Characteristics of a zoomable Fresnel lens (ZFL) used for solar concentration

This paper presents a zoomable Fresnel lens (ZFL) approaching to use tensile deformation to solve the problem of the focal length's shortening during the tilt solar incidence. A sample of ZFL with width of 95 mm and focal length of 240 mm was design and analyzed. It is found that the focal length would increase by 75% when the width of the ZFL is elongated by 30% and the variation is linear. The focus can steadily locate on the designed focal plane when tilt incident angle changes from 0° to 40° during the zooming. As for the 10 mm-width receiver, the minimum interception coefficient under normal incidence is 85%, when elongation is 20 mm. The thermal collection test shows that ZFL should be started to stretch when tilt incident angle reaches to 25°. The Maximum elongation, 20 mm, can satisfy the tilt incident angle about 50°, which is 2.5 times than the undeformed ZFL. Besides, the increase of lighting area during the stretching of ZFL can well compensate the energy loss of deformation. Generally, ZFL can significantly increase the acceptable tilt incident angle about 2.5times, which makes the linear-focusing Fresnel lens has about 6.7 h effective working time a day.