The direct production of electricity from sunlight with photovoltaic cells is a clean and inexhaustible source of electricity generation. Various photovoltaic cell technologies are commercially available, but the cost per watt still remains high for wide-scale practical implementation.
Concentrating photovoltaic (CPV) systems use optics to concentrate sunlight onto small solar cells (see Figure 1). The significant decrease of the required solar cell area provides a pathway to lower cost, as expensive semiconductor material is replaced by inexpensive mirrors or lenses by a factor roughly equal to the concentration factor.
Furthermore, high-efficiency multi-junction solar cells can be used to boost the conversion efficiency of CPV modules beyond 30%, making CPV the most efficient among the PV technologies. To be economically viable, the use of expensive multi-junction solar cells requires high concentration of sunlight - typically exceeding a concentration factor of 400. Achieving this level of concentration normally requires precise 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 2). These systems are suitable for utility-scale power plants, but are less adequate for providing electricity from mid-scale or smaller installations. 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 factor of about 300 for polar alignment, where the tracker axis equals the Earth's axis of rotation. This level of concentration may not be high enough for economic use of multi-junction solar cells.
In comparison to conventional CPV modules, we propose a novel approach that integrates part of the external solar tracking functionality into the CPV module (see Figure 3). 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.
To unleash the full potential of our proposed CPV system, a free-form (without rotational symmetry) optics design is absolutely essential (see Figure 4). We have developed a new analytic optics design method capable of calculating the surfaces of our movable free-form lenses directly. The ray tracing simulation results for the concentration performance show that the analytic moving design outperforms all preceding optical designs (see Figure 5).
Based on this new free-form optics design, it is possible for the first time to use more compact and flexible single-axis trackers without being forced to abandon high-efficiency multi-junction solar cells. This design strategy is essential to open small to mid-scale installation markets for CPV, due to the strong reduction of the external solar tracking effort.