Solar parabolic cooker with the mechanical mathematician!

Step 14 Experimental results and conclusions

As you know, I made the cardboard parabolic pannel cooker. Previously I also made a larger parabolic cooker with cob.
Saturday was beautiful and I was home nearly all day so I got some experimental results.
I found the cardboard parabola difficult to set up correctly because it is not quite stiff enough. So it needs a spine to back up the cardboard. (Perhaps just one 1by 2 about 4 ft long with pieces attached to keep the middle cardboard segments exact.
Results from the cardboard parabola heating up 1.2 liters of water in a black 0.9 kg teapot were as follows.
(I turned the parabola to follow the sun)
12.50PM 23 C
1.54 pm 33 C
2.16pm 43 C
2.49pm 50 C
3.05pm 55 C
3.31pm 60 C
3.55pm 61 C
(at this point the parabola was in partial shade and I stopped measuring)
At 6.12 it was back down to 34 C in complete shade
Measurement on the big cob parabolic oven went as follows.
I did not turn the parabola but did move the pot a little to follow the focal point
(There were 3.7 kg of wet soil in a 1.35 kg dark glass pot).
11.36 11 C
12.25 24 C
01.54 58 C ( it was already past directly focused at the sun!))
02.16 72 C
02.29 75 C
02.49 80 C
03.05 84 C
03.30 89 C
03.55 90.5 C
04.05 89.5 C
(I measured and the sun was coming in at about 50 degree angle at 4.05 so almost no heat being reflected!)
06.12 67 C cooling down stage
06.34 63 C
Conclusions The cardboard parabola was disappointing because it slightly lost its shape without a stiffening spine. so not correctly focussed on the pot. I didn't have an oven bag for that experiment and my plastic was too snug so energy was lost there too.
Must have a spine to keep the parabola correctly alligned!
The Cob parabola was a different story.
As you can see it continued to heat up long after the sun had passed the best focus (I did move it to follow the focus).
Even on less sunny days during the week when i came home from work, it was still at 55 C at 6.05 on sept 5th,
at 61.5 at 5.26 the next day
and at 61 C at 5.48 a day later.
Why the slow continued heat up?
Perhaps the heat migrates slowly up through the wet clay? Thermometer was in the middle of the pot so it probably took a while for the heat to get there.
Conclusions I do not need more than 70C in wet soil to kill weed seeds so I can use a bigger pot for more efficiency.
Perhaps twice as big! (volume wise) I get 3 gains, 1 bigger pot will stay in focus longer, has bigger volume to surface area ration so will retain heat longer and has bigger volume to pot weight ratio too so more heat usefully used.
The parabola maintains focus on the pot much longer than I thought possible.
3rd conclusion. Tracking or deformed parabola or 2 parabolas would be much better!
If the parabola was stretched along the line that the sun follows, it might maintain focus a lot longer. Or a simple tracking device to move a cardboard parabola round the focus. Or 2 parabolas!
First parabola is for midday to 2 pm and is short focus. Second is to one side of it and is same surface area but longer focus about 20 inches (so it does not interfere with direct sun at 12. By 2;30, this parabola is starting to hit the pot pretty well. The additional effort to make this parabola with the mechanical mathematician would not be great but the benifit would be an extra hour or more of good heat.