However, there are some problems- it is cold outside! Five degrees, and some clouds scudding the sky. But, I figured that I should get it done.
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| Outside temp was 5 degrees C. |
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| Water temperature before starting was 12 degrees |
Step 1: Prepare the target. I took a 350ml can from the recycling pile and spray-painted it matte black.
Step 2: Aiming the mirror. I have two strings running in an "X" over the top of the parabola. When the shadow X hits the small X I made on the base, it is focused.
Step 3. I filled the can with water and measured the temp- twelve degrees. Then I took some string and tied it to the ring-tab. Finally I suspended it from a 1x4 just above the "X" on the baseplate.
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| Can in place and ready to heat. |
Step 4. I made a spreadsheet to record the data and began to measure.
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| You can see the can shadow over the target. |
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| There were a few clouds in the sky. |
Over the course of an hour, the temperature of the 350ml of water in the can did manage to rise to 45 degrees C. Perhaps more constant vigilance in aiming could have improved that. I did have to end the experiment early. The original plan was to continue until a stable temperature was reached for 30 minutes.
Observations:
- The parabola is not very sturdy. It flops around and sags, causing a very soft focus I think.
- You really cannot use it in windy conditions. It will blow it around and the wind will steal heat from your target. perhaps insulating the can inside a clear glass jar would improve efficiency.
- Winter is not a good time for outdoor solar cooking in this thing. The sun is so low, that it is hard to suspend your target in the focal point, due to the angle. And the cold air also steals a lot of heat.
- It is fun to experiment.
So, any geeky types out there who can help me understand my data from the spreadsheet?
Also, any tips or hints, or requests for other data?
It looks, from the picture, like the can is sitting inside the hemisphere. Isn't the focal point actually outside?
ReplyDeleteI wonder how much better it would be if you put glass over the foil... Though I guess if you went that far you might as well build a frame and use mirrors. At least it wouldn't blow away easily!
I don't know if people have those big old-school satellite dishes in Japan, but I've heard they make butt-kickin' frames for parabolic cookers.
It is a deep focus parabola, which is much safer than a shallow focus- it would be very hard to get your head down there, while you can easily cause eye damage with a shallow focus. That was one reason I chose this design, with small children and all.
ReplyDeleteI don't think that putting glass over it would be all that effective- it would make it harder to suspend the target for sure. Rather I think that putting a dark target in an oven cooking bag or a large glass container would do wonders.
I dream of having one of those old parabolic antenna dishes. I saw a video on Youtube once where they waved a 2x4 into the focus point, and it exploded into flame instantly.