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44 Analysis and Design of Energy Geostructures
2.5.2 Efficiency of underground thermal energy storage systems
Underground thermal energy storage systems can be coupled to ground source heat
pump systems and can be realised with or without heat pumps or reversed heat pumps.
There are many ways to measure the performance of such systems (Lanahan and
Tabares-Velasco, 2017).
When heat pumps or reversed heat pumps are used in underground thermal energy
storage systems coupled with ground source heat pump systems and are the primary
driver for such technological systems, the COP of the pumps can be used for this pur-
pose. This approach is particularly favourable when the goal of the installation is to
improve heat pump performance by raising evaporator temperatures (Rad et al., 2013;
Nam et al., 2015; Omer, 2008; Wang and Qi, 2008).
In general, however, the performance of underground thermal energy storage sys-
tems is defined in terms of the efficiency
η UTES 5 Total amount of heat extracted from the energy system ½kWh ð2:3Þ
Total amount of heat injected into the storage ½kWh
When underground thermal energy storage systems employ solar thermal panels to
inject heat into the ground, two particular definitions of efficiency become useful and
these are the total system efficiency, defined as (Sweet and McLeskey, 2012):
η UTES;tot 5 Total amount of heat provided to the energy system ½kWh ð2:4Þ
Total amount of incident solar radiation on solar collectors ½kWh
and the internal system efficiency, defined as (Sweet and McLeskey, 2012):
η 5 Total amount of heat provided to the energy system ½kWh ð2:5Þ
UTES;int
Total amount of heat provided by solar collectors ½kWh
While the efficiency provides a better understanding of required energy storage
size, the total system efficiency characterises the overall performance of the system and
the internal system efficiency provides a better understanding of how well the system
meets an energy goal (Lanahan and Tabares-Velasco, 2017).
2.6 Application and development of energy geostructures
2.6.1 Historical facts
Energy geostructures have been increasingly applied worldwide since the 1980s
(Brandl, 2006). To date the highest number of energy geostructure applications
involves energy piles, whereas fewer projects involve energy walls, energy slabs and
energy tunnels. Only the advent of the ‘Grand Paris Express’ project in France, which
