Solar Ovens

(Updated 12/18/2007)
 

For additional information, see our other web pages on solar ovens:

--Our Current Solar Oven

--Solar Oven Design

 
This page describes our experience with solar ovens at Casa Juliana (in south Texas) in the 1990's, which lead to the development of a solar oven that had a high heat potential (nearly 600 degrees F), was easy to focus and allowed the cooking container to remain level regardless of oven angle. 

 

A variety of solar oven designs displayed at a Solar Oven Workshop at Casa Juliana in the mid-1990's.

 

 

During our years at Casa Juliana, the abundance of sunlight encouraged us to experiment with a variety of solar ovens. Over the course of several years, as we built and used various oven designs, we observed a familiar pattern of use. Typically, each new oven design was initially used with enthusiasm, but then gradually fell into disuse. Eventually, we looked closely at the reasons for disuse and it became evident that each of the oven designs we'd used had some or all of the following limitations:

-Insufficient heat potential to permit cooking on partly cloudy days.

-Cooking containers required re-leveling each time oven was focused.

-Focusing ovens was inconvenient, requiring propping up oven on one side with rocks, etc.

-Instability--ovens were prone to tipping over on windy days.

-Construction of non-weatherproof materials required oven to be carried indoors daily.

Eventually, we decided to design a solar oven with features that would encourage continued use. Each of the above problems was considered in the design process.

The resulting oven design we called the "Casa Juliana" solar oven. It overcame all of the problems listed above and incorporated additional convenience features as well.

 

The Casa Juliana Solar Oven

 

 

-It was capable of reaching temperatures of nearly 600 degrees F, allowing cooking even on partly cloudy days. Lower cooking temperatures were achieved by simply turning the oven somewhat away from the sun.

-Cooking containers were always level, regardless of oven orientation.

-The oven was quick and easy to focus. Focusing required only a few seconds.

-The oven was stable in all winds.

-The oven was constructed of weather resistant materials and was left outside in all weather.

-The oven was conveniently located just outside the kitchen door.

-The height of the oven door was convenient for putting food into the oven box and for taking it out.

 

A convenient hinged door on the back made loading easy.

 

 

Although the performance of the design was excellent, it also suffered from several significant drawbacks. One was inherent in the design: Although it was a very convenient oven to use, the design was sufficiently complex  that constructing it is outside the scope of most home builders (see Simplicity vs. Complexity on the Design Page).

Two other limitations were related to materials: The reflectors were constructed of heavy galvanized metal flashing which oxidized over time and required periodic re-covering with new aluminum foil. Also, some of the wood components began to rot after several years in the hot, humid climate of South Texas.

These problems notwithstanding, this design achieved the "critical mass" of performance and convenience such that it was used on virtually a daily basis for many years. With some repairs, it is, we're told, still in use.

The relatively high temperature potential of this oven is a result of the following design factors:

-Reflector size is as large as possible while still permitting access to the outside surface of the oven box glass for cleaning.

-Ratio of the area of sunlight intercepted to the area of the oven box glass is relatively large at almost 5 to 1. That is, nearly 5 times more sunlight is captured than would be the case without reflectors.

-The size, shape and relative angles of reflectors and oven box glass were calculated to insure that, when the oven is properly focused, all intercepted sunlight is reflected fully and directly to the oven box. Intercepted sunlight is reflected only once before entering the oven box, thus reducing the diffusion inherent in many designs in which a portion of the sunlight is reflected off two or more surfaces before entering the oven box.

-Heat in the oven box is conserved through double glazing on top and 3" of insulation on sides and bottom.

Plans:  No formal plans for this oven have been drawn up since it would be fairly time consuming and there haven't been enough requests to justify the effort.  However, for those who would like to build one of these ovens, I've tried to include enough information below to enable a skilled builder to do so.  Many details are necessarily left out as a complete description would require many pages of text and drawings. Referring to the photos above should help to make the explanation more understandable. If you decide to build one and have specific questions, I'll be happy to try to answer them.

-Oven Box and Reflector Size:

The size of the oven can be scaled up or down depending on anticipated cooking vessel size. Provided that the relative proportions given below are maintained, the high heat potential will be retained regardless of size.  Below is an explanation of how to calculate the size of the oven box and reflectors, followed by an example.

The first step is to determine the size of the inside of the oven box.  This is the part of the oven where cooking takes place.  This inner oven box is square on top and its depth is about 2/3 of the top dimension. 

Figuring the oven box size is a bit complicated since it depends on several factors including the size of your cooking vessel, the location of the cooking vessel tray (see below) inside the oven box and the fact that the box must be large enough that the vessel will not touch the glass or sides of the oven box regardless of what angle the oven is at. Figuring the oven box size for your vessel is a process of trial and error.  It's helpful to make a full size side view drawing showing the oven box at various angles relative to the cooking vessel.   

Although the formula for determining the size and angle of the reflectors relative to the oven box size is somewhat complex, it yields a basic geometry which is relatively simple. Once the size of the inner oven box has been determined, calculating the reflector sizes is easy. Add 1" to the top dimension of the inner oven box to get the dimension of the glass, then use the glass size to calculate the reflector dimensions. 

There are 8 reflector panels.  4 of them are rectangular and 4 are triangular. The width of each rectangular reflector is the same as the dimension of the glass.  The length of each rectangular reflector is twice its width. The width of the top edge of each triangular reflector is also equal to the glass dimension.  The the other two sides of each triangular reflector are equal to twice the width of the top edge. 

As an example, the inner oven box on the Casa Juliana oven is 17" square (by about 12" deep).  The glass is 18" square.  Each rectangular reflector is 18" wide by 36" long.  Each triangular reflector is 18" wide at the top by 36" long on the other two sides. 

If you decide to make a larger oven, be aware that reaching the glass to clean it may be difficult if the reflectors are more than 36" long.   

Note: The information below applies specifically to the Casa Juliana oven.  If you scale your oven up or down, the following dimensions will not apply. 

-Oven Box Details:

The sides and bottom of the inner oven box are made of sheet metal and the top is glass.  The inner oven box is insulated with at least 3" of fiberglass insulation on the sides and bottom.  We originally made the mistake of using cardboard covered with aluminum foil for insulation.  When the oven was first aimed at the sun, the cardboard quickly went up in smoke.  Don't use foam insulation or anything else flammable for that reason.

The outside of the oven box is made of 5/8" plywood siding and its dimensions are about 24" square by 16" high.  The door is about 11" high by 17" wide.  

-Reflector Details:

The reflectors are made of galvanized steel sheet metal and are fastened to each other along the edges with small machine screws and nuts.  A wood collar made of 2"x2" lumber provides support to the top of the reflector assembly.  The collar pieces are beveled at 69 degrees. 

-Cooking Vessel Tray:

Inside the oven box is a metal tray on which the cooking vessel sits. The tray is connected by two pieces of threaded rod to the ends of two horizontal pieces of pipe. Each pipe passes through a hole in opposite sides of the oven box and connects to the yoke outside the box.  The entire oven box and reflector assembly pivots on these pipes in a horizontal axis.  It is the fact that the tray is fastened to these pipes, which are in turn connected to the yolk, that keeps both the tray and the cooking vessel level regardless of the oven angle.

The pivot pipes mentioned above are each 3/4" galvanized pipe about 10" long.  They pass through holes in the sides of the oven box that are centered along the 24" dimension and are about 6" below the top of the box.  The tray is about 16" long by 12" wide and is located about 3" below the pipes.  The tray is connected to the pipes with 1/4" threaded rod that passes through holes near the end of each piece of pipe.  Each pivot pipe is secured to the yoke with 2 U-bolts  

-Yoke:

The oven box and reflector assembly is fastened to a yoke which permits the oven to be rotated horizontally and vertically. The yoke is made of 2" x 4" lumber and is mounted on a piece of galvanized pipe that is 1 1/2" in diameter by about 3' long.  The pipe is buried vertically in the ground with several inches extending above the ground. A 1 1/2" pipe base is bolted to the bottom of the oven yoke and is then threaded onto the top of the pipe. The rotation of the pipe base on the pipe permits the horizontal angle of the oven to be adjusted. 

-Vertical Angle Adjustment:

The vertical angle adjustment assembly is difficult to explain.  It consists of two 2"x2" pieces of wood.  One of these is fastened at one end to the oven box and at the other end to the wood collar around the top of the reflector assembly.  This piece has holes drilled into it every few inches. The second 2"x2" is fastened to the yoke by means of a hinge. Near the other end is a horizontal bolt which goes into one of the holes in the other 2"x2".  The hole is used that gives the best vertical angle relative to the sun.  The 2" x2" with the bolt and hinge has a spring near its base which provides tension to keep the bolt securely in the hole, yet permits quick and easy adjustment of the bolt from one hole to another. 

-Glazing:
Tempered glass must be used since ordinary window glass will break due to thermal expansion. Double glazing is used for thermal efficiency. In the Casa Juliana oven two 18" square pieces of double-strength tempered glass are used. The 2 pieces of glass are separated by 4 pieces of 1/8" wooden dowel between the 2 pieces of glass around the perimeter.  The dowels are held in place with a bead of 100% silicone caulk which also serves to seal the air space between the 2 pieces of glass.

 

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