ERC Geothermica Heatstore
Prof. Martin O. Saar, Zurich
The 2050 Swiss Energy Strategy aims to reduce carbon emissions by embracing renewable energy resources. In Switzerland and worldwide, the heating and cooling sector uses roughly half of the total energy consumed, which is generated primarily by fossil fuels (Quiquerez et al., 2017; Fleuchaus et al., 2018). Aquifer thermal energy storage (ATES) allows low-carbon and/or low-cost heat to be stored until the demand for heat rises, usually in the winter. ATES stores waste heat (e.g. from a power plant, industrial process, or incineration of domestic waste) by injecting hot water into aquifers, and extracting the heat at a later time. ATES is typically operated at moderate temperatures (<25 °C) in thick, unconsolidated aquifers (Fleuchaus et al., 2018). ATES has been shown to be technically and economically successful, and it is becoming widespread in countries such as the Netherlands, which have favorable geologic and legal frameworks (Hartog et al., 2013; Bloemendal et al., 2014; Fleuchaus et al., 2018).
There is interest in expanding ATES to higher temperatures and different reservoir types, like those found in Switzerland. As temperature and pressure increase in ATES systems, the potential for thermo- and poroelastic deformation also increases. We study thermo-hydro-mechanical-economic (THM$) effects in high temperature (HT) (>50 °C) ATES systems (Fig. 1). We hope to understand, mitigate, and avoid ground surface deformation, wellbore integrity problems, hydraulic fracturing, and induced seismicity, which is important due to the proximity that ATES typically has to cities and infrastructure. We also suggest practical engineering guidelines (e.g., optimal well spacing and flow rate) and tools for HT-ATES pre-assessment (i.e., the minimum economically-viable transmissivity).
Figure 1: Schematic of THM processes in a HT-ATES system. The temperature and pressure changes lead to mechanical deformations, such as ground uplift.
THM Numerical Model
Our work involves the use of coupled THM numerical models. We use the MOOSE framework to solve the thermo-poro-elastic equations (Gaston et al., 2015; Alger et al., 2019). It is a generalized, parallelized finite element software that facilitates the coupling between different physics. An illustrative example of the numerical model is shown in Figure 2. We collaborate with partners across Europe as part of Geothermcia HEATSTORE. At the national level, we work closely with other universities and industry partners in the Swiss HEATSTORE Consortium. Our models consider input from the energy systems scenario modelers and from the geological understanding at the pilot projects in Geneva and Bern. Likewise, our results inform the experimental design of lab and field work done for these sites.
Figure 2: Numerical model results. The 3D mesh (a) uses localized refinement near the wells. The pore pressure (b) and temperature (c) affect the deformation (d).
THM$ Analytical Model
We combine reservoir-engineering with economic calculations in our THM$ approach (Birdsell et al., 2021). We balance three constraints: (a) the size and thermal capacity of the reservoir, (b) avoidance of hydraulic fracturing, and (c) minimization of the levelized cost of heat (LCOH). This provides practical insights on the optimal well spacing, flow rate, and depth for HT-ATES wells. Perhaps more importantly, it gives the minimum economically-viable transmissivity (MEVT), which is the value of reservoir transmissivity that will surely lead to economic infeasability. The MEVT is 5*10-13 m3 and is useful for reservoir pre-assessment at a local or global scale. The THM$ approach is available on Github at: https://github.com/danielbi-ETHZ/THM-Econ.
Figure 3: Levelized cost of heat contours as a function of depth and transmissivity. The MEVT is shown by the dashed line.
REFEREED PUBLICATIONS IN JOURNALS
Alger B., Andrš D., Carlsen R. W., Gaston D.R., Kong F., Lindsay A. D., Miller J. M., Permann C. J., Peterson J. W., Slaughter A. E., and Stogner R.: MOOSE Web Page. https://mooseframework.org. (2019).
Bloemendal, M., Olsthoorn, T., & Boons, F.: How to achieve optimal and sustainable use of the subsurface for Aquifer Thermal Energy Storage. Energy Policy, 66, (2014), 104-114.
Fleuchaus, P., Godschalk, B., Stober, I., and Blum, P.: Worldwide application of aquifer thermal energy storage–A review. Renewable and Sustainable Energy Reviews, 94, (2018), 861-876.
Gaston, D. R., Permann, C. J., Peterson, J. W., Slaughter, A. E., Andrš, D., Wang, Y., … and Zou, L.: Physics-based multiscale coupling for full core nuclear reactor simulation. Annals of Nuclear Energy, 84, (2015), 45-54.
Hartog, N., Drijver, B., Dinkla, I., & Bonte, M.: Field assessment of the impacts of Aquifer Thermal Energy Storage (ATES) systems on chemical and microbial groundwater composition. Proceedings, European Geothermal Conference, Pisa, Italy (2013).
Quiquerez, L., Lachal, B., Monnard, M., & Faessler, J. (2017). The role of district heating in achieving sustainable cities: Comparative analysis of different heat scenarios for Geneva. Energy Procedia, 116, 78–90. https://doi.org/10.1016/j.egypro.2017.05.057