We present work at the Steward Observatory Solar Lab on a high concentration photovoltaic system in which sunlight focused by a single large paraboloidal mirror powers many small triple-junction cells. The optical system is of the XRX-Köhler type, comprising the primary reflector (X) and a ball lens (R) at the focus that reimages the primary reflector onto an array of small reflectors (X) that apportion the light to the cells. We present a design methodology that provides generous tolerance to mis-pointing, uniform illumination across individual cells, minimal optical loss and even distribution between cells, for efficient series connection. An operational prototype has been constructed with a 3.3m x 3.3m square primary reflector of 2m focal length powering 36 actively cooled triple-junction cells at 1200x concentration (geometric). The measured end-to-end system conversion efficiency is 28%, including the parasitic loss of the active cooling system. Efficiency ~32% is projected for the next system.

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Abstract. The emergent field of tracking-integrated optics enables a potentially low cost concentrating photovoltaic (CPV) implementation, where single-axis module tracking is complemented by an additional degree of freedom within the module [1,2,3,4,5]. Gross module tracking can take on multiple configurations, the most common being rotation about a polar or horizontal North-South oriented axis. Polar-axis tracking achieves >95% sunlight collection compared to dual-axis tracking [6], leaving the tracking-integrated optics to compensate for +/-23.5° seasonal variations. The collection efficiency of N-S horizontal axis tracking is latitude-dependent, with ~90% collection relative to dual-axis tracking at 32.2° latitude. Horizontal tracking at higher latitudes shifts an increasing burden to the tracking-integrated optics, which must operate between two incidence angle extremes: summer solstice sunrise/sunset to winter solstice noon. An important aspect of tracking-integrated lens design is choosing a suitable field weighting to appropriately account for annual DNI received at each angle of incidence. We present a field weighting model, generalized for polar or horizontal module tracking at any latitude, which shows excellent agreement with measured insolation data. This model is particularly helpful for the design of tracking-integrated optics for horizontally-tracked modules, where the correct field weighting is asymmetric and significantly biased away from the normal incidence.

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