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Greenhouse effect

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Glass and certain (but not all) "transparent" plastics will allow virtually all the visible and ultraviolet solar radiation striking their surface to pass through, but will absorb most of the infra-red radiation. That is, the visible sunlight comes through the glazing and is directly absorbed by the pot and the air inside, but the thermal energy reradiated by these will not pass directly outward through the glazing. To escape, it must heat the glazing so it radiates the energy outward. This process increases the temperature inside the cooker, as explained below. This greatly aids the cooking. Some facts:

  • Sunlight can be considered as having two parts roughly in the same proportion. Broadly the two parts are (1) the visible spectrum light and (2) the infra red rays.
  • Both parts can be directly absorbed by the pot and will heat it. However, the colour of the surface of the pot affects how much sunlight is absorbed, and how much is reflected.
  • Black paint is good at absorbing visible light, converting its energy into heat. It is also usually good at absorbing infra-red rays. When the pot becomes hot, the black surface re-radiates energy as infra-red rays (black body radiation).
  • Glazing materials are transparent to visible light, but fairly opaque to infra-red, especially "far" infra-red, which has wavelengths far from the visible spectrum. When they are hot, these materials radiate infra-red energy, like black bodies.
  • Some approximate calculations: Suppose the intensity of sunlight which strikes the glazing is represented by 2x, of which x is visible and the other x is i-r (infra-red). For the cooker to be in equilibrium, the glazing must be hot enough to radiate 2x outward. By symmetry, it also radiates 2x inward, to the pot. The pot therefore receives 2x of i-r, and also x of visible light which has passed unaffected through the glazing. To be in equilibrium, the pot must therefore radiate 3x of i-r outward. (Thus the glazing absorbs x in i-r from the sun, and 3x from the pot. This balances the 2x that it radiates in both directions.) If the glazing were not present, the pot would radiate 2x outward, to be in equilibrium with incoming sunlight. Thus the effect of the glazing is to raise the temperature of the pot so it radiates 50% more than if the glazing were not present. By the fourth-power law of radiation, the pot's temperature measured from absolute zero is raised by about ten percent, since \scriptstyle 1.1^4 \approx 1.5. At typical solar-cooking temperatures, this means that the glazing raises the temperature of the pot by about 35 degrees Celsius, or 65 deg. F. This is enough to make a large difference to the effectiveness of the cooker.
  • The last paragraph contains many approximations, but its conclusion is roughly right. It is true that glazing greatly improves the effectiveness of a cooker.
  • Note that the greenhouse effect depends on the glazing being hot (though not as hot as the cooking pot). If there is a wind which cools the glazing, the pot will be cooled too, even though it is protected from the wind.
  • Note also that this calculation makes no mention of reflection. It is often erroneously stated that glazing traps infra-red by reflecting it. This is not true. In fact, gases such as carbon dioxide produce a "greenhouse effect" without reflecting energy at all.

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