|Fig. 1: ATES for cooling and heating during summer and winter, respectively. (Source: Wikimedia Commons)|
As fossil fuel resources are rapidly diminishing, sustainable energy consumption is becoming increasingly important. [1,2] Although this depletion process can be slowed by reducing excess energy wastes and conserving energy, it is eventually necessary to substitute fossil fuel and nuclear energy with renewable energy alternatives. Energy storage is an important technology for both energy conservation and efficient large-scale utilization of renewable energy.  In particular, it effectively addresses the renewable energy's inherent nature of unsteady energy supply by storing excess energy during low demand periods for later extraction during peak demand periods. [2-5] For effective utilization of renewable energy, it is also desirable to have flexibility in storage capacity, either small- or large-scale, and time, either short- (weeks) or long-term (months to seasons). 
Underground thermal energy storage (UTES) is a form of energy storage that provides large-scale seasonal storage of cold and heat in natural underground sites. [3-6] There exist thermal energy supplying systems that use geothermal energy for cooling and heating, such as the deep lake water cooling (DLWC) systems which extract naturally cooled water under deep lakes as a source of cooling energy.  However, UTES differs from them in that it is an active energy storage system.  The underground is suitable for thermal energy storage, because it has high thermal inertia. [2,5,6] If undisturbed, below a depth of 10-15 m, the ground temperature is only weakly affected by local climate variations above ground and maintains stable temperature slightly above the local annual mean air temperature. [2,5,6] Also, the large storage capacity of natural underground sites makes UTES a common form of long-term and seasonal storage. UTES can efficiently store thermal energy from sources, including the summer and winter ambient air, solar energy and by-product waste heat from industrial and other cooling processes, underground for a long period of time.  During demand periods, it can supply space cooling/heating, ventilation air precooling/preheating and process cooling. 
There are currently three common types of UTES: aquifer thermal energy storage (ATES), borehole thermal energy storage (BTES) and rock cavern thermal energy storage (CTES). [2,4-6] The suitability of each type depends on the local site conditions including geological and hydrogeological conditions. [2,4]
ATES is an open-loop energy storage system that stores thermal energy in the groundwater and the porous matrix in aquifers. [3,4,6] It was unexpectedly discovered in China when cool water, injected into aquifers to address land subsidence issues from excessive groundwater extraction, was observed to have maintained its cold temperature for a long period of time.  In ATES, the groundwater acts as the thermal energy carrier, and the energy can be either injected into or extracted from aquifers through control of the temperature and flow direction of the groundwater. 
|Fig. 2: Small- (left) and large-scale (right) BTES systems.  (Source: S. Lim.)|
A typical form of ATES consists of a set of cold and warm wells, coupled through hydraulic pumps and heat exchangers, as shown in Fig. 1. [2-4,6] Depending on their volume and storage capability, multiple wells can be used to increase the energy storage capacity.  The wells must be separated by a large enough distance to suppress the occurrence of thermal breakthroughs within the storage time of the particular systems. [2,6] For seasonal storage systems, the separation has to be larger than the distance thermal energy flows through the particular aquifers in one season. The groundwater is pumped from the extraction well during demand periods, and the heat exchangers that are thermally coupled to the pumped groundwater extract thermal energy to supply cooling and/or heating. [2-4,6] The by-product groundwater, which has been cooled or heated, is then injected into the injection well for thermal storage.
An example of large-scale ATES is the use of winter cold during the summer and summer heat during the winter, as shown in Fig. 1. During the summer, the cold well containing cool water from the previous winter acts as the extraction well. The cold is extracted from the pumped groundwater to supply space cooling, ventilation air precooling and process cooling. The by-product heated groundwater is then injected into the warm well for extraction during the winter as a source of heating energy. The warm well reversibly acts as the extraction well during the winter, and the flow of the groundwater is reversed.
BTES is another common form of UTES with the same working principle as ATES, that thermal energy is stored underground for extraction during demand periods. Unlike ATES, however, BTES is a closed-loop system that stores thermal energy in the bedrock underground and, therefore, is not limited to locations with aquifers underneath. [3,4] Borehole heat exchangers (BHEs) are installed to penetrate into the storage medium, and the thermal energy carrier circulating through the BHEs is thermally coupled to the bedrock. [2-4] The liquid, carrying thermal energy from sources including the ambient air, solar energy and process waste heat, can either store or discharge thermal energy into or out of the bedrock, as shown in Fig. 2. 
Unlike ATES, which is favorable for large-scale energy storage, BTES is suitable for both small- and large-scale energy applications, depending on the number of installed BHEs. [4,5] As a small-scale system, BTES typically supplies cooling and/or heating to single-family houses. [4-6] Large-scale BTES, on the other hand, is more applicable toward providing seasonal thermal energy storage.  Fig. 2 illustrates these two cases schematically. Among geothermal energy technologies, BTES is the most common energy storage form for supplying cooling and/or heating to houses and buildings. 
CTES is not as common as ATES or BTES, and there are only a handful number of applications today due to its high construction cost. [2,4-6] It uses underground rock caverns including abandoned mines and oil reserves to store hot water underground.  Unlike BTES, which is suitable for base loading/unloading, CTES has the advantage of providing a high loading/unloading power simply by pumping water into and out of caverns faster. 
UTES is a promising environmentally friendly form of energy storage that can efficiently utilize renewable energy in large scales. For example, Sweden is currently one of the leaders in utilizing this technology, and UTES is expected to provide 13-15% of the total space heating power in Sweden.  However, there exist some potential harms to the environment from UTES, which include a leakage of thermal energy carriers, biochemical effects on the groundwater, ecological stress due to mechanical, chemical and thermal abuse and groundwater contamination.  With potential solutions to these concerns, UTES may bring the world closer to sustainable energy consumption.
© Se Joon Lim. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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