Kaleidoscopic Solar Oven / Cooker

by Curlydock

One of my earliest installments dealt with the theory of the best angle to use with the reflecting planes of the solar concentrators of the Box-type solar oven. Since then, I have come to prefer what I call a “Kaleidoscopic” type solar oven.

I feel I have many reasons for this preference, but the most important is simplicity or ease of construction. Roughly speaking, the 3-D description of a Box-type oven takes about 20 vertexes and 32 lines. For the Kaleidoscopic type, it is 8 vertexes and 11 lines. So the Box type is about three times more complicated than the Kaleidoscopic type.

image 92

Image 92 shows the Kaleidoscopic oven I used to bake many loaves of genuine sourdough bread over this past summer.

image 91

Image 91 shows a not-fully-risen loaf before baking. I consider it fully risen when the top of the loaf reaches the top of the bowl. The bread bakes in the oven-proof glass bowl which sits in the oven cavity. The cavity is detailed later.

image 82

image 83

Images 82 and 83 show a finished loaf.

image 89

Image 89 looks into the front of my Kaleidoscopic oven. Most of the essential parts are seen. Missing is the glass bowl that would sit inverted over the top of the black lid. The oven cavity is shown in position and ready to receive the bowl of dough.

The reflective concentrators are in four planes. Two that I will call R1 and R2 form a vertically hinged unit that opens like a book and sits at a 60 degree angle. The hinge for R1 and R2 is made with strapping tape. The oven cavity just touches R1 and R2 and sits on R3, which is a separate unit the shape of a triangle.

The R3 angles are all 60 degrees and the length of each side is three times the diameter of the oven cavity. R1 and R2 are as wide as the sides of R3 and considerably taller than that.

R4 is also a separate piece and extends from the open edge of R3 as if it were hinged horizontally to R3. It could be permanently hinged but I feel there is no need for it. A pole pivots from the outer edge of R4 and fixes on the ground. It is used to set the angle of R4 so that the oven cavity is the brightest you can make it. If the wind is not blowing, gravity and the angle adjustment pole will keep R4 in place.

If there is wind, then I fasten all the sail-away reflective panels to the table with shoestrings. The cardboard from which R1, R2 and R4 are made is reinforced along bottom edges with narrow wood strips and package sealing tape. The shoestrings go through holes punched in the cardboard, around the wood strips, and through the mesh of the table top.

The weight of the oven cavity keeps R3 in place.

When the wind is very strong I use sandbags to hold down the table legs.

Here is a diagram comparing the Box and Kaleidoscopic type solar cooker / ovens and labeling of the concentrator panels I have been describing:

oven types diagram

The Box type has only one side glazed. That is the side where the solar flux enters the box. The other five sides have to be well insulated to keep the heat in. The maximum reachable temperature will depend a lot on the effectiveness of this insulation and the quality of box construction.

The Kaleidoscopic type does away with this particular need altogether by making all sides glazed. So, solar flux would enter all around the oven cavity, in theory. In actuality, this will not be perfect. The reasons have to do with the positioning of the oven cavity among the reflecting walls. Some positions are better than others.

Here is a detailed semi-exploded diagram of the oven cavity:

oven cavity diagram

The oven cavity works like a green house to trap the heat from the focused solar flux. The ideal would be a series of concentric spheres. The outermost sphere is transparent glazing that passes light. The next sphere is an insulating jacket to keep the heat, for which a vacuum would be best but air is easier. The next inner sphere is flat black metal which absorbs light and converts it to heat. This heat ideally accumulates in the central sphere where the food cooks in its container.

The ideal is approximated here by the use of oven proof glass bowls and a stainless steel metal mixing bowl.

The outermost sphere consists of two glass bowls: (1) is inverted on top and (4) completes the bottom half.

The insulating air jacket is made by suspending the metal radiation absorber bowl (6) on a ring (7) cut from a double layer of heavy corrugated cardboard. The ring rests on the lip of outer glass bowl (4). The lip of the metal bowl (6) makes a snug fit in the ring (7) so that the metal bowl will not fall through. The metal absorber does not touch the outer bowls anywhere. It only touches the cardboard ring. The ring and air jacket are poor conductors of heat. They confine most of the heat to the cooking area.

The metal radiation absorber bowl is a stainless steel mixing bowl painted flat black on the outside with the kind of paint that withstands heat, or the paint you would use on a charcoal grill. Let the paint dry, cure under heat and air out for several days before using it for cooking. You probably would not like paint flavored bread.

I was lucky in finding a black metal cooking pot lid (2) that just fits over the lip of (6) and rests on ring (7). There are cake or pie tins that might also work if painted black on the outside.

Bowl (3) holds the food or bread dough. It does not have to be transparent. I have been using oven proof glass but recently found a ceramic bowl that should also work. Another metal pot identical to (6) would fit snugly and maximize cooking space and thermal conduction to the food, but I have not tried that yet. In fact, I suppose you could do without (3) altogether by putting the food in the radiation absorber bowl (6). But, since (6) is not easy to get on and off ring (7) and the cardboard of (7) should not be washed or get wet, I decided to use another bowl to hold the food.

On my wish list is some kind of thin wire handle to make food bowl (3) easier to get in and out of metal bowl (6). The handle would need to quickly and easily connect and disconnect from the edge of the food bowl and not compromise the thermal seals around the edges.

The whole cavity needs to be somewhat elevated so I put it on a transparent pedestal made by inverting the smallest glass bowl (5) near a corner of the bottom reflector, R3.

Most of the glass bowls I found and purchased as a nested set. I think perhaps the largest, (4), was not part of that set and had to be separately purchased, but I am not sure.

Why Kaleidoscope

To study the effect of the focal positioning and the angle of R1 and R2, etc., I decided to research the geometrical and mathematical aspects of multiple reflections in mirrors. From that, I realized the kinship between kaleidoscopes and this type of solar cooker. The next pictures should make the relationship obvious.

Fascinating as it was, I thought it might take too long, so I did an empirical study with a scale model instead of the exacting thought experiments. I gathered some pieces salvaged from a broken mirror (never throw anything away), tape, and construction paper. Also, I borrowed a large bead from a trusting and tolerant friend.

Image 74 is an overview of the apparatus:

image 74

The bead stands for the oven cavity or focus.

The mirrors that hinge on a vertical axis stand for reflecting planes or solar concentrators R1 and R2. R3, seen here on the bottom, will be moved in and out. R4 is not shown here but will be seen later.

image 57

image 60

image 61

image 62

Images 57, 60, 61 and 62 show how the number of reflections of the bead increase as the angle between R1 and R2 decreases. This inverse relationship says to me that the narrower this angle the better as far as solar flux concentration.

Surely, the more images of the bead (oven cavity) the sun “sees” then the more solar flux will concentrate on the bead.

But there are several trade-offs.

As you can see, the ring of bead reflections gets gradually larger as the angle decreases. To compensate for this, the sizes or areas of R1 and R2 need to progressively increase. At some point R1 and R2 are too large and cumbersome.

image 63

Image 63 shows how adding one more mirror, representing R3, doubles the number of bead images. Note how one of the images is lost because it is shadowed or hidden by the actual bead.

image 73

Image 73 shows how images are partially obscured when the bead is not elevated:

This is the reason that the oven chamber is elevated a bit by bowl (6).

image 72

Image 72 shows how the bead image count can be at least doubled yet again by adding the mirror that stands for R4. But, as the count and complexity of reflections increase, more and more images are obscured. There seems to be a threshold of diminishing returns.

image 66

Image 66 shows the concentrators at work. I used flash, which, I belatedly realized, is probably not good for a digital camera in a setup like this. Fortunately, perhaps most of the energy focused and dissipated on the bead instead of getting back into the camera lens.

If bead were bread, it baked.

How I Use the Kaleidoscopic Solar Oven

I use an angle of 60 degrees between R1 and R2. There may be a better angle. I have not tried others yet. I adjust the table orientation and the angle of R4 about once every 15 or 20 minutes. This needs to be done more often when the sun is high in the sky.

I frequently measure a temperature of 280 F between the top glaze bowl (1) and the lid (2), depending on the time of day. Morning hours, with the sun at a lower angle, seem to make the oven hotter than do the noon hours, probably because of the reflection obscuring effect already mentioned. Elevating the oven cavity even more when the sun is high in the sky might make the oven even hotter, but I have not needed to try that yet.

Either time of day works fine for baking my bread. The recipe for one loaf of sourdough calls for 45 minutes at 350 F in my conventional oven. I can bake 3/4 of that recipe in the Kaleidoscope solar oven in around 90 minutes. The crust browns nicely, especially on the top.

You might be tempted to let the finished bread cool just a little bit in the oven. But don’t do that. And don’t be fooled. The oven gets very hot. Be careful not to burn yourself.

While the oven is cooking, the moisture escapes as steam. As soon as the oven starts to cool, that moisture condenses on the lids and runs down to collect on the corrugated cardboard ring. The cardboard ring may dissolve if it gets wet. But, it can withstand the highest temperatures of the oven just fine. The high temperature helps keep the ring dry. As soon as I finish baking, I dump the bread on a rack to cool.

After a bit of practice, you can tell when the bread is finished baking by how it smells around the solar oven. Also, you will begin to see condensation on the inner side of glass bowl (1) when the bread is ready.

Outside of baking sourdough and cornbread, I have not yet cooked other things in this particular oven / cooker. I wonder if the condensation will be more of a problem if, for example, I make soup. I don’t know yet.

A Note on Construction Technique

Many instruct builders of these types of ovens to glue the aluminum foil to the cardboard with diluted white glue. I no longer do this.

I believe it is sufficient to bend the foil around the edges of the corrugated cardboard and fasten it in the reflective plane about every square foot using brass plated paper fasteners. Insert the fasteners through small holes prepared with a knife blade. These fasteners can be found where you get office supplies. They look like tacks with points that can be spread apart. This is much easier than working with glue. It is easy to repair.

But the main reason I do it this way is that the foil is easily removed from the cardboard when time comes to recycle them both. My red worms can eat the cardboard but the foil might not be good for them and would not be wanted in the vermicompost.

I do use white glue or carpenter’s glue to bond cardboard to cardboard where a panel needs more strength or a flap needs to be made rigid.

image 93

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39 CommentsLeave a comment

  1. I think that this solar cooker is pretty neat and i would like to now how to make one

  2. jasmine:

    The most complicated part of this oven is the oven cavity. An exploded view is seen if you click on the oven cavity diagram that is in this post. It is made of a set of nested glass bowls, a metal mixing bowl, a cardboard ring, a pot lid and some flat black paint.

    The rest of the oven is just aluminum foil held in place with cardboard panels.
    Thanks for your interest.


  3. I am interested in knowing why the box has to be so tall. I am presently using a windshield cooker to good effect (over 300F mostly sunny days); however, I have not yet tried to bake bread in it.

    I would be interested in your sourdough bread recipe, as well as knowing why your oven is so tall.

  4. GW:

    The height of the oven reflectors that stand open like a book is a function of the average height of the sun while you are using the cooker.

    The higher the sun in the sky, then the hotter the oven will get if the height of these reflectors is increased. This also means that when the sun is very low in the sky, then the oven is hotter when these reflectors are longer than taller.

    In general, the longest dimension of the reflector should extend in the direction of the sun.

    Based on this, a good design might change whether these reflectors hinge on their wide or narrow side. You could change the hinge depending on the angle of the sun or the time of day.

    My sourdough bread recipe was taken from “Sourdough Baking: How to Begin – The Basics By S. John Ross”.
    Check with him for details. The ingredient list is:

    2 Cups of sponge or starter
    3 Cups of unbleached flower (I use half whole wheat)
    2 tablespoons olive oil
    4 teaspoons sugar
    2 teaspoons salt

    This bread is especially good when toasted and eaten with eggs. I once left the oil and sugar out and thought the bread was fine. If you miss the oil, you won’t when you toast a piece of the bread in a skillet with butter. If you miss the sugar, you won’t when you heap some jelly or jam on the toast.

    Good Luck!

  5. so, I live at about 31 degrees, and have a pretty high sun with a southward ‘slant’, and very hot days. I would have thought the height meant outward reflection of heat, and not downwards? How far in would one want to move the hinge for better reflection to the pot?

    The recipe sounds pretty good~ certainly worth trying out. I enjoy ‘lean’ breads, and am quite sure a pain au levain without sugar and oil would work well.

  6. I meant to say I use a similar but simpler oven with my windshield cooker. I use two pyrex salad bowls, (12″ diameter) held together with 3 large binder clips, with a ten inch enameled aluminum cassarole and lid within. This actually holds three complete chicken legs and two cups of rice with veggies, or a smallish chicken over rice, or three one pound potatoes for baking, etc. About enough for three fairly easily. I would think with a small riser I can use this with your oven pretty well.

    The oven thermometer which allows me to see temperatures while baking usually indicate an internal oven temp of around 300F. With your cooker I might be able to get that up another fifty, and that would definately allow for good baked bread.

  7. GW:

    I am curious about the nature of your “windshield” oven. Do you use an automotive glass windshield that is covered with aluminum foil? I seems the curve of the windshield might work almost like a parabolic, producing a better focus.

    The good thing about parabolic focus is that you can get extremely high temperatures. The bad thing is the high temperature is focused on a very tiny spot. Taken to an extreme, a thought experiment shows that you could quickly blow-torch a hole in the bottom of an iron skillet without even frying the egg inside. I joke, of course, but your get the point. In a practical oven, the focus only needs to get most of the solar flux into an area the size of your pot assembly.

    Another problem with tight focus is that it takes almost constant attention to keep the oven aimed at the sun.

    If my memory is correct, with optimal conditions I measure about 380 F degrees between the black lid and top inverted glass bowl. Even without clouds, if there is a haze the condition is not optimal. I have never had trouble baking bread on cloud free days. On hazy or partly cloudy days it might take a little longer but the bread always bakes.

    The details of where to place the oven bowls with respect to the rear hinge or how high to place the bowls over the bottom reflector was looked into when I did some computer simulations of my type of oven. The graphed results of those simulations can be found elsewhere in this blog. Poke around a bit.

    I always place my bowl assembly as close to the rear hinge as I can get it. That means it will be just touching the vertical reflectors.

    It is almost always desirable to elevate the bowls over the bottom reflector. I do this by sitting my bowl assembly on another inverted glass bowl – the smallest from the nested set. Just how high to elevate depends on the height or angle of the sun above horizon which in turn depends mostly on time of day. How high the sun gets at noon depends, of course, on your latitude and the time of year. Since I do not have a way to continuously vary the elevation, I just let the one fixed amount do.

    When I am cooking, to compensate for the changed position of the sun, I tweak the reflector positions about every 15 or 20 minutes.

    Your oven bowl assembly does sound simpler. I would like to try that sometime myself. The concern I have is that the metal pans inside would seem to make contact with the outside bowls. This would seem to me to make a thermal leak that would keep the internal pan from getting as hot as it might if kept thermally insulated from the outside bowls. That is why I chose to support the inner pan with a cardboard ring around the top edge of the pan. The cardboard ring makes better thermal insulation. I suppose you could put a wooden support in the bottom, but that sort of support might shade the pan from the solar influx from the bottom reflector.


  8. the windshield cooker can be found at the solar wiki very easily. I think it is a form of modiifed parabolic cooker, but it acts more like a cookit. I do get quite high temperatures, which is why I use the pyrex salad bowls, they do allow for the heat one gets. But, the environment the bowls create do allow for 300F temperatures quite easily. I noted that with the condensation from the bread baking did reduce the heat, though, so will have to come up with another method, as I do wish to bake bread at 300F or higher. I think it makes better bread, actually…

    Biscuits, etc. should be able to be baked pretty easily, though, as one can really reduce liquid in a biscuit recipe.

    I have found a possible solution to creating a whole new box cooker, though, such as your kaleidoscope cooker. I will have to go find the authors and title again, but one of the cookers demo’d in the book was a windshield cooker on top of a cookit, and held with binder clips! It looks quite similar to your cooker, not as refined, but easy to do. I will want to try it out, and see how it actually performs, but it offers a higher back than my regular windshield cooker. I have three cookits, in two sizes (one about a hotpot sized)and it will be easy to try this out. I may have to buy more binder clips though! I use the extra large industrial sized ones…


  9. Gina:

    I hope you keep me posted on your results.

    It has been quite a while since I researched solar cookers on the Web. Guess I’ll have to do that again soon to see what is new out there. Lately, most of my time has been used in research on the design of simple radios that seem to run “forever” on “dead” batteries.

    This year has been cool and wet in Kentucky. Even if it does not rain, it seems the days are cloudy and overcast. So, this year I have been using the efficient wood-burning cooker called the “rocket stove” more than my solar oven. I discovered it is easy to pan-fry biscuits in an iron skillet over flaming twigs using this stove. I do cover the skillet with a lid to make it work more like an oven.

    Speaking of biscuits, I like them dry or soft. But, if you want to make them really soft, the secret to getting the dough to hold more moisture is to add sugar. It might not be good for you, but it sure makes them soft.


  10. The book where I found the picture of the windshield attached to the top of a cookit is “COOKING WITH SUNSHINE” by Lorraine Anderson and Rick Palkovic; pg 187. The recipes aren’t bad either.


  11. i’d love the instructions!

  12. Well, basically, they took a “cookit” (you will have to go hunt up the pattern for it over at solar wiki) and then took a windshield cover, one of the mylar or foil ones, and attached it to the top of the cookit with those large binders. I am still struggling with the photo, as there are no instructions! sorry…
    But it adds height to the cookit which then makes it appear a great deal more like the kaledoscope oven described above.

    Let me see what I can do, and when I get it working I will take pictures and send them here. Perhaps they can be posted? but it will take a week or two. No problem with cooking outdoors right now, our drought takes the form of few clouds whatsoever.

  13. I have a picture I can send you, jpg. Do you have a mail I can use to do so? Its b&w, not that large, and shows clearly the addition of the windshield to the Coookit, from ch 6 Cooking with Sunshine.

    Let me know!

    • i LOVE this oven and would LOVE to get directions and what is needed toto make one. what do u mean by “a mail” for the jpg? please send the link if u can.


  14. hey since you guys know a lot about solar ovens can you do my homework:

    1. How could you increase the efficiency of this cooker? Describe 2 or 3 design changes that would help this cooker get hotter faster, and keep its heat better.

    2. What is the purpose of having insulation (shredded paper) around the baking chamber?

    3. What would happen if you painted the inside of the baking chamber white instead of black?

  15. Nice

  16. I gotta favorite this internet site it seems invaluable invaluable.

  17. Alan, very , very eclectic material. You might consider putting your art on the blog also.

  18. Hi Curlydock,

    I know this is an old thread, but I have only just come across it.

    I’m at 50 deg N here in the south of the UK, so less insolation to play with, hence my interest in optimising any solar oven I build.

    I’m struggling with the ‘panel angle problem’ myself, and I find your reasoning fascinating, and I’m pleased to see that you’ve provided proof of concept by real world experimenting with the angles.

    The ‘kaleidescope’ argument seems compelling on the face of it – if the sun can ‘see’ more of the surface area of the bead, then collector efficiency will increase. This makes sense, as if you put a bead at the focus of a parabolic mirror, the sun would ‘see’ a very large bead that would take up the whole surface area of the mirror. Thanks for that insight.

    It’s windy round here, and tall thin structures disappear very quickly so some compromises are warranted.

    I’ve been thinking of Pyrex bowls myself, and struggling because I thought I would put a matte black cast iron casserole inside, but it’s difficult to find one with a decent capacity that will fit inside, mainly because the handles stick out – think Le Creuset. However, a blackened stainless steel container is is great idea. Thanks for that.

    I have a couple of suggestions that might improve your set up – but really, it’s the application of the same suggestion twice. Your small inverted base bowl could be replaced by some sort of wire trivet of suitable height. I think there is a lot of internal reflection going on in that inverted dish and a trivet might be better. I also think you could put a trivet inside your outer pyrex bowl to replace the cardboard – it would conduct very little heat because the legs would have very small cross section, and it would cast tiny shadows from the upgoing light reflected from the bottom mirror. I’m talking thin wire trivets here, not heavy cast ones.

    Thanks for the insights, and I’ll look forward to reading any further results you publish.

    Dave (UK)

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