To optimize the behaviour of an ATES (aquifer thermal energy storage), to estimate its efficiency and to identify the optimal well locations, the planned installation was simulated with a FE-FV (finite element-finite volume) simulator with realistic water properties, created on the basis of the CSMP++ (Complex Systems Modelling Platform) software library. Simulation results show that storage efficiency increases with the distance between injection and production wells and decreases with increasing injection temperature. Results also support a storage design where the storage wells are placed near the walls. Groundwater flow does not affect storage efficiency significantly, proving that the double concrete walls act as a sufficient thermal insulator. The full operation cycle of an ATES with an optimal well placement was simulated: 120 days of hot water injection (charging), 60 days of production (discharging). The predicted energy and exergy efficiency are 35.6% and 27.4%, respectively. The storage supplies between similar to 300 kW and similar to 120 kW of thermal energy the first 60 days of hot water production. (C) 2014 Elsevier Ltd. All rights reserved.