Almost everyone has had experience with a light concentrator. A hand lens focused on the ground on a sunny day demonstrates the ease with which sunlight can be collected and concentrated. What is not well known are the mechanisms for this concentration or the limits imposed by physics and thermodynamics. A survey of the literature over the past 20 years provides information on many different types of devices called concentrators. Some of these systems are as simple as the hand lens. Others, such as the fluorescent planar concentrator (FPC) seem, at first glance, to have radically different thermodynamic limitations. All of these systems, however different, are related in that they increase the number of photons on a surface or the irradiance (illumination) above the level occurring without the device. This is an advantage for solar-energy conversion and materials characterization since the receiver (e.g., a solar cell or thermal absorber) can be reduced in size relative to the total system. In this way, an area exposed to the sun can be covered by potentially cheaper, and technologically simpler, materials. High photon (flux) levels can also be used for the generation of high temperatures to produce steam, a photothermal reaction, or materials processing. In this chapter, the principles unifying geometrical optics and fluorescent concentrators are presented. General equations are developed and discussed in regard to their use in solar energy and solar concentrators.
A concentrator using only geometry, and not relying on a frequency shift, is called a geometrical, or passive, concentrator. A system that is concentrated only by a frequency shift is called a fluorescent, luminescent, or active system.
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