One of the most important actions to limit climate change is to decrease worldwide CO₂ emissions. A large contributor to worldwide CO₂ emissions is the production of heat. Therefore, the recently started transition from fossil based fuels to renewable heat sources is of great importance. Renewable heat sources like geothermal and solar energy often exhibit a temporal mismatch between the availability and demand of heat. Excess heat is available in summer while the heat demand cannot be fulfilled in winter. A solution for this problem is to use heat storage facilities that are able to bridge the gap between winter and summer. Given the needed storage capacity for these systems, high temperature aquifer thermal energy storage (HT-ATES) is one of the best options to do so. At the TU Delft, a combined geothermal well with a HT-ATES installation is currently being prepared. The system is designed to provide the district heating network of the university and possibly a part of the city with renewable heat, and, is set up as a research facility to foster future research.

The performance and feasibility of HT-ATES systems is affected by many factors as previous research showed. Understanding which factors are important, and how these factors impact the project feasibility, would provide a solid basis for future HT-ATES feasibility studies and foster the future use of HT-ATES systems, ultimately resulting in are more rapid reduction of CO₂ emissions. However, while low temperature ATES systems are regarded a mature technique, only limited experience is available with HT-ATES. Higher storage temperature and larger storage capacity cause technical challenges and variable performance, resulting in an uncertain business case.

Therefore, we determined the most important conditions that influence the feasibility of HT-ATES and performed a feasibility study for the TU Delft HT-ATES project. Our study shows that the integration of HT-ATES together with a geothermal well on the TU Delft campus is feasible, both technically and financially. Most importantly, the use of HT-ATES leads to twice as much CO₂ savings compared to the stand alone geothermal well.

At this moment, the project is in the next, more detailed phase, of the feasibility study to optimize the HT-ATES design and decrease project uncertainty. The feasibility of the project is strongly linked to the performance of the HT-ATES system, which is unclear because of uncertainties regarding the characteristics of the subsurface. Therefore, we are currently working on subsurface characterization by means of drilling, sampling and logging activities and aim to determine which layer(s) are most suitable for placement of well screens for the and determine appropriate, generic, methods for subsurface characterization for HT-ATES systems. In this presentation we will discuss the learning outcomes of the feasibility study for future studies and present our current effort in developing the HT-ATES project.

Agriport A7 is a large-scale greenhouse area in Middenmeer in the Netherlands. The local energy company ECW provides geothermal heat (92ºC, from 2 km depth) to the greenhouses through a heating network. The geothermal systems have significant overcapacity in the summer period while in winter they can provide only ~25% of the heat demand, resulting in a strong dependence on fossil fuels. ECW has built a full-scale High Temperature Aquifer Thermal Energy Storage (HT-ATES) system, which facilitates the large-scale storage of surplus heat (overcapacity in summer) and its recovery in winter time. HT-ATES improves the yearly net heat production of geothermal systems hence reduces GHG emissions.

The full-scale HT-ATES doublet well system allows the storage of heat in an unconsolidated sand aquifer at nearly 400 m depth, with a maximum flow rate of 150 m³/h. Each summer, up to 28,000 MWh of thermal energy (>100.000 GJ) can be stored, the bulk of which is recovered in winter.

The HT-ATES system is an innovation that takes place at the edge of technology. The depth (360-380 mbgs) and the temperature of 85 degrees Celsius offered technical and legal challenges that had to be overcome. On a technical level, the standardized well design for ‘regular’ ATES systems (< 25 ^oC, <200 mbgs) needed to be reconsidered entirely. Components of both the well and the surface installations must withstand the combination of high temperatures, saline groundwater and high pressures. Knowledge and experience from both the ATES and Geothermal sector were combined to get to a suitable design.

The reliability of the HT-ATES system finds root in the knowledge and experience available from thousands of lower temperature ATES systems that have been successfully built and operated in the Netherlands over the last decades. Risks identified in former small-scale HT-ATES pilot-projects were investigated within the HEATSTORE context, and the results contribute to the quality of the full-scale HT-ATES system. The test drilling performed in 2019 has offered a detailed image of the subsurface properties and the risks associated with it. A highly detailed system was designed, and successfully installed. Clogging risks are tackled by a special CO2 dosing unit and groundwater will be monitored on chemical and microbial changes.

In the second quarter of 2021, the installation of the HT-ATES has been completed and the system has been taken into operation. During the test period, samples were taken from the groundwater and analysed on the chemical composition and microbiological content. Also temperature profiles were made during the injection of heat into the aquifer. The first results are very promising and gives valuable information about the effects of an HT-ATES on the aquifer and environment.

The subsurface conditions of the Upper Rhine Graben are favorable for the development of novel geothermal utilization concepts. In particular, they allow optimization of energy use with flexible heat production and storage scenarios. A first potential analysis revealed an enormous storage potential of formerly used and well-explored oil fields. The involvement of former hydrocarbon reservoirs as components of geothermal concepts perfectly symbolizes the transition from the fossil-fuel age to the use of carbon-neutral renewable energies.

The proposed DeepStor concept takes advantage of these preconditions. The comprehensive geothermal concept is tailored to the Campus North of the Karlsruhe Institute of Technology (KIT) that is located in the central-eastern Upper Rhine Graben. It includes multi-level utilization with heat recovery from the deep Mesozoic reservoirs (associated GeoHeat project) and seasonal high-temperature heat storage in the Tertiary Sandstones above (DeepStor project). The KIT Campus North Campus offers good prerequisites for the concept implementation with extraction, seasonal storage and distribution of heat from deep geothermal energy: The underground of the campus is characterized by the largest known heat anomaly in Germany, with temperatures exceeding 100 °C at a depth of 2 km. An existing area-wide local heating network allows for heat distribution. In the long term, the concept provides for the coverage of a significant part of the basic heat load of the KIT Campus North in a climate-neutral way.

The scientific DeepStor storage project represents the first stage in the step-by-step development of deep geothermal energy utilization at the KIT Campus North. The targeted reservoirs involve the same Tertiary strata from which hydrocarbons have been extracted until the 1990s. Initially, the high-temperature thermal storage reservoir will be fed from cogeneration as well as current renewable waste heat from scientific infrastructures such as the biomass pilot facility "bioliq". The overarching scientific goal of the first DeepStor phase is the establishment of a scientific demonstrator to validate the technical feasibility of high-temperature heat storage in the deep underground. In the associated GECKO project, a transdisciplinary approach with natural and social sciences is pursued to develop concepts for deep geothermal energy usage on KIT Campus North in a co-design process with the local population.

Geothermal energy extraction through closed systems is a secure approach responding to the global heating demand without contaminating subsurface water and causing seismic events. However, the generated power of conventional closed systems is much lower than those of open systems. Therefore, this study is dedicated to planning a novel closed system, which can produce a significant amount of thermal power. For this purpose, the performance of a single closed-loop deep system with a lengthy horizontal extension is preliminarily assessed. Based on the achieved results, it is feasible to produce roughly 3 MW thermal power while operating with thermosiphon flow. It is a big step forward in designing a new type of closed geothermal system that operates without pumping power and produces a considerable amount of thermal power, comparable to the power generation of open systems. Nevertheless, the low ratio of generated power to the total length of the wellbores and long payback period are big barriers to the spread of this system. Therefore, in the next step of this research project, enhancing the lateral heat exchange area by designing multilateral closed deep systems is proposed to increase this ratio. It is demonstrated that operation with multilateral systems can remarkably improve the performance of the system. Hence, working with multilateral systems is more reasonable than operating with several single systems to generate the same amount of power. However, it requires an extensive sensitivity analysis for different numbers of lateral wellbores and flow rates to identify the best operation scenarios. Additionally, some criteria are set as functions of extraction temperature, produced power, and relative drilling expenses to define successful cases. The interpretation of the results revealed that a successful project requires a specific relation between local vertical and horizontal flow rates. Finally, it is found that the long-term performance of a multilateral system can be predicted as a function of its short-term behavior.

Climate change requires immediate action, and for sustainable development, and uninterrupted energy supply is necessary. Since anthropogenic emission of CO2 in the atmosphere has a major role in climate change, carbon negative energy solutions are the necessity of the time. Geothermal energy is one such renewable source that can assist in achieving an economic solution to low carbon energy. Engineered geothermal systems or enhanced geothermal systems (EGS) are more suitable from an industrial perspective and can supply uninterrupted energy supply for a long duration. In conventional EGS systems, water is the heat transfer fluid. However, the use of supercritical CO₂ as the heat-carrying fluid has significant advantages over water including less chemical reactivity, low fluid viscosity, and comparatively higher thermal conductivity for shallow systems. Fluid loss is the major issue in any EGS operation. However, CO₂ loss during the EGS operation could lead to carbon geosequestration, and therefore a carbon-negative energy solution is possible when using CO₂ in EGS operations. A case study of Soultz-sous-Forêts geothermal site is considered in this work to investigate the feasibility of CO₂ usage as the heat-carrying medium. Soultz-sous-Forêts is present in the Upper Rhine Graben, France. Geologically Soultz-sous-Forêts geothermal site comprises three layers: 1.5 km of thick quarternary and tertiary sediments, 350 m thick Buntsandstein and the basement is granite. Presently, three wells (GPK-3, GPK-4: injection wells, and GPK-2: production well) are operating at this site up to a depth of approximately 5 km. In this work, a three-dimensional Soultz-sous-Forêts site is considered with five major faults. In the present model, supercritical CO₂ is injected through GPK3 and GPK4 and produced using GPK-2. This work investigates the coupled hydro-thermal processes occurring in the fractures and the rock matrix. The local thermal non-equilibrium (LTNE) approach is considered to account for the heat exchange between the rock matrix and supercritical CO₂ flowing through the faults. Recent studies have reported fluid loss along the wellbore casing in all three wells. Therefore, a wellbore leakage model is also coupled along these well trajectories and its impact on final production temperature is assessed. Results obtained from different injection rate strategy at different injection temperature indicates that even 100 years of geothermal energy extraction operation will not have much impact on the production well temperature and therefore, a sustainable energy supply is feasible at the Soultz-sous-Forêts site.