Thermal Energy Storage (TES) is an important technology for achieving more efficient and environmentally acceptable energy systems. Where space is limited, thermochemical TES has the highest potential to achieve compact thermal energy storage. Here, a general closed thermochemical TES is investigated using energy and exergy analyses. An illustrative example is presented, and the energy and exergy efficiencies are determined for the overall TES system to be 50% and 9%, respectively. These results indicate that the efficiency of the closed thermochemical TES based on exergy is much lower than that based on energy, and that there is a significant margin for loss reduction and efficiency improvement.