To build a useful solar oven, we must pay attention to certain design features. The most important features are size, shape, insulation, interior color, reflectors, and heat retention. For this article, let’s consider the size and shape of the oven.
Why is size important? because of the square-cube law. As an object of the same shape increases in size, the interior volume increases much more rapidly than the exterior surface area. The volume is 3-dimensional (cubic feet), but the surface area is 2-dimensinal (square feet). For a solar oven, the interior volume holds the heat, and the surface area loses heat. The larger the solar oven, the easier it is to reach a higher temperature (provided that the rest of the design features are also well-chosen).
This point is counter intuitive. If the oven has the same proportions, the same r-value for insulation, in short everything is the same except size, why would it reach a different temperature? You are letting in more energy in the larger oven, since the window is larger. The surface area of the oven, through which you lose heat, compared to the surface area of the window, through which you gain heat, is proportionately the same as the smaller oven. But the interior volume versus exterior surface area is not proportional. And volume is one factor that retains heat. Let’s take the simplest case of a cube sized oven:
Oven A: 1 foot on each side is 1 cubic foot of volume, and 6 square feet of surface area. The top window is 1 square foot.
Oven B: 2 feet on each side is 8 cubic feet of volume, and 24 square feet of surface area. The top window is 4 square feet.
Oven B has 8 times the interior volume of Oven A, but only 4 times the surface area.
I’ve run the math on solar ovens of the same design and proportions, but of increasingly larger size, and the difference is substantial. Increasing the size of the oven can increase its maximum temperature by over 100 degrees Fahrenheit. Counter-intuitive, but true.
There is another design consideration that also increases the efficiency of the solar oven: shape.
In the above example, the window area increases proportionally to the surface area. A solar oven gains heat through the window and loses heat through the sides. So if we increase the size of the oven and keep the same shape, we gain no advantage on this design point. We have an advantage on the proportion of interior volume to surface area, but no advantage on the proportion of window area (heat gained) to the area of the sides and bottom (heat lost). To gain this second advantage, we must change the shape of the oven.
We cannot do much about the surface area of the bottom of the oven, since it is going to be the same as the surface area of the top window of the oven. The larger the top window, through which we gain heat, the larger the bottom surface area, through which we lost heat. But having higher sides gives us no advantage; we lose heat through the sides of the oven, but there is no heat gain from an oven with high sides. So beyond the practicality of having some height to allow food and containers to fit in the oven, we should not increase the height of the oven as the size of the oven increases.
The ideal shape of a solar oven is probably a circular design, with low sides. This minimizes the surface area of the sides, reducing heat loss without reducing the size of the top window. However, curved sides are harder to build. And mathematically, the reduction in surface area of the sides of a circular oven is small compared to the surface area of the sides of a square oven. Taking into account this practical consideration, the ideal shape of a solar oven becomes a pizza-box shape: a square design with low sides.
But now I’m going to throw one more design change at you, based on another practical consideration. A square shaped oven will require you to constantly adjust the reflectors on the oven, as the sun moves across the sky. This means constant attention to the oven, or if you forget because you are busy with some other issue, it means a lower temperature for the oven as the sun moves and the reflectors do not.
The solution to this problem is an oven shape that is a long rectangle. This shape change means that you have a greater surface area along the sides, through which you lose heat, compared to a square design. But if the oven is well-insulated, the practical advantage that you gain in not having to constantly adjust the reflectors is worth the small increase in heat loss.
Why a rectangle? because you can set up the oven so that its length runs east-west. As the sun moves from east to west, it retains the same solar declination: the angle of the sun relative to an east-west line. The angle of the sun relative to a north-south line changes constantly as the sun moves across the sky, but this only affects the reflectors at the small ends of the rectangular oven. The longer the oven, the less those reflectors matter. You can omit reflectors there, or have small reflectors that are adjusted in angle only a couple of times a day. The angle of those reflectors is less crucial because the length of the oven will catch light from the end reflectors at almost any angle.
So there you have it, the ideal yet practical solar oven shape is a long rectangle with a low height. And the ideal size is large. An example size would be 4 feet in length by 2 feet in width by 1 foot in height. Once you have enough height for cooking purposes, a larger oven would retain the same height as the smaller oven. The increase in length is most useful, but some increase in width lets in more light and makes it easier to position the reflectors to focus light into the window. So some other example sizes would be 6 feet by 2 feet by 1 foot, or 8 feet by 3 feet by 1 foot.
More on solar oven design in future posts.