Frost Protection - UTL Repository

MECHANISMS OF ENERGY TRANSFER
Sensible heat is energy that we can “sense”, and temperature is a measure of the
sensible heat content of the air. When the sensible heat content of air is high, the
molecules have higher velocities and more collisions with each other and their
surroundings, so there is more kinetic energy transfer. For example, a
thermometer placed in warmer air will have more air molecule collisions,
additional kinetic energy is transferred to the thermometer and the temperature
reads higher. As sensible heat in the air decreases, the temperature drops. In frost
protection, the goal is often to try to reduce or replace the loss of sensible heat
content from the air and plants. Sensible heat flux density (H) is the transfer of
sensible heat through the air from one place to another. The flux density is
expressed as energy per unit time per unit surface area (e.g. W m -2 ) that the
energy passes through.
Latent heat is released to the atmosphere when water is vaporized and the
latent heat content of the air depends on the water vapour content. Latent heat
changes to sensible heat when water changes phase from water vapour to liquid
water or ice. As water vapour moves, the flux density is expressed in units of
mass per unit area per unit time (e.g. kg m -2 s -1 ). Multiplying by the latent heat
of vaporization (L) in J kg -1 converts the water vapour flux density from mass
units to energy units. Therefore, the flux is expressed in energy per unit time per
unit surface area or power per unit surface area (e.g. W m -2 ). The water vapour
content of the air is a measure of the latent heat content, so humidity expressions
and the relationship to energy are discussed in this chapter.
Energy balance
Sign convention
Positive and negative signs are used in transfer and balance calculations to
indicate the direction of energy flux to or from the surface. Any radiation
downward to the surface adds to the surface energy and therefore is considered
positive and given a “+” sign. Any radiation away from the surface removes
energy and it is considered negative with a “-” sign. For example, downward
short-wave radiation from the sun and sky (R Sd ) is positive, whereas short-wave
radiation that is reflected upward from the surface (R Su ) is negative. Downward
long-wave radiation (R Ld ) is also given a positive sign since it adds energy to the
surface and upward long-wave radiation (R Lu ) is given a negative sign. Net
radiation (R n ) is the “net” amount of radiant energy that is retained by the
surface (i.e. the sum of all gains and losses of radiation to and from the surface).
These relationships are illustrated for (a) daytime and (b) night-time in
Figure 3.2. Note, in the equation, that net radiation is equal to the sum of its
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