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City-fied Self-Sufficiency

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Posts Tagged ‘solar oven’

TEST of the (so-far) Solar Oven

Not being able to complete the solar oven until I located a suitably large piece of cardboard from which to make the lid, I decided to see what sort of temperatures I could get from it as it is.  I have since procured a piece of cardboard, thanks to my uncle, so hopefully I can get to the lid this weekend.  :)

I remembered that there was a piece of glass sitting unused around the house, so I cleaned it and took it outside.  I set the oven base on the grass and put a cotton pad in it, and a cast iron skillet on the pad, and then an oven thermometer in the skillet.  I put the glass on top.  There was no angling the box so the sun entered it more directly, there was no reflector to direct more sunlight into the oven, and no black pan to help collect heat.

The skillet was necessary to help convert the sunlight to heat.

You know how your car heats up on a sunny day?  A solar oven works on the same principle.  The car is closed, and the sunlight enters through the windows.  It strikes the surfaces inside the car and some of it is converted to infrared light.  Most infrared light is heat.  The darker the surface the light strikes, the greater the amount of light that is converted to heat.  Once the light is converted to infrared, it cannot pass back through the windows.  Hence, black vinyl seats = burned tush.

In a solar oven, you want the heat concentrated where the food is.  This is why the inside of the box is reflective.  You want the light to keep bouncing around until it hits the dark items, which are the tray and the pots.  The oven will heat up, as well.

Now, I had meant to get the oven outside earlier in the day, like about 8:30 or 9:00, but I forgot about it until 11:30.  In spite of that, the temperature of the oven at 2:00 pm was around 210*-215*!

Not the greatest angle, but you can see the needle is pointing just about at 200* at this angle. From straight on, you would be able to see it was a little higher.

Not bad at all for an unfinished solar oven!!  Yeah!!!

It is good to have the oven reach at least 250* for cooking, to provide the safest temperature rise and not allow too much bacterial growth.  This is about the temperature a crock pot operates at.

Some solar ovens are all black in the interior.  They work on the same principle, just preferring to have all surfaces of the oven involved in converting light to heat.  I don’t know if either is superior, and there seems to be disagreement on that point.  Ah, well, as long as the thing cooks!  :)

Building a Solar Oven, Part 2

Well, now that the kids are feeling better, maybe I can continue on about the solar oven.  :)

After I foiled the insides of both of the boxes, it was time to put them together.  I am using dry lawn clippings from the yard as insulation.  Apparently, dried grass makes a rather good insulation.  I did some poking around online, and found a number of sites that mentioned using it as insulation in solar ovens, as an ingredient in walls, as emergency survival stuffing in clothing, and even as a normal part of going outside to play in Alaska (until recently) — it would be stuffed into shoes, and wrapped around the feet.

The first step was putting some sort of spacers in the bottom of the outer box to help hold the oven box up off of the bottom.  I could just put an inch or two of grass in the bottom and set the oven box on top of it, but I figure the grass would compact over time and become less of an insulator (due to reduced trapped air), and, if the grass compacted, then the oven box would sink below the top of the outer box, causing problems its seal with the lid.

So I considered several ways of making spacers, and finally decided to simply cut small squares of cardboard and stack them.  I sprayed adhesive on one side of the square and affixed it to the bottom of the outer box, then did the same for three more squares, figuring that four stacks should give good support.  I added a second layer, then a third, and so on until I had about eight or so layers (a little over an inch high).  I rotated the squares so that the corrugation crossed the corrugation of the previous layer, hoping that would help further prevent compression of the squares.  The stacks of squares are roughly 2″x3″, but they vary a bit, as I didn’t worry about getting them all exactly the same size.

If I had had my brain plugged in, I probably would have tried to put a fifth stack in the middle, since that is where the weight will be concentrated. It may not have worked out, though, because the flaps of the box don't meet in the middle, making the middle lower than the places where the other stacks are. I think it will be alright, though, because my oven box is double-walled, meaning I will have four layers of corrugated cardboard making the floor of the oven, and spreading the weight more evenly across the bottom.

Once I had the stacks done, and as much of the adhesive removed from my hands as possible (that’s sticky stuff!!), I started putting the dried grass in.  This is mostly St. Augustine grass, with some weeds interspersed.  I feed this same dried grass to the rabbits, though it isn’t their favorite, and is only of mediocre forage quality… but it’s FREE and right here in my yard!  Anyway, I continued to add grass, lightly pressing on it to see where it was too thin, until I had a nice, firm (but not packed) layer of grass that just barely covered the support stacks.

I tried to make sure I had a nice, even layer that would give good support to the oven box.

I then concentrated on the oven box, using Duck tape to seal all the seams and also to reinforce the corner supports.

I'll never have the chance again, so I pretty much taped it to death.

Finally, I had reached the point at which it was time to put the oven box into the outer box.  Once that was accomplished (a little harder than it sounds, because of the foiled flaps), I began stuffing grass around the sides.

Again, I went for a nice, firm packing, but not hard pressed. It was a bit difficult to keep the oven box perfectly centered, but that isn't really as important as making sure that there is some space between the boxes on all sides. The walls of both boxes ended up bulging a little, but that would be remedied as time went on.

Once it was suitably stuffed, it was time to fold the flaps down.

The extra height of the oven box will be used to secure it to the outer box.

Once I had the flaps taped down, I cut the oven box flaps apart just to the top of the outer box.  Some of my corner supports extended above the top of the outer box (oops… shoulda measured that), so I had to cut through them down to the top of the outer box.  Then I sealed the gap along each side of the oven box to the outer box with Duck tape.  Using the outside of a pair of scissors, I made a dent along the outside of the oven box flap so that I could fold it over neatly.  The idea was to fold it over the top of the outer box, and then fold it again down the side, and then tape it down.

I stopped and thought for a while, though, looking at it and thinking about how thick that was going to be, and how it was going to be more difficult to make a lid that would fit well.  I finally sprayed one flap with adhesive and folded it down, but it wasn’t working well because it was so thick.  Then my mom, who had joined me outside at this point, suggested peeling some of the thickness away.

Since I’m going to have plenty of layers of cardboard at the top of the box anyway, I thought that was brilliant!  So I cut the Duck tape seals I had just made, and Mom helped me peel a layer of cardboard from the flaps, including the little bits of the corner supports that were too high.  I had to be very careful not to cut all the way through the flaps.  I then cut the flaps down so they wouldn’t go over the edge of the outer box.  This would make a tight-fitting lid much easier to make.

Peeling away the second layer of cardboard, since the double-walled construction of the oven box wasn't advantageous here.

This side has had the second layer of corrugated cardboard peeled away, and has been cut down.

I then sprayed the surfaces to be glued — the top of the outer box, and the peeled side of the oven box flap — using a piece of scrap cardboard to catch overspray.  I don’t want to taste adhesive in my food!  Maybe I’m just picky.

I waited 30 seconds for the adhesive to set, and then I pulled the top of the outer box and the top of the oven box together tightly, and folded the flap down and pressed it into place.  I held it to be sure the two surfaces adhered well.  Then I scored the foil on the flap and peeled most of it off, since I don’t need foil on the flaps, and I need the flaps to stick well.  Then I taped the flap down.  This is where the bulging was mostly rectified.

It is important to not have any Duck tape on the inside of the oven, because it fumes a bit when heated. I kept the tape about 1/4" away from the edge where it drops off into the oven.

Once I had all of the flaps peeled, cut, glued, and taped, I taped diagonally across the corners.

Since I had peeled the oven box flaps, I didn't have large spaces in the corners that needed filling.

The base of the oven is now complete!  :)

Building a Solar Oven, Part 1

My materials:

  • two cardboard boxes in good shape, one a couple of inches larger than the other on all sides
  • aluminum foil
  • spray adhesive (can also use diluted white glue)
  • dried grass clippings
  • a few other various pieces of cardboard
  • turkey-sized baking bag (can also use clear Plexiglas or glass, two layers are ideal)
  • wire clothes hanger
  • flat black grill paint
  • probably some Duck tape (duct tape)

With our recent move, it was surprising to me how hard it was to find the two perfect boxes for this project.  The inner box is the oven itself, and the outer box provides insulation.  You don’t want boxes that are too deep, but your outer box still needs to be large enough to accommodate the oven box.  When it came down to it, I had to decide between an oven box that was shallow but of a nice length and width, and another box that was deeper, but not as long.  I chose the shallow box, and decided to modify it so that the flaps would stand up rigidly and provide more depth.

First, I closed the larger box and centered the oven box on top of it, and then traced around the oven box.

Tracing around the smaller oven box onto the top of the larger outer box.

Yes, I did have a large box devoted to hangers.  Didn’t you?  And… I have never had Direct TV… where did that box come from?

Next, I cut all the way through all of the flaps with a mat knife (box cutter).

All layers of flaps cut through, and I have them weighted down to show you the resulting hole. Unfortunately, I forgot to take pictures until I was several steps along, which is why the inside is already shiny. :)

Then I opened the box and applied foil to the inside, using the adhesive spray.  It isn’t absolutely necessary to foil the inside of the outer box, but it reflects more heat back inside, making the oven more efficient.  White school glue diluted to half strength can also be used to apply the foil, but I didn’t want to deal with the difficulties associated with applying and using it.  And… the spray adhesive will adhere the foil better, as well.  Not very important for the outer box, but for the inner box, which will get more wear and tear as the oven box, having the foil stick as well as possible is a good thing.  The spray adhesive was about $10.  It was the more expensive of two spray adhesives available by 3M at Wal-Mart, but it was a larger can, and was the only one that mentioned gluing foil.

I applied foil to the inner flaps instead of the outer ones, forgetting that they don’t meet in the middle.  So I have applied foil to the part of the outer flaps that bridges the gap.  Should be good enough, but I’ll remember that if I ever build another one.

I have since added some more foil to close the gaps in the corners and along the flaps.

There’s less trouble with keeping the foil shiny if you spray the back of the foil and apply it, rather than spraying the box.

My oven box wasn’t as deep as I wanted, so I cut some cardboard strips from extra material and bent them.  To get them to stick and do their job, I needed the strongest possible bond.  This was accomplished by spraying both the strip and the corner on the box, and then waiting half a minute.  When pressed together, the adhesive acts like a contact cement, and bonds quite well.  I did have to go back around when I was finished and hold each one for a few seconds again.

I will probably reinforce the corners with Duck tape all the way around, to make sure I exclude air from outside. I'll concentrate on the added flap supports, to prevent them from popping off, just in case they're tempted to.

Then, I applied foil to the inside of the oven box, using smaller pieces so I could keep them a bit bit flatter for better reflection.

I think I ended up with two layers of foil over most of the surface.

Tomorrow, I hope to get a good bit of assembly done!  :)

In defense of solar cooking

First, why would I go to the effort of building a solar oven?  After all, studies show that cooking uses only 4 – 10% of a home’s total use of electricity.  Couldn’t I save more money faster by putting my efforts elsewhere?

Maybe.  We already keep the temperature around 75* in the summer (okay, it’s not the suggested 78*, but here, where it’s so very humid most of the time while it is so very hot, 78* is just not much relief a lot of the time).  That’s higher than many keep it, which is 68 – 72*.  So we save there some already.

We have two refrigerators and a chest freezer, so that doesn’t help the bill much.  It would be nice to move down to one fridge, but I doubt that will happen anytime soon, especially since their presence here predates our moving in with my uncle.  And we do use all the space in them, somehow.  I’m not sure where we would put everything for us all in one fridge.  And that freezer’s not going anywhere, sorry!!!

So then there’s the water heater.  Well, that’s gas, and the gas isn’t that expensive for now.  And the laundry?  We wash nearly all of our laundry in cold water.  The dryer, on the other hand, has only one remedy:  line drying.  I’ll be getting a clothesline shortly to see how we do with that.  I have pretty active allergies, our daughter has somewhat active allergies, and she, our son, and my mom all have asthma.  So clothes full of pollen might not work out very well, but we’ll see.  We’re willing to try.

Since the powers that be have openly admitted that the changes coming down the pike will cause the cost of electricity to “necessarily skyrocket”, I figure any attempt to save electricity is called for, including solar cooking.  Like I said, I’m no environmentalist.  I don’t believe all the scary doom-and-gloom-man-is-killing-the-planet stuff (and I have read quite a bit on both sides of the subject!).  Not that that means that I think we are free to pillage the earth and leave devastation in our wake.  God said for man to be fruitful and multiply, and to have dominion over the earth and to subdue it.  Dominion includes the responsibility of caring for that which you have dominion over, and wisely managing what you have.  I’m not an environmentalist, but I do like saving money.

What can you cook with a solar oven?  Apparently, quite a bit.  Bread, cakes, and cornbreads, pretty much anything you can cook in a crock pot, and many things you cook on the stove.  Some things merely require modification of the process.  Rice, for instance, requires you to heat the water and rice separately in the solar oven before putting them together to cook.  Potatoes need to be cut pretty small in order to cook properly.  Most, if not all, foods benefit most from being cooked inside dark cookware so they will more efficiently collect heat.

In order to do other things like making reductions, frying, or sauteing, you would need your conventional stove or a solar concentration cooker like a parabolic cooker.  These things are very sensitive to the movement of the sun across the sky, though, and require adjustment of the mirror(s) frequently.  I don’t want to babysit the thing, so I’m going with a plain solar oven.

A solar oven can be as cheap or as expensive as you want.  You can spend nothing on it and build it out of scraps, or you can spend several hundred on one that is ready to use and will supply supplemental electric heat should the sun duck behind a cloud for too long.  I’m spending about $15.  Only about $5 of that did I really need to spend.  The rest of it I spent on some spray adhesive that can adhere foil.  It will come in handy for other projects, too.  :)

Out(side)sourcing some of the cooking

I am not an environmentalist, and do not believe in global warming, but I do believe in reasonable conservation and wise use of our resources, as well as wasting as little as possible.

When you come down to my embracing of the idea of solar cooking, though, it has nothing to do with my views on conservation and resources. It has to do with frugality — I’m trying to save money, here. We don’t have a lot of it, after all, and I could swear I hear some of our pennies whimper when I squeeze them particularly hard. But I have to.

I already had one solar oven, but it wasn’t quite finished when we moved. My beloved husband had built it from scrap siding, duct board, and the glass door from an old entertainment center. The last thing it needed was a way to keep the reflector open, and a lock to keep the neighborhood kids from adding unexpected ingredients (like chameleons) to the food. It got up to 225* in spite of the fact that I couldn’t keep the reflector open, so I was really hoping to see what it could do once it was finished.

Well, it didn’t make it into the moving truck. :( So I’m building one. Maybe it’ll be temporary, or maybe I’ll have it quite a while. I’m building it out of two cardboard boxes, foil, and dried grass. I just started, so I’ll be posting pictures. I’m more or less following these plans: http://www.thefarm.org/charities/i4at/surv/solarbox.htm