A long-term thermal energy storage system has been implemented at the University of Utrecht campus in the Netherlands. The project will extend co-generation operation into the summer beyond heat demand periods, resulting in increased electricity generation. The stored heat (90 °C) is used in winter for space heating of several university buildings. In this demonstration project experience is being gained, among others, with groundwater treatment and flow-rate control in an aquifer thermal energy plant.The storage system has been in full operation since May 13, 1991. In this paper a short review is presented of the technical problems that were met during the design stage and one year of operation.The general conclusion is that during the first operational period too much water had been treated so that there was a risk of clay-swelling. This problem was caused by a technical failure of the spit-control for water treatment. Since the control system has been altered the high temperature ATES has been functioning perfectly.