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Forum Index : Solar : Solar Box Cooker
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MacGyver Guru  Joined: 12/05/2009 Location: United StatesPosts: 1329 |
Crew Here's what I've been up to today. I built another solar cooker. All I want it for is to bake cake. I designed a parabolic trought cooker last week, but it's too hard to keep focused and it gets too darned hot at the focal length. In not wanting to come up short, I designed it with a bit of overkill, but it proved too much, so I scrapped it. I used some of the reflective material from the parabolic trough mirror in this project. Solar Box Cooker This all started because Harbor Freight put their medium-sized aluminum technical tool box on sale for $19. I couldn't resist; it was my civic duty to buy the silly thing. The first thing I did was rip out the tool-holder part in the lid. I've placed it back in place temporarily so you can see which tool box I bought: 
Here's where we're headed: 
I have it perched on top of my new workmate table I purchased online for $29; another deal I couldn't resist. 
The insides are nothing but chunks of Styrofoam building insulation. I cut the 1/2" sheets so as to have them "wedge" fit against each other on the inside of the box all the way around. I overlayed these panels with pieces of reflective material cut from a dashboard reflector -- $5 at Pep Boys Auto Parts. 
I left the two inside end pieces of reflective material a bit loose. The idea here was to allow its flexibility to redirect sunlight making it in that far, towards the center of the cooker where the cake is. 
I cut out a piece of Lexan plastic sheet the right size to cover the inside of the cooker with enough left over on its edges to allow it to rest on the reflective-covered sides. My tape measure is sitting on the Lexan sheet since being clear, it otherwise doesn't show out much. 
This is another sheet of Lexan, which I've yet to cover with reflective material (tomorrow). I drew a line on it so you can see it better in the picture. I'll construct two of these, one for each side and they will be set at about a 50* angle from vertical so as to redirect sunlight to the center of the cooker. The whole rig will sit propped up at an angle aiming right at Mr. Sunshine. By the way, I cut the Lexan so all three sheets fit inside the lid where formerly tools had been housed. 
Here it is all packed up and ready do go on a picnic. As soon as I cover both side reflector wings and figure out a way to prop them in place while things cook, I'll do just that. I'll bake a cake and come back here with an update. On a clear day, this little solar box cooker should sustain 350*F. . . . . . Mac Nothing difficult is ever easy! Perhaps better stated in the words of Morgan Freeman, "Where there is no struggle, there is no progress!" Copeville, Texas |
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Tinker Guru Joined: 07/11/2007 Location: AustraliaPosts: 1904 |
It may do that Mac, but can your styrofoam insulation or the lexan top cope with that temperature? 
Try something not too delicious on the first time baking test, I somehow imagine lexan topping on a cake does not do much for the taste buds
Perhaps rockwool insulation and an inverted Pyrex glass baking dish as a lid might do better in the long run  Klaus |
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| MacGyver Guru Joined: 12/05/2009 Location: United StatesPosts: 1329 |
Tink Hmmmm . . . Gosh; I don't know. I've made one of these before and used Styrofoam then with no problems, but I used a glass top rather than Lexan. I've used Lexan on one of my solar water heater panels and it didn't melt or deform, so we'll just have to wait and see. If it does impart a taste to things, I could maybe market it as a new flavor. I'll call it "Plastic Fantastic" and maybe all the former hippies will purchase it just to see what it's like, eh? 
If this turns out to be a problem, I'll rebuild the sucker and post the changes. I did this project mainly just to have a solar oven handy and not have to put up with storing something the size of a beach umbrella unfolded. As you know, I live on my land yacht and space is a premium. . . . . . Mac Nothing difficult is ever easy! Perhaps better stated in the words of Morgan Freeman, "Where there is no struggle, there is no progress!" Copeville, Texas |
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| MacGyver Guru Joined: 12/05/2009 Location: United StatesPosts: 1329 |
UPDATE I've made a few changes and will make a few more, but I thought I'd update things a bit to show my progress: 
I decided to use real glass mirror instead of the plastic stuff. The glass gives a more-efficient reflection. 
I also changed the size of the side-mirror supports. Seen here, small Velcro squares were attached to both the back of each side mirror as well as to the piece of sheet aluminum. I bent each piece to achieve the reflection targeted on my baking pan inside the cooker. The Velcro steadies the sheet of glass mirror in the wind. The bottom edge of the glass is 'wedged' between the case and the insulation; the Velcro is just for good measure. 
Since the angle of the rear mirror is somewhat "fixed" because it's sitting against the tool case lid, I decided to use it in it's long-vertical- side position; it gives a better reflection target. One thing to keep in mind here is WIND can be somewhat problematical and if you're baking cake on a windy day, you might want to babysit it whilst it cooks. 
This little solar box cooker seems to get only to 300*F. The directions on my cake mix box say 325*F. I guess it will just take a little longer. There were some comments from the crew about using the reflective plastic inside this thing, but even at a sustained temperature of 300*F I noticed no discoloration, smells or smoke from anything. The Lexan sheet I'm using as an insulation shield got a little droopy in the middle and I may replace it with a sheet of glass, but then again, maybe not. When I pack this up, all three mirrors fit within the top lid and I pack them securely using the former lid reflector sheet of platic. A double-insulated center would likely allow things to get hotter, but I'm happy with 300*F for now. All I want this for is to bake cake. One thing else I'm going to construct is a beach-sand-filled base for this whole rig to sit on. The reason is, beach sand is silicon and is about the BEST insulation material on the planet. I noticed when I went to pack things up for storage, the back side of the case (the bottom when it sits on a table top) was pretty roasty-toasty. Apparently, a lot of heat is escaping there and that may be why this little cooker peaked out at 300*F. Tomorrow, if I'm home between 10 a.m. and 2 p.m. I'll bake a chocolate cake and post the results. I think I've found a way to garner free energy for baking, now if I could only do as well finding some free cake!
Edit: typo . . . . . Mac Nothing difficult is ever easy! Perhaps better stated in the words of Morgan Freeman, "Where there is no struggle, there is no progress!" Copeville, Texas |
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| Tinker Guru Joined: 07/11/2007 Location: AustraliaPosts: 1904 |
So, how was the chocolate cake? 
I do a far bit of baking on my boat in rather primitive conditions, a trick I learned is that the heat needs to be as little fluctuating as possible for a consistent result. For that I would suggest including some additional thermal mass, like an outer copper dish to hold the cake tin. My boat oven is double walled, inner wall made completely out of 1mm copper sheet and a ss outer skin, about 12mm gap between these. 300 deg F is OK for cake and cup cakes (if you find a dish to fit) but its too low to bake decent bread. Have fun. Klaus |
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| MacGyver Guru Joined: 12/05/2009 Location: United StatesPosts: 1329 |
Cake Update I baked a chocolate cake today in my little solar box oven. By the way, I replaced the clear Lexan sheet with a sheet of glass. I also mixed up about a half gallon of plaster of Peris and poured it into the center chamber. This insulates the bottom of the cooking chamber and keeps the heat from migrating as much as it did without it. I was going to use beach sand, but got lazy and did the quick-&-dirty. Here's a rundown the best way I know how: Good News: It "looked" done, tasted great, did not burn or stick to the pan and was an easy clean up. Not So Good News: Only the top and outter third turned to cake. The rest stayed batter, the consistency of which resembled baby excrement!
I'm tryin' it again tomorrow, but I'm going to adjust the recipe. Today's batter had too much moisture in it. There was so much in fact, little water drops formed on the underside of the clear cover glass and dripped back onto the top of the cake. That made for a very lovely decoration, but it hindered things getting hot enough. The recipe said cook it for 35 minutes at 325*F. Well, okay; it was 325*F when I popped it in there, but the temperature stayed an even 225 for the entire baking time. Hey, that's only 33% off the mark; what do you want, eggs in your beer? We had a bit of overcast today, so I had to wait until about 12:30 to start. That meant the little oven only had about a 2 hour window for good heat. Tomorrow, I'm thinkin' of tryin' brownies, because they have less moisture to start with. I'll also try to get in more baking time, but that depends a lot on the weather. I think with a little recipe adjustment and a longer baking session, this puppy will fly. It may not be the E-ticket ride for cake, but in a jam, it would surely cook your supper! . . . . . Mac Nothing difficult is ever easy! Perhaps better stated in the words of Morgan Freeman, "Where there is no struggle, there is no progress!" Copeville, Texas |
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| Tinker Guru Joined: 07/11/2007 Location: AustraliaPosts: 1904 |
Mac, my advice would be to follow the instructions on the cake packet to the letter - I take it you used a packet cake mix? Re the moisture, it indicates your oven is too airtight. You need some vent holes on top to let the steam out by the sound of it Klaus |
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| MacGyver Guru Joined: 12/05/2009 Location: United StatesPosts: 1329 |
Tink Yep, I'd agree with you there. Today I tried making Solar Brownies. Again, I used a box mix and dumped the whole thing into the baking pan. Things went great and it looked like a yummy batch of brownies was coming my way. More like a yummy batch of BRICKS!
Now I understand the science behind making clay bricks. The brownies tasted just fine, but the top quarter inch as well as around the rim was stone hard, so much so, that I was affraid the plastic fork I used to cut things into squares was going to break under the pressure it took to cut through the silly things. Hmmm . . . . . Tomorrow, I'll tryi it again, but I'm going to use only half the mix. I'm also going to add a vent hole to let off the moisture as well as cut the baking time in half. Whereas yesterday I baked for about 2 1/2 hours, today I let it go a full 5 hours! I'll get the hang of this yet and when I do, I'll publish the CORRECT way to do this. Bricks or not, it's still lots of fun cooking with only the sunshine for heat. . . . . . Mac Nothing difficult is ever easy! Perhaps better stated in the words of Morgan Freeman, "Where there is no struggle, there is no progress!" Copeville, Texas |
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| MacGyver Guru Joined: 12/05/2009 Location: United StatesPosts: 1329 |
Solar Brownies I took Tink's advice and added some metal to the interior of the cooker. I used a piece of tempered aluminum sheet, painted flat black with stove paint. This increased the static (standing alone unloaded) temperature from 325*F to over 350*F so I figured solar brownies were now a shoe-in. It would seem my affinity for "small" has gotten me into trouble yet again. This little solar box cooker, though it works really, really well, is just not large enough to effectively "bake" stuff. Even inverting my $8 Onida baking pan over another metal baking pan holding the batter didn't help much. A picture's worth a thousand words, so here are 2,000: 
This is the result of 3 hours at 250* (as hot as it would get) when the box said to bake the brownies at 325*F for a lilttle over half an hour! Hey, they "look" good. 
I ran a plastic knife through them before they cooled (I know that's a no-no, but I was anxious). Unfortunately, as you can see from the picture, my solar brownies are a bit too much like me; soft in the middle! I think I'm going to give up on baking stuff and use this for stews and roasting chicken and the like. I suppose it's in my best interest that it wasn't a huge success, cause lord knows, I really don't need to be munching on cake and brownies 24/7. Still, it's a compact and useful tool that will be used at beach parties to cook hot dogs as well as around here to make "crock pot" stuff now and then. . . . . . Mac Nothing difficult is ever easy! Perhaps better stated in the words of Morgan Freeman, "Where there is no struggle, there is no progress!" Copeville, Texas |
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NathanM Newbie  Joined: 23/08/2011 Location: United StatesPosts: 5 |
I was thinking it would make a good crock pot too. It's not hot enough for bread or brownies but perfect for cooking some meat all day in it. I'm sure at the end of a good days sun, it would be perfect. Also you wouldn't have to worry too much about the moisture as that would help keep it tender and not dry out. Need San Francisco electrician |
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| Georgen Guru Joined: 13/09/2011 Location: AustraliaPosts: 462 |
Mac, From what you've got looks that reducing volume of mix, eventually you would give good result. Definitely mix drains too much heat. George |
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philb Regular Member  Joined: 05/07/2008 Location: United StatesPosts: 96 |
Good job! You are almost there! You might try to put a small rack under your pan. That way the hot air can circulate around it. My sun oven had a rack. It seemed to cook everything evenly. It also had a black interior to help keep the heat in that area better. On a sunny day, it would boil the beans over while sitting in the snow. philb |
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