An Off-Axis Cassegrain Optimal Design for Short Focal Length Parabolic Solar Concentrators

The present work addresses an off-axis Cassegrain optical concentration system. The specific primary collector analyzed, a short focal length parabolic concentrator, is at the University of Florida’s Energy Park. A secondary hyperbolic reflective element was designed to redirect the solar radiation from the primary focal plane to an off-axis target on the polar axis of the primary concentrator. This ground level target will be required for planned experimental work. The analysis was performed using a numerical ray tracing procedure that incorporates both random and systematic errors due to slope and surface irregularities. The optimization process varied secondary element size, curvature, and offset angle, and yielded information required for optimum design. As a single secondary element was found impractical, three elements were designed for use at various times of the year. The numerical analysis predicts that typically 70 to 75 percent of the solar flux incident on the primary concentrator aperture was focused within a 0.5-meter radius. During the design, it was found that this type of compact concentration system is a practical alternative. The optical system is also shown to have advantages that are generally applicable for problems involving short focal length primary concentrators, or when the solar apparatus is to be placed outside the primary collector aperture.