In this study we deal with the methods and applications of describing, assessing and using thermal energy Storage systems. as A ell as economical. energy conservation and environmental aspects of such systems. The energetic and environmental impacts of thermal energy storage (TES) systems are discussed and highlighted with a number of illustrative examples. The main emphasis is laid on sensible TES, since it is internationally accepted as the most economical and practical energy storage technique. An energy and exergy modelling is presented for TES systems as a key component in the above-mentioned aspects. Illustrative examples are also given to practically demonstrate how exergy analysis provides a more realistic and meaningful assessment than the conventional energy analysis of the efficiency and performance of a sensible heat storage system. It is believed that the results will be useful to engineers and designers seeking to improve and optimize TES systems. Copyright (C), 2002 John Wiley Sons, Ltd.