Off-axis Aberration Of Non-Imaging Focusing Heliostat-the Analysis And Correction

Scientists and engineers and often encounter situation in which concentrator has to be stirred to an off-axis position relative to the incident light.This is particularly so when the concentrator has to track a moving incident source,for instance,to track sun in concentration solar power(CSP) application using fixed target focusing heliostats.In another situation,such as in moving object tracking of radar surveillance,it would bring good possibility in designing a type lower cost and powerful 3D surveillance radar if it was possible to realize an antenna consists of a high gain.large aperture scanning reflector and a fixed position transceiver.In these situations,the resulted image of the concentrating device subjects to off-axis aberration due to which the resulted spot tends to spread or becomes larger thus reducing the concentration ratio. It is also known that the severity of off-axis aberration depends greatly on the amount of offset from on-axis condition,or in other words,the magnitude of incident angle. The challenge of off-axis concentration,particularly in respect of solar energy collection,has been first partially answered by Harald Ries in 1990 when he proposed a concept of concentrator which has two different radii of curvatures in tangential and sagittal directions.A more prominent answer was given by Chen et al.in 2001 with the announcement of a new type of heliostat,called the non-imaging focusing heliostat.The new heliostat has a mirror array capable of line-tilting in order to realize a type of dynamically change optical surface which has different radius of curvature in both tangential and sagittal direction.It is found that the new design can significantly reduce the off-axis aberration to the first order.Two prototypes of non-imaging focusing heliostat have been reported by Chen et al.in 2002 and 2005, detailing its principle design and the test applications in high temperature solar furnace and solar potatoes peeling.The ability of correcting the first order of off-axis aberration is the key purpose of the development of non-imaging focusing heliostat. The study of off-axis aberration of a non-imaging focusing heliostat with line-tilting mirror array has been first discussed in Chen et al..2003 and Chong,2001 while the effect of off-axis aberration of a non-imaging focusing heliostat with fixed geometry mirror array has been disclosed in Chen et al..2004.Chen el al.,2006 further extend the study of off-axis aberration correct towards using a single continuous piece of mirror with higher order surface.There,however,lacks of a generalization of the characteristics of off-axis aberration of non-imaging focusing heliostat as a whole among the different designs and thus the optimization for the different purposes of application.The purpose of study within the scope of this dissertation has been on the study of the optical characteristic of non-imaging focusing heliostat with respect to off-axis aberration correction and means of optimization so that it would be possible of the engineers and scientists to choose of the most suitable type of non-imaging focusing heliostat for their application needs.