Parabolic Trough Solar Concentrator

 
Introduction

This page describes the design, construction, and operation of a trough-type parabolic solar concentrator.

Mathematics/Geometry

The trough has a parabolic cross-section consisting of that portion of the parabolic curve between its intersections with the latus rectum. It is sized so that the length of the parabolic curve is the same as the width of the available mirror material (48" / 1219.2mm). It should be noted that there is nothing "magical" about this choice of geometry - it simply seemed a reasonable trade-off between getting the most out of expensive mirror material and ease of construction. A flatter parabolic section would have provided a slightly larger capture area at the cost of much more awkward construction.

The shape of the back side of the ribs is unimportant. The shape used was chosen because I found it pleasing.

The length of the trough was chosen to be the same as the length of the available mirror material (96" / 2438.4mm).

Materials

The mirror is 3/32" (2.38125 mm) rear-silvered polycarbonate plastic purchased as a full sheet measuring 48"x96" (1219.2x2438.4 mm). The mirror comes with a peel-away protective plastic film.

The seven ribs were cut from a half sheet of 3/4" pressure-treated plywood.

Two L-shaped side rails were ripped from common 2x4x96" construction-grade fir lumber using a table saw.

Two 1x4x96" pine stringers were used as purchased.

Construction

Construction consisted of the following five steps:

  • Screwing the rails to the ends of the seven ribs
  • Fitting the mirror into place and screwing the mirror ends to the outer ribs
  • Screwing the two stringers to the seven ribs
  • Screwing the collector tube support brackets to the rails
  • U-bolting the collection tube to the support brackets

Here you see the rails screwed to the ribs at the end of step 1.

The "lip" on the rail captures the edge of the mirror material to ensure that the mirror edges are straight and that the mirror is held tightly against the ribs.

The prototype trough complete with the mirror installed - ready for the initial test (rear view to show construction).


Testing

Protective film peeled back from the portion of the mirror used for this test.

A target board was placed across and about 3/4" above the mirror's focus.

With the target 3/4" above the focus, the remote sensing thermometer indicates that the target is being heated to 127°F. Note that in this and the next photo, I've adjusted image brightness and contrast so that the temperature display can be read.

That went well, so the target was lowered to the focal line - and this time the thermometer registered a warm 724°F (384°C). This is near the high end of the hoped-for range.

Not being quite grown up yet, there was this strange compulsion to see if it'd be possible to start a fire...

Perforated angle has been attached across the face of the trough and a collection pipe installed with U-bolts at the focal line.

Another photo of the concentrator and collection pipe installation. This photo and the previous should delight any ray tracing readers.


 
WARNING!

Solar concentrators can produce temperatures high enough to produce nearly instantaneous deep burns. Care must be taken to avoid burning yourself and to keep children, pets, and wildlife from injury and death. When you build something as dangerous as a concentrating collector, please never forget that you are assuming responsibility for more than just your own safety.

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