Science of Solar Cooking

Box cookers heat using the same principles that heat the interior of a closed car parked in the direct sun. The reflectors concentrate the sun's rays through the glass top. The dark cookie sheet placed on the bottom of the oven absorbs the sunlight and gets hot. Once the oven thermometer indicates the oven is hot (225 - 250 degrees Fahrenheit), food is placed in the oven. Food takes longer to cook at low temperatures, but the slow cooking makes for tasty meals.

The solar box cooker is an excellent tool to demonstrate physical science concepts to a group of students. Introduce your students to Newton’s Law of Heating and the following principles of heat gain, heat loss and heat storage before they experiment with the solar box cooker.

Newton’s Law of Heating. Food placed in a 250 degree oven will take longer to cook than the same food placed in a 350 degree oven. Newton’s Law of Heating provides a more precise description of how the rate at which items cook is affected by the starting temperature of the food and the temperature of the oven. The rate at which food cooks is proportional to the difference between the temperature of the food to be cooked and the temperature of the oven. In equation form, the law relates [Rate at which food cooks] = k (Oven temperature - Food temperature). The same law demonstrates that the rate at which the oven heats is better on a hot day than a cold day. Students can collect data from the thermometers and graph the data. Students in higher level physics or math classes can work with modeling the data with equations.

Heat gain. The heat gain inside a solar box cooker is due to “the greenhouse effect.” Sunlight passes easily into glass covered enclosures. Once light is absorbed by materials within the enclosure, it is transformed into longer wavelength heat energy. Dark materials placed in the bottom of the oven will absorb more sunlight and generate more heat. If the heat energy builds up faster than it is lost, the oven will become hotter  [Mark Aalfs]. A poorly insulated oven will lose heat too fast to attain temperatures hot enough to cook. The solar box cooker has a dark cookie sheet in the bottom and is wrapped with good quality insulation to allow for adequate heat gain and retention.

The orientation of the glass to the sun also affects the heat gain. Figure 5 shows that the greatest amount of sunlight passes through the glass when it is perpendicular to the sun’s rays. The solar box cooker built with the enclosed instructions places the glass facing straight up to the sky, as shown in box 1 of Figure 5. As a result, it works only during the middle of the day in the mid-summer months.

Heat loss. The loss of heat from a solar oven is described by the Second Law of Thermodynamics, which explains how heat travels from hot to cold. Heat loss is a combination of conduction, radiation, and convection. Heat is lost from the solar oven by conduction, when the heat travels through the molecules of the physical box to the outside air [Mark Aalfs]. Radiation is also a factor in heat loss, when hot air radiates through the glass lid. The aluminum foil and foil tape used throughout the construction slow the heat loss due to conduction and radiation.

The greatest amount of heat loss is through convection, when warm air rises and pushes its way out through the spaces between the inner box lid and the glass. Strips of adhesive-backed foam weatherstripping added to the edges of the glass slow the heat loss due to convection considerably.

Heat storage. The capacity of a solar box cooker to hold heat increases when more mass is placed inside the oven. The oven takes longer to heat with heavy materials inside, but will hold heat longer through periods when the sun is covered with clouds. The section on “Using the Solar Cooker” has ideas for using bricks to experiment with this idea.

For more details and the source of the excellent drawings on heat gain, heat loss and heat transfer in solar cooking, see "Principles of Solar Box Cooker Design" by Mark Aalfs at www.accessone.com/~sbcn/sbcdes.htm.  A Physics textbook may have more information on Laws of Thermodynamics and Newton’s Law of Heating. Visit the  Internet Resources included in this website for sources of  on-line information.

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