Depiction of a solar cooker, how it works

Word Document: our-solar-cooker

Application of principles in solar cooking (1)

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)

Principles behind solar cooking (1)

Refraction

Another property of light is that it refracts, which means that it bends when passing from one medium to another. Moreover, when light enters a more dense medium from one that is less dense, it bends towards a line normal to the boundary between the two media. This is illustrated in the figure below.

The greater the density difference between the two materials, the more the light bends. One place where this is used is in lenses for a variety of optical devices, such as microscopes, magnifying glasses, and glasses for correcting vision. An example of an image formed from a lens is shown below.

In this case the light from the object passes through the lens and is bent, forming an image on the other side of the lens which is magnified and inverted.

Many types of optical illusions are due, at least in part, to the refraction of light. One such example is the fact that if you look down while standing in a swimming pool, your feet appear closer to the surface than they actually are. This is due to the fact that light is bent when passing from water to air, as indicated below. Note that since air is less dense than water, the light bends away from the normal as it emerges.

The illusion comes from the fact that our eye doesn’t know that the light has been refracted when it comes from water into air, and so thinks that it has originated from a point closer to the surface.

No Author. (1999). Refraction. [on-line] Available from: http://theory.uwinnipeg.ca/mod_tech/node113.html (Last Accessed 1st March 2008)

Total internal reflection

An effect that combines both refraction and reflection is total internal reflection. Consider light coming from a dense medium like water into a less dense medium like air.

 

When the light coming from the water strikes the surface, part will be reflected and part will be refracted. Measured with respect to the normal line perpendicular to the surface, the reflected light comes off at an angle equal to that at which it entered at, while that for the refracted light is larger than the incident angle. In fact the greater the incident angle, the more the refracted light bends away from the normal. Thus, increasing the angle of incidence from path “1” to “2” will eventually reach a point where the refracted angle is 90^o, at which point the light appears to emerge along the surface between the water and air. If the angle of incidence is increased further, the refracted light cannot leave the water. It gets completely reflected. The interesting thing about total internal reflection is that it really is total. That is 100% of the light gets reflected back into the more dense medium, as long as the angle at which it is incident to the surface is large enough.

Fiber optics uses this property of light to keep light beams focussed without significant loss.

The light enters the glass cable, and as long as the bending is not too sudden, will be totally internally reflected when it hits the sides, and thus is guided along the cable. This is used in telephone and cable TV cables to carry the signals. Light as an information carrier is much faster and more efficient than electrons in an electric current. Also, since light rays don’t interact with each other (whereas electrons interact via their electric charge), it is possible to pack a large number of different light signals into the same fibre optics cable without distortion. You are probably most familiar with fibre optics cables in novelty items consisting of thin, multi-coloured strands of glass which carry light beams.

No Author (1999). Total Internal Reflection. [on-line]. Available from: http://theory.uwinnipeg.ca/mod_tech/node114.html (Last accessed 1st March 2008)

Radiation

The third and last form of heat transfer we shall consider is that of radiation, which in this context means light (visible or not). This is the means by which heat is transferred, for example, from the sun to the earth through mostly empty space – such a transfer cannot occur via convection nor conduction, which require the movement of material from one place to another or the collisions of molecules within the material.

Often the energy of heat can go into making light, such as that coming from a hot campfire. This light, being a wave, carries energy, as we saw in the last chapter, and so can move from one place to another without requiring an intervening medium. When this light reaches you, part of the energy of the wave gets converted back into heat, which is why you feel warm sitting beside a campfire. Some of the light can be in the form of visible light that we can see, but a great deal of the light emitted is infrared light, whose longer wavelength is detectable only with special infrared detectors. The hotter the object is, the less infrared light is emitted, and the more visible light. For example, human beings, at a temperature of about 37 ^o Celsius, emit almost exclusively infrared light, which is why we don’t see each other glowing in the dark. On other hand, the hot filament of a light bulb emits considerably more visible light. We shall discuss in more detail the nature of light in Chapter 10.

No Author (Sep. 1999). Radiation. [on-line]. Available from: http://theory.uwinnipeg.ca/mod_tech/node77.html (Last accessed on 1st March 2008)