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Old 08-02-2016, 07:03 AM   #1
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Joined: May 2004
From: Central Arkansas

Posts: 4,005
Solar cooking

{In reply to BigBassMan's question about solar cookers; found an old thread on another forum}

We’ve used a couple box-style cookers for years now. Haven’t lately as we’ve been silly busy with stuff, but it does work surprisingly well. Made our first one out of wood and it worked great until my then-young son left it open in the rain for two days and it molded irrecoverably before I found it.

First one, made from wood scraps, a mirror, and plain styrofoam insulation. On a cloud-free day it would usually reach 370-380 degrees and more than once broke 400:

Used a table-leg bracket and eye-bolts for the mirror brace and adjustment:

Closed. Prop & all fit down inside for storage:

Don’t have pics of the second one, but when I tried to make better I ended up making it worse. Made it larger and heavier, angled the top to face the sun more directly, and insulated it much thicker with cellulose blow-in type insulation. Turned out to be too much mass and it never got above 200 degrees. Lesson learned – mass is the enemy. Obvious once I thought about it, but I just thought about it too late.

Third one, I made out of an old fire-alarm control backbox. Sealed up the conduit entry holes with sheet metal and epoxy, insulated it with two layers of ductboard, and added the mirror & prop similar to the first one. Actually, the mirror is the mirror from the first one; not an exact fit for this new one, but close enough that I used it. This one reaches 300 pretty regularly, but has never broke 375 afaik. The construction makes it much more durable than the first two wood ones, but more mass to try & heat up and makes it heavier to carry around as well.

Most-used pan is a simple aluminum pan with snap-on lid, painted with high-temp flat black grill paint:

Inside the pan, a cooking bag keeps the moisture contained. This is important on a flat-glass-top box cooker, as escaping moisture causes condensation on the glass lid, greatly reducing light entry and cooling the oven down below the cooking point. This is a ~3 lb roast, three potatoes, & half an onion. Usually add carrots, but for whatever reason, not that particular day:

Four & a half hours at ~250 degrees that day; slow-cooked like this, the meat is almost literally too tender to cut. It almost falls apart with just a fork:

I've been intrigued by the commercially-made box cookers, as they should be more efficient than my home-made, but never have taken the plunge on one yet.
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Old 08-02-2016, 07:18 AM   #2
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Joined: May 2004
From: Central Arkansas

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One of the big pluses of solar cooking is that if you can use it every day, then great; but even if the weather is such that you can only use it half the time, that still means that your other (stored) fuel only gets used half as much, and so lasts twice as long.

Also, check out a "WAPI" (water pasteurization indicator) made out of a plastic tube (closed on both ends), with a glob of soy wax inside it, that gets placed in the water. It has a line & weight on it, and you hang it vertically with the wax at the top of the tube. When the wax melts & goes to the bottom, you know that a safe pasteurization temp has been reached.

Another low-tech is the "sodis bottle", a solar distiller made from a soda bottle. Paint a third to a half of the circumference black as a heat gatherer, fill with water, cap and lay in the sun (with the black side down. The sunlight enters the clear side and because of the black side (bottom), it gathers enough heat to pasteurize.

Never personally tried this one, as I don't have a good (low-tech, non breakable) way to check the temperature & confirm pasteurization temp, but they're supposedly in fairly widespread use in poorer countries to kill biologics in locally-gathered water.

IIRC, pasteurizing water doesn't require as high a temperature as pasteurizing milk, so it's relatively easy to achieve. I believe it's only something like 150-155 farenheit for water, where milk (iirc) is 160 or higher. But don't take those temps as gospel by any means; I'm going by memory and could be way off.

Fwiw, this page has a lot of different plans and approaches. I like the box cookers obviously, but there are other approaches that use parabolic reflectors, fresnel lens magnifiers, etc. Lots of ways of harnessing & collecting solar radiation.
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Old 08-02-2016, 07:41 AM   #3
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Joined: Jun 2015
From: Girard, IL

Posts: 1,157
Looks like a project for later this week! Thanks for the ideas.
Old 08-02-2016, 07:51 AM   #4
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Joined: May 2004
From: Central Arkansas

Posts: 4,005
One thing to really watch and be mindful of is the fact that "mass is the enemy". My second one was greatly insulated with cellulose, but even though it seems paradoxical, the added mass of all the insulation worked against it. If you've got enough solar BTU's to heat 5 lbs to 300 degrees, those same BTU's simply can't heat 7 or 8 lbs to the same temperature.

Best insulating material I've found is ductboard, basically fiberglass insulation in sheet form with foil on both sides. It's lightweight, doesn't melt like some foam boards do, and if it gets wet, once it dries out it's fine again.

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