Impact of operating temperatures on the thermal balance of low-temperature ATES coupled with a water-to-water heat pump

Aquifer Thermal Energy Storage (ATES) systems provide a sustainable solution for seasonal heating and cooling by utilizing subsurface aquifers as thermal energy buffers. During winter, warm groundwater is extracted from the aquifer and its heat is transferred via a heat exchanger to supply space heating; the cooled water is then reinjected into a cooler portion of the aquifer. Conversely, in summer, the colder aquifer water can absorb excess heat from buildings, serving as an efficient cooling source. This cyclical operation enables seasonal storage and reuse of thermal energy; however, discrepancies between heating and cooling demands can result in thermal imbalances in the aquifer. This study investigates the integration of an ATES system coupled with a water-to-water heat pump (WWHP) serving a small-scale energy district in Naples, Italy. The district comprises eight buildings with residential, commercial, and educational uses, connected to a low-temperature district heating and cooling network. Dynamic simulations were conducted in TRNSYS 18, using a 1.5-minute time step. Three well-temperature configurations were analysed: C#1 (26/10 °C), C#2 (27/9 °C), and C#3 (28/8 °C) to evaluate the effect of the temperature lift on seasonal WWHP performance. Results show that the annual COP remains nearly constant (3.99) across all scenarios, whereas summer EER decreases progressively with increasing well temperature lift (from 5.16 to 4.97), confirming the sensitivity of summer performance to source temperature. The 26/10 °C configuration minimizes electric energy demand (29.32 MWh/year), primary energy use (69.5 MWh/year), CO2 emissions (10.05 tCO2/year), and operating costs. However, ATES analysis reveals a significant seasonal imbalance between injected and extracted heat (68.3 vs. 39.5 MWh/year), attributed to the predominance of heating load and limited cooling demand. These findings indicate that, while the lowest temperature lift scenario is energetically advantageous, sustainable operation requires optimized control strategies and consideration of dynamic aquifer behaviour to prevent long-term thermal inefficiencies.