Last updated: February 18, 2014
The initial intent of the Cob Solar Box Cooker was to create a box solar oven that could be made almost completely with the locally available materials of sunny developing nations. The project was funded by Jewish World Watch as research effort, and the designer of the oven was Joe Kennedy. The cob structure for the oven walls is made from combining a mixture of straw and mud, and letting it cure in the sun. These same materials and techniques are currently used by the local women to create housing while living in the Darfur refugee camps in Chad, in eastern Africa. A clear glazing top is made from polycarbonate sheet held in a simple wooden frame.
However, tests have revealed that the earthen mass of the cob absorbs too much of the heat generated inside the box oven for it to attain an internal temperature above 93° C (200°ºF) (a temperature sufficient for pasteurizing water, but not hot enough to cook food in a reasonable period of time). Even lining the oven with strips of Mylar and blackening the interior with charcoal did not significantly increase the internal temperature of the oven. By creating an insulated oven liner (with Home Depot Reflectix foil bubble wrap) the builders were able to increase the internal oven temperature by an additional fifty degrees to 121°C (250ºF) (the average cooking temperature of the CooKit solar cooker used by the women in the camps).
The hope is that by reducing the internal volume of the oven by several inches the design will be able to attain an internal oven temperature above 149°C (300ºF). This will entail adding more cob to the oven interior to decrease its dimensions; constructing a smaller Reflectix liner for the oven; and reducing and refitting the polycarbonate window and frame to create a tight seal with the Reflectix liner.
Current manufactured solar box cookers range from $200 USD more than $400 USD, a price that is out of range for refugees, and for NGOs wanting to provide distressed populations with low cost, donated solar cookers. The great advantage of solar box cookers is that they are more insulated than solar panel cookers (like the CooKit) and can thus reach temperatures between 300º-350ºF. This allows them to stew, roast and bake much faster than a panel cooker. Also most box cookers can hold more than one pot, which is a great advantage for larger families or for families that need to cook two dish meals. The cob box cooker we are designing and testing can hold two twelve-inch traditional cooking pots.
With its soft but durable insert acting as an oven door, this design will allow the women to built a cob oven shell for the insert, creating a solar box oven inside their family compound. If they have to move, the lightweight insert can be removed from the cob shell, carried to a new location and put into another locally constructed cob oven, as cob design requires having the oven wall structure remain in one location, and in one orientation once constructed.
It is estimated that the Reflectix insulating oven insert, the polycarbonate window and frame, the metal sheet flooring for the oven and the removable reflector can likely be produced (in bulk) for less than $30. The rest of the materials needed to make the cob solar box oven (mud, straw and fabric for the pillow) can be obtained and made locally. The reflector can be made in the camps by the women who are currently making the Cookits using the materials they already have (foil, cardboard, glue and metal grommets). The six, attached panels of the reflector are held together by 12 small, inexpensive plastic zip ties that are run through grommets in the corners of each panel. Since the reflector is only used during the cooking process, and it is never exposed to moisture, it is not necessary that it be waterproof. The oven itself, should be able to withstand the wind and the limited rain that falls.
Audio and video
Construction process of the Cob Solar Oven - Oriol Balliu
- See Pat McArdle.