Introduction: Portable Fresnel Solar Oven

Picture of Portable Fresnel Solar Oven

Those sun rays traveled 150 million kilometers to get here, the least you can do is put them to good use. If you are looking to harness the power of the sun to cook your favorite casserole or crumble, then you need only to build a Fresnel Solar Oven. This is not the first solar oven featured on Instructables (see this or this), nor is it the first use of a Fresnel lens to cook (see here), but I wanted to try to make a rig for camping or for a picnic. The whole build folds nicely into a flat pack and is relatively robust; the oven-safe glass bowls and pot stack nicely as a separate piece to carry.

I was very pleased with the initial results: the pot reached 425°F in 10 minutes and the chicken rolls were beautifully done within half an hour. So put away that CO2 spewing BBQ once in a while and use the sun's freakish power to feed your family.

SAFETY WARNING:

HEAT: You are concentrating massive amounts of energy with these big lenses (YOU are the ant on the sidewalk in this case). You can easily burn yourself or curious children; see this example for illustration of power.

FIRE: You will easily set fires with anything combustible if you make the point of light small enough (example). Consider the consequence of leaning it on your house, or new car while to you go for lunch.

LIGHT: You WILL burn your corneas if you do not shield your eyes and the curious onlookers. Wear appropriate eye protection, keep the beam wide and shoo away kids. You will nor realize it while they are burning, your eyes get desensitized quickly, but can get damaged permenantly.

FOOD: Make sure any meat is cooked through before serving. There are many factors that will affect the speed of cooking: including clouds, angle of the sun, size of the lens size of your cut of meat etc. Use a meat thermometer and enjoy.

Step 1: What Is a Fresnel Lens and How Do I Get One?

Picture of What Is a Fresnel Lens and How Do I Get One?

Many of you are wondering what the heck is a Fresnel Lens. It basically has the same functionality as the lens of a magnifying glass, except that it cheats on the geometry to reduce the volume drastically. See the sketch above; in the lens I used, the ridges are barely perceptible except for texture: one side is smooth and the other feels grooved. You can buy these lenses from ebay, but most people get them from obsolete rear projection TV left out in the street. Each unit will be different but please check out the great video by Tool Using Animal too see the process of extracting the lens.

Please heed the safety warnings in previous step when playing with these lenses.

Great Fresnel Experimentation Instructable

Step 2: ITEMS and TOOLS

Picture of ITEMS and TOOLS

For this project I used:

  1. One Fresnel Lens from a 52 inch rear projection TV.
  2. 16' long 1" by 2" of pine
  3. Seven small brass hinges
  4. Eight corner brackets to hold the frame together
  5. One small tube of silicone (any flexible adhesive will do)
  6. Two identical Pyrex or equivalent glass bowls about 8 inches in diameter
  7. One old black stove-top pot
  8. Various screws

Tools required:

  1. Drill, with a few small bits
  2. Screw drivers
  3. Utility knife with fresh blade
  4. Measuring tape
  5. Square (L-shaped ruler)
  6. Angle grinder (to take handle off the pot)

Step 3: Wooden Frames

Picture of Wooden Frames

The lens you have on hand will define the size of your frame. You want to make two identical frames that together fit perfectly within the confines of the lens. Measure twice, cut once. I messed up a few times with the pieces of the frame and orientation; if in doubt go back to the lens and put the pieces down on top of it. Use the corner brackets on the inside of the frames, fastened with screws.

Prime and paint your frames to protect them for years to come.

Step 4: Legs, Hinges and Closure

Picture of Legs, Hinges and Closure

Ideally, you will want to test the focal length of your lens (each will be slightly different): measure the height of lens off the ground required to obtain a beam around 3 inches wide. You will need a friend to this and its best to wait till midday. There is no point of creating a single point of light because you want to cook your food throughout, not burn a point and leave the remainder raw. Unfortunately the length of the legs are restricted to the size of the frame to fold back into a tight package. In my case the rear legs were slightly too short for the optimal focal distance, so I am boosting them up 4 inches with whatever I have on hand in the field (firewood, food cans).

Again, check the geometry of the frame with respect to the lens prior to screwing in the hinges (I screwed up here again). I used the screws that came with the hinges except for those that went into the ends of the legs where I used higher gauge and longer screws. Make sure to pre-drill the screw holes, particularly for the wider screws into the legs. The rear legs are longer and should be installed on the outer back of the frames, this way they can be pointed outward and oppose lateral force to stay standing. The third leg was installed at an angle to have a slight lateral force towards the rear legs.

I wanted the frame to stay closed on its own, so in decided to use some Nd-magnets I had on hand from a previous project. I drilled small holes for the magnets, filled them with silicone and hammered them in with a bolt. On the opposite side of the frame I installed small screws. The result was underwhelming... but it keeps together very well with friction from the four hinges and tacky paint.

Step 5: Cutting the Lens

Picture of Cutting the Lens

I know what you're thinking: "You cant cut the lens... you are breaking it!" It turns out that the lens does't mind and neither do the sun rays. I am losing some of the power due to the edge of the frame at the center, but not much more than the rest of the frame along the edges. It is a sacrifice for portability.

Put the lens down on some wood with a straight piece of steel clamped into place. Use a sharp utility knife to to repeatedly score the lens. After 5 passes you will likely be able to pick up and bend it backwards and it should cleanly snap it two.

Step 6: Glue the Lens Onto the Frame

Picture of Glue the Lens Onto the Frame

I just used a bead of silicone on the frame to attach the lenses.

Groovy side up or down? It actually doesn't make much of a difference light-ray wise, because you are not trying to concentrate to a point of light. I found that the groovy side was very prone to scratches so I decided using it on the inside to protect it.

Step 7: Cooking Vessels

Picture of Cooking Vessels

I bought a set of oven-safe glass cookware my local hardware store (Canadian Tire). The pot was a spare from my camping set. I used an angle grinder to remove the handle from the pot (along with a hammer and bolt to get the pins out).

The two glass bowls act as a greenhouse to keep the hot air inside, much like your car on a sunny day. The pot is black and non-reflective on the inside, it is relatively massive and will keep some heat if sun hides behind clouds for minute or two. I used a towel from my hockey back to insulate the base of the cooking unit and limit the heat loss in this direction.

Step 8: Final Comments

Picture of Final Comments

I haven't experimented much yet, but I would not try anything that is very temperature sensitive (cake, souffle) or requires several hours to cook (roast beef, whole chicken). Even though the pot reached 425°F and stabilized there, I doubt the air reached that temperature. I recommend using a meat thermometer to make sure your food is well cooked (also check all the warnings on the first step: heat, fire, eyes)

Obviously results are going to vary with:

  1. Size and quality of the lens; mine was 52 inches diagonal.
  2. Cloudiness and angle of sun in the sky; noon in summer is best.
  3. The mass of what you are tying to cook; the lighter the faster.

I was pleasantly surprised by the result and I have been looking at the forecast everyday since I built this thing to see which days are going to be sunny. Please leave a comment below or to let me know if you have attempted this and share your stories.

Comments

foxposte (author)2016-02-15

Hi Renard--Thanks for sending me your link. You did a great job with this project! I would be reluctant to cut the lens, too, but nothing ventured, nothing gained, and it worked! It occurred to me that you could use telescoping legs such as seen on some photo tripods to solve the issue of too-short legs, but your solution works fine. I watched another video where the guy was making lava into basalt rock--but with a spot lens--this is a VERY powerful and potentially dangerous lens--but then, sticking your hand in a fire is dangerous and people quickly learn not to do it! I could see the spot lens being used for spot welding, although it would be clumsy, and take some practice. Back to cooking if you don't have sunlight, what about using a Coleman gas lantern? Any kind of light can theoretically be used--I just don't know whether a lens of this size will need sunlight to provide powerful enough light for the purpose of cooking. The issue of tracking the sun is an interesting one--some flowers will track the sun during the day--I wonder what the physics is that allows them to do that? Doing it manually, I can envision a ferris wheel contraption, with the food in an enclosed pyrex ball, so that as the Fresnel lens pivots on one side, the food in the pyrex ball pivots on the opposite axis. The wheel need be only half a wheel, of course, or less, depending on your latitude. In the Arctic in the summer, the sun is always at approximately 20 degrees above the horizon, so there you would need to track it horizontally, instead of vertically--fascinating concepts to think about!

Renard_Bleu (author)foxposte2016-08-18

Thanks for the comment, Sorry for the delay. I have since upgraded with telescopic legs from a scrap tripod. Tracking the sun is a science onto itself. I have attempted to make a Heliostat with mixed success https://www.instructables.com/id/ICARUS-The-Analog-... . As for using without sushine, I think any solution will be overridden by using your artificial power source directly (coleman stove, electric heat...). Thanks for your enthusiasm!.

headache (author)2016-03-24

I bought 2 much smaller lenses. They are something like 10x8. I got a picture frame and just slid the lens in between the two pieces of glass. It protects the lens and doesn't affect it's heating ability.

Renard_Bleu (author)headache2016-03-25

Very smart. You must be getting 2 focal point though. How is the heating capacity?
Thanks for the comment,

Guy

foxposte (author)2016-02-15

Hi Renard--Thanks for sending me your link. You did a great job with this project! I would be reluctant to cut the lens, too, but nothing ventured, nothing gained, and it worked! It occurred to me that you could use telescoping legs such as seen on some photo tripods to solve the issue of too-short legs, but your solution works fine. I watched another video where the guy was making lava into basalt rock--but with a spot lens--this is a VERY powerful and potentially dangerous lens--but then, sticking your hand in a fire is dangerous and people quickly learn not to do it! I could see the spot lens being used for spot welding, although it would be clumsy, and take some practice. Back to cooking if you don't have sunlight, what about using a Coleman gas lantern? Any kind of light can theoretically be used--I just don't know whether a lens of this size will need sunlight to provide powerful enough light for the purpose of cooking. The issue of tracking the sun is an interesting one--some flowers will track the sun during the day--I wonder what the physics is that allows them to do that? Doing it manually, I can envision a ferris wheel contraption, with the food in an enclosed pyrex ball, so that as the Fresnel lens pivots on one side, the food in the pyrex ball pivots on the opposite axis. The wheel need be only half a wheel, of course, or less, depending on your latitude. In the Arctic in the summer, the sun is always at approximately 20 degrees above the horizon, so there you would need to track it horizontally, instead of vertically--fascinating concepts to think about!

sclausson (author)2014-08-17

I like your innovation of cutting the lens in half.

Renard_Bleu (author)sclausson2014-08-18

Thanks, it was an eureka moment for me!

David the R (author)2014-08-14

NIce job! Those lenses are clumsy to deal with full size if you are going to move them around. Simply folding it is an 'elegant solution'.

Sweet.

Renard_Bleu (author)David the R2014-08-14

Thanks man. I had to get over the fact that I could actually cut it without losing too much functionality.

puggirl415 (author)2014-08-13

This is neat:) I'm building an off grid outdoor kitchen: rocket stove, no mortar brick pizza oven, solar dehydrator...I love this for my oven. I'd like an iteration with the ability to move the lens to the angle of the sun. I wonder if you'd have to move the food to the same angle as the lens/sun? I'll be thinking on that one.

Renard_Bleu (author)puggirl4152014-08-14

Thanks for the comment. This setup is easy to follow the sun with, you can merely nudge the bowl to keep them under the beam. To keep maximum efficiency you should adjust your lens to exaclty perpendicular to the sun, although you will still generate lots of heat if it is off a bit. Good luck on your project?

Renard_Bleu (author)2014-08-13

Doh... thanks for the comment. I'm sure that they are going to get harder to find as people transition to flat screens.

totszwai (author)2014-08-12

I need to make one and be ready for zombie apocalypse.

Renard_Bleu (author)totszwai2014-08-12

I hope for your sake that he zombie apocalypse is not accompanied by dark ominous clouds or infectious clouds, or its cold beans for you!! Thanks for the comment.

totszwai (author)Renard_Bleu2014-08-13

Oh snap, you gotta come up with something that starts fire that don't require sunlight!!!!

ghismo (author)2014-08-13

Nice project.

Please check out this video :

It may be a good alternative to Fresnel lense. Maybe cheaper and safer (as it doesn't focus light when it has no water inside of it).

What do you think ?

Renard_Bleu (author)ghismo2014-08-13

Thanks for the encouragment. I really like the Green Power Science guys; they come up with some really cool out of the box ideas, and actually follow through. This example is particularly "out-there", and surprisingly effective. However, it doesn't seem very practical and prone to failure (there is a a lot of weight up there). The geometry of the water lens is fixed at horizontal, which is great in California at noon, but up here you would lose much of the concentrating power due to the incoming angle of the sun (even at noon in June the sun is off to the south). The size of the lens (only the puddle) is also less than with a Fresnel, so you aren't focusing as many of the incoming rays.

jkimball (author)2014-08-12

Have you used your lens for anything besides cooking?

Renard_Bleu (author)jkimball2014-08-12

I've burnt paper and melted plastic, but it is awkward with the current setup because the legs are shorter than the optimal focal length (good for cooking though, and safer).

mrandle (author)2014-08-12

I built one similar to this a few weeks back and man is it powerful! I find you have to get just the right focal distance to work though. Many a project was completed with the help of canadian tire, they should sponsor a contest one of these days!

Renard_Bleu (author)mrandle2014-08-12

Thanks for the comment. There will be some fiddling with the leg height for sure. CT is definitely my favorite flyer of the week, always great deals on things I don't actually need...

michaelgc (author)2014-08-11

Nice, maybe a good project for my middle school stem students.

Renard_Bleu (author)michaelgc2014-08-11

Great, there is a lot of great ways to sneak science learning into a project like this. Lots of wow factor too, check out Youtube... I wouldn't put out more than a couple at a time to enable proper supervision and avoid trips to the nursery though: you'll see what I mean after a couple Youtube vids.

buck2217 (author)2014-08-11

Cool ---- or rather hot! good ible

Renard_Bleu (author)buck22172014-08-11

Thanks for the comment, have a good Monday.

fred3655 (author)2014-08-10

Does this heat water for chicken soup, or does it work better on solid food? Have you tried heating bricks and using the resulting heat for a convection oven? Thanks, I will definitely try this.

Renard_Bleu (author)fred36552014-08-10

It will definitely heat anything you put in there. I boiled water as a test, so no problem for chicken soup. As for using the heat for a convection oven... I haven't tried that, but given a big enough lens I'm sure it would work.

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