Claire Paulus’s scientific contributions

Since salt cannot always be used as a geophysical tracer (because it may pollute the aquifer with the mass that is necessary to induce a geophysical contrast), and since in many contaminated aquifer salts (e.g., chloride) already constitute the main contaminants, another geophysical tracer is needed to force a contrast in the subsurface that can be detected from surface geophysical measurements. In this context, we used heat as a proxy to image and monitor groundwater flow and solute transport in a shallow alluvial aquifer (< 10 m deep) with the help of electrical resistivity tomography (ERT). The goal of our study is to demonstrate the feasibility of such methodology in the context of the validation of the efficiency of a hydraulic barrier that confines a chloride contamination to its source. To do so, we combined a heat tracer push/pull test with time-lapse 3D ERT and classical hydrogeological measurements in wells and piezometers. Our results show that heat can be an excellent salt substitution tracer for geophysical monitoring studies, both qualitatively and semi-quantitatively. Our methodology, based on 3D surface ERT, allows to visually prove that a hydraulic barrier works efficiently and could be used as an assessment of such installations.

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.

In the last decades, aquifer thermal energy storage (ATES) has been proven to be a reliable renewable energy source. Yet, most of the ATES running systems are designed for seasonal or monthly storage and recovery applications. In the context of demand-side management, we have investigated the ability of such systems to perform short-term thermostatically-controlled load-shifting (storing thermal energy during off-peak periods and recovering it during peak periods) directly in aquifers at real-time, intraday, and interday frequencies. In this study, we mainly focused on the assessment of energy recovery rates for single low- and high-temperature ATES cycles at these typical frequencies. An aquifer thermal energy storage and recovery experiment was first set up and performed in a shallow alluvial aquifer in Wallonia, Belgium, and monitored through hydrogeological measurements and 4D electrical resistivity tomography. The investigated site is typical of Walloon alluvial aquifers, as being productive and presenting slow ambient groundwater flow (12 m/year). Moreover, such alluvial aquifers are the main target for open-loop geothermal systems in Wallonia, Belgium. With this experiment, we have shown that short-term ATES (here, after a 72 hours storage phase) should be considered for DSM applications since up to 90 % of the stored thermal energy could be recovered. A three-dimensional groundwater flow numerical model was then conceptualised and calibrated under variably saturated conditions, with coupled heat transport processes, in FEFLOW. With this predictive model, 77 simulations of single low- and high-temperature ATES cycles were performed to assess the applicability of short-term DSM applications at low or high temperatures. Simulated low-temperature ATES (-4 < T < 11 K) has 78 to 87 % energy recovery rates, while high-temperature ATES (T > 35 K) has lower energy recovery rates (53 to 71 %) (T being the difference in temperature between initial and injected water). Energy recovery rates decrease with increasing storage duration, this decrease being faster for high-temperature compared to low-temperature ATES. Recovering the absolute injected temperature is barely feasible with a single cycle since exergy lowers quickly. However, the thermal equilibrium and exchange processes between groundwater and the porous medium matrix run over multiple cycles is an optimisation lead. Our study shows that preheating (or precooling) the alluvial aquifer could significantly increase the coefficient of performance of ATES systems, allowing for example to store heat during off-peak periods and to recover it during peak periods. The direct use of heated water without the need of a groundwater heat pump (high exergy) is not possible with a single ATES cycle and further developments are needed to optimise the system at higher temperatures.

Appropriate design of geophysical experiments combined with common hydrological measurements offer opportunities to use geophysical data as hard data in hydrological models, regarding their conceptualisation or their calibration. Two study sites located in Wallonia, Belgium, were investigated. In the first case (fractured limestone aquifer), streaming potentials were linked to piezometric measurements, allowing us to better conceptualise the local groundwater flow model and calibrate it. In the second example (alluvial sandy aquifer), the use of 4D electrical resistivity tomography and temperature measurements appeared to be a reliable methodology to predict heat storage and recovery cycles in hydrological models with a better constrained calibration process.

Dans les contextes de la flexibilité de la demande électrique (EDSM) et du stockage de chaleur en aquifère, le dimensionnement et le fonctionnement de systèmes géothermiques ouverts (ATES) sont étroitement liés à la géologie du site, hétérogène et complexe par définition. Afin de pouvoir développer des outils de modélisation prédictive fiable, nous avons développé une méthodologie de collecte de données spatialisées et à haute résolution temporelle de la subsurface combinant de la tomographie de résistivité électrique (ERT) en 4D, des mesures hydrogéologiques et une expérience de stockage de chaleur à court terme dans un aquifère confiné de plaine alluviale. Les résultats démontrent la capacité de l'ERT 4D à délimiter la distribution du panache thermique (volume et estimation des températures) lors des phases d'injection, de stockage et de récupération de la chaleur dans l'aquifère. L'expérience montre que minimum 70% de l'énergie stockée pouvait être récupérée à condition de disposer de stratégies de récupération adéquate, ce qui apparaît intéressant dans le contexte de la flexibilité de la demande électrique. ABSTRACT. In the context of energy demand side management (EDSM) and heat storage in aquifers, the design and functioning of aquifer thermal energy storage (ATES) systems have strong interconnections with the geology of the site which may be complex and heterogeneous, making predictions difficult. In this context, we developed a subsurface monitoring methodology to better characterize heat transfer in the subsurface with the help of 4D electrical resistivity tomography (ERT), multiple hydrological measurements in wells, and a short-term heat storage experiment conducted for the purpose in a confined alluvial aquifer. Results clearly show the ability of ERT to delimit the thermal plume growth during injection, the diffusion and decrease of temperature during storage, and the decrease in size after heat recovery. Energy balance shows that up to 70% of the energy can be easily recovered with an adapted strategy in the context of DSM. Short-term heat storage in alluvial aquifer is efficient and ERT is a valuable tool to image and estimate the temperature distribution in the subsurface. MOTS-CLÉS : géothermie, hydrothermie, stockage de chaleur, flexibilité de la demande électrique, suivi temporel, géophysique

In the context of energy demand side management (DSM), energy storage solutions are needed to stock energy during high production periods and recover energy during high demand periods. Among currently studied solutions, storing energy in the subsurface through heat pumps and/or exchangers (thermal energy storage) is relatively simple with low investment costs. However, the design and functioning of such systems have strong interconnections with the geology of the site which may be complex and heterogeneous, making predictions difficult. In this context, local temperature measurements are necessary but not sufficient to model heat flow and transport in the subsurface. Electrical resistivity tomography (ERT) provides spatially distributed information on the temperature distribution in the subsurface. In this study, we monitored, with 4D ERT combined with multiple hydrological measurements in available wells, a short-term heat storage experiment in a confined alluvial aquifer. We injected heated water (ΔT=30K) during 6 hours with a rate of 3 m³/h, stored during 3 days, and then we pumped it back to estimate the energy balance. We collected ERT data sets using 9 parallel profiles of 21 electrodes and cross-lines measurements. Inversion results clearly show the ability of ERT to delimit the thermal plume growth during injection, the diffusion and decrease of temperature during storage, and the decrease in size after pumping. Quantitative interpretation of ERT is difficult at this stage due to strong spatial variations of the total dissolved solid content in the aquifer, due to historical chloride contamination of the site. Energy balance shows that up to 75% of the energy can be easily recovered with an adapted strategy in the context of DSM. Short-term heat storage in alluvial aquifer is efficient and ERT is a valuable tool to image and estimate the temperature distribution in the subsurface.