Concentrator optics

The past few years have witnessed a paradigm shift in photovoltaic (PV) power generation. It stems from the confluence of (a) dramatic advances in commercial high-efficiency multijunction solar cells, now capable of 40% conversion efficiency [1-4], and (b) optical designs capable of delivering flux levels of hundreds to thousands of suns at high collection efficiency - (1 sun = 1mW/mm²) [5-8]. In these high-concentration systems, even with cells that are two orders of magnitude more expensive on an area basis than conventional PVs, the cost contributed by the cell becomes attractively small. The burden then shifts to the optical design to provide a cost-effective and practical system. This chapter focuses on new classes of high-flux, ultracompact, practical optics, traced from the initial concepts through commercial realization (Fig 6.1). The target concentration levels of previous generations of high-efficiency PVs were constrained by the optical and thermal limitations of lens-based and large-dish collectors. Miniaturized mirrored concentrators overcome these hindrances. The issue of establishing optimal flux levels is then transferred from that of concentrator design to material engineering and cell architecture. In tailoring optical devices to PV cells, production, material and implementation cost realities impose severe constraints that demand pragmatic solutions without compromising high concentration or collection efficiency.