Concentrating Photovoltaics (CPV): The Path Ahead by Harry Apostoleris Marco Stefancich & Matteo Chiesa

Author:Harry Apostoleris, Marco Stefancich & Matteo Chiesa
Language: eng
Format: epub
Publisher: Springer International Publishing, Cham

where θ1 is the same as the acceptance angle θA since it represents the angular range from which the device can accept light and concentrate it onto the exit surface. But if this light then passes into a medium with a refractive index n, its angular divergence will be reduced due to refraction by a factor of n 2. Then the light can be further concentrated by this factor, leading to the concentration ratio limit above.

This is a typical starting point for discussions of CPV optics, but it is only part of the story. Some optical devices can come close to this ideal concentration—the most well-known example of this is the compound parabolic concentrator, which was the first optical device to be optimized for energy collection rather than for forming images and so is called a nonimaging optic [5]. Nonimaging optics typically have a major drawback for applications to solar collectors: their length is proportional to the concentration ratio, so in high-concentration systems they are completely impractical to use [6]. Therefore they can be useful for low-concentration applications [7], but high-concentration CPV systems must use modifications of older, imaging optics to collect light. These have a much smaller acceptance angle than the ideal limit [6, 8]. Figure 4.1 illustrates this by plotting the thermodynamic limits of concentration in air and in dielectric media, along with the acceptance angles and concentration factors of real CPV systems.

Fig. 4.1Thermodynamic concentration limit as a function of acceptance angle. We point out the ultimate concentration limit for the solar disk (divergence angle 0.27°) and the typical acceptance angle of CPV optics (~1°). The range of concentrations achieved by commercial CPV systems is several times below the limit for these acceptance angles

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