Frost Protection - UTL Repository

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F R O S T P R O T E C T I O N : F U N D A M E N T A L S , P R A C T I C E A N D E C O N O M I C S
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Radiation
Electromagnetic radiation is energy transfer resulting from oscillation of electric
and magnetic fields. A good example is sunlight or solar radiation, which
transfers huge amounts of energy to the Earth’s surface. Most of the distance
between the Sun and Earth is a vacuum (i.e. empty space), so one property of
radiation is that the heat transfer occurs even through a vacuum. Although much
cooler, objects on Earth also radiate energy to their surroundings, but the energy
content of the radiation is considerably less. The energy radiated from an object
is a function of the fourth power of the absolute temperature:
E′
= ε σ
4
T
K
W m -2 Eq. 3.15
where ε is the emissivity (i.e. the fraction of maximum possible energy emitted
at a particular temperature); σ =5.67 × 10 -8 W m -2 K -4 , the Stefan-Boltzmann
constant; and T K is the absolute temperature (T K = T a + 273.15). Assuming that
ε = 1.0, the radiation flux density from the surface of the sun at 6000 K is about
73,483,200 W m -2 , whereas radiation from the surface of the Earth at about 288
K is approximately 390 W m -2 . However, because irradiance (i.e. radiation flux
density in W m -2 ) that is received by a surface decreases with the square of the
distance from the Sun and the mean distance between the Earth and Sun is
about 150 660 000 km, the solar energy has reduced to about the solar constant
(G sc = 1367 W m -2 ) by the time it reaches the upper atmosphere of the Earth.
As the radiation passes through the atmosphere, some is reflected and some is
absorbed, so, on a clear day, only about 75 percent of solar radiation reaches the
surface. Because the earth receives solar energy on a surface area (πr 2 ) of a disk
perpendicular to the sun’s rays with a radius (r) the same as the earth but it emits
from a surface area of a sphere (4πr 2 ), the input and output of radiant energy are
in balance and the Earth’s temperature is relatively stable.
Radiant energy can be described in terms of wavelength of the radiation.
Bodies with higher temperature emit shorter wavelengths of the electromagnetic
energy. Energy emitted by a perfect emitter at 6000 K falls within the range of
0.15 to 4.0 µm, where 1.0 µm = 1.0 × 10 -6 m. Much of the high-energy (short
wavelength) radiation is absorbed or reflected as it passes through the
atmosphere, so solar radiation received at the Earth’s surface mostly falls in
wavelength range between 0.3 to 4.0 µm. The wavelength of maximum emission
(λ max ) is calculated using Wein’s displacement law as:
2897
λ max = µm Eq. 3.16
T K
60