Page 66 - Analysis and Design of Energy Geostructures

36 Analysis and Design of Energy Geostructures

heat is typically extracted from the superstructure and injected into the ground in
warm climates or during hot seasons.
Two possible uses of the energy geostructures are possible for the heat exchange
operation mode:
1. Heating and/or cooling only can be employed when the natural thermal recharge
occurring in the ground during nonoperating periods of the energy geostructure
system is sufficiently high to keep the shallow temperature field in the subsurface
undisturbed (except for the influence of climatic conditions) over time. This situa-
tion generally characterises energy geostructures in permeable soil with significant
groundwater flow.
2. Heating and/or cooling combined with heat storage has to be employed when the natural
thermal recharge occurring in the ground during nonoperating periods of the
energy geostructure system is insufficient to keep the shallow temperature field in
the subsurface undisturbed (except for the influence of climatic conditions) over
time. This situation generally characterises energy geostructures in low permeable
soil with negligible groundwater flow.

2.3.3 Heat storage operation
In the heat storage operation the primary purpose of energy geostructures is to use the
ground as a storage medium. Waste heat and solar heat is typically injected into the
ground. While solar heat is usually injected in warm climates during hot seasons for
subsequent heating use in cold seasons, waste heat (involving elevated or low tempera-
tures) can be stored for subsequent use in the ground for both heating and cooling
purposes in cool climates or cold seasons and in warm climates or hot seasons, respec-
tively. Heat storage is often required when heating or cooling needs do not match the
heating or cooling productions.

2.4 Ground source heat pump systems
2.4.1 General
Ground source heat pump systems comprise a primary and a secondary circuit that
allow the heat to be exchanged between the ground and any considered built envi-
ronment via energy geostructures. The primary circuit includes the ground heat
exchanger system. The secondary circuit characterises the built environment to be
heated or cooled.
In the primary circuit heat is exchanged between the ground and the geostructure,
and is collected to be transferred to the built environment. In the secondary circuit
heat is transferred to the built environment for heating or cooling purposes. In
between these two circuits electrically driven machines such as heat pumps or reversed