Frost Protection - UTL Repository

CHAPTER
3
MECHANISMS OF
ENERGY TRANSFER
MASS AND ENERGY IN THE AIR
To know the concepts of frost protection, it is important to have a good
description of the constituents of air and their relationship to energy content.
Numerically, nitrogen (N 2 ) and oxygen (O 2 ) molecules are the main
constituents of the atmosphere, with water vapour (H 2 0) being a minor (and
variable) component. Within a cubic metre of air there are more gas molecules
than stars in the universe (about 2.69×10 25 ), but the volume occupied by the
molecules is less than about 0.1 percent of the total volume of the air
(Horstmeyer, 2001). Therefore, while the number of air molecules within a
cubic metre of atmosphere is immense, the Earth’s atmosphere is mostly empty
space. However, the molecules are moving at high velocity, so there is
considerable kinetic energy (i.e. sensible heat) in the air. In this chapter, the
methods of energy transfer that control sensible heat content and hence air
temperature are discussed.
Energy transfer rates determine how cold it will get and the effectiveness of
frost protection methods. The four main forms of energy transfer that are
important in frost protection are radiation; conduction (or soil heat flux);
convection (i.e. fluid transfer of sensible and latent heat properties) and phase
changes associated with water (Figure 3.1).
Radiation is energy that comes from oscillating magnetic and electric fields
and, unlike the other transfer mechanisms, can transfer through empty space.
Good examples are the energy one feels from sunlight or from standing near a
fire. Radiation that is intercepted by a surface is commonly expressed in terms of
energy per unit time per unit surface area (e.g. W m -2 ). In frost protection, the
net radiation (R n ) is an important factor. The components that determine R n ,
including short-wave (solar) radiation downward (R Sd ) and upward (R Su ), and
long-wave radiation downward (R Ld ) and upward (R Lu ), are discussed later in
this chapter.
Conduction is heat transfer through a solid medium, such as heat moving
through a metal rod (Figure 3.1) or through the soil. Technically, soil heat can be
measured with a thermometer, so it is sensible heat, but it moves mainly by
conduction (i.e. from molecule to molecule) through the soil. When energy
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