Page 155 - Analysis and Design of Energy Geostructures

Heat and mass transfers in the context of energy geostructures 127

ð
λ 5 nλ a 1 1 2 nÞλ s 5 0:25 W=ðm CÞ
The thermal conductivity of water at ambient temperature is

λ w 5 0:58 W/(m C). Hence, for the case in which the sand is saturated
with water the effective thermal conductivity reads

λ sat 5 nλ w 1 1 2 nÞλ s 5 0:48 W=ðm CÞ
ð
The above explains why a thermal conductor present in soil pores,
such as water, strongly increases the heat exchange between, for example
an energy pile and the surrounding soil, in contrast to what happens with
a thermal insulator like air.
n. The effective thermal conductivity can be defined as

λ 5 nλ a 1 1 2 nð Þλ s

The thermal conductivity of air at ambient temperature is

λ a 5 0:025 W/(m C), hence λ 5 nλ a 1 1 2 nð Þλ s 5 0:1 W/(m C), which is
five times less than the value obtained for the saturated sand. Therefore
in general, the heat exchange is less favourable for an energy geostructure
adjacent to a saturated clayey deposit than to a saturated sandy deposit.
o. Thermal conductivity for concrete goes from about 0.15 to about 2 W/
(m C), depending, for example on its structure and mix design.

p. Energy is transferred by the macroscopic motion of the fluid, in addition
to energy transfer due to random molecular motion (diffusion). The fluid
motion is associated with the fact that large numbers of molecules are
moving collectively or as aggregates. This motion, in the presence of a
temperature gradient, contributes to heat transfer. Since the molecules in
the aggregate retain their random motion, the total heat transfer is then
due to a superposition of energy transport by the random motion of the
molecules and by the bulk motion of the fluid. The term convection is
usually used when referring to this cumulative transport, and the term
advection usually refers to transport due to bulk fluid motion.
q. Convection heat transfer can be classified according to the nature of the
flow. Forced convection characterises heat flows caused by external means,
such as a pump, a fan or atmospheric winds. In contrast, free (or natural)
convection characterises heat flows induced by buoyancy forces, which are
due to density differences caused by temperature variations in the fluid.
r. Thermal radiation is energy emitted by matter that is at a nonzero tem-
perature. Radiation that is emitted by the surface originates from the
thermal energy of matter bounded by the surface and the rate at which