The role of thermal energy storage (TES) in district energy (DE) system is assessed. The Friedrichshafen DE system is considered as a case study and exergy analysis is utilized. The TES is designed to complement and to increase the effectiveness of the solar panels included in the district energy system. The TES stores the surplus solar energy until is needed by thermal energy users of the Friedrichshafen DE system. The results quantify the positive impact of the TES on the performance of the Friedrichshafen DE system, and demonstrate that the overall energy and exergy efficiencies of the TES are 60% and 19%, respectively. It is also shown over an annual period that the temperature, energy, exergy and energy efficiency of the TES exhibit similar trends and that the TES exergy accumulation and exergy efficiency exhibit similar trends. Crown Copyright (C) 2014 Published by Elsevier Ltd. All rights reserved.