Well now you know the theory. So perhaps you would be wondering how it applies to solar cooking. That is good thinking on your part. But back to the point – in this post, I will explain how refraction, reflection, and radiation apply in solar cooking.In solar cooking, a solar box cooker is sometimes used. It cooks because the interior of the box is heated by the energy of the sun. Sunlight, both direct and reflected, enters the solar box through the glass or plastic top. It turns to heat energy when it is absorbed by the dark absorber plate and cooking pots. This heat input causes the temperature inside of the solar box cooker to rise until the heat loss of the cooker is equal to the solar heat gain. Temperatures sufficient for cooking food and pasteurizing water are easily achieved.Greenhouse effect: This effect results in the heating of enclosed spaces into which the sun shines through a transparent material such as glass or plastic. Visible light easily passes through the glass and is absorbed and reflected by materials within the enclosed space. Hence, you can see that solar cooking is based on the radiation of visible light waves from the sun into the solar cooking box. The light energy that is absorbed by dark pots and the dark absorber plate underneath the pots is converted into longer wavelength heat energy and radiates from the interior materials. Most of this radiant energy, because it is of a longer wavelength, cannot pass back out through the glass and is therefore trapped within the enclosed space. The reflected light is either absorbed by other materials within the space or, because it doesn’t change wavelength, passes back out through the glass.
Critical to solar cooker performance, the heat that is collected by the dark metal absorber plate and pots is conducted through those materials to heat and cook the food.
Reflectors, additional gain: Single or multiple reflectors bounce additional sunlight through the glass and into the solar box. This additional input of solar energy results in higher cooker temperatures. Again, you see the properties of light being used in solar cooking.
However, despite the heat gain from the sun heat is also lost. Radiation is one of the ways in which it is lost: Things that are warm or hot — fires, stoves, or pots and food within a solar box cooker — give off heat waves, or radiate heat to their surroundings. These heat waves are radiated from warm objects through air or space. Most of the radiant heat given off by the warm pots within a solar box is reflected from the foil and glass back to the pots and bottom tray. Although the transparent glazings do trap most of the radiant heat, some does escape directly through the glazing. Glass traps radiant heat better than most plastics.
Aalfs, Mark (no date). Principles of Solar Box Cooking Design. [on-line]. Available from: http://www.solarcooking.org/sbcdes.htm (Last accessed on 15th March 2008)
