Page 156 - Analysis and Design of Energy Geostructures

128 Analysis and Design of Energy Geostructures

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energy is released per unit area [W/m ] is termed the ‘surface emissive
power’.
s. The emissivity of a surface E [ ], with values in the range 0 # E # 1, pro-
vides a measure of how efficiently a surface emits energy by radiation rel-
ative to a blackbody. It strongly depends on the surface material and
finish.
t. The main heat transfer mode within the pipes of energy geostructures is
forced convection associated with the pumping of the heat carrier fluid
circulating in the pipes.
u. Conduction governs the heat exchange between the walls of the pipe and
the concrete. Within the pipes, forced convection governs the heat
exchange.
v. In these conditions forced convection occurs within the pipes of the
energy pile, whereas conduction occurs between the pipes, the concrete
constituting the energy pile and the soil. The contact resistance between
the outer surface of the energy pile and the soil deposit may play an
important role in the heat transfer phenomenon.
w. In these conditions forced convection occurs within the pipes of the
energy pile, whereas conduction occurs between the pipes, the concrete
constituting the energy pile and the soil. Since there is no thermal insula-
tion between the slab and the soil, the system can be affected by sur-
rounding environmental variations, for example related to convection
heat transfer with the surface. However, if the underground car parking
were to be characterised by an almost constant temperature field
throughout the year, the influence of environmental variations, for
example by convection should not be significant.
x. The first metres of pipes beyond the surface are thermally insulated
because the soil temperature field in the first metres below the surface is
not stable throughout the year: radiation and convection phenomena
caused by the sun or by external air temperature flows can significantly
affect the thermohydraulic behaviour of the energy pile. The interplay
between these phenomena with rainfalls and other climatic events can
also significantly influence the thermal properties of the soil with a note-
worthy impact on the thermohydraulic behaviour of the energy pile.
Thermally insulating the pipes in this soil region allows the limiting of
the influence of the aforementioned phenomena on the thermohydraulic
behaviour of the energy pile.
y. In a dry sandy soil deposit the main heat transfer mode is conduction. If
the soil would be fully saturated with water and characterised by a