Simulating surface water and groundwater flow dynamics in tile-drained catchments

Pratique agricole répandue dans les champs sujets à l’accumulation d’eau en surface, le drainage souterrain améliore la productivité des cultures et réduit les risques de stagnation d’eau. La contribution significative du drainage sur les bilans d’eau à l’échelle de bassins versants, et sur les problèmes de contamination dus à l’épandage d’engrais et de fertilisant, a régulièrement été soulignée. Les écoulements d’eau souterraine associés au drainage étant souvent inconnus, leur représentation par modélisation numérique reste un défi majeur. Avant de considérer le transport d’espèces chimiques ou de sédiments, il est essentiel de simuler correctement les écoulements d’eau souterraine en milieu drainé. Dans cette perspective, le modèle HydroGeoSphere a été appliqué à deux bassins versants agricoles drainés du Danemark. Un modèle de référence a été développé à l’échelle d’une parcelle dans le bassin versant de Lillebæk pour tester une série de concepts de drainage dans une zone drainée de 3.5 ha. Le but était de définir une méthode de modélisation adaptée aux réseaux de drainage complexes à grande échelle. Les simulations ont indiqué qu’une simplification du réseau de drainage ou que l’utilisation d’un milieu équivalent sont donc des options appropriées pour éviter les maillages hautement discrétisés. Le calage des modèles reste cependant nécessaire. Afin de simuler les variations saisonnières des écoulements de drainage, un modèle a ensuite été créé à l’échelle du bassin versant de Fensholt, couvrant 6 km2 et comprenant deux réseaux de drainage complexes. Ces derniers ont été simplifiés en gardant les drains collecteurs principaux, comme suggéré par l’étude de Lillebæk. Un calage du modèle par rapport aux débits de drainage a été réalisé : les dynamiques d’écoulement ont été correctement simulées, avec une faible erreur de volumes cumulatifs drainés par rapport aux observations. Le cas de Fensholt a permis de valider les conclusions des tests de Lillebæk, ces résultats ouvrant des perspectives de modélisation du drainage lié à des questions de transport.Tile drainage is a common agricultural management practice in plots prone to ponding issues. Drainage enhances crop productivity and reduces waterlogging risks. Studies over the last few decades have highlighted the significant contribution of subsurface drainage to catchments water balance and contamination issues related to manure or fertilizer application at the soil surface. Groundwater flow patterns associated with drainage are often unknown and their representation in numerical models, although powerful analysis tools, is still a major challenge. Before considering chemical species or sediment transport, an accurate water flow simulation is essential. The integrated fully-coupled hydrological HydroGeoSphere code was applied to two highly tile-drained agricultural catchments of Denmark (Lillebæk and Fensholt) in the present work. A first model was developed at the field scale from the Lillebæk catchment. A reference model was set and various drainage concepts and boundary conditions were tested in a 3.5 ha tile-drained area to find a suitable option in terms of model performance and computing time for larger scale modeling of complex drainage networks. Simulations suggested that a simplification of the geometry of the drainage network or using an equivalent-medium layer are suitable options for avoiding highly discretized meshes, but further model calibration is required. A catchment scale model was subsequently built in Fensholt, covering 6 km2 and including two complex drainage networks. The aim was to perform a year-round simulation accounting for variations in seasonal drainage flow. Both networks were simplified with the main collecting drains kept in the model, as suggested by the Lillebæk study. Calibration against hourly measured drainage discharge data was performed resulting in a good model performance. Drainage flow and flow dynamics were accurately simulated, with low cumulative error in drainage volume. The Fensholt case validated the Lillebæk test conclusions, allowing for further drainage modeling linked with transport issues

How informative are single well tracing experiments? : an assessment using Bayesian evidential learning

ATHEN

Assessment of short-term aquifer thermal energy storage for demand-side management perspectives : experimental and numerical developments

In the context of demand-side management and geothermal energy production, our proposal is to store thermal energy in shallow alluvial aquifers at shorter frequencies than classical seasonal aquifer thermal energy storage. We first conducted a one-week experiment in a shallow alluvial aquifer, which is characterized by a slow ambient groundwater flow, to assess its potential for thermal energy storage and recovery. This experiment has shown that up to 90% of the stored thermal energy can be recovered and would therefore suggest that aquifer thermal energy storage could be considered for demand-side management applications. We then conceptualized, developed, and calibrated a deterministic 3D groundwater flow and heat transport numerical model representing our study site, and we simulated 77 different scenarios to further assess this potential. This has allowed us to demonstrate that low-temperature aquifer thermal energy storage (temperature differences of −4 K for precooling and 3, 6, and 11 K for preheating) is efficient with energy recovery rates ranging from 78 to 87%, in a single aquifer thermal energy storage cycle. High-temperature aquifer thermal energy storage (temperature differences between 35 and 65 K) presents lower energy recovery rates, from 53 to 71%, with all other parameters remaining equals. Energy recovery rates decrease with increasing storage duration and this decrease is faster for higher temperatures. Retrieving directly useful heat (without upgrading with a groundwater heat pump) using only a single storage and recovery cycle appears to be complicated. Nevertheless, there is room for aquifer thermal energy storage optimization in space and time with regard to improving both the energy recovery rates and the recovered absolute temperatures

Using thermal recycling to optimise short-term high-temperature aquifer thermal energy storage for demand-side management applications

Coupling electrically-driven heating, ventilation, and air-conditioning (HVAC) systems with thermal energy storage (TES) in buildings is seen as a promising tool for demand-side management (DSM) in the low-voltage grid, mainly thanks to the ability to decouple electricity and heating demand. So far, TES strategies consider the thermal envelope of a building or a water tank or both as buffers for thermostatically-controlled load-shifting. Thermal energy is then stored during off-peak periods and recovered during peak periods. With this study, we further assess and optimise aquifer thermal energy storage (ATES) to improve the overall energy efficiency of an open-loop geothermal system connected to a building and a smart-grid with a groundwater heat pump (GWHP). If we only consider space heating (or domestic hot water production), two ATES strategies can be developed. The first one considers the preheating of the aquifer to improve the coefficient of performance of the overall system, which includes a GWHP. Such ATES is known as a low-temperature (LT) one with a ΔT (difference of temperature between initial and heated groundwater) mainly ranging between 3 and 11 K. The second strategy considers a direct use of the stored heated water, using only a heat exchanger for space heating. Such ATES is known as a high-temperature (HT) one and the main goal is to retrieve a minimal absolute groundwater temperature of 45°C (space heating systems operate at this temperature). To assess and optimise such ATES strategies, we first conceptualised, constructed, and calibrated a 3D groundwater flow and heat transport model in FEFLOW that represents a typical productive shallow alluvial aquifer of Wallonia, Belgium, where ambient groundwater flow is slow (~12 m/year). This model was calibrated with PEST and the pilot points method with the help of data coming from an experimental design mimicking a 72 hours ATES cycle. A previous study demonstrated that single ATES cycles (at real time, intraday, and interday frequencies) already presented suitable energy recovery rates ranging between 78 and 87 % for LT-ATES, but only sufficient rates ranging between 53 and 71 % for HT-ATES. Obviously, higher energy recovery rates correspond to shorter storage periods. In terms of exergy, it was impossible to recover an absolute temperature of 45°C with a single ATES cycle, demonstrating the need for better control strategies. With this study, we consider the joint use of thermal recycling and consecutive ATES cycles at typical DSM frequencies to improve both energy recovery rates and exergy for HT-ATES. The use of a production well close enough to the injection well favours thermal recycling and as a consequence the local increase of groundwater temperature. Thanks to an adequate design of the well doublet, taking into account the local hydrogeological conditions, and thanks to adequate control strategies, a training phase of less than 10 weeks allows for an optimal exergy since groundwater can be maintained at 45°C and energy recovery rates of at least 80 %. This study shows that considering ATES in shallow alluvial aquifers for DSM applications is feasible for both LT- and HT-ATES.ATHEN

How Informative Are Single Well Tracing Experiments? An Assessment Using Bayesian Evidential Learning

ATHEN

Bayesian Evidential Learning: a field validation using push-pull tests

Recent developments in uncertainty quantification show that a full inversion of model parameters is not always necessary to forecast the range of uncertainty of a specific prediction in Earth sciences. Instead, Bayesian evidential learning (BEL) uses a set of prior models to derive a direct relationship between data and prediction. This recent technique has been mostly demonstrated for synthetic cases. This paper demonstrates the ability of BEL to predict the posterior distribution of temperature in an alluvial aquifer during a cyclic heat tracer push-pull test. The data set corresponds to another push-pull experiment with different characteristics (amplitude, duration, number of cycles). This experiment constitutes the first demonstration of BEL on real data in a hydrogeological context. It should open the range of future applications of the framework for both scientists and practitioners.ATHEN

Field and Numerical Experiments of An Urban Aquifer Thermal Energy Storage System

ATHEN

10 years of temperature monitoring experiments using electrical resistivity tomography : what have we learned?

The electrical resistivity of the subsurface is dependent on the temperature. This makes electrical resistivity tomography a good candidate for monitoring temperature variations within the context of aquifer thermal energy storage. In this contribution, we review the advances made in the development of ERT for monitoring heat storage and heat tracing experiments during the last ten years. We highlight the common limitations related to ERT such as the need for a petrophysical relationship for a proper survey design, as well as the concerns related to noise and inversion. We also point towards the solutions available to overcome those limitations and guidelines for successful monitoring experiments. We think this contribution will help practitioners and scientists to make the appropriate choice when designing or exploiting shallow geothermal systems

Investigations into the First Operational Aquifer Thermal Energy Storage System in Wallonia (Belgium): What Can Potentially Be Expected?

In the context of energy transition, new and renovated buildings often include heating and/or air conditioning energy-saving technologies based on sustainable energy sources, such as groundwater heat pumps with aquifer thermal energy storage. A new aquifer thermal energy storage system was designed and is under construction in the city of Liège, Belgium, along the Meuse River. This system will be the very first to operate in Wallonia (southern Belgium) and should serve as a reference for future shallow geothermal developments in the region. The targeted alluvial aquifer reservoir was thoroughly characterized using geophysics, pumping tests, and dye and heat tracer tests. A 3D groundwater flow heterogeneous numerical model coupled to heat transport was then developed, automatically calibrated with the state-of-the-art pilot points method, and used for simulating and assessing the future system efficiency. A transient simulation was run over a 25 year-period. The potential thermal impact on the aquifer, based on thermal needs from the future building, was simulated at its full capacity in continuous mode and quantified. While the results show some thermal feedback within the wells of the aquifer thermal energy storage system and heat loss to the aquifer, the thermal affected zone in the aquifer extends up to 980 m downstream of the building and the system efficiency seems suitable for long-term thermal energy production.ATHEN