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Solar Energy Engineering by Soteris A. Kalogirou

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Chapter two

Environmental Characteristics

The sun is a sphere of intensely hot gaseous matter with a diameter of 1.39 × 109 m (see Figure 2.1). The sun is about 1.5 × 108 km away from earth, so, because thermal radiation travels with the speed of light in a vacuum (300,000 km/s), after leaving the sun solar energy reaches our planet in 8 min and 20 s. As observed from the earth, the sun disk forms an angle of 32 min of a degree. This is important in many applications, especially in concentrator optics, where the sun cannot be considered as a point source and even this small angle is significant in the analysis of the optical behavior of the collector. The sun has an effective black-body temperature of 5760 K. The temperature in the central region ...

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