This factsheet describes thermal energy storage in an aquifer (abbreviated ATES), a technique used for heating and cooling of buildings. It describes an individual system for one or multiple buildings (‘individual’ means there is no heat/cold transport network present to transport heat/cold over large distances such as in district heating). The technology is applicable to buildings with a relatively high cooling demand, which are found in the services sector (e.g. an office or university building). The reason for this is that heat and cold storage must be in balance to retain the heating/cooling capacity of the system. The applicability and energetic performance of ATES strongly depend on site-specific hydrogeological conditions (Department for Business, Energy and Industrial Strategy, 2016).
ATES comprises low temperature sensible heat storage in water bearing (e.g. sand) layers in the subsurface (aquifer). Depending on type of system (at least) one or two thermal wells are required for extraction and injection of water (since water is extracted from the subsurface it is called an ‘open system’). The wells are typically between 30 and 150 meters deep (Agentschap NL, 2011). At a depth of more than 500 meters, heat storage would comprise the heat of the interior of the earth, which is called geothermal energy (ECW, 2019). The technology furthermore consists of pipes, pumps and controls. Using heat exchangers, heat is transferred to the heating system in a building. A heat pump is needed to upgrade the temperature to useable levels for space heating. The heat pump is taken into account in this factsheet. The heating/cooling distribution system inside the building (costs and efficiency) is not taken into account in this factsheet. Also, peak supply heating systems (e.g. gas-fired boiler running on winter days) are not taken into account in this factsheet.
The overall temperature range of water storage is ±5ᵒC to ±25ᵒC. In summer, cold water (typically 5-8ᵒC) is extracted from the aquifer in order to provide space cooling (Bloemendal et al., 2017). The warmed-up water is returned to the aquifer. The cooling part is essential to regenerate the heat source. The ATES system must be thermally balanced over the year otherwise heating or cooling capacity will deteriorate. In winter the proces is reversed. The stored water (typically 14-18ᵒC) is supplied to buildings for heating purposes and the cooled water is returned (Bloemendal et al., 2017). A heat pump (water/water heat pump) raises the temperature to the temperature necessary for space heating. Buildings with ATES are heated using a low temperature heating system using underfloor and/or wall heating. This means that the heat distribution system for space heating inside the building works at a relatively low temperature (supply temperature to heat emitters is in range 30 to 50ᵒC). This requires a well insulated building (i.e. a building with a good energy label). Direct cooling (space cooling without using a heat pump) can be provided by pumping cold water directly through the building. In case there is also a demand for hot tapwater, this needs to be at least 60ᵒC and can be supplied for instance by a separate gas boiler, an electric boiler, or a solar water heater.