Mirror Assemblies That Simply Warp Flat Glass Mirrors

In our first attempt to eliminate the backing glass layer and use flat mirrors alone, we mounted them directly to a parabolic surface. We had developed a technique to accurately curve aluminum extrusions by systematically hitting the outboard leg with a ball peen hammer. Increasing the number of hits per length or the force used reduces the focal length of the parabola until it matches a template. The photo below shows a truss with a curved top member.

Aluminum Truss With Parabolic Top Member and Discretely Bent Bottom Member

A number of these trusses made up the concentrator structure by attaching them to two girders that were discretely bent so the parallel trusses formed a parabola in the transverse direction. This parabolic surface was then lined with corrugated polycarbonate roofing material, see photo below.

Parabolic Concentrator Structure, with Left Side Covered with Corrugated Roofing

We directly attached mirror facets to the corrugated sheet using a variety of methods but none were satisfactory. Also, solar images reflected by these mirror facets mechanically attached to the corrugated sheet did not all superimpose at the target focal length hit each so did not concentrate sunlight enough to warrant continuing this option. Another drawback to this approach: we found no way to ascertain which mirrors needed correcting nor was there any way to tweak mirrors that did not direct sunlight to the target center.

Mirror Facets Attached to the Corrugated Parabolic Surface

We next tried two methods to form plastic materials into mirror panels: heat-forming thermoplastic structural sheets and molding fiberglass and resin, see photo below. Although both these methods may work in dedicated mass-assembly lines with multiple copies of dedicated tooling, neither approach proved workable as a boot-strap process because they required too much labor that would take too long to produce adequate results. Also, these materials were expensive and would probably not weather well for thirty years.

Thermo-formed Twin-wall Polycarbonate Panel-Based Mirror Assembly with Reflection on Target

Male Mold for Both Heat-forming and Casting Mirror Assemblies

A series of mirror assemblies that used the hammering technique to stretch aluminum material behind the parabolic surface delivered ever-improving performance.  These fastened curved mirror support members between straight end pieces with features that enabled stacking mirror assemblies one on top of another.

Mirror Assembly Intensifies Sunlight to Target the Same Size

Mirror Assembly Using Aluminum Rod Pullback Supports

Rear View of a Mirror Assembly with Four Curved Mirror Support Members Between Straight Ends with Stacking Features, One Foot Square Mirror Facets and Angle Mirror Pullback Supports

 

Stacked Mirror Assemblies with Candidate Mirror Pullback Support Options

One Foot Diameter Image of 3 x 3 Array of Warped One Foot Square Mirrors

Computer Model of a Dish Solar Collector Showing Where Concentrator Offset Alignment Features Are Required at Each Corner to Attach Mirror Assemblies to Underlying Trusses: Corners of Four Separate Mirror Assemblies Meet at a “Quad”

Mirror Assembly Fabrication Fixture That Insures Mirror Supports Form Parabolic Surface with a Mirror Assembly Clamped In Place

Stacked Mirror Assemblies With One on Top That Has Three 1 x 3 Foot Mirrors and Offset Alignment Features That Enable One Person Align and Fasten Them Close Together from the Front

A 3 x 3 Foot Mirror Assembly with Two 1 x 3 Foot Mirror Facets Mounted Against Their Respective Curved Members Using Three Pullback Fasteners (Bicycle Spoke Ferule + Machine Screw)

Mockup of a “Quad” Mirror Assembly Alignment Feature That Allows One Person to Mount and Adjust Each of Four Corners So That a Reflected Image Hits a Target at the Concentrator Focal Length. The Curved Mirror Support Member Shows Hammer Marks that Stretched the Leg Behind the Mirror Supporting Surface. Alternate Columns of Mirror Assemblies Are Offset So That One Can Be Removed, Replaced and Realigned Without Moving Another