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Aquifer thermal energy storage (ATES) is the storage and recovery of thermal energy in the subsurface. ATES is applied to provide heating and cooling to buildings. Storage and recovery of thermal energy is achieved by extraction and injection of groundwater from aquifers using groundwater wells. Systems commonly operate in a seasonal mode. The groundwater that is extracted in summer, is used for cooling by transferring heat from the building to the groundwater by means of a heat exchanger. Subsequently, the heated groundwater is injected back into the aquifer, which creates a storage of heated groundwater. In wintertime, the flow direction is reversed such that the heated groundwater is extracted and can be used for heating (often in combination with a heat pump). Therefore, operating an ATES system uses the subsurface as a temporal storage to buffer seasonal variations in heating and cooling demand. When replacing traditional fossil fuel dependent heating and cooling systems, ATES can serve as a cost-effective technology to reduce the primary energy consumption of a building and the associated CO2 emissions.

System types
In its basic form, an ATES system consists of two wells (called a doublet). One well is used for heat storage, and the other for cold storage. During winter, (warm) groundwater is extracted from the heat storage well and injected in the cold storage well. During summer, the flow direction is reversed such that (cold) groundwater is extracted from the cold storage well and injected in the heat storage well. Because each well serves both as an extraction and injection well, these systems are called bi-directional. There are also mono-directional systems. These systems do not switch pumping direction, such that groundwater is always extracted at the natural aquifer temperature. Although thermal energy is stored in the subsurface, there is usually no intention to retrieve the stored energy.

Thermal energy storage can also be achieved by circulating a fluid through a buried heat exchanger, that usually consists of a horizontal or vertical pipeline. As these systems do not extract or inject groundwater, they are called closed systems and are known as borehole thermal energy storage or ground source heat pumps. Another thermal application that uses the subsurface to provide thermal energy is geothermal energy production, which commonly uses the deeper subsurface where temperature is higher.

History
The first reported deliberate storage of thermal energy in aquifers was in China around 1960. There, large amounts of groundwater were extracted to supply cooling to an industrial facility. This lead to substantial land subsidence. To inhibit the subsidence, cold surface water was injected back into the aquifer. Subsequently, it was observed that the stored water remained cold after injection and could be used for industrial cooling. Storage of thermal energy in aquifers was further suggested in the 1970s which led to field experiments and feasibility studies in France, Switzerland, US and Japan. There are no official statistics on the number and size of ATES systems worldwide. However, the Netherlands and Sweden are considered to dominate the market in terms of implementation. In Sweden there were approximately 104 ATES systems in 2012 with a total capacity of 110 MW. The number of ATES systems in the Netherlands in the same year was 2740, with a total estimated capacity of 1103 MW.

Typical dimensions
Flow rates for typical applications in the utility sector are between 20 and 150 m$3$/hour for each well. The total volume of groundwater that is stored and recovered in a year generally varies between 10 000 m$3$ and 150 000 m$3$ per well. Depth at which ATES is applied varies commonly between 20 and 200 meter below surface. Temperature at these depths is generally close to the annual mean surface temperature. In moderate climates this is around 10 $o$Celsius. In those regions cold storage is commonly applied between 5 and 10 $o$Celsius and heat storage in the range 10 to 20 $o$Celsius. Although less frequent, there are also some projects reported in which heat was stored above 80 $o$Celsius.

Legal status
The legal status of shallow geothermal installations (<400 m) is diverse among countries. Regulations for installations of wells concern the use of hazardous materials and proper backfilling of the drilling hole to avoid hydraulic short circuiting between aquifers. Other legislation concerns protection of groundwater areas for drinking water supply. Some countries adopt limits for minimum and maximum storage temperatures. For example Austria (5 - 20 $o$C), Denmark (2 - 25 $o$C) and Netherlands (5 - 25 $o$C). Other countries adopt a maximum change in groundwater temperature, for example Switzerland (3 $o$C) and France (11 $o$C).

Hydrogeological constrains
Energy savings that can be achieved with ATES are strongly depending on the geology of a site. Mainly, ATES requires the presence of a suitable aquifer that is able to accept and yield water. Therefore, thick (>10 m) sandy aquifers are selected. Natural groundwater flow may transport (part of) the stored energy outside of the capture zone of a well during the storage phase. To reduce advective heat loss, aquifers with a low hydraulic gradient are preferred. In addition, gradients in geochemical composition should be avoided, as mixing of water with different geochemistry can increase clogging, which reduces the performance of a well and lead to increased maintenance costs.