Pushing the envelope: Advanced 3D secondary concentrators for high-temperature solar power

Three-dimensional secondary concentrators positioned at the entrance of solar thermal receivers, in tandem with point-focusing primary concentrators have been a subject of study and experimentation since the 1980s owing to their demonstrated ability to enhance the optical and thermal performance of Concentrating Solar Power (CSP) systems operating at high temperatures. They have been also proposed to reduce costs by rendering the overall optical system more tolerant of primary concentrator tracking and surface slope error. In certain CSP and thermal (CST) applications and configurations, the integration of 3D secondary concentrators is vital to meeting temperature and flux density requirements. This paper presents a comprehensive review of the prototypes that have been tested thus far, detailing their characteristics and enumerating the engineering, material science, testing, modeling and optimization challenges. Measures to address thermal management and related fabrication problems are discussed. The underlying theory of non-imaging optics and essential optical design considerations are summarized. Finally, the ongoing developments of reflective materials suitable for high-temperature applications in 3D secondary concentrators are examined, along with an evaluation of the most promising candidates. Priorities for filling research gaps and comprehensive design approaches are identified (based on testing of prototypes reported in the literature).