Stanford Geothermal WorkshopFebruary 9-11, 2026

This study proposes a component-based model of a heating and cooling system comprised of an Aquifer Thermal Energy Storage (ATES) and a heat pump system for a commercial building, developed using Open Modelica. The heating system is divided into the following sub-models: water-to-water heat pump, ATES system, building load, chiller, domestic hot water, and control panel. The present study simulates four operating scenarios under identical loads and boundary conditions in order to evaluate the limits of both constrained and unconstrained aquifer extraction temperatures, as well as different Heat Pump (HP) to district heating energy ratios (60/40% and 90/10%) for heating supply. Across the four scenarios, the ATES supplied between 29–50% of the annual heating load and 41–57% of the annual cooling demand. However, performance proved highly sensitive to the ATES energy extraction/injection ratio. In the unconstrained 60/40% scenario, the imbalance led to an extracted-to-injected ratio of 0.74 and increases of 69% and 21% in the pumped volume per unit of heating and cooling energy, respectively, compared to the 90/10% cases. In contrast, the 90/10% scenarios achieved a balanced operation, with ratios close to 1.27–1.24 only incurring a 3% reduction in the cooling energy injected in the ATES when constraints were applied. The highest Monthly Performance Factors (MPF) were obtained in both 90/10% scenarios, achieving 4 and 55 points for heating and free cooling, considering the HP+ATES boundary level. Free cooling values in the 90/10% scenarios are 10 points higher than both 60/40% scenarios. The results also indicate that over-extracting energy in the ATES while heating is more tolerable than over-injection during cooling for long-term aquifer sustainability and system performance. System-level results further showed that mass flow modulation improved ATES-side heat exchanger effectiveness to values as high as 0.95, improving the quality of the ATES energy and reducing risks of thermal breakthrough. The model also confirmed the value of smart control strategies, and instantaneous Key Performance Indicators (KPIs), including extraction temperature thresholds and switching between free and machine cooling, reducing electricity use and pumping energy. Overall, the study demonstrates that a balanced GSHP–ATES operation with high heat pump participation can significantly reduce district heating reliance, improve efficiency, reduce the risk of thermal breakthrough, and ensure sustainable long-term operation of the ATES.

Topic: Low Temperature