Novel ATES triplet system for autarkic space heating and cooling

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.