ATES SYSTEms

Heat storage in aquifers

A double-well, low-temperature Aquifer Thermal Energy Storage (ATES) system uses two permeable reservoirs in the subsurface. The reservoirs are used cyclically to provide a supply of thermal energy in the winter and a thermal sink in the summer. The system is paired with a heat pump at the surface which can raise the temperature of the aquifer fluid to meet the heating demand in the winter, and can also drop the temperature of the aquifer fluid to meet the cooling demand in the summer. These systems provide a low-carbon solution for space heating and cooling.

Schematic of an ATES system used to buffer the heating and cooling demands of a healthcare facility

Seasonal modes of operation

During the summer, the working fluid is extracted from the cold reservoir to provide the cooling source for the heat pump system at the surface. As such, the working fluid gains thermal energy. The now warmer working fluid is injected into the hot reservoir for storage.

During the winter, the system is reversed and fluid is extracted from the hot reservoir to provide the heat pump system with a heat source. In doing so, the working fluid cools down at the surface before being injected into the cold reservoir for storage.

In this operational mode, the excess thermal energy of buildings in the summer is captured and used to recharge the hot well. The thermal sink in the winter is also captured and used to recharge the cold well. As such, the system does not use any other types of external energy to recharge the wells and so the injection temperatures are continuously evolving over time.

Operating temperatures

Aquifers in the shallow subsurface have been used for thermal energy storage mainly in northern Europe. The use of aquifers involves the direct supply and removal of water from the permeable rock, and so an ATES system can access a much larger volume of rock than a borehole system.

The temperature of the water in an ATES system depends on the depth of the aquifer. Cold water stores may have typical temperatures of order 5 – 15°C, while higher temperature systems may contain water as hot as 50 – 60°C, stored at depths up to two kilometres below the surface.