TES (Thermal energy storage) can enhance energy systems by reducing environmental impact and increasing efficiency. Thermochemical TES is a promising new type of TES, which permits more compactness storage through greater energy storage densities. In this article, closed and open thermochemical TES is investigated using energy and exergy methods. The latter method enhances assessments of made using the former. Efficiencies based on energy and exergy are determined for the overall storage cycle and its charging, storing and discharging processes. Examples using experimental data are presented to illustrate the analyses of closed and open thermochemical TES. The overall system energy and exergy efficiencies, respectively, are determined to be 50% and 9% for the closed storage, and 69% and 23% for the open storage. The results suggest that there is a significant margin for loss reduction and efficiency improvement for closed and open thermochemical storages, since the exergy efficiencies of both are significantly lower than the energy efficiencies. (C) 2011 Elsevier Ltd. All rights reserved.