Frost Protection - UTL Repository

MECHANISMS OF ENERGY TRANSFER
through a metal rod if one end is placed in a fire, where the heat is transferred
from molecule to molecule to the other end of the rod. Conduction is an
important transfer mechanism for energy storage in the soil and therefore it is
important for frost protection.
The rate that energy transfers by conduction depends on the capacity for the
material to conduct energy (i.e. thermal conductivity) and the gradient of
temperature with distance into the material. The thermal conductivity of a soil
depends on the type and relative volume occupied by soil constituents. Air is a
poor conductor of heat, so dry soils with more air spaces have lower thermal
conductivities. The thermal conductivity of dry soils varies, but it is
approximately 0.1, 0.25 and 0.3 W m -1 °C -1 for organic, clay and sandy soils. If
the soils are nearly saturated with water, the conductivity is approximately 0.5,
1.6 and 2.4 W m -1 °C -1 for the three general soil types.
There is positive conduction into the soil when the surface is warmer than the
soil below and the conduction is negative when heat conducts upward to a colder
surface. As the sun comes up, the surface is warmer than the soil below, so heat
conducts downward and is stored in the soil. As net radiation decreases in the
afternoon, the surface will cool relative to the soil below and heat is conducted
upwards towards the surface (i.e. negative flux). This negative heat flux continues
during the night as soil heat conducts upwards to replace lost energy at the cooler
surface. On an hourly basis, the soil heat flux density can change considerably
but, on a daily basis, the amount of energy going into the soil is generally about
the same as the quantity leaving the soil. In the longer term, there is a slight
deficit each day during the autumn, so the soil gradually loses energy and cools.
In the spring, there is a slight increase in energy receipt and storage each day, so
the mean daily soil temperature will gradually increase. One should always
remember that soil selection and management has both short-term (i.e. daily) and
long-term (i.e. annual) effects on soil temperature.
Soil flux heat density (G) is estimated as:
G = –K ∂T
s W m -2 Eq. 3.11
∂z
where K s is the thermal conductivity (W m -1 °C -1 ) and the second term on the
right hand side is the change in temperature with depth (°C m -1 ) called the
thermal gradient. It is not possible to directly measure soil heat flux density (G)
at the surface. If a heat flux plate is placed on the surface, then sunlight striking
the plate will cause considerably higher flux density data than the real
conduction through the soil. Burying the flux plate within 0.01 to 0.02 m of the
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