For efficient operation of heating and cooling grids, underground thermal energy storage (UTES) can be a key element. This is due to its ability to seasonally store heat or cold addressing the large mismatch between supply and demand. This technology is already available and there are many operational examples, both within and outside a district heating network. Given the range of available UTES technologies, they are feasible to install almost everywhere. Compared to other storage systems, UTES have the advantage of being able to manage large quantities and fluxes of heat without occupying much surface area, although the storage characteristics are always site specific and depend on the geological and geothermal characteristics of the subsoil. UTES can manage fluctuating production from renewable energy sources, both in the short and long term, and fluctuating demand. It can be used as an instrument to exploit heat available from various sources, e.g., solar, waste heat from industry, geothermal, within the same district heating system. The optimization of energy production, the reduction in consumption of primary energy and the reduction in emission of greenhouse gases are guaranteed with UTES, especially when coupled with district heating and cooling networks.

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Larger well diameters allow higher groundwater abstraction rates. But particularly for the construction of wells at greater depth, it may be more cost-efficient to only expand the borehole in the target aquifer. However, current drilling techniques for unconsolidated formations are limited by their expansion factors (<2) and diameters (<1000 mm). Therefore, we developed a new technique aiming to expand borehole diameters at depth in a controlled manner using a low-pressure water jet perpendicular to the drilling direction and extendable by means of a pivoting arm. During a first field test, the borehole diameter was expanded 2.6-fold from 600 to 1570 mm at a depth of 53.5 to 68 m and equipped with a well screen to create an expanded diameter gravel well (EDGW). In keeping with the larger diameter, the volume flux per m screen length was two times higher than conventional 860 mm diameter wells at the site in the subsequent 3 year production period. Although borehole clogging was slower on a volumetric basis and similar when normalized to borehole wall area, rehabilitation of particle clogging at the borehole wall was more challenging due to the thickness of the gravel pack. While jetting the entire borehole wall before backfilling holds promise to remove filter cake and thus limit particle clogging, we found that a second borehole (expanded 4.1-fold to 2460 mm) collapsed during jetting. Overall, the EDGW technique has potential to make the use of deeper unconsolidated aquifers economically (more) feasible, although further understanding of the borehole stability and rehabilitation is required to assess its wider applicability.

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Low temperature (<25 °C) Aquifer Thermal Energy Storage (ATES) systems have a world-wide potential to provide low-carbon space heating and cooling for buildings by using heat pumps combined with the seasonal subsurface storage and recovery of heated and cooled groundwater. ATES systems increasingly utilize aquifer space, decreasing the overall primary energy use for heating and cooling for an urban area. However, subsurface interaction may negatively affect the energy performance of individual buildings with existing ATES systems. In this study, it is investigated how aquifer utilization levels, obtained by varying well placement policies, affect subsurface interaction between ATES systems and how this in turn affects individual primary energy use. To this end, a building climate installation model is developed and integrated with a MODFLOW-MT3DMS thermal groundwater model. For the spatial distribution and thermal requirements of 26 unique buildings as present in the city centre of Utrecht, the placement of ATES wells is varied using an agent-based modelling approach applying dense and spacious placement restrictions. Within these simulations ATES adoption order and well placement location is randomized. Well placement density is varied for 9 scenarios by changing the distance between wells of the same and the opposite type. The results of this study show that the applied dense well placement policies lead to a 30% increase of ATES adoption and hence overall GHG emission reduction improved with maximum 60% compared to conventional heating and cooling. The primary energy use of individual ATES systems is affected at varying well placement policies by two mechanisms. Firstly, at denser well placement, ATES systems are able to place more wells, which increases the capacity of their ATES system, thereby decreasing their electricity and gas use. Secondly, aquifer utilization increases with denser well placement policies and thus interaction between individual ATES increases. At subsurface utilization up to 80%, individual primary energy use does not change significantly due to subsurface interaction. At aquifer utilization level > 80%, both negative and positive interaction is observed. Negative interaction between wells of the opposite type leads to an increase of gas or electricity use up to 15% compared to spacious well placement. On the other side, buildings may experience a maximum decrease of 15% electricity use at dense well placement due to positive interaction between wells of the same type. Local conditions like building location, plot size, distance to other buildings and heating/cooling demand determine the specific effect per building. The optimal well placement policy result from the aquifer utilisation levels discussed above. Maximum GHG emission reduction while maintaining individual ATES system performance, is achieved with well distances of 0.5–1 times the yearly average thermal radius for wells of the same type (cold-cold and warm-warm). Opposite well types (cold-warm) should be placed apart ∼2 times the thermal radius to prevent negative subsurface interaction.

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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). @en

In the energy transition, multi-energy systems are crucial to reduce the temporal, spatial and functional mismatch between sustainable energy supply and demand. Technologies as power-to-heat (PtH) allow flexible and effective utilisation of available surplus green electricity when integrated with seasonal heat storage options. However, insights and methods for integration of PtH and seasonal heat storage in multi-energy systems are lacking. Therefore, in this study, we developed methods for improved integration and control of a high temperature aquifer thermal energy storage (HT-ATES) system within a decentralized multi-energy system. To this end, we expanded and integrated a multi-energy system model with a numerical hydro-thermal model to dynamically simulate the functioning of several HT-ATES system designs for a case study of a neighbourhood of 2000 houses. Results show that the integration of HT-ATES with PtH allows 100% provision of the yearly heat demand, with a maximum 25% smaller heat pump than without HT-ATES. Success of the system is partly caused by the developed mode of operation whereby the heat pump lowers the threshold temperature of the HT-ATES, as this increases HT-ATES performance and decreases the overall costs of heat production. Overall, this study shows that the integration of HT-ATES in a multi-energy system is suitable to match annual heat demand and supply, and to increase local sustainable energy use.@en

Open bodemenergiesystemen dragen bij aan de overgang naar duurzame energie en kunnen bijna overal in Nederland worden toegepast. Lokale ondergrondse condities en systeemeigenschappen bepalen echter de efficiëntie van het systeem. Er is nog geen gedetailleerde kaart van de invloed van deze eigenschappen op het rendement van bodemenergiesystemen beschikbaar. In dit artikel wordt het verlies in terugwinefficiëntie door geleiding en achtergrondstroming geanalyseerd. Hiermee wordt voor elke plek in Nederland inzichtelijk met welke van deze verliezen rekening moet worden gehouden bij het ontwerpen van bodemenergiesystemen.@en

Energy consumption for space heating and cooling of buildings can be decreased by 40-80% by use of Aquifer Thermal Energy Storage (ATES). ATES is a proven technique, however, it is not known how efficient currently operating systems are recovering stored energy from the subsurface and how this can be determined with available data. Recent research suggests that storage conditions have a large influence on the recovery (e.g. shape and size of stored volume). In addition, literature and previous research show that other aspects of ATES system are often unfavorable (e.g. subsurface energy imbalance, small ΔT). Therefore, the main goal of this research is to define a framework to determine overall performance of ATES systems by analysis of monitoring data from operational ATES systems. The province of Utrecht was selected for this. Monthly operational data of 57 ATES systems (40% of the ATES systems in the province) was provided by the authorities and pre-processed accordingly. Results showed that recovery efficiency is positively correlated to system size (stored volume) and that ambient groundwater temperature is site-specific and should be determined for each ATES system individually. Ambient groundwater temperature can vary more than 4 °C and are spatially correlated. Next to this, a large part of the analyzed systems are not equally storing heat and cold in the subsurface. More than 80% of the studied ATES systems have an subsurface heat imbalance larger than 10%. Altogether, results indicate that a big part of the ATES systems in the province of Utrecht can substantially improve their ATES system (management) to increase long-term energy savings. This research provides an useful assessment framework to determine if an ATES system is performing correctly and what aspect of the specific ATES system needs most improvement.@en

Aquifer Thermal Energy Storage (ATES) systems combined with a heat pump save energy for space heating and cooling of buildings. In most countries the temperature of the stored heat is allowed up to 25-30°C. However, when heat is available at higher temperatures (e.g. waste heat, solar heat), it is more efficient to store higher temperatures because that improves heat pump performance or makes it unnecessary. Therefore, interest in HT-ATES development is growing. Next to developing new HT-ATES projects, there is also a large potential for additional energy savings by transforming ‘regular’ low-temperature LT-ATES systems to a HT-ATES. Such a transformation is tested for a greenhouse system in the Netherlands. This greenhouse has a LT-ATES system operational since 2012, and from 2015 onwards heat is stored in the warm well at temperatures up to 45°C. In this HT-ATES transformation pilot, water quality parameters are closely monitored as well as temperature distribution in the subsurface (using DTS). Together with the operators, the results from the ATES monitoring are used to continuously improve system performance. Numerical groundwater and heat flow simulations of actual and expected well pumping data are used to evaluate how well operation can be optimized. In this paper, the optimization using monitoring results and simulations is discussed as well as general and site specific lessons/conclusions for such transformations.@en

Aquifer thermal energy storage (ATES) is a technology with worldwide potential to provide sustainable space heating and cooling using groundwater stored at different temperatures. The thermal recovery efficiency is one of the main parameters that determines the overall energy savings of ATES systems and is affected by storage specifics and site-specific hydrogeological conditions. Although beneficial for the optimization of ATES design, thus far a systematic analysis of how different principal factors affect thermal recovery efficiency is lacking. Therefore, analytical approaches were developed, extended and tested numerically to evaluate how the loss of stored thermal energy by conduction, dispersion and displacement by ambient groundwater flow affect thermal recovery efficiency under different storage conditions. The practical framework provided in this study is valid for the wide range of practical conditions as derived from 331 low-temperature (<25 °C) ATES systems in practice. Results show that thermal energy losses from the stored volume by conduction across the boundaries of the stored volume dominate those by dispersion for all practical storage conditions evaluated. In addition to conduction, the displacement of stored thermal volumes by ambient groundwater flow is also an important process controlling the thermal recovery efficiencies of ATES systems. An analytical expression was derived to describe the thermal recovery efficiency as a function of the ratio of the thermal radius of the stored volume over ambient groundwater flow velocity (Rth/u). For the heat losses by conduction, simulation results showed that the thermal recovery efficiency decreases linearly with increasing surface area over volume ratios for the stored volume (A/V), as was confirmed by the derivation of A/V-ratios for previous ATES studies. In the presence of ambient groundwater flow, the simulations showed that for Rth/u <1 year, displacement losses dominated conduction losses. Finally, for the optimization of overall thermal recovery efficiency as affected by these two main processes, the optimal design value for the ratio of well screen length over thermal radius (L/Rth) was shown to decrease with increasing ambient flow velocities while the sensitivity for this value increased. While in the absence of ambient flow a relatively broad optimum exists around an L/Rth-ratio of 0.5–3, at 40 m/year of ambient groundwater flow the optimal L/Rth-value ranges from 0.25 to 0.75. With the insights from this study, the consideration of storage volumes, the selection of suitable aquifer sections and well screen lengths can be supported in the optimization of ATES systems world-wide. @en

Bodemenergiesystemen worden veelvuldig toegepast om energie te besparen. De warmtepomp van zulke systemen gebruikt echter nog altijd veel elektriciteit, waardoor voor grootschalige toepassing ook grootschalige netverzwaring nodig is. Daarom is het voor de verduurzaming van de gebouwde omgeving belangrijk om alternatieve duurzame technieken voor verwarming en koeling van gebouwen te vinden. Het WKO-‘triplet’-systeem vermijdt elektriciteitsverbruik door de warmtepomp door warmte en koude op het gewenste temperatuurniveau in te vangen en in de bodem op te slaan met (bijvoorbeeld) zonnecollectoren en droge koelers. Het primair energieverbruik van een tripletsysteem is ongeveer 5% van een conventioneel systeem.@en

To be able to overcome water shortages, Abu Dhabi Emirate started an Aquifer Storage and Recovery (ASR) project with desalinated seawater (DSW) as source water near Liwa. It is the largest DSW-ASR project in the world (stored volume ~10 Mm³/year), and should recover potable water for direct use. DSW is infiltrated into a desert dune sand aquifer using "sand-covered gravel-bed" recharge basins. In this study, we evaluate the hydrogeological and hydrogeochemical stratification of the (sub)oxic target aquifer, and water quality changes of DSW during trial infiltration runs. We predict water quality changes of DSW after 824 d of infiltration, during 90 d of intensive recovery (67% recovered) without storage (scenario A), as well as after 10 years of storage (scenario B, with significant bubble drift). Monitoring of preceding trials revealed a lack of redox reactions; little carbonate dissolution and Ca/Na exchange; much SiO₂ dissolution; a strong mobilization of natural AsO₄ ^3-, B, Ba, F, CrO₄ ^2-, Mo, Sr and V from the (sub)oxic aquifer; and immobilization of PO₄, Al, Cu, Fe and Ni from DSW. The Easy-Leacher model was applied in forward and reverse mode including lateral bubble drift, to predict water quality of the recovered water. We show that hydrogeochemical modeling of a complex ASR-system can be relatively easy and straightforward, if aquifer reactivity is low and redox reactions can be ignored. The pilot observations and modeling results demonstrate that in scenario A recovered water quality still complies with Abu Dhabi's drinking water standards (even up to 85% recovery). For scenario B, however, the recovery efficiency declines to 60% after which various drinking water standards are exceeded, especially the one for chromium.

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Managed Aquifer Recharge (MAR) is a promising method of increasing water availability in water stressed areas by subsurface infiltration and storage, to overcome periods of drought, and to stabilize or even reverse salinization of coastal aquifers. Moreover, MAR could be a key technique in making alternative water resources available, such as reuse of communal effluents for agriculture, industry and even indirect potable reuse. As exemplified by the papers in this Special Issue, consideration of water quality plays a major role in developing the full potential for MAR application, ranging from the improvement of water quality to operational issues (e.g., well clogging) or sustainability concerns (e.g., infiltration of treated waste water). With the application of MAR expanding into a wider range of conditions, from deserts to urban and coastal areas, and purposes, from large scale strategic storage of desalinated water and the reuse of waste water, the importance of these considerations are on the rise. Addressing these appropriately will contribute to a greater understanding, operational reliability and acceptance of MAR applications, and lead to a range of engineered MAR systems that help increase their effectiveness to help secure the availability of water at the desired quality for the future.

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The efficiency of heat recovery in high-temperature (>60 °C) aquifer thermal energy storage (HT-ATES) systems is limited due to the buoyancy of the injected hot water. This study investigates the potential to improve the efficiency through compensation of the density difference by increased salinity of the injected hot water for a single injection-recovery well scheme. The proposed method was tested through numerical modeling with SEAWATv4, considering seasonal HT-ATES with four consecutive injection-storage-recovery cycles. Recovery efficiencies for the consecutive cycles were investigated for six cases with three simulated scenarios: (a) regular HT-ATES, (b) HT-ATES with density difference compensation using saline water, and (c) theoretical regular HT-ATES without free thermal convection. For the reference case, in which 80 °C water was injected into a high-permeability aquifer, regular HT-ATES had an efficiency of 0.40 after four consecutive recovery cycles. The density difference compensation method resulted in an efficiency of 0.69, approximating the theoretical case (0.76). Sensitivity analysis showed that the net efficiency increase by using the density difference compensation method instead of regular HT-ATES is greater for higher aquifer hydraulic conductivity, larger temperature difference between injection water and ambient groundwater, smaller injection volume, and larger aquifer thickness. This means that density difference compensation allows the application of HT-ATES in thicker, more permeable aquifers and with larger temperatures than would be considered for regular HT-ATES systems.

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The use of multiple partially penetrating wells (MPPW) during aquifer storage and recovery (ASR) in brackish aquifers can significantly improve the recovery efficiency (RE) of unmixed injected water. The water quality changes by reactive transport processes in a field MPPW-ASR system and their impact on RE were analyzed. The oxic freshwater injected in the deepest of four wells was continuously enriched with sodium (Na⁺) and other dominant cations from the brackish groundwater due to cation exchange by repeating cycles of 'freshening'. During recovery periods, the breakthrough of Na⁺ was retarded in the deeper and central parts of the aquifer by 'salinization'. Cation exchange can therefore either increase or decrease the RE of MPPW-ASR compared to the RE based on conservative Cl^-, depending on the maximum limits set for Na⁺, the aquifer's cation exchange capacity, and the native groundwater and injected water composition. Dissolution of Fe and Mn-containing carbonates was stimulated by acidifying oxidation reactions, involving adsorbed Fe²⁺ and Mn²⁺ and pyrite in the pyrite-rich deeper aquifer sections. Fe²⁺ and Mn²⁺ remained mobile in anoxic water upon approaching the recovery proximal zone, where Fe²⁺ precipitated via MnO₂ reduction, resulting in a dominating Mn²⁺ contamination. Recovery of Mn²⁺ and Fe²⁺ was counteracted by frequent injections of oxygen-rich water via the recovering well to form Fe and Mn-precipitates and increase sorption. The MPPW-ASR strategy exposes a much larger part of the injected water to the deeper geochemical units first, which may therefore control the mobilization of undesired elements during MPPW-ASR, rather than the average geochemical composition of the target aquifer.

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Groundwater systems are increasingly used for seasonal aquifer thermal energy storage (SATES) for periodic heating and cooling of buildings. Its use is hampered in contaminated aquifers because of the potential environmental risks associated with the spreading of contaminated groundwater, but positive side effects, such as enhanced contaminant remediation, might also occur. A first reactive transport study is presented to assess the effect of SATES on the fate of chlorinated solvents by means of scenario modeling, with emphasis on the effects of transient SATES pumping and applicable kinetic degradation regime. Temperature effects on physical, chemical, and biological reactions were excluded as calculations and initial simulations showed that the small temperature range commonly involved (ΔT < 15 °C) only caused minor effects. The results show that a significant decrease of the contaminant mass and (eventually) plume volume occurs when degradation is described as sediment-limited with a constant rate in space and time, provided that dense non-aqueous phase liquid (DNAPL) is absent. However, in the presence of DNAPL dissolution, particularly when the dissolved contaminant reaches SATES wells, a considerably larger contaminant plume is created, depending on the balance between DNAPL dissolution and mass removal by degradation. Under conditions where degradation is contaminant-limited and degradation rates depend on contaminant concentrations in the aquifer, a SATES system does not result in enhanced remediation of a contaminant plume. Although field data are lacking and existing regulatory constraints do not yet permit the application of SATES in contaminated aquifers, reactive transport modeling provides a means of assessing the risks of SATES application in contaminated aquifers. The results from this study are considered to be a first step in identifying the subsurface conditions under which SATES can be applied in a safe or even beneficial manner.@en