determining the performance of the collectors, and it should be carefully monitored and optimized to achieve maximum efficiency in solar water heating systems.
The results presented in Fig. 12 show a direct relationship between the temperature of the collectors and the inlet temperature of the water. As the inlet water temperature increases, the temperature of the collectors also increases. This trend is also reflected in Fig. 13 and to a lesser extent in Fig. 14, which show the temperature variation of the window and insulation, respectively. The increase in temperature of these components leads to a decrease in efficiency, as seen in Fig. 10. This is because the higher temperature of the glass increases heat transfer to the outside, while the higher temperature of the insulation contributes to thermal energy loss from the collector.
3.4 Effect of the wind speed on the performances of the collectors
To analyze the impact of wind speed on the performance of the collectors, measurements were conducted at a constant ambient temperature of around 22°C and solar irradiation level of 900 W/m2, for different wind speeds. The findings, illustrated in Fig. 15, indicate that as wind speed increases, the efficiency of the collectors decreases. This decline in efficiency can be attributed to the increased heat loss caused by the wind, leading to a decrease in the collector's temperature and ultimately resulting in lower energy recovery from the water. Figure 16 displays the effect of wind speed on the temperature difference between the collector's inlet and outlet, demonstrating that as wind speed increases, the temperature difference decreases. These results emphasize the significance of accounting for the impact of wind speed during the solar collector design process and implementing appropriate measures to mitigate its negative influence on the collector's performance.
Wind speed has a significant impact on the performance of solar collectors, as demonstrated by the results in Fig. 15 and Fig. 16. The decrease in efficiency and temperature difference between the inlet and outlet of the collectors with an increase in wind speed can be attributed to increased convective heat transfer, which leads to more heat losses from the collector. This effect must be taken into account during the design and operation of solar collectors to minimize the negative impact of wind on their performance. The observations made in this study are consistent with those reported in other studies, such as Hakem et al. (2008) and Agbo and Okoroigwe (2007), indicating the importance of considering the effect of wind speed on solar collector performance.