Solar Box Oven

Intro: Solar Box Oven

I fiddled around with parabolic dishes and fresnel lenses but in terms of cooking, I couldn't do much more than fry an egg or boil a couple cups of water. So I decided to take a shot at a more efficient way of solar cooking reasonable amounts of food like cakes and poultry. To do so, I chose a box design with reflectors placed all around it to gather more light and energy into the cooking area. It was a relatively easy build and it cost me no more than 30$ and I was thrilled by the results it gave me!

STEP 1: Materials

A large mojority of the materials I used were gathered from friends who had them lying around or who wanted to dispose of them.
What I used:
Tools

• A circular saw and its guiding rail
• A glass cutter
• A jigsaw (with metal cutting blade)
• A compact drill
• A grinder

Materials

• 8 mm thick plywood
• 18 mm thick plywood
• glass pane from a picture frame
• Glass or arcylic or stainless steel mirror
• Silicone caulk
• Metal bars
• Screws
• 2*4 or other wood pieces
• A length of seal (strip seal for doors)
• High termerature black spray paint

STEP 2: Choosing the Dimensions

I chose my dimensions taking into account the size of box I wanted to allow for enough cooking space. I used rimstar's super cool reflector size and angle calculator to determine how my reflectors would be set up once I'd decided the size my box would be:
http://rimstar.org/renewnrg/how_design_solar_cooker_sun_reflector.htm
I believe my box is cubic and roughly 35 cm and my reflectors 65 cm long.

STEP 3: Making the Box

This step is simple, you make the box according to the dimensions you chose. I made mine out of 18 mm thick plywood and assembled it with screws and sealed the seams with the silicone caulk to allow as little air leaks as possible. Now, in the pictures you see that the wood planks were painted with high termerature black spray paint but you will later see that this was of no use after a modification was done inside of the box (see in the final steps)

STEP 4: Making the Reflectors

I chose to make 8 reflectors and not 4 because when using only 4, part of your reflector is useless. They were all cut from the 8 mm thick plywood.
So 4 were rectangular: the width of the box and the lenght I had chosen (65 cm)
The 4 other were triangular: They are the same length than the rectangular but the width of the base had to be calculated
 To calculate the width, the math come to the rescue:
 We first calculate the dimensions of the dotted length (see diagram on 2nd picture!). Now that we have the angle (ß) at which our reflectors will be tilted (step 2) we can determine this dotted length using the cosine function:

 Cos(ß) = dotted length / reflector length

     In my case:

 Cos(68°) = dotted length / 66
 dotted length =  Cos(68°) * 66 = 24.7 cm

Now, using the Pythagorean theorem inside the orthogonal projections of the reflectors (diagram on 3rd picture), we can determine the width of the triangular reflector's base:

(Base of triangle)² = 2*(dotted length)²
Base of triangle = 35 cm

(If this is not clear, go ahead and ask your question in the comment section)

STEP 5: Making the Reflector Cone

Now that you have your 8 reflectors cut out, you need to assemble them into a cone. To do so, I cut 16 pieces of 2*4's at a 67.5° angle long of approximately 20 cm each. I screwed them onto the reflectors being careful to align the beveled surface of each piece. Once the 16 guide pieces of wood are srewed, you now have a way to easily connect the reflectors to each other: you screw the guide pieces together from the side!

STEP 6: Adding the Mirrored Surface to the Reflectors

I used glass mirror that I cut myself with a diamond glass cutter. To attach the cut mirrors to the wood reflectors, I used little screws to be able to replace the mirrors if needed. Glass mirrors are heavy and a pain to work with so if ou can, go for stainless steel or acrylic ones!

STEP 7: Praparing the Box for the Reflector Cone

Here, you need to bend metal bars at the correct angle to be able to attach the cone to the box. To attach the bars to the box, I screwed and siliconed them. The photos are probably expilcit enough for you to understand.
The bars were bent using my manly muscles!

STEP 8: Making the Rotating Food Tray

This plate is very important because it allows you to keep the food level when tilting the oven towards the sun. It was cut from some aluminum sheet metal with a jigsaw, it was then painted black and screwed in place inside the box using washers to allow the tray to rotate.

STEP 9: Making the Glass Door

First, glue the seal strip all around the top of the box. Cut the glass from the picture frame to the correct size, the glass I used was quite thin (2.5mm) but it works fine for me. I tried making some double glazing but it didn't turn out quite well... I'll try again some other time.
To secure the glass to the box I didn't use any hinges, I just used pins at all four corners.

STEP 10: Mods and Tweaks

The inside of the box was initially black but I decide to test it mith stainless steel mirror instead of the black paint and for me, the mirrors gave way better results.
Using a progam called Crocodile Physics I saw why it was probably better with the mirrors inside: all the solar radiation is redirected towards the cooking pot (more simply, the food), heating it faster.

STEP 11: It's Time for Solar Cooking Now!

With the mirrors inside, I manged to cook a brownie in 45 minutes in early april, in a normal oven it's 30 minutes so this oven a fast cooker!

If you have any questions, don't be afraid to ask!

Comments

[Frederik74]

how about bread?

[rimar2000]

Very good design, and well made.

At intro you says that parabolic gave poor results, but I think you didn't insist enough with it. I have cooked meat, noodles, fruit, hamburgers and sausage using a mirrors array of about 1m2. All in quantities enough for 3 or 4 persons.

The advantage of parabolic is that you can roast food, not only boil it. A small disadvantage is you need reorient it each 10 or 15 minutes, except that you have a solar seeker.

[rimar2000]

These are images of 2008. Today the cooker is disassembled. The net surface of mirrors is about 1 m2. The weather was cold, midwinter.

[Gael - Tim]

Wow! That's a nice solar cooker. I've never made my own parabola, I've only used TV satellite dishes covered with reflective plastic.

[rimar2000]

Aluminized plastic is treacherous: it seems so bright (and it is!) but makes little wrinkles anywhere. And each wrinkle is like a black spot, bigger that it seems. Glass mirrors are far better, although the problem is the weight. In the last photo you can see that the sun is low, because the lower edge of the array barely catch sunlight. It is midday in midwinter.

[jobias1]

How about water proofing the interior side of your glass window. There are a few products in the market that does that, just to show you it works, try the basic material , wax or paraffin candles, rub it on the inside of your glass window ,make sure that you coat the surface evenly, the tendency of the moisture is drip down from the glass pane preventing the moisture cover on your glass window.

[Karakia]

Great Instructable guys. I suspect the reason you're getting condensation is because the glass door is at a much lower temperature than the food-induced moist air inside your oven. A couple of ways to reduce the amount of condensation...

1. Double or even triple glaze your door - you were already on this track. Multiple glazing will do two things: It will help keep heat inside the oven and therefore speed-up your cooking. The innermost glazing will attain a much higher temperature and therefore any moisture in the air will struggle to condense on it.

2. Cook your food inside an oven bag; keep the moisture contained :-)

Cheers.

[georgep4]

I have found that once the glass has been completely covered in condensation the food should almost be done.

On another note. I am looking to build a large solar oven. I was given a large double pained piece of glass from a window. It has a blue tint to it and I am not sure if it is a true tint to block the sun or if it is just a glaze. I can't peel it off.

Do you think it will work?

[Karakia]

Gidday GeorgeP4,

1. Technical Response

From my high school physics days... the 'colour' or 'color' of anything, including your blue-tinted window, is dependent on the wavelength / frequency of light being REFLECTED off it. Your glass appears to be 'blue-tinted' because it's reflecting the blue wavelengths of the white light that is being shone on it.

Your eyes pick up these reflections, and your brain says "that glass is 'blue'". Something that appears to be 'white' is actually reflecting all visible light, and something that appears to be 'black' is actually absorbing all visible light (i.e. NO visible light is being reflected).

In order for a solar oven to work, the glass being used MUST allow infra-red light (aka 'heat') to pass through it and be absorbed by the black box enclosure, which then re-radiates that infra-red light or heat, heating the air inside the box.

infra-red light is near (actually outside of) the red-end of the visible spectrum, whereas blue light is towards the other end of the visible spectrum (remember 'ROY G BIV' for the colours of the rainbow). Therefore, in theory, there is a REASONABLE chance your glass will work.

2. Practical Response

Get a cardboard box and paint the inside of it black. Put the open cardboard box directly behind the blue-tinted glass, just like you were building a solar oven, and put the whole lot outside in direct sunlight. OR, even simpler, place the glass between something that is already black or near black in colour and direct sunlight.

If the blue-tinted glass allows infra-red light or heat to pass through it then your cardboard box enclosure, or your black object, should get warm/hot.

Cheers.

[Gael - Tim]

I'd have to disagree with this. If you see the glass blue, as you said it's because it reflects or reradiates only blue light; the other wavelenghts are absorbed (green and red and most likely infrared). If your pane of glass happened to be red there would be more chances of it NOT absorbing the infrared wavelengths (which are near to the red ones in the spectrum).
I may be wrong here...

[Karakia]

Gael,

Clearly you were paying attention in Physics. In my (limited) defence, it's been many years since I took Physics classes, although it's coming back to me... :-)

GeorgeP4,

While my general comments about why your eyes / brain perceives an OPAQUE object as being of a particular colour is correct (i.e. due to reflected light), I haven't followed the physics / logic through when it comes to semi-transparent objects such as coloured / tinted glass.

If an object appears blue in colour, then all visible light other than blue light is being absorbed by that object. The blue-coloured light is being reflected off the surface of the object AND, with blue-coloured but otherwise transparent objects, is passing right through the object.

The mistake I made was assuming that with semi-transparent objects, such as tinted-glass, the colours you can't see (i.e. the colours that aren't being reflected) are passing right through the object. As Gael has pointed out, the light you can't see is being ABSORBED (by the tint). Only the colour you can see is being reflected off, or passing right through, the tinted glass.

Therefore, in theory, your blue-tinted glass is likely to be at the wrong end of the (electromagnetic) spectrum and there is a REASONABLE chance your glass will NOT work, in not allowing infra-red light or heat to pass through it...:-(

The above said, given you've already got the glass and it's pretty easy to test, forget 'the theory' and give it a go.

OK, so that's two bum steers from me now!

Cheers.

[georgep4]

Here is the glass I am talking about. Its about 4'4" x 4'2". I am guessing that its not going to work due to the comments above but I just thought a visual would help with the decision. Thanks for all the input.

[Karakia]

I'm not a glazier, so I can't tell just by looking at the glass - sorry. Someone more familiar with tinted-glass, such as a glazier, MAY be able to tell you.

I see you're using imperial measurements for your window size, so I assume you're living somewhere in the States. If you're living in an area that sees extreme temperatures (e.g really hot or really cold, or both) then there's a fair chance the glazing is designed to stop heat either entering or leaving your house (which is no good for a solar oven). Tinted-glass for houses is generally designed to reduce glare, keep heat inside or outside the house, and/or keep UV light outside the house (UV light fades curtains, lounge suites etc.).

I'm only using light theory to GUESS what the glass is likely to do, which I originally screwed-up in any event, so go figure. Even if I do know, or think I know, the theory, I usually test to confirm.

It's a relatively simple thing to test so, if it was me, I'd Nike it.

Hope this helps.

Cheers.

[Gael - Tim]

Thanks a lot for the tips, Before advnturing myself into making some double glazing, I'll try the oven bags (great and simple idea by the way). If I ever do make the double glazing, I'll let you know how it worked out for me :-)

Gael

[Karakia]

Sorry to give you a bum steer, but it occurred to me AFTER posting my comment that you can't use an oven bag after all. The bag would expand with all the steam generated from the food and would eventually explode... :-)

[diyer8]

+1 for solution 2. => quick and easy!

[souichi]

To remove the water vapor.. you may place a fan from the computer or such... since it is going to be sort of air tight it needs an exhaust..

[Gael - Tim]

The oven has to be air tight in order to function correctly since it gradually gathers energy and heat inside the air tight box. Whenever the glass door isn't shut well and that air leaks, the oven almost stops working because of the convection going on between the hot air inside the box and the cold outside air. So the fan is probably not the best option, however thanks a lot for the comment :-)

[The Green Gentleman]

Just blu-skying here, but what about a schrader valve from a bike tire? The amount of air let in should be minimal, and it as the steam would rise, it may allow for some venting if placed near the top of the oven.

Alternatively, a water scavenger, like damp rid (aka calcium chloride) could work. If you don't want to buy something, you can turn a non-toxic hydrate, like epsom salt or even baking soda, into a anhydrous form by baking at the appropriate temperature (wikipedia can be a good info source for this). Leave the anhydrous form in an open dish in the oven when you cook, and it should dehydrate the air fairly well.

[batonas]

maybe double glass would help you to avoid condensation like in the real oven's, first glass would get hot and would be less condensed, simple physics. hope I helped, good luck on your new projects.