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.

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Aquifer thermal energy storage (ATES) is a technology to sustainably provide space heating and cooling. Particularly in The Netherlands the number of ATES systems has grown rapidly in the past decade, often with the (re)development of urban areas. To meet objectives for greenhouse gas emission reduction the number of ATES systems is expected and required to further rise in future both in The Netherlands and elsewhere. To evaluate the lessons learned and the role of practical aspects in the Dutch development of ATES systems, in this study the geohydrological conditions and well characteristics for 331 (~15% of total) Dutch ATES systems are evaluated with respect to optimal well design for maximal thermal energy recovery. The study shows that well design of most (70%) ATES systems is suboptimal. The well design criteria that have been used thus far in practice, focus on allowing maximum flow/capacity, disregarding the effect of groundwater flow on efficiency and the effect of well design on subsurface space use. Instead, well design should be based on a more representative value for the storage volume that takes into account . Based on monitoring data and analysis of variations and uncertainties of the actual storage volume, a guideline is defined to reflect these in the storage volume used for design. Also a guideline for well design is introduced that accounts for both conduction and dispersion losses as well as advection losses in case of high ambient groundwater flow. @en

Water quality deterioration is a common occurrence that may limit the recovery of injected water during aquifer storage and recovery (ASR) operations. This limitation is often induced by the oxidation of the reduced aquifer components by the oxygenated injection water. This study explores the potential of aquifer pre-oxidation using permanganate to improve the quality and volume of the recovered water during ASR. An experimental ASR column setup was developed to simulate the oxygenated water injection and recovery cycles. Undisturbed sediments from an anoxic brackish aquifer at a pilot ASR site were used. A series of 4 conventional ASR cycles injecting oxygenated tap (drinking) water was initially performed. These experimental trials showed a persistent Mn(II) production due to the dissolution of a Mn-containing carbonate that was triggered by pyrite oxidation reactions, as shown by the observed sulfate production. The rise in the Mn(II) concentrations above the drinking water standards would limit the recovery to 15–30% of the injected water without treatment of the recovered water. To a lesser extent, arsenic production resulting from the oxidative dissolution of pyrite posed a water quality threat to the ASR operation. Consequently, a second series of experiments was performed with an oxidation cycle using a dilute (5%) potassium permanganate (KMnO4) solution, aimed at deactivating the reactive phases responsible for the acidity-triggered Mn(II) production. This pre-treatment cycle improved the net recovery ratio to 84% during a conventional ASR cycle using oxygenated tap water. The extent of pyrite oxidation was decreased by 63% after the permanganate treatment. The increased competition for oxygen by the adsorbed Fe(II) and Mn(II) on the newly precipitated Mn-oxides combined with the pyrite “deactivation” by removal of the most reactive iron-sulfide crystals during the permanganate flush were primarily responsible for the observed decrease in the pyrite oxidation. The stability of the Mn-oxide precipitates was tested by flushing the columns with native groundwater before performing a final ASR cycle, simulating an increase in the reducing conditions during static periods in the ASR system. The Mn-oxide reduction by ferrous iron in the native groundwater released substantial amounts of Mn(II), which adversely affected the ASR operation in the subsequent cycle. In these cases, repeating the permanganate treatment should be considered.@en