The advice for warmer climates does not work here. I've built ovens out of boxes, clay pots, Fresnel lenses, and various cooking supplies - but I've finally worked out the details to succeeding in colder locations.
In order to succeed at solar cooking in a northern latitude - you must do the following:
1. Use a parabola shaped sun reflector - all others are inefficient
2. Take advantage of multiple layers of heat trapping
3. Do not attempt in the winter, cloudy days, windy days, or when the sun is low (before 9am or after 4pm)
4. Plan ahead - especially if you need to cook meat - and allow for a multi-hour cook time.
Take a look at our solar oven:
mylar blankets, or in a worst case scenario - mylar packaging (like chip bags or juice pouches.)
largest size pyrex bowl.) This is covered with a clear glass pyrex lid (from a baking casserole dish.) The clear lid and bowl allow sunlight to enter the cooking space, they also keep the heat in. The black bowl absorbs the heat.
You can cook directly in this set up, or you can place a smaller black pan (wrapped in a cooking bag) or lidded pyrex inside to provide an additional layer of heat trapping.
On June 22nd at 9am, 80F - we set the oven out to pre-heat. By 9:45 it was at 310F. Opening the top to place the food inside dropped the temperature to 250F. It quickly heated back up to 310F and that seemed to be our maximum temperature inside the two pyrex glass pieces. We cooked blueberry rhubarb bars in the oven for 50 minutes. The recipe calls for 350F for 30 minutes.
The initial temperature read: 77 F and after 1/2 hour it read 225 F - daytime temperature was still 80F. It did not get above 225. Would this be hot enough to cook eggs?
I capped it with a pyrex lid to see if we could raise the temperature. We did not get a higher temperature and started to run out of sun. I will reattempt a lidded skillet in the future.
A lidded, cast iron dutch oven could also work and would probably benefit from being wrapped in an oven bag or placed inside a large pyrex bowl with lid.
In the shade, the deck boards registered temperatures between 76 and 80F. In the sun - 136-150 F.
With an outside temperature of 80F - it took 140 minutes to bring the water to a boil. For comparison - a direct flame will usually heat the same quantity of water to boil in 15-20 minutes. But the sun was free.
This means we could cook rice or noodles easily in the solar oven (with enough time to plan - actually: I think it would be better to heat the water in the solar oven and finish the cooking in a thermos like I did here.) It also means we could easily sanitize water and cook stews/soups, braise meats over a long period of time, and much more!
This is the most successful solar oven set up we have used in a northern climate. It is small and portable. It heats up well and maintains a consistent temperature. With the clear glass and the grill thermometer, it is easy to see the temperature inside the oven.
It cooked well with slightly longer cooking time compared to an indoor oven (but with free heat from the sun and no electricity.) The max temperature seems to be 310 for our climate. I will be adding a fresnel lens to a portion of the top (above the black bowl but beneath the glass lid) in another rounds of tests to see if I can up the temperature even more. I will also attempt to cook meat. At 300F, it's going to take a few hours, and I'll have to rotate the oven to chase the sun.
In case you were wondering, the box style solar oven worked okay in our area. This is one I built from two shoe boxes (one shoe, one boot,) some shredded paper, duct tape, black spray paint and a fresnel lens.
We baked cookies inside this oven one year. The cookies were mostly done and burned in random areas (due to the fresnel lens focusing beams of light in certain places.)clay pot for the base instead of the box - as long as it has a glass top (pyrex lid!) and there is a solid base of pebbles/sand holding and insulating the cooking vessel. That oven was dismantled so I could grow plants in the clay pot.
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