Figure 3.11: Temperature of Fresh Pomelo Peel and Carbonized Pomelo Peel Samples under an artificial sun
The photo-thermal conversion ability is evaluated based on the studying on the carbonized pomelo peel surface temperature when placed under an artificial sun with the power P=1 kW.m-2. The temperature of the sample is calculated based on the analysis of data obtained when taking infrared images of the sample at different times. Figure 3.11 compares the temperature of the fresh pomelo peel with carbonized pomelo peel in different thickness conditions. When receiving the sunlight from the artificial sun, the surface temperature of the fresh pomelo peel increases slowly and reaches about 400C after 300 seconds. In contrast, the temperature of the carbonized pomelo peel samples changed significantly after receiving artificial sunlight. They increase rapidly in the first 100 seconds, then reach a steady temperature. The maximum temperature that carbonized pomelo peel can achieve is 700C, 780C, 860C, and 930C, with thicknesses of 1 mm, 3 mm, 6 mm, and 10 mm respectively. When the thickness of the material increases from 1 mm to 10 mm, the saturation temperature of the material also increases from 700C to 930C.
Especially, when the thickness of the sample is greater than 10 mm, the saturation temperature of the sample also nearly unchanged and maintained at 930C. Figure
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3.12 reveals the infrared image of a carbonized pomelo peel model with a thickness of 10mm when placed in the artificial sunlight. The surface temperature of the sample is highest in the central region and slightly reduced at the edges.
Figure 3.12: Infrared image of sample carbonized pomelo peel placed under artificial sunlight
To clarify the above phenomena, the mechanism of light absorption and photo- thermal conversion was considered. As demonstrated above, the carbonized pomelo peel material has strong absorption in sunlight. Therefore, most of the light emitted from the artificial sun and reached the sample is absorbed. The photon energy is quickly transferred to molecules within the material and can be transformed into thermal energy through the thermal vibration of molecules. The more photon energy absorbed, the stronger the thermal vibration of molecules, and the greater the sample temperature. This causes the material's temperature to increase rapidly in a short time. When the sample temperature is sufficiently large, the process of heat loss due to exchange with the environment should be considered. The higher the temperature, the greater the heat exchange with the environment, therefore, the growth of the material’s temperature decreases. After 100 seconds, the material’s temperature is almost unchanged due to the equilibrium in the photo-thermal conversion process with the heat exchange process. When the material is thicker, the heat exchange with the environment will be limited by the vibration-sharing of molecules inside the material. On the other hand, heat exchange with the environment only occurs on the surface of materials, therefore, the temperature at the center of the sample is a little higher than the temperature at the edge due to less heat exchange. Besides, when considering the ratio of surface area to the weight of
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the sample, the sample with the larger the coefficient will have the stronger the process of heat exchange with the environment. For the sharing of the thermal vibration of molecules, only the neighbor-molecules exchange process is preferred.
Therefore, when the thickness of the sample is larger than 10 mm, the saturation temperature of the sample when placed under artificial sunlight does not increase anymore due to the constant in the total energy sharing. Thereby, 10 mm is also chosen as the optimal condition for the thickness of the material.
Figure 3.13: Temperature of Fresh Pomelo Peel and Carbonized Pomelo Peel Samples under some real conditions (a): affected by wind; (b): affected by solar
intensity; and (c): affected by cloud
Figure 3.13 lists several factors that affect the temperature of the material. The presence of wind promotes the heat exchange of materials with their surroundings, leading to a decrease in temperature. However, when the wind disappeared, the material temperature increased incontinently and still reached a high temperature.
Besides, solar intensity and cloud affect the total photon energy that reaches the material in a unit of time. As the light flux decreases, the total heat energy converted from light energy also decreases, hence the temperature of the sample
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also decreases. Because of the above factors, wind, solar intensity, clouds should be considered to improve the performance of Solar Steam Generation system in the reality.