From the author of the original " How to Make a Solar Power Generator for Less Than $300" (free)

Phil Heiple's
Fresnel Sun Ovens

Two Illuminating Designs

Introducing Fresnel Sun Ovens! Capable of reaching 430 F Degrees! They pasteurize drinking water! They boil water! They cook! They bake! Some collapse flat! Some are backpackable! And they can be made in a couple of hours with cheap and recycled materials.

Most sun ovens operate by reflecting several beams of light onto a single spot. These new designs work on the principle of magnification with a Fresnel lens. Without wing-like reflectors, this design is much more wind-friendly, lighter, and more compact.

With the simplest design, the single-lens "Pasteurizer,"starting at 8:30AM on a cool 50 degree January morning in Southern California, it pasteurized drinking water in sixty minutes (160 degrees F) and brought water to a rolling boil in three hours. Once it started to boil, it was easy to keep close to that temperature (212 degrees) the rest of the day. I started the rice at noon. Anything that is cooked in boiling water can be cooked in this little oven: couscous, eggs, rice, beans, pasta, pasta sauces, soups, oatmeal, and any of those freeze-dried packaged meals backpackers like.

With the eight lens, seven mirror "Octalens," with "Double Whammy" (bottle inside a big pickle jar) water was pasteurized in fifteen minutes and brought to a boil in twenty-five minutes! In about forty-five minutes the oven thermometer inside it read 430 F degrees!

These results are possible because of a simple Fresnel lens. A Fresnel lens is basically a flat magnifying glass made out of acylic. There are cheap 8 1/2 x 11 inch sizes available. I got mine at the local 99 Cent store. I combine them with small 12' x 12" mirrors. This doubles the abouit of sunlight being delivered compared to the usual four reflector sun box. Then the Fresnel lens make more efficient use of the increased sunlight. Plans for larger systems can be found here. This is an easy to make non-polluting renewable essentially free-running carbon-barefoot gizmo that actually works and is gosh-darn fun.

Prevent deforestation, desertification, even violence against women (do your reserch). Not even wood is used (other than that which made the recycled cardboard).

Single Lens Box Design "The Pasteurizer"

You will need:

One 8 1/2" x 11" Fresnel lens.
One cardboard box
50 cent aluminum cookie sheet
A sheet of clear acrylic, glass or plastic to cover the top
something to cut cardboard with
12 oz. brown glass bottle or
flat black stainless steel water bottle (best)
a cooking thermometer

Get yourself a Fresnel lens. Get a cardboard box. Bend the cookie sheet into a "U" shape and line one end of the cardboard box. (This reflects light back as well as keeping the walls from catching fire.) Put your bottle in the protected end and then experiment with the sun to see where to mount the lens. It should be thirteen inches from the surface of your bottle, perpendicular to the sun, and at about 60 degrees. Cut slots in the box and mount the lens, or tape in place and cut away any cardboard you don't need. You will move the whole box as the sun rises and lowers. You should check it every fifteen minutes to make sure the white hot bead of light is heating your bottle and not setting your box (and house) on fire.

I took a piece of clear acylic I had and laid it over the top, so I could watch the thermometer in the bottle. A sheet of glass or several layers of clear plastic wrap would also work. Most of the heat loss is through the top (heat rises).

But then I left it alone and it self-immolated. Now I use a sheet of glass I got from a thrift store frame. But the main problem was that I had a small mirror, not the recommended cookie sheet, behind the bottle. The white hot bead of light hit it, shattered it, and moved on to the cardboard. The bead of light will shatter any empty glass bottle, and clear and green glass do not stand up to the punishment as well as brown glass.


I use masking tape to seal all the seams and openings. It's cheap, easy to cut with a knife to break down, and after several layers works even better. You can glue tin foil to the inside walls and floor of the box (but do not leave out the cookie sheet--the lens will set fire to the box through the tin foil).

Enter the "Octalens" Eight Lenses, Seven Mirrors
It Boils! It Cooks! It Bakes!

Our next revolutionary design tears down the walls, literally. It is an oven without walls. Seven Fresnel lenses stand vertically in cardboard feet. Each has a mirror in a cardboard mount that reflects light to it. At the southernmost point of the Octalens is a "Master" Fresnel lens in a horseshoe mount (a big "U") that focuses directly on the center.

You will need:

Eight 8 1/2" x 11" Fresnel lenses.
Seven 12" x 12" mirrors
Masking tape
Clear tape
Flat black stainless steel water bottle
Stainless steel cooking bowl or black sauce pan
A cooking thermometer
A big glass pickle jar

Mount seven lenses on rectangles of cardboard with slots cut in them. They should all stand vertically. Surround your cookware with them, each at thirteen inches from its target. Make a horseshoe mount (a big "U") out of cardboard and fit a lens in it set to focus several inches off the ground. This is your hottest lens. It goes in the southernmost part of the set up. You can put a rock in it if the wind blows it over.

Then tape pieces of cardboard to the edge of the mirrors and then fold the cardboard to make the correct angle for each mirror. The northernmost lens is next strongest, then the northern-east and northern-west, so do them in this order. By the time I had five in place, reflecting their beams through their respective lens, the water had started boiling. Write on the back of each one what its position is. Then you just remove the tap, straighten out the card board and they will stack flat, each mirror protected by its own piece of cardboard.


How to Cook with the Octalens

Cooking with the Octalens is a simple two-step process. First you get water boiling in your black stainless steel water bottle. Then you snatch out the bottle with an oven mit, and pour the water and food stuffs into a stainless steel bowl and put the bowl back into the eye of the furnace. As soon as the thermometer reaches 160 F it is safe to eat. Couscous and hard-cooked eggs are not returned to boil.


1 cup couscous
1 1/2 cup water
1/2 teaspoon salt

Get the water boiling. Pour it into a bowl with the couscous and salt. Cover and wait five minutes. Fluff with a fork and serve. Serves four.

Morrocan Couscous Soup with Chicken and Peas

1/4 cup dry couscous or one cup cooked
2 cups water
8 oz. tomato sauce
1 cup chickpeas or green peas (canned or precooked)
1/4 cup cooked chopped chicken
1/2 tsp. onion powder
1/2 tsp. cinnamon
1/2 tsp. cumin
1/2 tsp. tumeric
1/2 tsp. garlic powder
1/4 tsp. red pepper
1 tbsp. chopped fresh cilantro
juice and zest of one lemon

Get the water boiling. I use a small 3-cup sauce pan black on the outside with a glass cover. I unscrewed the handle from the glass lid, giving me a small hole to stick my thermometer in. (Thermometer in photo shows 200 F degree, soup is boiling around the edges.) Dump in the couscous. If dry, bring to a boil by itself, then add all the other ingredients. Bring to a boil, or at least 180 F degrees. Serves four.

Macaroni and Cheese

1 cup elbow macaroni
2 cups water
1/2 tsp. salt
1 cup shredded cheese

Boil water. Pour water, pasta, and salt into a stainless steel mixing bowl or black saucepan and put it back into the eye of the Octalens. Cook for 10-12 minutes or until tender. Drain the pasta. Add cheese. Stir. Serve. Serves four.

Perfect Hard-Cooked Eggs

3 eggs
saucepan with lid
wide-mouth thermos bottle

Put the eggs in the water and bring to a boil. Eggs start to firm up at 158 F Degrees, and pasteurize at 140-150, so if you can't get the water any hotter, you can still cook eggs (it will just take a lot longer). Using my little black sauce pan w/lid I got the water boiling in about twenty-five minutes. Then I let it boil a minute, removed from the heat, and poured the eggs and hot water into the thermos, and then fastened the top back on. Wait twenty minutes. Remove, cool, peel. Then enjoy hard-cooked eggs with bright yellow flakey yokes and tender soft whites. I like them with a pinch of sea salt and fresh ground pepper.
Another good way to cook eggs with the octalens is to poach them in boiling water.

Cheddar Polenta
1 Cup Corn Meal
3 Cups water
tsp salt
tsp Olive oil
tsp butter
half cup graded cheddar
Get water boiling. Wisk in corn meal gradually. Stir in everything else. Stir up once really good then put back into the eye of the octalens. Bring back up to a boil and then remove and cover. Wait 20-30 minutes and then dig in Comes out nice and creamy and perfect without all the stirring constantly on a stovetop. Spices to taste: red pepper, chopped jalapenos, fresh chopped herbs, chopped olives, garlic, bacon, or pimentos. Again, four servings.

Sun-Baked Cornbread

1/2 cup baking flour
1/2 cup corn meal
1 tbsp sugar
1 tsp. baking powder
1/4 tsp. garlic salt
1 tbsp olive oil
1 tsp. milk
1 beaten egg

I used a one-quart cast iron sauce pan for this. Put the pan in the eye of the octalens to warm up and then go inside and prepare the batter. Combine dry ingredients. Stir in milk, oil, and egg. Pour batter into hot, greased cast iron pan dusted with corn meal. Put it in the eye of the octalens with the white-hot beads of light focused on the walls, except for the master lens, which I focus through the glass lid. I took the handle off the lid and inserted a thermometer. The temperature quickly reached 220 F degrees. Ninety minutes later it was done. Firm, sweet, moist, hot cornbread with a nicely browned crust. This small recipe makes one big round disc for one, or 2-4 smaller bits. Larger amounts would require longer cooking times.
Cornbread! Baked without any petrofuel, gas, electricity, charcoal or firewood!

"The Double Whammy"

Got a big pickle jar? Put it in the eye of the Octalens and then put a smaller jar or bottle inside it. Keep pliers handy for getting the bottle out. It gets amazingly hot. One test got the oven thermometer up to 430 F degrees. Do not focus the beam directly on the pickle jar, but rather on the jar inside. Focusing directly on the pickle jar will break it.

CAUTION: Do not reach into the eye of the octalens with your bare hand. It will burn your hand. Do not stand inside the octalens. It will burn holes in your trousers and then your legs. Watch out for little beams of light being reflected everywhere. Always have something sitting in the middle of the octalens when the sun is shining or else it will act like a firing squad arrainged in a circle, each one shooting the soldier directly across from him. (A tea pot works good.) Do not stare at the little white hot beads of focused light, especially you use a stainless steel cooking bowl, least you scar your retina.

Not mechanically adept? No time? Okay, get a commercial sun oven from one of these fine dealers:

A typical commercial sun oven. They work really good. But they cost about $240 each. They reach 300 degrees F and higher. And are fun.

Instructional Videos for the Fresnel Sun Oven

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    Copyright 2011 Phil Heiple. All rights reserved.