Page 73 - Analysis and Design of Energy Geostructures

Energy geostructures 43

2.5 Underground thermal energy storage systems
2.5.1 General
Underground thermal energy storage systems allow the heat collected from solar
thermal panels or in excess from built environments to be exchanged for storage
purposes in the ground. Different storage strategies can be achieved depending on
the technology or approach used for this storage, resulting in so-called (1) hot
water energy storage; (2) gravel water thermal energy storage; (3) aquifer thermal
energy storage; (4) borehole thermal energy storage; and (5) energy geostructure
storage. The latter systems are of particular interest herein and involve the heat
exchange between any considered built environment and the ground via energy
geostructures.
The storage can be targeted on a daily or seasonal basis, the coupling between
these solutions being characterised by showing the highest promise. In fact since
seasonal storage might have slow charging or discharging rates, coupling seasonal
storage with diurnal storage might bridge this gap (Lanahan and Tabares-Velasco,
2017).
In ground source heat pump systems the heat exchange between energy geostruc-
tures and the surrounding ground should be maximised. In contrast in underground
thermal energy storage systems the heat exchange between energy geostructures and
the surrounding ground should be minimised to preserve heat storage. Underground
thermal energy storage systems are often considered to hold little promise if applied
via a limited number of energy geostructures such as piles (Ingersoll et al., 1954).
However, where the site conditions are favourable and significant amounts of heat can
be stored via substantial (or multiple) energy geostructures, underground thermal
energy storage systems can represent an advantageous solution. The ground occupa-
tion of these systems is limited and, among different storage technologies, they can be
characterised by approximately 100% round trip efficiency of thermal energy storage
and recovery, compared to the 80% efficiency batteries possess (Denholm et al., 2012,
2015; Evans et al., 2012).
Underground thermal energy storage systems established via energy geostructures
can be particularly effective as compared to other storage systems achieved via aquifers
or gravel water systems, because they are not limited to specific formations as per the
former solutions (Dincer and Rosen, 2002; Rad et al., 2013; Kalaiselvam and
Parameshwaran, 2014; Xu et al., 2014; Rad and Fung, 2016). However, as reported
by Lanahan and Tabares-Velasco (2017), limitations of underground thermal energy
storage systems applied with elements such as energy piles include the comparatively
large amount of heat loss compared to insulated water tank or gravel tank systems
(Schmidt and Mangold, 2006; Rad and Fung, 2016).