View Full Version : Bolster's Solar Oven Log
Just took delivery of a solar oven called the Global Sun Oven. Going to record some of my learnings in this thread. Hopefully you folks with experience will jump in when I'm off track.
So Calif is blessed (and cursed) with lots of sun. Thinking through potential scenarios, it seemed that having an off-grid oven would be a worthy prep. For food of course, but also for water purification. As you know I'm a bit of a 'solar still' nut, and while Calif can run out of fresh Northern Calif water in a heartbeat, we have loads of salty ocean water. A solar still would turn that into fresh. Two of the ovens (below) had slanted-top doors that might allow them to do double-duty as a dandy solar still. We'll see.
Wasn't easy to research, not many people comparing the ovens head to head. One of the few:
The Sport Solar Oven (http://www.solarovens.org/buy.html) was attractive to me because of its simplicity, low profile, and angled double-pane lid. A reported potential for warpage (all plastic construction...my plastic tub solar still warped immediately, so I was leery of that) and more in the crock pot range with top heat of 250-300F. Cost $200 w reflectors.
The other alternative that caught my interest was the Global Solar Oven (http://www.sunoven.com/sun-cooking-usa), angled lid, top heat 350-400F, and an anodized aluminum interior. I really wanted that anodized aluminum interior, and the additional heat didn't hurt, so I ante'd up the $260 and purchased on Amazon.
Showed up in about 4 days, well packed. Simple construction, and I'll admit to a wave of regret when I thought about how I might have built something myself, like Quake did (http://www.glocktalk.com/forums/showthread.php?t=1342755). There are parts of the oven that are surprisingly low tech, like the poplar wood frame to which the oven door and reflectors are screwed with wood screws. But then other parts are far beyond my ability to create, such as the formed, anodized aluminum tub, and the plastic outer tub that is sized to it. There's insulation between the plastic and aluminum walls. The folding reflectors would not be easy to duplicate either; they are polished and then anodized aluminum.
A nice touch is the gimballed bottom to the stove, so the pot/pan stays level despite the tiltage of the oven. And on the other hand, the reflectors are kept in place by sliding over an oval thumbscrew head (ultra low tech and certain to mar the reflector in that area), and the reflectors are latched down with a loose snap strap (that needs to be shortened). There's a spring-loaded leg in the back that tilts the oven to match the sun. This leg is crotchety enough that even the promo videos (http://www.sunoven.com/sun-cooking-usa/how-to-use) could not make it look smooth, but it works. The oven becomes a tripod when you use it.
There are complaints about this oven stinking on eBay. All the greenie chemophobes think it's a deadly "plastic smell." I recognized a smell immediately upon unpacking...it's the oil-based walnut stain they use on the wood frame. I've smelled it during dozens of projects. So that needs to dry out. Sun Oven should probably switch to a water or alcohol based stain and follow with a polyurethane. That would take care of at least some of the smell.
However there is a "de-stink" process we are to follow, to make certain the insides of the oven are clean; basically you cook a pot of soapy water inside the oven.
As luck would have it the oven arrived on a day of heavy rain, and today, following, are high winds. So I set the oven facing the eastern sun INSIDE the house at 9am, and it attained 200 degrees within an hour, and 300 in an hour and a half. Now to make my first tasty dish: Soapy water.
The condensate is sheeting off the glass door and dripping into the anodized tray below, so this looks like it will be a very efficient solar still, probably beating the relatively efficient solar still I already built (http://www.glocktalk.com/forums/showthread.php?t=1358199).
The oven hovered happily @ 330 degrees for several hours, the water at a slight boil inside. At 1PM I called it, and measured out the amount of distilled water produced...
...amazingly, ONLY 1/2 cup! My little self-built bus-tub solar still made 1-1/2 cups in that same amount of time, with considerably less surface area!! I pondered this for awhile and came up with two possible answers:
(1) The dark anodized aluminum interior gets so hot, that condensed water re-evaporates. The aluminum interior may be the hottest element in the oven, so the water's got nowhere to safely exist as a liquid. Whereas in my bus tub design, the water tray is definitely the hottest element, and the water is certainly cooler when it hits the plastic tub bottom.
(2) As I was cleaning the interior, I noticed that the aluminum was NOT formed like a sink, with all four sides and base from one seamless piece of material. Actually there are four riveted seams along the four corners of the interior. If these are not water tight, then water and vapor may be escaping the interior and leaching into the insulated area between the aluminum and the plastic. Will check this with the manufacturer.
For the time being, the oven gets plenty hot, 330F, but it is NOT doubling as a solar still in its current configuration. That's a big disappointment.
Day the second.
The morning was clear and the sky cloud free, so I preheated the solar oven to its preferred 330, where it seems to stabilize, stirred up some cornbread batter and put it in a dark pie tin. Into the oven at 10 am, and within minutes, clouds. Since the sky was clear in every direction except between my oven and the sun, I figured it was just one stray cloud. It drifted away and another took its place. Then another, and another, and finally it dawned on me...the clouds were FORMING in that position. They weren't blowing in from the ocean (clear sky to the west), they were being born overhead!
Oven dropped as low as 170, then gradually and painfully inched upward over the next two hours to 250. Cornbread percolated along. How long would it need? The recipe called for 1/2 hr at 400, what timing for the solar oven with clouds? No idea. After 2.25 hrs I thought I detected a browned top, so tested it with a toothpick and it was done.
I wish there were a low-heat cooking conversion factor.
If I were to guess based on this one experiment it would look like this:
(ideal temp/actual temp) ^2.3 * ideal time = actual time.
That formula would indicate if my temp were 300 degrees, it would take only 1 hour to bake. Which, by the way, is the estimate I found for cornbread in a solar oven on a clear, sunny day.
Lunch today was dad's treat; cumin crockpot beans and solar cornbread. The family scarfed down the cornbread, but could not tell any difference between the solar version and regular; same texture, same taste, same moist/dryness, same everything. Just took 2 hours instead of a half hour in the oven. Normal day, it would have been an hour. I didn't mind leaving the oven on for twice as long, since it was costing me nothing to keep it running.
Did the solar oven evaporate all the jalapenos in that cornbread? Because I don't see any in that picture.
Ah, good point. We have a little jalapeno holdout in our midst; so my cooking is often to the "shortest" common denominator...who would prefer that I had mixed in Lucky Charms instead.
...I preheated the solar oven to its preferred 330, where it seems to stabilize, stirred up some cornbread batter and put it in a dark pie tin.
...Oven dropped as low as 170, then gradually and painfully inched upward over the next two hours to 250.
...I wish there were a low-heat cooking conversion factor.
Good point; one that I failed to bring up. When you put the food in the hot oven, it will cool the oven down substantially, simply by virtue of the added, colder mass suddenly in there. It'll sometimes take a couple hours to get back "up to speed". The good news is that at most anywhere over 200 degrees, it'll still cook, and there's almost no worry of over-cooking or burning something as with a powered oven. (At least not that I've encountered; could be an issue in Arizona or Ecuador maybe..?) So as long as you put your stuff in early enough, no worries really. Even if it is "done" in three hours, there's typically no harm done at all leaving it in for four or five. Kind of like a slow-cooker in that regard.
Added thought - may not get the chance in your area, but it's a golden moment when you get to solar-cook a hot meal in an ice-covered yard during a power outage in the middle of winter.
In some situations it might be a bad thing from an opsec perspective, but the look on my neighbor's face was "truly priceless" as they say... :cool:
Cornbread looks good.
It will be interesting to see how well the cooker works for you for various foods.
Going to try to put some data and information up here next.
First, a map of average solar radiation. Tells you your status for "access to sunlight."
This map takes into account both latitude and cloud cover, giving average solar radiation. Easiest places to solar cook are darker grey/black. However, the solar cook book I got this from says that anything between the 60th parallels of latitude mean you can solar cook. That's basically mid-Canada to the tip of South America. No solar cooking for: Iceland, Alaska, N. Canada, Russia, Antarctic.
Prime Cooking Days @ latitude:
60 lat = 50 days
50 lat = 100 days
40 lat = 150 days
30 lat = 200 days
20 lat = 250 days
Here are easiest-to-hardest to cook foods with solar:
Easy: Fish, chicken, egg, cheese, white rice, fruit, above-ground veggies. Takes 1-2 hours.
Moderate: bread, brown rice, root veggies, lentils, most meat. 3-4 hours.
Difficult: large roasts, soups and stews, most dried beans. 5-8 hours.
This is great information Bolster, thanks for providing it. It appears in SW Missouri that I may have 150+ days a year. One question I have is where you set up the oven. If you are using covered cookware it's not a problem, but if you're cooking in an open dish how do you keep the critters and bugs out of the stew - or is that just added protien ;-O
Today's experiment, pink lentils (much less gas than the brown variety). Well, they were pink when they went in. First batch I ruined with not enough water; second batch turned out well and just in time for lunch. 1c lentils: 3c water (plus buillon, olive oil, garlic, onion, salt, curry powder). 1 hour 20 min, around 330 degrees. These were tender and delicious, couldn't ask for more. Leftover solar cornbread rounded out the meal.
Solar cooking wants thin dark bakeware, and I am waiting for a GraniteWare covered casserole dish from Amazon, which will make life much easier. In the meantime I'm faking it by using page binders to clip together two black pie tins. It's a kludge solution but it works.
@Carry16: My solar oven (the Global Sun Oven (http://www.sunoven.com/around-the-world/sunovens/global-sun-ovens)) has a hinged glass lid, and a flexible seal, to keep in the heated air. Within the oven you can cook breads in regular open pans (but they need to be dark/black); but MOST solar cooking is done in these lidded GraniteWare items, which increases heat and shortens cooktime even further. I hope to have mine soon.
Setup is the back yard. Easier if placed on the picnic table. When done, folds up and goes inside the house.
BIG TIP: Wear sunglasses when cooking!!!
There is a very simple "panel solar cooker" of which I was completely unaware, until I started reading. It would be an ideal way to experiment because the cost is so low. You can make one from a windshield shade (or even out of a cardboard box you face with aluminum foil), a high-temp oven bag, and a dark pot. Some sources say it'll do 200-275 degrees, others report as high as 350. You could probably make it for under $10 depending on the materials you used.
The amazing thing about these "panel cookers" is...they're lightweight, foldable and portable. I gotta make me one.
They seem like an ideal water pasteurization method.
Download that last one...you might need it later!!
Check out a "WAPI", a pasteurization indicator made out of a plastic tube (closed on both ends), with a glob of soy wax inside it, that gets placed in the water. It has a line & weight on it, and you hang it vertically with the wax at the top of the tube. When the wax melts & goes to the bottom, you know that a safe pasteurization temp has been reached.
Another low-tech is the "sodis bottle", a solar distiller made from a soda bottle. Paint a third to a half of the circumference black as a heat gatherer, fill with water, cap and lay in the sun (with the black side down. The sunlight enters the clear side and because of the black side (bottom), it gathers enough heat to pasteurize.
Never personally tried this one, as I don't have a good (low-tech, non breakable) way to check the temperature & confirm pasteurization temp. The good news is that pasteurizing water doesn't require as high a temperature as pasteurizing milk, so it's relatively easy to achieve. IIRC, it's only something like 150-155 farenheit for water, where milk (iirc) is 160 or higher.
I want a WAPI! Saw them on the sunoven.com site:
"A WAPI is a transparent tube which contains wax. The wax melts and drops to the bottom of the tube when it reaches 150°F (65°C) for 6 minutes indicating that the water has been pasteurized and is now safe to drink."
I think any of these solar oven designs could hit 150 without breaking much of a sweat. It's a big relief not having to wonder where I'll be hunting for firewood to purify filtered water.
I haven't tried SODIS yet. Interesting though.
Not my intent to show you photos of every single dish cooked in the solar oven, but this one's interesting because it browned. And it did so in about 2/3 the time the solar recipe called for. The oven was running around 300 today, because of high thin clouds. I thought solar ovens didn't brown, but both the cornbread and now this mac-and-cheese-from-scratch dish browned!
I've had trouble with cooking utensils, though. The deal is, you need lightweight, dark-colored cookware, and unless you're baking bread, you want a tight lid. GraniteWare is universally recommended, yet they have precious few sizes that will fit this solar oven. My Amazon-ordered roaster was a little too large so got sent back. I had to call the company, and get their product spreadsheet, from which I could pick out a few items that would fit.
This oven is limited by a 12 x 9 x 7 internal capacity...the inside is larger than that, but this is what will fit on the gimballed "swing" inside.
I reordered a 4-quart pot/steamer that will fit (for soups, and for pasteurizing water) but am still SOL regards normal cookware. I have yet to find a decent two-quart sauce pan with lid.
For the time being I have been using two of my wife's black 9" cake pans, one turned upside down on the other, and held together with spring-type paper binders. One of those cake pans is what you see in the photo.
Great posts Bolster! I really like reading this thread and seeing the photos of your food, please keep them coming.
Well thanks, Uneasy! Although I'm not sure if kind words are allowed here on GT S/P, are they? If I get responses on this forum other than that I'm a fool or a coward, I don't know quite how to respond.
Today's experiment: "Peach Cobbler Cake." Thought it time to try a dessert. Oven hit a max of 340 degrees around noon. Notice once again, the food browned nicely. A little cream and turbinado sugar on top.
I don't know where the mfgr, Sun Oven, gets its range of 350-400 maximum, though. Maybe if you were cooking in hell. Still its normal temp of 330 gets the job done.
In the background you see my salvation, a GraniteWare roaster that fits. If you get the same oven I did, that 9x12 measurement pertains to the tray at the BOTTOM of the oven. All the measurements given from GW are taken at the TOP of the utensil. So this 9-3/4 round roaster fits just great. Capacity about 2.5 quarts. And that thin-walled, black-coated cookware really does help with the cooking...!
On that granitewear pot, you could make it work somewhat more efficiently with a coat of high-temp flat black grill paint. It's non-toxic, although I'd still cook it empty a few times to air it out before using it with food in it. The flat black will eliminate the reflected (and therefore lost) light, which means reducing lost heating potential. I use flat black for everything in the cooker.
On the Mac and Cheese, did you cook the pasta first?
...a coat of high-temp flat black grill paint. ...
A most excellent tip, thanks!!
On the Mac and Cheese, did you cook the pasta first?
No, throw it all in at once, as per the _Cooking with Sunshine_ cookbook. Which incidentally has an awesome "build it yourself" section in the back.
You may be able to tell I'm only interested in simple recipes. I figure if there's an "event" I won't be getting fancy.
The WAPI arrived...update soon, weather willing. Socked in fog this morning.
I forgot to tell you guys one of the joys of outdoor cooking...I had pulled out my cobbler to check it, and an overflying bird shait upon the lid of my cook pot. It baked on instantly. Also I have contended with curious hornets and flies, and the crows keep a constant vigil on my oven. Ah, the good life.
The WAPI promises to be a decent water purification technique!
Oven at its typical 300+ degrees, put in a gallon of cold water.
Float the WAPI at the top of the water. The little vial pokes into the bottom of its storage container, which floats on the top of the water, with the WAPI hanging beneath like the rudder of a boat.
In 45 minutes, the wax has melted and flowed to the bottom of the WAPI, indicating that a gallon of water's been pasteurized and is safe to drink. No boiling, so the water doesn't taste flat.
No chemicals, no filtration system. Dead viruses, protozoa cysts, and bacteria.
If you were working the oven from 10am to 3pm, you'd be able to process around 7 gallons a day.
:thumbsup: Fun stuff to learn isn't it...
Fwiw, remember that as good as pasteurization is, it's only addressing biologics as you list. There's still the risk of chemical or other contaminants, so a good filter is still important.
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