Page 70 - Analysis and Design of Energy Geostructures

40 Analysis and Design of Energy Geostructures

ground via the pipes of the energy geostructures to warm up again. The refrigerant
gas, at low pressure and relatively low temperature, then passes to the compressor.
In the compressor, this gas is compressed by using external energy (e.g. electrical
power) to a higher temperature. The refrigerant gas, now at a relatively high pressure
and temperature, afterward passes to the condenser.
In the condenser the resulting hot gas supplies the gained heat to a heat carrier
fluid circulating in the secondary circuit by condensing (at a much higher tempera-
ture than at which it boiled). Eventually the hot liquid refrigerant at high pressure
passes through an expansion valve that returns the pressure and temperature of the
liquid to its original conditions prior to reentering the evaporator where it starts a
new cycle.
The aforementioned process is reversed when reversed heat pumps are used, the
refrigerant condensation heating the heat carrier fluid circulating in the primary circuit,
which is reinjected in the ground to cool down again.
Water-to-water heat pumps used in energy geostructures applications are available
in numerous sizes for different possible uses. Depending on the use the heat pump
must be supplied with a constant water flow coming from the source. The required
water flow can range from a couple of cubic metres per hour for relatively small heat
pumps characterised by a peak power of 4 6 kW up to 60 cubic metres per hour for
300 kW heat pumps. The optimal required source water flow is usually provided in
the heat pumps’ technical information. The considered water flow corresponds to that
of the header pipe collecting the several pipes included in the various subsystems that
constitute the energy geostructure(s) (e.g. piles and tunnel rings), the manifold(s) and
other connections.

2.4.4 The secondary circuit
Heat exchange in the built environment is typically achieved in the secondary circuit
through heat exchangers such as radiant heating floors or ceilings, radiators, fan coil
units, etc. Temperature values that are adequate to reach comfort levels in living spaces
and advantageous for engineering applications (e.g. deicing of infrastructures) can be
achieved through energy geostructures with a highly efficient use of primary energy
(Batini et al., 2015).
The distribution schemes employed for the secondary circuit are typically the same
as conventional systems and details of these may be found in standard heating, ventila-
tion and air-conditioning references (see, e.g. ASHRAE, 2011). However, one must
note that ground source heat pump systems may deliver hot air or water at tempera-
tures marginally lower than that of other conventional heating, ventilation and air-
conditioning systems (Narsilio et al., 2014): hence due consideration should be taken
for the design of ducting and piping in the secondary circuit.