Pneumatic Photo-concentrator for Space Applications

Overview

Space industry will require heat and power sources. Gathering and concentrating large amounts of sunlight with lightweight devices is an economical way to produce heat and power. A large concave reflector can be constructed with minimal mass by using the natural tendency of inflated films to assume the shape of a section of the surface of a sphere. Such a device is shown in Figure 1, below.

Figure 1: Pneumatic photo-concentrator for space applications.

The device consists of two polymeric films, one clear, and the other metalized to reflect light or other electromagnetic radiation. The two films are stretched over a box sectioned ring structure, and gas is introduced between them, causing them to bulge into spherical shape. If the bulge is kept small relative to the ring diameter, the spherical aberration of the photo-concentrator is manageably small. Figure 2, below, shows the plan and elevation views of the pneumatic photo-concentrator:

Figure 2: Plan and elevation views of the pneumatic photoconcentrator.

The concavity, and hence, focal length, of the concentrator is adjusted by regulating the interior gas pressure.

The concept sketches of the pneumatic photo-concentrator are not shown to scale. The ring section dimensions relative to the overall diameter of the pneumatic photo-concentrator will vary with size, which can be on the order of tens or hundreds (or more) of meters. The structure is inherently stable in zero-g: the membrane tensions tend to hold the ring in a plane, so only compressive stress is experienced by the ring, with no tendency toward buckling. The membrane tensions also hold the circular shape of the ring. Hence, the pneumatic photo-concentrator is an extremely efficient structure.

The metalized reflector surface will operate over a wide range of photon frequencies, from radio waves, through infrared, visible, and ultraviolet light.

Fabrication and Space Deployment

A pneumatic photo-concentrator larger than a few meters will have to be folded (or disassembled) on the ground and deployed (or assembled) in space. A number of folding schemes can be conceptualized. Attention will have to be paid to the folding of the polymeric film membranes for space deployment, which can be automatic or astronaut aided.

Key Features

The following are key features of the pneumatic photo-concentrator:

Clear polymeric film is used on the light source side while metalized film is used on the opposite side for photo-reflection.
A box cross-section ring structure provides sufficient stiffness to hold the ring shape while minimizing system weight. Because the ring is loaded in pure compression, with the circular shap being maintained by the thin film, and its in-out-of plane bending being likewise opposed by the lens-shaped thin film, the ring section can be made quite small.
A pneumatic pressure regulator subsystem is mounted to the ring to maintain the degree of curvature desired. In very large concentrators, the gas pressure will be quite low in order to maintain a reasonably low stress in the film. Therefore, the lensing effect of the gas will also be quite low.

Prior Art

No large photo-concentrator for industrial space applications has yet been built. Existing concepts for photo-concentrators use rigid mirror structures and do not take advantage of the natural spherical curvature of an inflated polyermeric film.

Applications

Possible space applications of a large photo-concentrator include the following:

Heating or melting of materials for research or fabrication purposes.
Power generation.
Optical magnification for imaging.
High gain radio antenna.
Solar sail for spacecraft propulsion.

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