[Turtle]

I ran across this idea in reading links on the 'terra preta' soils. That discussion aside, making charcoal out of yard debris rather than composting it appeals to me so I have started building an oven.
So far, I have a 2 foot length of black-painted galvanized steel stove pipe as the vessel which is suspended at the parabolic trough's focal point. (Photo attached below).
As I have a bit of work to do building the trough, I don't want to make it too small and so not achieve the 450F to 500F temperatures I need inside the tube. So, any ideas of how big I need to build the trough? The width I have fixed at 20"; this gives the pipe 4" of extension to accomodate further modules.

Parts/costs: 4" diameter galvanized steel stovepipe ($4.49), & a galvanized steel cap ($2.79), both painted with high-temp flat black paint ($2.19). Yard stick marked in inches for scale.

Drawings & links to come.

[Turtle]

Here's a rough description of what I have in mind. Four partitions of double-wall corrugated paperboard as support.
I bought 2 pieces of posterboard ($4.00 US) yesterday that already has a silvered surface.
I measured a temperature of 95 to 100 F in the closed black tube lying on the ground & perpendicular to the Sun.

[Racoon]

I have plenty of reflection stuff turtle, more than I can use.
Like the things for car windshields...


I have a sleeping bag sized roll of it...

I will be looking forward to your progress enthusiastically!

[Turtle]

I have settled on a size based on the available corrugated board on hand. Using a standard 'wardrobe' box, I cut 4 clean pieces 42" x +-18". From the large systems I looked at, the focus is nested below the level of the parabola's edges, but because I am using flammable paperboard I didn't want the tube too close.
Where p is the distance from the Focus to the Vertex of a parabola, I have settled on p=10". Then we have x^2=4py for the standard equation of an up pointing parabola. This gives me x^2=40y as the equation for my table of points to use in the layout.
This then gives me a 42 " trough 11" deep, leaving the tube 3/4 of the way inside the reflector.
I have in mind to glue some 11x17 1/4" size graph paper on 1/2 of one corrugated sheet to plot my points on, then cut that out & use it for a template on the rest of the partitions.

[GAHD]

You can also use those AOL CDs you got in the mail for so many years. CDs are pritty reflective, so they offer a cheap alternative.

Hot idea, let me know how it turns out.

[Turtle]

I taped rather than glued the graph paper on & plotted 20 points which I joined free-hand.
One frame/partition done & 3 to go. I have some scrap 2x2 for longerons & some scrap aluminium angle stock for tube supports.

[Turtle]

Last post of the night. All 4 partitions cut & notched. Held in place by temporary papaerboard longerons & reflective posterboard laying loose.
Back sore & eye blurry 'til tomorrow.

[Turtle]

The frame is complete except for some glue fillets yet to dry. I used corrugated paperboard instead of wood for the longerons, and everything is glued with Elmer's white glue.
My saving grace is that it's cloudy & mizzling here so I feel no pressure to hurry faster than the glue allows.
eek2:
PS This may function as a parabolic microphone as well!? If I don't achieve 500 deg F I may still have high enough temp to bake a spud in it!?

[Turtle]

While the glue dries, I thought to review some other design suggestions by gost from the terra preta thread.

Originally Posted by gost

...building a solar oven for making low temp charcoal. He seemed to feel that the trough of the reflector needed to be about 8 to 10 feet wide in order to achieve the 470 F needed to make the charcoal. It almost seems like he is guessing at this, but I can't tell for sure..... Are you basing your design on an existing concept that has calculated the temps for different latitudes? Is there a simple formula for calculating the parabola?
My thought was to cut two pieces of half inch plywood into the parabolic shape, making the ends of the trough, then simply use a staple gun to attach the mylar posterboard to the edges of the plywood, thus eliminating the need for any ribs. The pipe itself would then go through oversized holes in the plywood, at the height of the focal point, where it would then be kept from touching the wood by the use of 3 adjustment screws.

I had also been planning on using galvanized stovepipe, of the kind used for gas water heaters. Does anybody know if the galvinizing might introduce anything toxic, when heated, that might absorb into the charcoal?

I am not using an existing design & I simply picked a size suitable to material on hand.
I gave the parabola formula above; it is in every high school algebra book (And they say you'll never need it! )
The idea of using the 'positive' piece of the cut is good. Since I have these as left-overs from my 'negative' form trough, I can use them for another reflector.
I have no idea about metals migrating from the pipe to the charcoal; galvanization is done with zinc & the pipe is steel of some type. You may want to wash the pipe with warm soap & water before use.
While the glue dries I have the struts under construction. By tonight I plan to have the reflective panels glued on. With some cooperative weather I may have temperature measurements from the oven this coming week.

Oooooo! Idea! Popcorn! Oh yeah...

[Colby]

Be careful with that, those can get incredibly hot if properly designed. I used a little foil one as a camp stove once and made a nice baked potato, no problem.