Energy storage technologies have a large role to play in a low-carbon society. For instance, energy storage helps to address renewable energy intermittency. Storing either electrical or thermal energy prolongs the period in which renewable energy can deliver its energy, and deliver it when the demand is there. Moreover, energy storage technologies can be used as an energy efficiency measure in structures by making smart use of heat and cold storage. This reduces the need for heating and cooling in the structure.
Energy can be stored in a variety of ways. For instance, the technology of phase change materials stores thermal energy, and batteries or flywheels can store electrical energy. Underground Thermal Energy Storage can be performed in two main ways: Aquifer Thermal Energy Storage (ATES) and Borehole Thermal Energy Storage (BETS). ATES is illustrated in Figure 1, which provides the possibility of balancing energy demand between summer and winter. This is in contrast with phase change materials, which can only balance between shorter time periods of day and night. Heat and/or cold is stored in underground reservoirs and extracted when demand for the thermal energy is there.
Next to borehole and aquifer storage other methods for underground thermal energy storage are available. These include cavern storage and pit storage. With cavern storage and pit storage, large underground water reservoirs are created in the subsoil to serve as thermal energy storage systems. These storage technologies are technically feasible, but the actual application is still limited because of the high level of investmentWhich of these technologies is selected strongly depends on the local geologic conditions. This CTW description focuses on BETS and ATES.
Countries in temperate climates experience four seasons. In a technology known as Underground Thermal Energy Storage (UTES), energy sources charge a subsurface store for use at a later season. An example is the use of winter's cold to charge a store which will be used in summer to cool a building. Similarly, solar energy can be stored in summer for use in winter. Such seasonal storage of thermal energy (Seasonal Thermal Energy Storage-STES) can be accomplished in rocks, caverns, tanks and gravel beds. In North America and Europe, as well as northern China, winters are relatively cold and summers are relatively warm. This seasonal variation in temperatures is ideal for UTES.
Thermal energy storage may refer to a number of technologies that store energy in a thermal reservoir for later reuse. They can be employed to balance energy demand between day time and night time. The thermal reservoir may be maintained at a temperature above (hotter) or below (colder) than that of the ambient environment.
Aquifer Thermal Energy Storage
The most frequently used storage technology, which makes use of the underground, is Aquifer Thermal Energy Storage. This technology uses a natural underground layer (e.g. a sand, sandstone, or chalk layer) as a storage medium for the temporary storage of heat or cold. In ATES, groundwater is pumped from an aquifer for addition or extraction of thermal energy. The groundwater at the changed temperature is then injected back into the same or another aquifer for storage in the aquifer medium (soil or rock). In the opposite season, the stored thermal energy from the aquifer is recovered by pumping out the groundwater, using the stored energy and re-injecting the groundwater at a changed temperature back into the aquifer. Of course, to minimize thermal mixing within the aquifer, the supply and injection wells have to be spaced an appropriate distance apart.
There are three essential requirements for the successful application of ATES:
1. A suitable aquifer with low groundwater flow velocities (to reduce energy losses);
2. High quality, high efficiency groundwater production and injection wells;
3. A suitable source of low cost thermal energy with a later demand for stored energy.
Borehole Thermal Energy Storage (BETS)
With borehole storage, vertical heat exchangers are inserted into the underground, which ensure the transfer of thermal energy towards and from the ground (clay, sand, rock, etc.). Meanwhile about a dozen of projects has been completed in the participating countries. Many of these projects are about the storage of solar heat in summer for space heating of houses or offices. Ground heat exchangers are also frequently used in combination with heat pumps, where the ground heat exchanger extracts low-temperature heat from the soil.
Similar conditions to the ATES apply for BETS. However, since BETS depends on drilled boreholes, it is not necessary to have an underground reservoir in the form of an aquifer.
Several hundred of ATES projects have been realized (IEA, no date). A major condition for the application of this technology is the availability of a suitable geologic formation.
The technology is especially suitable in combination with renewable thermal energy, such as solar thermal. During the summer, solar energy is collected through solar collectors and stored. In winter, when demand for heating is highest, the stored energy can be used to heat the building.
Both ATES and BETS are capable of achieving considerable energy savings. For instance, reductions in electricity consumption for cold production up to 80 % have been achieved (Paksoy et al., 2004). For heating, percentages of 20-30 % have been achieved (Paksoy et al., 2004). Not only does the implementation of UTES result in the conservation of energy and a better energy economy, it will also improve the environment by reducing emissions associated with electricity or heat production.
There are a number of factors that influence the cost of an energy storage technology. Storage tends to be an application-specific resource and therefore the costs (and benefits) can vary greatly. One of the complications in developing detailed cost estimates of energy storage technologies is that the costs of a given technology are greatly influenced by the particular application in which that technology is deployed. Thus, any generalized cost estimates are of questionable value.
An energy storage system’s size varies on two dimensions: power (how much energy can be discharged at one time) and capacity (how many hours can be discharged continuously). Energy storage system costs are impacted by system efficiency (how many useable kWh, or equivalent unit of energy, can be discharged compared to the amount charged). The frequency of how often and deeply the system is discharged also impacts costs. All of these factors (size, efficiency, and frequency) mean that an energy storage technology’s cost cannot be meaningfully estimated independently of the way in which it is used.
The lifecycle cost of an energy storage system is made up of two basic components: capital costs and operating and maintenance (“O&M”) costs. O&M costs include the cost of buying the energy used to charge the system, fixed costs that do not depend on how much or often the system is used, and variable costs, the bulk of which are replacement costs.
Amrath, no date. Image retrieved from: http://www.amrathamsterdam.com/en/about-us/Environment/ 
IEA, no date. Information retrieved from the International Energy Association Energy Conservation through Energy Storage (IEA ECES) website: http://www.iea-eces.org/energy-storage/storage-techniques/underground-thermal-energy-storage.html 
Paksoy et al., 2004. "Cooling in all climates with thermal energy storage". Presentation at the International Energy Agency FBF Conference of June 21-22 of 2004. Document can be found online at: www.iea.org/work/2004/cooling/HalimePaksoy.pdf 
For pumped hydro and electrical energy storage see: http://www.electricitystorage.org/ESA/technologies/ 
California Energy Storage Alliance (CESA). Information retrieved from the CESA website: http://storagealliance.org/whystorage-resources.html 
Texas Energy Storage Alliance (TESA). Information retrieved from the TESA website: http://www.texasenergystorage.org/ 
Energy Conservation through Energy Storage (ECES). International Energy Agency workgroup. Information retrieved from the IEA ECES website: http://www.energy-storage.org/energy-storage/success-stories/sustainable-heating-and-cooling-by-utes.html 
Energy Storage Systems (ESS) research at Sandia national Laboratories. Information retrieved from the ESS Sandia website: http://www.sandia.gov/ess/