Primary advantages would be light weight and tolerance of shape errors.
Inflatable Fresnel lenses are being developed for use as optical concentrators in solar power systems. These lenses are of two types: dome (point-focus) lenses and cylindrical (line-focus) lenses (see Figure 1). Originally intended for supplying power to spacecraft, these lenses might also be adaptable to some terrestrial solar-power systems in cases in which optical aberrations caused by gravitational and wind-loading distortions of lens shapes could be tolerated.
Figure 1. Fresnel Lenses would be supported on the interior surfaces of spherical and cylindrical balloons. The spatial intervals between Fresnel-lens prisms are greatly exaggerated in these views.
The main structural element of a dome lens concentrator would be a spherical balloon. The balloon would be made of a polymer (e.g.
, polyethylene terephthalate) film about 12 μm thick, possibly coated with aluminum on both sides everywhere except in the dome lens region. The concentric prisms that constitute the Fresnel lens elements could be molded into a sheet that would be bonded to the inner surface of the balloon; typically, this Fresnel-lens sheet would be cast from clear silicone rubber and would be about 250 μm thick. In practice (at least initially) it could be desirable to approximate the desired spherical shape by assembling the balloon and Fresnel lens from gores. At a latitude chosen consistently with the focal length and the radius, the balloon would be anchored on a back plane that would support the solar receiver and would serve as a radiator for dissipating waste heat.
Figure 2. A Symmetrical-Refraction Design (in which the angle of incidence equals the angle of emergence) maximizes the tolerance to slope error in that it minimizes the deviation of the angle of refraction for a given slope error. An additional benefit of the configuration shown here is that the blunt prism peaks are tucked out of the ray paths and thereby prevented from scattering light.
A paraboloidal dish reflector is the main competitor to an inflatable dome lens concentrator. The most obvious advantage of an inflatable dome Fresnel-lens concentrator is that a large concentrator surface can be established and maintained readily by inflating a lightweight balloon, whereas a paraboloidal dish rigid enough to maintain the required shape weighs considerably more. Another less obvious but equally important and concomitant advantage is that in comparison with a paraboloidal reflector of nominally equal area and relative aperture, a dome Fresnel lens can be made considerably more tolerant of shape error (see Figure 2); for a typical slope error at the outer edge of the lens, the spreading of the solar image is less than a hundredth of that caused by the same slope error at the outer edge of the reflector.
A cylindrical Fresnel-lens concentrator would be essentially a simplified, two-dimensional version of a dome Fresnel-lens concentrator. The dome Fresnel-lens concentrators would be well suited for applications in which there is a requirement for aperture sizes of the order of meters for photovoltaic, thermophotovoltaic, and various types of thermal solar receivers. The cylindrical Fresnel-lens concentrators would be suitable primarily for aperture widths of 5 to 10 cm for linear arrays of photovoltaic cells.
This work was done by Mark J. O'Neill and A. J. McDanal of ENTECH, Inc., for Lewis Research Center. No further information is available.
Inquiries concerning rights for the commercial use of this invention should be addressed to NASA Lewis Research Center, Commercial Technology Office, Attn: Steve Fedor, Mail Stop 4-8, 21000 Brookpark Road, Cleveland, Ohio 44135. Refer to LEW-16662.