|Natural Energy and Vernacular Architecture: Principles and Examples with Reference to Hot Arid Climates (UNU, 1986, 172 pages)|
|Part 1. Man, natural environment, and architecture|
|2. Architectural thermodynamics and human comfort in hot climates|
All matter emits electromagnetic waves which are generated by the thermal motion of molecules composing the material. Such radiation is called thermal radiation. The intensity and wavelength distribution of this radiation depend on the nature and temperature of the material.
A perfectly opaque material with a totally absorbing and therefore totally non-reflecting surface, which is usually called a black body, emits radiation at the maximum possible rate for any given temperature. This black body is a convenient concept used as an idealised standard, but which should not be confused with an actual object with a black-colored surface. For such an object, the rate of radiation emission depends only on the fourth power of its absolute temperature.
As the temperature of the radiating object increases, the wavelength of maximum radiation intensity becomes shorter, and the distribution changes so that a greater proportion of the energy is radiated at shorter wavelengths (i.e., with higher energy). At temperatures below about 500 °C (about 900 °F), the emission consists almost entirely of wavelengths too long to be observed as light. At about 700 °C (about 1300 °F), the object glows with a dull red color. As the temperature increases further, the wavelength of maximum emission decreases, and the color shifts successively to bright red, yellow, and white.
The energy emitted by a radiating body ultimately impinges on other matter, which absorbs it, reconverting the energy into heat. In this way heat is transferred from one place to another by radiation.
At ordinary temperatures, most nonmetallic surfaces, including painted surfaces, radiate virtually as black bodies-their emissivity is high, and they are good absorbers for long wavelength radiation. Thus, various paints ranging from black to white are found to be indistinguishable as regards heat radiation at temperatures up to 100 °C (212 °F). However, whereas dark paints absorb most of the short wavelength radiation received from the sun, white pigments reflect most of it. And, at temperatures up to 100 °C (212 °F) aluminum and other metallic paints have an emissivity only about one-half that of a black surface. On the other hand, highly polished metals are strong reflectors of radiation, and many such surfaces are almost perfect reflectors of the long wavelength (low-energy thermal) radiation emitted by bodies at ordinary room temperature.
Emissivity, Absorptivity, and Reflectivity
Reference has been made to the importance of surfaces for heat transfer by radiation. To evaluate their emissive, absorptive, and reflective properties, surfaces can be compared with the properties of a black body, which absorbs all radiation falling on its surface and therefore reflects none.
The emissivity of a surface at a given temperature is equivalent to its absorptivity for radiation from another body at the same temperature, since two bodies at the same temperature will remain in thermal equilibrium with each other. The emissivity, and hence the absorptivity, of a black body has by definition, a value of unity, with the values of all real surfaces being in practice less than this value. Radiation falling on an opaque surface is partly absorbed, and the remainder is reflected. Since the incoming radiation can only be absorbed or reflected, the sum of the absorptivity and reflectivity must equal unity. For example, at normal temperatures, an aluminum foil may have an emissivity of 0.05, and thus its absorptivity will also be 0.05, but its reflectivity will be 0.95. This means that it emits by radiation only 5% of the amount a black body emits at normal temperatures. Also, it absorbs only 5% of the radiant energy falling on it (from another body at normal temperatures), and it reflects the other 95%.
The emissivity of a surface at normal temperatures (10-38 °C or 50-100 °F) is not necessarily the same as its absorptivity for radiation received from the sun. Emissivities at normal temperatures are important when considering heat losses from buildings through cavity-wall, floor, or roof constructions. For external surfaces, the absorptivity for solar radiation is important when considering heat gain from the sun. Table 1 gives these characteristics for some common surfaces.
Table 1 shows that the emissivities of white and dark paints are about equal at normal temperatures but that white paint has a much lower absorptivity for solar radiation. A roof coated externally with white paint gains less heat from the sun than if it were a dark color.
Table 1. Average emissivities and absorptivities for some common building surfaces under relevant conditions
|Surface||Emissivity or Thermal Absorptivity at 10-38 °C (50-100 °F)||Absorptivity for Solar Radiation|
|Black nonmetallic surfaces||0.90 0.98||0.85-0.98|
|Red brick, concrete, and stone, dark paints||0.85-0.95||0.65-0.80|
|Yellow brick and stone||0.85-0.95||0.95-0.70|
|White brick, tile, paint, whitewash||0.85-0.95||0.30-0.50|
|Gilt, bronze, or bright aluminum paint||0.40-0.60||0.30-0.50|
|Dull copper, aluminum, galvanized steel||0.20-0.30||0.40-0.65|
|Highly polished aluminum||0.02-0.04||0.10-0.40|
Source: Heating and Air Conditioning Guide, American Society of Heating and Ventilating Engineers.
Table 2. Reflectivities of various materials and paints
|Material or Paint||Reflectivity (%)|
|Red brick or stone||30-50|
|Asphalt bituminous felt||10-20|
|Galvanized metals (new)||36|
|Whitewash or white paints||80-90|
Source: N. S. Billington, Journal of the Institute of Heating and Ventilating Engineers 19, no. 190 (June 1957).
Table 2 gives the reflectivities of various materials and paints.
Some substances, such as glass, rock salt, liquids, and gases, are more or less transparent to radiation of certain wavelengths. Glass is transparent to wavelengths within the visible range of the spectrum, but absorbs radiation in the infrared or thermal region, while rock salt transmits a high percentage of infrared radiation. Most solids, however, are opaque to thermal radiation, and in such cases the emission and absorption of radiation are surface phenomena. Thus, the low emissivity of a burnished metal surface depends on the cleanliness of the surface. A very thin film of non-metallic material, e.g., transparent varnish or grease, will increase the emissivity of the metal surface almost to that of a black body.
Clothing and human skin radiate virtually as black surfaces. For radiation at the wavelengths encountered in buildings and other living spaces, the absorption of clothing and skin approximates that of a black object. Indoors, white clothing has no advantage over black. But outdoors in the sun, although both materials radiate heat freely, white clothing reflects most of the solar radiation, while black clothing absorbs the sun's rays.
If the human body emits more radiant energy than it receives from its surroundings, it is, on balance, losing heat by radiation. If, on the other hand, the radiation received exceeds that emitted, there is a net heat gain by the body.