Illumination characterization of glass and metal secondary concentrator elements in a photovoltaic solar concentration system

Using illumination modeling, we provide a comparison of glass Total Internal Reflection Concentrators (TIRC) and metal Hollow Reflective Concentrators (HRC) used as secondary concentrator elements in a dish-based highconcentration photovoltaic (CPV) system. Comparisons of optical efficiency, flux uniformity, off-axis acceptance angle, and cost are vital to choosing an ideal secondary concentrator element for a CPV system employing multi-junction (MJ) cells. In many CPV systems, a free-form optic or sharp-cornered rectangular TIRC composed of glass is used to increase the geometrical flux concentration at the surface of the MJ cell, and may also serve as homogenizers to mix the light to increase flux uniformity. We have demonstrated in on-sun testing that an electroformed metal HRC can be used in place of a glass TIRC of the same geometry, eliminating the need for polymeric bonding to the MJ cell surface, and providing a side-contact surface pathway for active cooling. Although geometrically equivalent, we show that glass TIRC's achieve superior off-axis performance (higher etendue from surface refraction) and are generally acknowledged to have less degradation than optics with over-coated silver, yet metal HRC's employing over-coated silver are superior in spectral absorption characteristics under high solar flux (no losses from glass absorption or Fresnel surface reflections) and don't require accurate glass pressing into many shapes. To better understand the trade-offs between optical efficiency, off-axis performance, mechanical tolerances, cost and reliability, metal (HRC) and glass (TIRC) tapered funnels are analyzed at the surface of equal irradiance in a Kohler-Illumination concentrator system, and a trade study is presented.