There are several different designs of solar cookers you can make, but the most effective uses a Fresnel lens. Fresnel lenses are flat plastic magnifying glasses. If you’ve ever used one of those sheet magnifiers, you’ve used a Fresnel lens. They consist of a flat plastic sheet that has concentric rings molded into the back side.
In order to gather and focus enough sunlight to cook with, you need a fairly large Fresnel lens. Smaller ones won’t gather enough light and using a bunch of smaller ones together really isn’t any better. It’s extremely difficult to focus the beam from several together.
Basically, you need a Fresnel lens that’s about 30″x 40″. New lenses that size are hard to come by, but you can find a used one very easily on eBay. Another option is to keep your eyes open for one of the old-style big screen TVs that someone is throwing out in their trash. Those had a large Fresnel lens, just like you need, located just behind the screen. It was part of the system to magnify the image and project it onto the back of the screen.
With that lens in hand, you’re halfway to making a solar cooker. The lens needs to be mounted into a frame, in order to hold it straight and steady. The easiest way to make that frame is out of two layers of 1″x 3″ wood. One layer will need to be routed slightly, to match the thickness of the lens. The lens can then be clamped between the two layers, holding it in place.
The framed lens needs to be mounted in a stand to hold it, as you’d get tired holding the lens in place while you are cooking. As you can see in the picture included ion this post, this frame doesn’t have to be complicated. The triangular pieces in the corners provide rigidity, so that the frame doesn’t collapse. The extra cross-piece at the bottom is just to make sure that the uprights stay at the same distance from each other. The feet can be made in a number of different ways, such as circles of plywood. In this case, I attached three foot long pieces of wood at a 90 degree angle to the uprights, which provides a lot of stability.
The entire stand is made of 1″x 2″ lumber, with the exception of the feet, which are 1″x 3″. And the triangular stiffeners, which are made of scrap plywood.
It is important that the frame with the Fresnel lens in it can move inside the stand, so that the angle can be adjusted. This is easily accomplished by putting a threaded insert into the middle of both sides of the frame and using a knob with a threaded stud as a screw to attach and tighten it. A one inch diameter rubber washer is inserted between the frame and the stand to provide some resistance, causing the framed lens to stay at the angle it is placed at.