Instructables
Picture of The Office Worker's Portable Solar Oven
In keeping with the grand tradition of creating ways to warm my lunch at work with readily available office supplies, I present for your review, dear reader The Office Worker's Portable Solar Oven.

 
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Step 1: The Situation

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Due to circumstances beyond my control, my cubicle at work is close to both the break room and a small loading area on the second floor. Because of this I usually have a wall of boxes filled with copier paper situated somewhere behind me. Aside from this being a nuisance when it comes to concentrating on my job it also affords me the opportunity to scavenge as many copier boxes that a man can get his hands on.

This is a perfect situation to improve upon my earlier solar cooking ideas.
See here: http://www.instructables.com/id/Dashboard-Chilli/

Step 2: The Result

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Behold! My dashboard cooking days are over. I present to you The Office Worker’s Solar Oven. This project was built completely with materials laying about the office, with the exception of tinfoil, which I picked up at the dollar store.

Step 3: Gathering the pieces

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Above you can see all the precut pieces to form the main cooking chamber. I cut the box at an angle of 20 degrees to allow for maximum exposure to the sun. In my latitude the lowest elevation that the sun gets is 26 degrees above the Southern horizon.

Be sure to cut your box accordingly if you plan to make one.

Step 4: A little design consideration

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I designed the oven to allow for one inch of insulation on each side of the box. To do this I placed slots in the inside liner. This is the back piece.

Step 5: More design consideration

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These are the sidepieces, slotted as well. I also cut these with an angle of 20 degrees.

Step 6: Some Assembly Required

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As you can see the slotted pieces fit together nicely.

Likewise the liner fits nicely into the oven box.

Step 7: The Insulated Floor

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Once I was sure that everything fit together, I removed all the pieces and put a one-inch spacer in the bottom of the box to keep the bottom of the oven above a layer of insulation.

The insulation I used was just shredded documents I salvaged from the office shredder.

Step 8: The Reflective Surfaces

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Once properly stuffed I rapped the bottom liner of the oven in tinfoil and installed it into the oven.

I then wrapped each of the side liners in tin foil and assembled them.

Everything fits together perfectly.

Step 9: More Insulation

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A little more insulation around the sides.

Don't forget all the nooks and crannies.

Step 10: The Window

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For the port that allows the sunlight in I basically just created an insert that would fit into a regular box lid with a hole cut into it. I used the same trash bags they use to hold all the shredded documents and stapled it to a piece of cut out box.

This when gets inserted into the box lid with a hole cut about the same size in it.

Viola! The Office Worker's Portable Solar Oven



Step 11: A Question of Efficiency, oh and a little math to.

This is all fine and dandy but what kind of power and efficiency are we talking about here. I mean can we bake bread, pasta, and stew?

I figured that for the first “light” of this oven it might be a good idea to run a test before I place a bunch of food in it and end up with a big mess and a half a dozen people laughing at my ruined lunch.

This will require some thought.

I will need to know a few things:
• How much potential wattage I can get out of a given area of sunlight
• How much wattage I AM getting out of a given area of sunlight
• How efficient is the set up based on these two values

To get potential wattage is easy.

The dimensions of the port that allows sunlight in are 40.64cm by 21.59cm. This equates to 877.4176 sq cm.

Each square meter of sunlight has a potential of 1000 watts of energy in it when it reaches the Earth. Each sq meter is 10,000 sq cm. So 1000 watts divided by 10,000 sq cm gives up .1 watts per sq cm.

So if we factor in the viewing area of the solar oven 877.4176 sq cm multiplied by .1 watts we get 87.74176 potential watts for the solar oven.

Potential wattage I have come to find out is often a pipe dream left for those that believe in endless amounts of power that can be conjured through a philosopher's stone at the stroke of midnight when moons are properly aligned. So I don’t readily buy into the thought of my copier paper box being able to harness 87 watts of power by merely being pointed at the sun.

I needed a way to measure the true wattage of this oven to be able to determine cooking times and more importantly what I could cook.

The easiest way to achieve this is to measure calories. This is done by multiplying the temperature change in Celsius by the mass of pure water heated in grams.

So I need to heat some water and measure the temperature change. However temperature change does not happen instantaneously, it takes time so that will need to be factored in down the road at some point.

Each calorie is equal to one degree centigrade increase in one gram of water. So with a given amount of water and the temperature change in the oven over a given amount of time, it should be enough to calculate its power.

Step 12: The Test Subject

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For this I needed a small enough vessel for water. The container for the V8 I drink everyday will work fine. However I will need to paint it black to absorb the maximum amount of heat within the oven.

Here we see my test setup. The can has been painted black and filled with 163ml of filtered water. I plugged up the opening with a cork and stuck the probe of an oven thermometer in the center of this.

Step 13: The Test

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And finally we see the entire test setup sitting out in the sun. The test consisted of leaving the entire apparatus in the sun for one hour and recording the temperature every moment.

This actually caused a small panic at my place of employment. Apparently the warehouse manager saw what looked to him as a strange device sitting out by one of the vehicles and it aroused a bit of concern. Lucky for me I managed to get a hold of him before anyone was called.

Step 14: The Numbers and Of Course Math

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This is a graph of the progress.
The base temperature of the water was 77 degrees F. The time was 12:01 EST on 05/29/2008. The maximum elevation of the sun for that day was 71.8 degrees above the horizon. As can be noted there seemed to be a linear progression in regard to temperature increase. As I sat and observed I saw that the temperature went up one degree every two minutes. By the end of the hour the water was at 113 degrees. I would assume that it would have been higher if it were not for some clouds in front of the sun at the end of the hour.

To calculate the power of the oven I first needed to get the temperature change in Celsius.

77 F = 25 C
113 F = 45 C
Temp change = 20 C

163ml of water = 163 g

20deg * 163g = 3,260 Calories

Next we need to convert calories to joules. 1 Calorie = 4.1868 Joules.

3,260 Cal * 4.1868 = 13648.968 Joules

Now that we have the power in Joules we need to factor in the time to get wattage. Se we divide 13,648.968 Joules / 3,600 seconds (1 Hour)

3.971 Watts. Not very much. Hell even an Easy Bake oven uses a 100-Watt light bulb. But in the end one must remember that the heat is cumulative. The better the insulation the more heat will acquire and the better things will cook. It’s sad to report that out of 87 potential watts from the sun for the given area only a little fewer than 4 watts was produced. It would appear the overall the oven was only about 4.5% efficient

If nothing else this taught me a valuable lesson in the design considerations and power calculations necessary to build a more robust unit.

But will it cook food? Build one and see.

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diy_bloke11 months ago
with regard to the reflective aspects of alu foil, that is a much 'heated' discussion. Apparently it is abt 50% and with that it is less than a white surface. Perhaps it would have been better to just line yr oven with white paper. That would have saved a run to the dollar store. :-)
z0rb (author)  diy_bloke11 months ago
You are correct, and in reality the list I can now form that would improve this project is far too long for me to even consider at this point seeing as how I have moved on to other things, namely
http://zerosumhacker.blogspot.com/
:D
carl87gt2 years ago
Fantastic instructable! I wonder if you would get more heat by painting the interior black to absorb light and heat to the box would work better than using foil. You still see people (thankfully not in my neighborhood) using foil on home windows to reflect light and heat away. You could use an outside hood (outside the main box) covered in foil to direct the light in (like a satalitte dish focusing on a smaller point) and use black paint on the inner box to absorb the heat.

I may have to try this and report back my results.
z0rb (author)  carl87gt2 years ago
I think it all depends on where you want the heat to go and what you want it to heat. It was my intention to bounce the light around inside the box and have it ultimately hit a black target that would absorb the heat.

In retrospect I see several areas for improvement of the model I presented here. But in reality I wanted to limit myself to what I have available around the office and of course I wanted to have fun.

Thanks for reading
yanksguy3 years ago
Fantastic Instructable! Thanks for all the pics, the well explained directions, the quantification, and most importantly, for your time.

I would like to echo what Haw1horne said: Heating plastic is a bad idea unless it is designed for heating. You may not see or taste the outgassed products, but they are there. Other sites also say that if you can't use glass then the baking bags will work. I think a layer on the inside of the cover sealed with a cardboard edge, and another layer on the outside of the cover also sealed. Of course, pull tight to reduce diffraction.

An aside to the previous; careful what paint you use. Some are not safe to be heated. Look for those labeled as non-toxic stove paint.

A glass top would help keep the lid sealed because of its additional weight. If you go with the turkey bag window route, I'm thinking paperclip and rubber band anchoring system around the outside would keep things tight and allow for less heat loss. My guess is this is where your heat loss came from.

An angled lid lined with foil (aluminum, tin, lead, mylar, whatever) would catch more light and protect the works when all finished and folded down.

I'll start my oven tomorrow. Again, fantastic job with this and thank you so much for your time and effort.
Great...



have a nice day
elescape4 years ago
I found a typo, "rapped" should be "wrapped".
_Scratch_4 years ago
if the insulation is to keep the heat in, i would suggest the paper shreds AND, a  layer of mylar emergency blanket cut accordingly to fit the walls. because it reflects heat it is a good material to use for this, you can get them for like 7bucks online.
this really is great thinking and i think that we should also try and develop more because  in this era we have to give much importance to solardevices as they're cheap and best
ddmg4 years ago
 in the end, does this oven work ?
TRIPLEC4 years ago
you, sir, are EXTREMELY good at making instructables. you were able to show how to make an oven (although you didnt really provide enough detail, but im sure the pictures are good enough for someone to tell what to do) andyou even did the math for us to show how hot it gets and all the important things the oven does. i salute you  =]]
Aluminum foil. Tin foil was around when our grandparents were children. :)
When I was a young whippersnapper.... My 1955 encyclopedia lists the three most common uses for lead: lead solder, lead paint, and lead foil. Ha!

I always call aluminum foil "tin foil" though, just like my father always called the refrigerator the "ice box."

Good lunch warmer, Z0rb. We get lots of sun here. It will work well.
z0rb (author)  StrangeRover4 years ago
Everyone is a critic :P
ranmawolf z0rb4 years ago
Hehe - yep o_~


True! haha - although for information's sake (in case there was ever any kind of specific need for *tin* foil as opposed to aluminum), they actually still make tin foil these days - you can find it at florists shops that carry it. It's often preferred over aluminum at such places for some reason or other.


Or even Aluminium if you don't hail from the US. Just thought of it after reading the syllables comment below :)
It's quicker to say 'tin foil'; then you have three extra syllables to spend on something more important!
FYI the term aluminium foil is not the common terminology in every part of the world
It's not quite the same principle, and meant for slightly different application, but have a look at this:

http://solarflower.blogspot.com/2009_09_01_archive.html#6825927391729345306

It's a compound parabolic concentrator. If you scaled it up a bit, insulated it and put your food in a black container at the focus it would act as an oven.

The angle range will give you about 4 hours without having to reposition and the degree of concentration is about 10 times when it's pointed directly at the sun, down to about 6 two hours either side.

I spent quite a bit of time with a cad package, this is the most efficient design I could come up with.

And use reflective mylar, you can get it cheap from hydroponics stores and is about 50% more reflective than tinfoil.

HobbyistX4 years ago
 I probably don't have to warn you that that temperature range is the one they incubate petri dishes in.  :D
Oh-HO! GOOD POINT! ahahahaha

Instead of cooking food, it might accelerate microbial growth! ICK! lol


ranmawolf4 years ago
I wonder how much hotter it would've gotten in there, if say, instead of the trash bag plastic, you used one of those cheap, acrylic/plastic, 8.5x11" Fresnel lens thingies - they don't jjust focus light, but heat as well... I think I'll try that at some point between instrumennt projects.

Dude, I really gotta say (in somewhat paraphrased agreeance wth fishhead445 had to say), I thoroughly enjoyed this entry! Clear, concise, highly informative, well documented data, and witty as alll freakin' hell! THANX! ^_^

HAIL!!!! ^_~ 


I think you'll find that lenses DO just focus light. The heat is created as a result of that light, so obviously by focusing the light to a smaller area > higher intensity > greater heat. However, the advantages of that increase in light intensity would be restricted to the focal point, I think, as it's the same amount of total light going through the window/lens.

The real trick here, I suspect, is maximising insulation on all sides and the area of the window, while minimising total volume. Afterall, the more air inside, the more air to heat up.
You can only make it so small, though, before you're not collecting enough light to make the efficiency worth it. Wouldn't a more finely-tuned reflection chamber help more? Most solar ovens I've seen use a parabolic surface to not only collect sunlight, but also focus it on the object to be heated. I haven't seen one that's just  a reflective box before.  (Not that I know much about solar ovens or have researched or anything...) Is there a reason that wasn't done here, I wonder?

I wonder how much light gets lost from diffusion by the trash bag, too. Probably not much, but it would be interesting to compare it with glass. (There's gotta be a broken flatbed scanner around the office somewhere...)


Yes, true - and as a result, it does focus heat as well as light... a bit of an "accidental quality" perhaps, but yeah. Seems you and both I agree on this, it's just that I'm just saying things kinda weird here hehe ^_~

But yeah, what with the way a Fresnel lens focuses light into a concentrated beam (much as lighthouses do), so does the heat concentrate, as well. Like with one of those Fresnel lens panels that can be gotten through Edmund Scientifics that can melt asphalt, for instance. As far as a focal point, with a Fresnel lens, it's a beam (again, like with lighthouse lenses, which are of the Fresnel type), so there's not much as far as spacial restrictions go, I would imagine. But I could be wrong. I'm no scientist lol

True that, on the insulation... but with a focused beam (as opposed to an apex/singular focal point) and spacially-throughout-heat-intensity, there *may* be a need for more heat resistant materials... hmmm... insulative fiberglass, perhaps? Like the kind one can find in one's attick? Maybe... 


aaronjehall4 years ago
seems like you could just paint it white inside, instead of using tinfoil.  you could use white out...
desertdog4 years ago
Very well executed and explained.  I wish some of my friends who rely on data for experiments, that most of the time is not accurate in the real world, will take the time to read yours.
Mig Welder4 years ago
haha I built one of those welders too.
you should see if you can finish it. they're awesome! especially when they're cheap;)
z0rb (author)  Mig Welder4 years ago
 Well unfortunately it would simply not have enough power to do the trick.
Mig Welder4 years ago
exactly what fishhead455 said!
hey that rhymes!
anca4 years ago
Fabulous. I especially love the math at the end.
Mig Welder anca4 years ago
yeah same I love the math
M F anca4 years ago
you would have gotten much highter temperatures faster, if you had painted the inside black; instead of using reflective foil. Give it a try and give us the new data figures.
peacenique4 years ago
Ha!  Too funny.  Good to know someone was concerned enough to worry though.  I'd like to be his next-door neighbour!
Haw1horne4 years ago
Looks like you're having fun!

There's 3 complications with your implementation of the solar box oven (variation of the Kerr/Cole Oven patented in the 1980s)

One is the plastic glazing. If the oven did get hot enough, that sheet would very likely melt, warp, and/or outgas. This could be bad for your lunch! In any event, its probably not a food-safe grade of plastic if you found it around the work place. One work around is to use food-grade turkey bag plastic. Stretch it tight across a frame, and mount the frame in the lid.

Second is the plastic glazing. Yes, I know that sounds redundant. The biggest source of heat leaking is going to be out through that plastic. You can improve this by either using glass or lexan for the glazing, or by creating a double layer of food-grade plastic with a 1/4"  gap in between the layers.

Third, most box ovens made for climates outside the Tropics have an additional reflector to help boost the heat  up to cooking temperatures. Cookers that don't have the additional reflector are (1) Made for the tropics (2) Use double glazing (3) Use high-R polyisocyanurate insulation in the walls.

the grocery store has inexpensive oven bags- if you put your food container (painted black) inside the bag, then capture air around it and close off with a rubber-band, you get excellent insulation (and easy cleanup in the event of a spill).

also painting the interior black might help, as would raising your food container so that the light reflected off of the foil will hit all sides of the container
sleepydog4 years ago
Nice build! I'm impressed that you made this with all reclaimed materials (and time) from work. There's nothing like a little creative goofing off on the job to keep your mind fresh. Thanks for doing the math. It was a very professional touch and it makes others think about their project in terms of measurable success Keep up the good work..
Kaiven4 years ago
Very good math sir! A project I should look into, if I ever need it :P
z0rb (author)  Kaiven4 years ago
Well the math is actually pretty simple, and the concept only took a little research to find. I just laid it out. But thanks :)
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