Compact Nested Aplantic Optics for Concentrator Photovoltaics
Abstract
Aplanatic optics were originally invented for improved imaging of telescopes and microscopes (Schwarzschild, 1905). Recently, these optics were considered for compact solar concentrators to be used in photovoltaic concentrator (CPV) designs (Gordon and Feuermann, 2005). Compactness is of paramount importance as it reduces manufacturing and shipping costs substantially. However, the two-mirror system – in the compact version of the design – causes shading of the primary mirror by the secondary and limits the level of practically achievable concentrations and/or forces the focus to be placed inside the optic. Passive cooling of the solar cell from within the optics (ruling out active cooling, for practical and economic reasons) is problematic as a large cooling fin (the size of roughly the aperture) in contact with ambient air is required for passive cooling. Thus, either a terminal dielectric concentrator to extract the concentrated sunlight into the solar cell located behind the primary mirror (Fig. 1) or a conical heat-conducting cone is necessary to permit positioning of the heat sink behind the primary from where heat can be rejected passively to the environment. It turns out that the heat-conducting cone is problematic causing relatively large temperature gradients along the cone and with it a relatively high temperature of the cell with the concomitant reduction in cell efficiency. Though the dielectric element does the job of extracting the light to the cell quite well, in fact permitting further concentration, there is the need for an optical bond to the cell without which additional Fresnel losses and totally internal reflected rays at the dielectric-air interface would result in intolerable losses. This optical bond has to withstand temperature variation and moisture over at least 20 years, a feat not easily achieved and of significant conern to the manufacturers. Nevertheless, the design of figure 1 has in fact been adopted for large-scale CPV systems and successfully deployed (Fig. 2, SolFocus, 2008-2011).