Governments and companies have set high targets in avoiding CO2 emissions and reducing energy. Aquifer Thermal Energy Storage (ATES) systems can contribute by overcoming the temporal mismatch between the availability of sustainable heat (during summer) and the demand for heat (during winter). Therefore, ATES is an increasingly popular technique; currently over 3000 low temperature ATES systems are operational in the Netherlands. Low-temperature ATES systems use heat pumps to allow the stored heat to be supplied at the required temperature for heating (usually around 40-50°C) and for cooling in summer. Although on average a conventional low-temperature ATES system produces 3-4 times lower CO2 emissions when compared to gas heating, the heat pumps still require substantial amounts of external electricity, causing over 60% of the remaining primary energy use. In the ATES triplet system, the temperatures in the hot and cold wells of an ATES system are increased and decreased respectively to match the required delivery temperatures and a third well is added at an intermediate temperature. With this strategy, other sources of sustainable heat and cooling capacity can supply the subsurface close to the temperatures required in the hot and the cold well. However, the return temperatures from the building systems do not conform with either of the hot or cold wells and an additional well is used to store water at the return temperature. Additional components are then required to supply the hot and cold wells (from the third well) by increasing the temperature in summer (e.g. solar collectors) and decreasing it in winter (e.g. dry coolers). In this study the feasibility of this concept is evaluated. Simulations and an economical evaluation show significant potential for triplet ATES with economic performance better than conventional ATES while the CO2 emissions are reduced by a factor of ten. As the temperature differences are larger, the volume of groundwater required to be pumped is considerably lowered, causing an additional energy saving. Ongoing research focusses on analysing the energy balance and energy loss in the subsurface, well design requirements, working/operational conditions of each well, as well as the integration of building system components, such as the influence of weather conditions on performance of system components.

Aquifer Thermal Energy Storage (ATES) is mostly used to store heat and cold in groundwater at relatively low temperatures for heating and cooling buildings. These systems emit 3-4 times less CO2 when compared to gas heating, but still require substantial amounts of electricity to run due to the use of a heat pump ( 60%). In typical ATES systems in the Netherlands, when there is a cooling demand, groundwater is pumped from the cold well for cooling, raising the temperature of the water to 18°C which is then injected in the warm well. When there is a heating demand water is pumped from the warm storage well and concentrated using a heat pump to the required heating temperature of the building (40-50°C). This process typically cools down the water to 7°C which is then injected into the cold well. Storing groundwater at a temperature that matches the required heating and cooling temperature can reduce or eliminate the need for a heat pump. This can be achieved by using sustainable sources to supply the heat and cold from the environment (e.g. solar panels, dry coolers). However, the availability of these sources can be insufficient to reach the required temperature level. Therefore a third well is added where water at the return temperature after building heating or cooling is stored, until it can be again heated or cooled to temperatures matching the demand. This is the concept of an ATES triplet. Initial simulations are presented which showa factor of 10 reductions in CO2 emissions compared to conventional systems, while the systems are calculated to have an improved economic performance, although require a higher initial investment. Further research will investigate the subsurface and above ground system layout and operational conditions which impact the economic and environmental performance (CO2,thermal efficiency and pollution).