Today it rained in Austin. All day. Which is good, since we are in a severe drought. It was not good for measuring the effective of our solar grill, however.
Elliot and I did work a solid seven hours, however, and accomplished two things. Elliot tested three different adhesive systems for gluing foil to posterboard. These were rubber cement, heated rubber cement, and contact cement. All of them look good when first constructed, but our initial experience is that rubber cement eventually wrinkles when you bend it to shape and put it in the sun. I believe that contact cement will keep this from happening, but of course as scientists we don't accept any hypothesis fully until tested---and even then it remains provisional.
The other major work was the completion of our "pressure conformity system." Previously we had glued the flat panels to the parabolic ribs, creating the shape of a parabola. Unfortunately, this also damaged the ribs, making them essentially unreusable. Our current system shows great promise. We constructed aluminum brackets (with a pair of tin snips) that hold the bottom edge of the panel in two little lips, and then we apply pressure downward to the top of the panel with a mirror-holder wired to a turnbuckle. The pressure pushes it solidly against the ribs, apparently producing a nice parabola.
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| Clip-and-turnbuckle system (wide angle.) |
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| Mirror clip with wire going over bolt to put force downard and into rib. |
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| Twisted wire through base as a turnbuckle attachment point. |
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| Top of mirror clip pressing down on panel. |
We then performed a "green paper test" of our resulting system using our
unpolished panels. The results are pictured below. I personally think
the drop-off in intensity outside the "cross" pattern is very troubling
for our overall concentration. Next weekend we are hoping to
experiment with this more completely.
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| Green-paper test with ceiling light. Note strong "cross" pattern and mirror clips on panels. |
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| Another view of the "green paper" test. Note troubling silver voids in the corners. |
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| Green paper test from less than acceptance angle. We believe we should be seeing green everywhere! |
Our biggest questions now are:
- How efficient is our concentrator in absolute quantifiable terms?
- Are very small deviations from a perfect parabola enough to cause major decreases in efficiency?
- How much would panels polished to a mirror-like finish improve our concentration?
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