Page 120 - Analysis and Design of Energy Geostructures

92 Analysis and Design of Energy Geostructures

Forced convection dominates heat transfer in geomaterials when significant
groundwater flow occurs, such a condition typically characterising coarse-grained
soils (i.e. highly permeable materials, such as sands or gravels) and rarely fine-
grained soils (i.e. low permeable materials, such as clays). For energy pile applica-
tions, values of groundwater flow velocities in the range of v rw 5 0.5 1 m/day
have been suggested to distinguish cases in which the role of convection can be
considered negligible (for lower velocities) or relevant (for greater velocities) in the
global heat transfer problem (Chiasson et al., 2000; SIA, 2005). However, these
values need to be considered with caution.
Free convection typically occurs in soils characterised by a pore size diameter of
several millimetres (Farouki, 1986). Furthermore, it may be considered significant
for temperature levels greater than 30 C or temperature gradients greater than or

equal to 1 C/cm (Martynov, 1963).

• Either free or forced convection can characterise heat and mass transfer over
geomaterials and reinforced concrete. These phenomena may typically charac-
terise the ground surface or the interface between energy geostructures and
adjacent built environments. In this context, air is the fluid responsible for the
heat transfer via its mass flow. Forced convection characterises the heat carrier
fluid flowing in the pipes of energy geostructures. In this context, water or the
general heat carrier fluid is the fluid responsible for the heat transfer via its
mass flow.

3.6 Radiation
3.6.1 Physical phenomenon and governing equation
Radiation (e.g. electromagnetic radiation) is the mode of heat transfer emitted by mat-
ter at nonzero (absolute) temperature solely on account of this variable. This mode of
heat transfer involves a motion of thermal energy through waves of the electromag-
netic field propagating in a medium (or a vacuum). When the material medium is a
solid or a liquid, the amount of heat transferred by radiation is usually negligible com-
pared to when the medium is a gas. The emission of thermal energy is attributed to
variations in the electron configurations of the atoms and molecules constituting any
medium (Bergman et al., 2011).
The physical phenomenon of radiation can be explained by considering the inter-
action of a medium at a given temperature with a much larger bounding surface at a
different temperature. This problem is represented in Fig. 3.10 considering the interac-
tion between the ground surface at a temperature, T s , and a much larger surface theo-
retically bounding a relevant portion of the atmosphere at a different constant
temperature, T N .