Compact, high-efficiency sunlight harvesting

Abstract

A new free-form optics design method could unleash the full potential of tracking integrated solar concentratorsConcentrating photovoltaic (CPV) systems use optics to concentrate sunlight onto solar cells. By increasing the concentration of light that reaches the solar cell, CPV systems allow the area of the solar cell to be reduced. Solar cells are made of expensive semiconductor material, and CPV systems consist of inexpensive mirrors or lenses. Thus, a CPV module-consisting of the CPV system, and the cell-can be cheaper than the solar cell alone, but equally efficient. High-efficiency multi-junction solar cells can boost the conversion efficiency of CPV modules beyond 30%, but their expense means they require a high concentration of light (>400 ) to be economically viable. Achieving this level of concentration normally requires dual-axis tracking of the sun's diurnal and seasonal movements.Most CPV manufacturers work with very large modules and pedestal-mounted dual-axis trackers: see Figure 1. These systems are suitable for utility-scale power plants, but are less adequate for providing power to mid-scale or smaller operations. In contrast, photovoltaic (PV) modules with single-axis trackers are already in use on flat rooftops. Currently, CPV systems designed for single-axis trackers are limited to a concentration of about 300 for polar alignment, where the tracker axis equals the Earth's axis of rotation. 1 This concentration is not sufficient for economic use of multi-junction solar cells.Existing CPV system designs vary, but almost all treat the CPV modules and the external trackers separately: see Figure 2(a). In contrast, our approach integrates part of the external solar tracking functionality into the CPV module: 2 see Figure 2(b). The laterally-moving optics arrays are mounted on a polar-aligned single-axis tracker, and combine the concentration and steering of the incident sun light. Tracking integration can be used to reduce the external tracking effort in favor of compact installation size, [3][4][5] or to fine-tune the total tracking functionality, allowing coarse external solar tracking. 6 Figure 2. (a) Schematic assembly of a conventional concentrating photovoltaic (CPV) module for pedestal-mounted dual-axis trackers, and (b) our tracking integrated CPV module for polar-aligned single-axis trackers (b).While a benefit of dual-axis trackers over fixed installationswhich allow for the modules to always be pointed at the sun-is the increased yearly insolation, comparing the potential annual energy yield for polar-aligned single-axis trackers with dual axis trackers shows only moderate differences. 7 The tracking integration of our single-axis system covers an angular range of˙24• for incident direction vectors (pointing at the sun) within a single plane. If symmetry is considered, it is evident that rotational symmetric lenses are not an optimal solution. We developed a new design algorithm, 8 based on the simultaneous multiple Continued on next page

SMS freeforms for illumination

“…One of the interesting aspects of the SMS freeforms is that they can be not only designed for nonimaging applications but also for imaging applications [17]. However, the imaging SMS is outside of the scope of this paper.…”